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Drainage_and_Flooding_Report-dated_10_03_2008
k I o t z 0 1674, 0associates 1160 Dairy Ashford, Suite 500 Houston, Texas 77079 T 281.589.7257 F 281.589.7309 houston. off ice@klotz. corn 31.r. Ron Bottoms City Manager City of La Porte 604 W. Fairmont Parkway LaPorte, Texas 77571 RE: City of LaPorte, Texas City Wide Drainage Study Draft Study Letter Report #3 Klotz Associates Project No. 0127.008.000 Klotz Associates, Inc. was authorized to proceed with the City Wide Drainage Study (CWDS) for the City of La Porte (City) on January 28, 2008. Letter Report No. 1 and Letter Report No. 2 for this study have been previously submitted and accepted by the City. Klotz Associates is submitting this Letter Report No. 3 to present the results of tasks 4.1 to 4.4 of our Scope of Work. 1. Purpose The purpose of tasks 4.1 to 4.4 of the CWDS is to 1) define standards and criteria for the CWDS, 2) prioritize drainage and flood control problems, �) identify conceptual solutions to high priority problems, and 4) identify funding sources for drainage projects. Discussions about criteria and standards as well as recommendations made in this report are for new development and areas of redevelopment. The City provides road sided ditches and culverts and stor -in sewer design criteria for new construction perforined within the City's jurisdiction in Chapter 5, Storm Sewer Design Criteria (see Appendix A), of its Code of Ordinance. These storm drainage criteria cover the design of new drainage facilities such as storm water drainage systems and storm water detention ponds, along with a summary of the accepted method of calculating peak flow and detention volume. The City currently requires that construction of new roadside ditches and storm sewer facilities convey the 3-year storm. For drainage areas greater than 100 Mr. Ron Bottoms, City Manager October 1, 2008 Page 2 of 31 klutz associates The City currently requires that construction of new roadside ditches and storm sewer facilities convey the 3-year storm. For drainage areas greater than 100 acres, ditches and culverts are sized for the 25-year event. Bridges and major drainage conveyances, (e.g., channels and creeks) are required to provide conveyance for the 1 00-year event. However, it is noted that significant portions of the City were developed prior to the adoption of the drainage criteria in 1987. The City's current criteria also specify that all drainage design should be performed using the Rational Method. The time of concentration and rainfall intensity are determined using the City's data and National Weather Service Technical Paper 40 and Hydro-35 data. These design guidelines cover the City's minimum needs for storm water drainage that is to be designed to prevent flooding of structures during smaller storins. In addition to having storm drainage requirements, La Porte also addresses the need to control the downstream effects of any changes to the drainage pattern in an area through the use of storm water detention ponds. In general, the City's guidelines call for detention ponds on all systems which outfall into Harris County Flood Control District (HCFCD) channels F212, F216, B 106, B 109, B 112 and A 104. Storage volumes for these facilities are determined using one of two methods depending upon drainage area size: predetermined storage factors or HCFCD criteria. For smaller drainage areas, the City's drainage criteria specify that 0.20 ac-ft/ac and 0.45 ac-ft/ac of detention storage be provided for drainage areas from 0 to 3 acres and from 3 to 10 acres, respectively. Detention for any area greater than 10 acres but less than 100 acres is to be designed using HCFCD criteria. Larger areas required specific review by the City Engineer. Outfall pipes are designed using an orifice equation with the minimum restrictor size being six inches. La Porte's design guidelines also list specific limitations on the location of storm water drainage structures in order to assure that the storm water is cleared from the surface efficiently for street systems with curb and gutters. For curb and gutter systems, the guidelines call for a maximum storm water travel distance of 300 feet along major thoroughfares and in commercial districts and 400 feet in single family residential areas. It is to be recognized that inlet spacing and capacity must be designed in conjunction with sufficient storm sewer capacity for the sewer to which the inlet drains; if the sewer capacity is misufficient, then proper drainage will not occur, irrespective of the inlet capacity and spacing. The City's design manual also requires that a storm sewer be designed to satisfy specific limits for the design storm. The maximum depth of flow cannot exceed the smaller of the top of curb or road crown. For collector streets, maximum storm water depths must be such as to allow one clear lane of traffic (i.e., a 12 foot wide zone), while the clear zone must be at least 24 feet wide on arterial Mr. Ron Bottoms, City Manager October 1, 2008 Page 3 of 31 19NININ0131402.1 fig k I o t z associates ! 1! 13 1 11111rivirmll iliiirimlt mi� miretrill 1� 101111MIJ habitable structure must provide a limshed tloor slab which is at least 1) 12 inches ,• the top •i curb at its highest •• along the • the curb • the building site; 2) 18 inches above the crown of the road elevation along the road • the building site; and 3) 12 inches .•• the ground • • all sides of the building site. Provision is provided for alternative methods of protection with approval of the City Engineer. A review of the design criteria for the City of La Porte and surrounding areas, including Harris County, Pearland, Galveston County, Chambers County, and Brazoria, County, found a mixture of municipal • water or • sewer design criteria. Both the City of Houston and the City of La Porte have standardized stonn sewer design requirements. The nearby cities of Deer Park and Baytown do not have specific standards that are to be used by the public or engineering • for storm water • Other cities in the region, such as Sugar Land, Pearland, Friendswood, and Missouri City, have • sewer design requirements. Some cities, such as Mont Belvieu, utilize county standards as surrogates for city -adopted standards. HCFCD has design guidelines available on its website. RUNNFNFITIVW��, �� Chapter •4 • the City's Code of Ordinances addresses flood hazard and potential construction in •- flood hazard areas. Standards applicable to new construction or substantial improvements in special flood hazard zones require a variety of measures for sound construction of structures and installation of utilities to eliminate or reduce damages from floods. These standards also include the following requirements: • Lowest floor elevation • new residential structures must •- I -foot •r 01 •. flood (100-year) elevation • Lowest floor elevation • new non-residential structures must • at or abo the base flood elevation (or equivalent structure protection must be provided] Special requirements also apply for manufactured homes to prevent floatation, collapse, or lateral movement. These .r• requirements for construction in •` flood hazard areas are consistent with many communities in the Houston •#• area. Mr. Ron Bottoms, City Manager October 1, 2008 Page 4 of 31 o t z associates Encroachment into floodways cannot occur unless it can be certified by a professional engineer that such encroachment shall not result in increase in base flood levels. The following are criteria and standards considered particular pertinent to the CWDS; potential changes or modifications are recommended to the City for inclusion into its storm drainage design criteria and standards for new development and redevelopment. • The design frequency for storm sewer design, drainage ditch, and drainage channels • Sheet flow provision • Construction in special flood hazard areas • Bridge clearances • Capacity of storm sewer systems • Capacity of open ditch systems • Allowable depths of ponding in streets and adjacent properties • Amount of minimal detention • Regional detention as alternative mitigation • Types of mitigation • Encroachment certification These issues are addressed in the following subsections; specific recommendations are made when appropriate. Recommendations are made for the purpose of application to new development or redevelopment; application to existing developments and facilities is not intended. Criteria or standards which might be considered for existing development requires case -by -case evaluation and cannot typically be imposed, required, or achieved once development has already occurred. The City criteria currently require that storm sewer facilities convey the 3-year storm. Ditches and culvert and other crossing structures for drainage areas more than 100 acres are to be designed for the 25-year event. These levels of storm sewer design capacity are consistent or even slightly more stringent than some other communities, including the City of Houston. However, since the City does experience continuing flooding and drainage problems with small storm events, consideration should be given to the possibility of generally using or using in selected situations a 5-year frequency storm event as the basis for sewer design. Mr. Ron Bottoms, City Manager October 1, 2008 Page 5 of 31 k 1 0 t Z a s s o c i a t e s Use of a 5-year frequency for sewer design would lessen the likelihood of street flooding for small storm events; however, sewer system development or i■ costs would ■` increased. In application of the frequency standards (whether they be a 3-year or a 5-year frequency), caution should be exercised in selection of rainfall intensity -duration - frequency parameters. Different entities have different parameters describing rainfall characteristics, and some data does not currently recognized information developed from the Tropical Storm Allison Recovery Project (TSARP). The parameters for describing rainfall for Harris County, City of Houston, La Porte, Harris County Flood Control District (HFCD), and the Texas Department of Transportation (TxDOT) are compared in Appendix B. The most significant difference in these data are the rainfall intensity duration data for TxDOT as compared to that for Harris County and the City of Houston. As part of storm sewer design, tailwater levels need to be recognized. The City criteria uses a 25-year frequency level for the outfall channel. For non -tidally influenced outfalls, this level is considered to be adequate unless the 25-year frequency positions the hydraulic grade line (HGL) at the outfall to be less than the top of outfall pipe, in which case the calculated hydraulic grade line should be set at the top of the pipe. Minor losses should be included in the sizing of and HGL calculations for existing and proposed storm sewer systems. For tidally influence outtalls, recogruition should be aiven to the effects of tUe—sori sewer design for those sewer facilities discharging directly to tidal -influenced waters (i.e., directly to Galveston Bay or major drainage ditches which are signcantly affected by tidal levels). For outfalls subject to tidal variation, it is recommended that the outfall tailwater used for sewer design purposes be set at the higher of 1) 25-year frequency level in the receiving water (i.e., the level based upon non -tidal conditions), or 2) the mean high tide (also referred to as the mean high water). Based upon tidal variation at Morgans Point on Barbours Cul (based upon data from National Ocean Service, National Oceanic and Atmosy,heric Administration, the mean high tide is wild'! feet above mean sea level. 2.4.2 Provision of Sheet Flow Paths The City drainage criteria do not specifically address sheet flow paths for drainage relief of accumulated waters which cannot be readily drained by a combination of sewers systems with curbed streets or roadside ditches. Sheet flow paths allow for drainage via overland flow along a directed, controlled [#,,athway which does not adversely affect residential or commercial structures. These pathways are dedicated for this purpose using easements or rights -of -way. Mr. Ron Bottoms, City Manager October 1, 2008 Page 6 of 31 k I o t z associates �4 I Consideration should be given by the City to requiring that for new development or re dr ina� ,qe relief of excess 3ondin_4 during extreme storm events be provided if significant localized ponding (as determined by engineering evaluation) is to be expected in view of the topography of the drainage area. Such relief would be required to be provided by sheet flow paths or equivalently effective alternate means. If the drainage relief were provided by a sheet flow path, the development or redevelopment would be required to identify and provide drainage pathways and pathway easement(s) which would not adversely affect nearby structures. INIF11111 !111 111111111 I -Ki I 0_7 The limits on construction in floodways should be reconsidered. Infringement on floodways has a cumulative effect that is difficult to identify on an individual the cumulative effect of Nadraulic blockingiiLD-um alters the energy losses -in a channel and thus eventually impacts floodway capacity). In addition, some construction in floodways, such as construction on piers in wide, shallow floodways, while not increasing flood levels, still poses significant hazard to the constructed structure because of the force of flowing flood waters and the large debris that may be carried in floodway waters. Thus it is generally preferable and is recommended that new construction in floodways not be allowed. However, if the proposed construction can be shown with ay-p".ai9Ae.vo*mo,* nisomlmw�anger to other structures and facilities, the City can make a decision on a case by case basis as to whether such construction could be allowed. In those cases where the floodway or those portions of a floodway are very wide and shallow, the City Engineer should have the option, based upon appropriate and engineering evaluation, to allow variance to limits on construction in a floodway, provided such variance is consistent with the City's floodplain management ordinances and would not adversely impact the City's compliance to the Federal Flood Insurance Program. A submittal to the Federal Emergency Management Agency (FEMA) may be required as part of the approval for such a variance. The City should consider clarifying the requirement that there should be a minimum 18-inch clearance of the low chord of a bridge above the 100-year flood level when 1) changes in design flood levels are proposed, 2) when new bridges are built, or 3) existing bridges are rehabilitated. Mr. Ron Bottoms, City Manager k I o t z 'L4 0 a s s o c 'i a t e s October 1, 2008 Page 7 of 31 2.4.5 Capacity Improvements in Storm Sewer Systems A significant limitation to storm sewer capacity, as discussed in regard to sources of drainage problems, is believed to derive from inadequate number or size of storm sewer inlets in sewered areas. The City's drainage criteria spees that maximum water travel distance in the street to a curb inlet shall not exceed 400 feet in residential areas. The common recurrence of residential street flooding in sewered-areas suggests that the 400-feet criteria may be inadequate. A reasonable sense of the potential problems arising from inadequate storm sewer inlet capacity because of too few inlets can be obtained by computing the average length between inlets along storm sewered-streets (obtained by dividing the total length of streets in sewered-areas by the number of street inlets in the same area). Representative examples of such calculation are shown in Exhibit 12. The subdivisions were arbitrarily chosen as representative examples and predate the current criteria of 400 ft. Values commonly appear to exceed 400 feet in existing residential areas, suggesting that inlets are typically spaced too far apart along streets. A detailed analysis for the storm sewer would have to be made to determine if there are any benefits for adding inlets for current storm sewer systems. However, consideration should be given to requiring that maximum inlet spacing be 300-ft for new residential developments, unless it can be shown by engineering calculation that the maximum ponding depth in the gutter upstream of the inlet will not exceed the lesser of the top of street crown or the top of curb along the drainage path leading to the inlet from the farthest point draining to the proposed inlet. Also to be avoided are the long stretches of no street inlets at the upstream end of a sewer line run. Slight changes in street slope due to construction procedures may reduce the planned gutter flow capacity toward the last upstream inlet in a line of street inlets. It would be recommended that engineering for new developments be required to assess inlet spacing and location in light of the realities of very flat street slopes and inaccuracies in construction, and identify an appropriate maximum distance of gutter runs without sewer inlets which would recognize inaccuracies in slope estimate and construction. Capacity limits can also be significant at outfalls if outfalls are too small. Small outfalls (i.e., small diameter outfall pipe) produce large energy losses. In addition, small outfalls are quite susceptible to blockage by debris, either from upstream or backup from receiving waters. It would be recommended that the minnnum outfall size be 24-inches in diameter (or equivalent diameter). Mr. Ron Bottoms, City Manager k 1 0 t Z a s s o c i a t e s October 1, 2008 Page 8 of 31 In new development situations or where drainage ditches are being improved in existing or infill areas, open ditches should be designed to handle the design storm flow. In addition, design criteria should require the following: • Minimum bottom width of ditch (recommend 2 feet) • Maximum side slope of ditch (recommend 4:1 slope) • Minimum bottom slope of ditch (recommend 0.05% slope) • Minimum freeboard under design flow conditions (recommend I foot) Engineering evaluations for new development and redevelopment, if roadside ditches are to be used, should account for energy loses at culverts, including driveway culverts. Discharges to open ditches from adjacent properties should not exceed the capacity of the ditch if 1) the ditch is adequate for generally accepted areas of localized runoff (for the design storm) adjacent to roadways, or 2) specific inclusion of existing large, off -roadway drainage areas are recognized in the design of the original ditch when the natural (i.e., not altered by man) topography drains the area toward the roadway. Generally accepted areas of localized runoff to roadways and the ditches adjacent to them consists of 150-ft wide strips adjacent to the roadway right-of-way on each side of the road. Drainage from beyond this commonly accepted width zone is to be discouraged and should be directed to areas and/or drainage pathways to which natural drainage occur. If such drainage cannot be safely directed to riatural drainage pathways, on -site detention should be required to limit the peak cates of discharge to an acceptable level for discharge either to the roadway or friatural drainage paths. Allowable street ponding depth of storm waters poses one of the most difficult choices for establishing drainage design criteria, particularly 'in coastal or near - coastal areas where slopes are quite low and natural drainage is poor. Because of the naturally poor drainage conditions, use of streets as part of the storm water drainage, system is common in coastal Texas. However, such use sometimes poses inconveniences to residents. Such use may also prevent access by emergency vehicles or ordinary citizen vehicles dealing with an emergency. The City currently allows street ponding to depths reaching the top of curb or street crown, with the proviso that one clear lane (of 12-feet width) be maintained �n collector streets and two clear lanes (of total 24-feet width) be maintained in Mr. Ron Bottoms, City Manager k I o t z a s s o c i a t e s October 1, 2008 Page 9 of 31 arterial streets. Allowing street ponding to depths that keep water within curb lines is not inconsistent with drainage criteria in other communities; that is, allowing limited street ponding is a generally recognized strategy for drainage management in poorly drained areas. P l•ug I tolmi 4 4 J-+2 4 a tie -61 - TAF 1417-IMW4440WIN I • Requiring that the HGL be at or below the street gutter line level. In addition, during extreme flood events (e.g., the 100-year event), in light of the difficulty of drainage in the City because of limited ground slope, we would reconunend reduction in the allowable maximum depth of street ponding, as follows: During a 100-year event storm, the maximum water surface in the street • on the structure is recommended to be the following: • No more than that 3-inches above the top of curb at the highest point of th(i curb adjacent to the lot; 2) no more than 6-inches above the natural ground a!j the highest point of the natural ground along the backside of the curb line; 3) • no • than 3-inches ,.• the roadway crown at the highest ,• within the limits • the lot • • • than that 6-inches ..•.- the top • curb at the lowest •• of the curb • to the lot; 2) no •- than • ,•s the natural ground at the lowest point of the natural ground along the backside of the curb line; 3) and no more than 6-inches ,•• the roadway crown at the lowest •• wiffiin the limits • the lot Data, including applicable calculations and profile plots, should be provided tit the City Engineer demonstrating compliance to these criteria when new development or redevelopment is proposed. NF9,110WITTMIMi 80tWIM1207-711m, 17 Me MinimaT onT ITIT =VV'TF'giVTT1 LW—db'St site mitigation detention storage requirement for drainage areas less than 10 acres, as specified by the current unit storage factors of 0.20 ac-ft/ac and 0.45 ac-ft/ac of detention storage for areas from 0 to 3 acres and from 3 to 10 acres, respectively. In view of the drainage problems currently • faced by the City, these factors rnay be too low to achieve the intended mitigation. Studies previously conducted by Klotz Associates have shown that such unit storage factors in some Mr. Ron Bottoms, City Manager k I o t z I 'kF, I a s s o c i a t e s October 1, 2008 Page 10 of 31 communities in the Houston metropolitan area tend to be in the range 0.4 to 0.8 ac-ft/ac. New developments should use such storage factors. Engineering review of specific conditions in the City and possible adjustment of these factors need to be considered. For mitigation by detention, the City specifies storage coefficients which depend upon developed area for areas less than 10 acres (as noted in section 2.1 above). For larger areas, HCFCD criteria (10 to 100 acres) or review by the City Engineer (greater than 100 acres) are applied. We would recommend that regional detention alternatives be allowed (but not necessarily required) where conditions would make regional detention feasible and effective, for which the regional detention facility would have to provide for the proposed development area in question the following information: • The allocated storage in the regional detention facility for the developed area would at a minimum have to meet the storage criteria as currently defined in City criteria for on -site detention basins. • Conveyance of storm waters from the developed area to the regional detention must be shown to the satisfaction of the City Engineer that such conveyance could be constructed and will be maintained by the development owner (or other identified party acceptable to the City) and that the conveyance would not adversely affect existing flood levels. Common mitigation techniques are detention ponds, over -sizing of sewers, and, less frequent but of increasing interest, subsurface detention systems. While such detention storage measures are a heavily relied upon mitigation techniques, the City should give consideration to the use of what are termed low impact development (LID) techniques, which are on -site detention devices or systems which limit runoff at or very near the point where runoff begins. Bio-retention, swales, rain barrels, special types of pavement and surfacing, and similar techniques can be used to promote infiltration and lessen direct runoff. ft is recommended that low impact development techniques be allowed to be used to the satisfaction of the City Engineer that a proposed technique or collection of techniques will, with frugh likelihood, not only achieve mitigation which equals or exceeds that required for mitigation if a detention pond were to be used but also will, with high likelihood, function as intended in light of required maintenance for such techniques. The demonstration must recognize the potential reduction in Mr. Ron Bottoms, City Manager fC l 0 t Z a s s o c i a t e s October 1, 2008 Page 11 of 31 effectiveness that limited ground slopes may have on the effectiveness of such LID techniques. If the City were to consider allowing use of LID techniques, the City should consider defining specific types, criteria, and effectiveness measures that should be -• when LID • are proposed. The City currently • •a into •*• provided that it can be certified by a professional engineer or architect that such encroachment will not result in increase in base flood levels. It is recommended that such certification can • •- provided by a registered professional engineer who is also a certified ••r manager 3. Prioritization of Drainage and Local Flooding Problems (Task 4.2 of Scope of Work) As part of the CWDS for the City, Klotz Associates was tasked with ranking the recommended projects for implementation to address drainage and flooding problems, both in the short and long term. Short ter -in problems and conceptual ways to address them for some areas have been discussed in Report #2. This report, Report #3, focuses upon the longer term problems requiring more extensive effort, usually reflected in significant capital improvements, to • Because of their greater cost or time to implement, a prioritization of such longer terin problems is useful because it helps the more critical problems to be addressed more rapidly and promotes cost effective use of available capital improvement flinds. Consequently this section focuses upon procedures used to define priorities for capital improvement projects to address longer term, more extensive drainage and flooding problems in the City. I . I Drainage problems can have their root causes in a variety • sources, • combination • • Solutions to problems should •- keyed to the source • the problem, but for conceptual planning purposes, inferences about the sources of problems must commonly be used as initial surrogates for clarifying problem • When drainage problems arising during periods of extreme rainfidl are located in floodplain areas, it is likely that insufficient channel capacity is the • • a Mr. Ron Bottoms, City Manager October 1, 2008 Page 12 of 31 k I o t z associates significant cause of the flooding problems. Currently known information on floodplain areas in the City comes from two sources: existing FEMA flood models and survey of channels developed specifically for the CWDS. Only primary channels and bayous are including in these studied channels, so inferences from out -of -bank flooding is restricted to the areas along these studied channels. Exhibit I I shows the location of the studied channels. Section 4 below discusses drainage problems arising primarily from insufficient capacity in major drainage channels. The following section 3.2 examines flooding and drainage problems based not on channel modeling but upon where flooding and drainage problems apparently occur based upon flooding reports, irrespective of the reason for the flooding or drainage problem. The prioritization process presented in this section for resolving flooding and drainage problems recognizes that apparent flooding and drainage problems arise from inadequate storm sewerage capacity, insufficient drainage ditch capacity, capacity limitations at structures, or a combination of these factors. Particular remedies for these inadequate capacities are discussed in Section 5. For the prioritization process discussed utilizes actual flooding data to distinguish both the severity of the flooding problem and the magnitude of the benefit that remedy of the problem will yield, irrespective of the precise cause of the drainage or flooding problem. 1JW 11 a 0 1 0 1 If The City has collected and Klotz Associates has assembled for evaluation the following types of flooding reports: • Report Type 1: Reports on severely damage residences • Report Type 2: Repetitive loss reports on structural (residential) flooding • Report Type 3: Tropical Storm Allison flooding in 2001. (It is noted that in the area of the Texas Medical Center at peak rainfall conditions, 8.5 inches of rain fell within one 2-hour period. This would exceed a I 00-year storm event; see Appendix B) • Report Type 4: Tropical Storm Erin flooding in 2007. (It is noted that this storm event had a frequency of a 100 to 500 year event, depending upon location, for the maximum 3-hour period of rainfall during the storm event.) may become available which provides flooding data. Thus, to deal with such potential, there is also introduced: Mr. Ron Bottoms, City Manager October 1, 2008 Page 13 of 31 k I o t z associates Of these types of flood data, Type 1 and Type 2 deal exclusively with structural flooding. The remaining types may deal with structural flooding, street ponding, or other miscellaneous flooding problems. Exhibits 1, 2 and 3 (adapted and updated from Report #1) shows the locations of these flooding reports (segregated by type of flooding incident) across the City. Clustering of the flooding reports is evident. Available data do not provide information sufficient to determine whether various reports of one type are possibly a duplicate of another type (e.g., a report of flooding of a structure by Tropical Storm Allison might duplicate a repetitive loss report of the same structure). To account for such possibility, less weight is given to Type 3 and Type 4 reports of flooding. As a first step to quantitatively describing this clustering and assess its implications for prioritization of projects to remedy long term flooding problems, the location of each report can be spatially evaluated using GIS tools to create a "flooding problem intensity" map (much like a elevation contour map). The to create a parameter representing the number of flooding incidences (as based upon the above four listed types of flooding reports) per unit area (i.e., a flooding intensity). As the number incidences increase in an area, the flooding intensity increases. As this flooding problem intensity increases, the more serious is the flooding problem in an area. That is, this intensity is a measure of flooding problem severity. ese foo ing intensities can Me vista as 077=7 MTirits Since it is only the relative magnitude of the intensities that is important to assessing priorities, the intensities are presented only in relative terms. Review of this flooding intensity map shows there to be "hot spots", i.e. areas of high flooding problem intensity. These are prime areas for remedy of long term flooding problems. It is seen from the exhibits that the hot spots can be easily associated with a particular subdivision or major area within a particular subdivision. Thus it is convenient to identify a hot spot by the subdivision *in which the hot spot is concentrated. As presented in Exhibits 4, 5 and 6, the flooding problem intensities assume that each type of flooding report is of similax importance. However, some types of flooding reports imply a potentially more serious flooding condition than others. For example, a report of repetitive structural flooding is expected to represent a more serious flooding problem than a citizen's single report on a flooding incident associated with Tropical Storm Allison (because of the severity of this storm event) for which the flooding may or may not have been a structural flooding report. Consequently, if more weight is given, from a planning standpoint, to Mr. Ron Bottoms, City Manager October 1, 2008 Page 14 of 31 k I o t z associates reports on severe flooding damage than to incidents of repetitive structure which are, in turn, given more weight than flooding reports (whether for residences or streets) arising from tropical storms, a modification of the flooding intensity map can be created by giving different levels of importance to different types of flooding reports, as given in the following table: Policy Weight Factors for Flood Problem Intensity Type of Flooding Report Policy -based Weighting Factor Report ,Tye _l -, Reports on severely damage residences 5 Report Type 2: Repetitive loss reports on structural 4 (residential) flooding Report Type 3: Tropical Storm Allison flooding in 1 2001 Report T.Y.De 4: Tropical Storm Erin flooding jn_2006 I '_Report Type 5: Miscellaneous but reliable data 1 The choice of different weights is a policy -based decision. Our primary concern is structural flooding, but street flooding may also be indicative of potential structural flooding. Thus for evaluation of long term flooding problems, the weightings listed in the table above are considered reasonable When the above weightings (for the first 4 types only; no data are available for Type 5) are applied to each flooding incident report and the flood intensity map is rescaled to have the same average intensity as the un-scaled map (in order to provide comparison between policy -scaled and un-scaled intensities), the intensity maps of Exhibits 7, 8 and 9 results. Table 1 shows problem intensity assuming that each type of flooding occurrence has the same importance and assuming that the policy -weightings given in above table are applied. The floodinR problem intensity maps of Exhibits 4, 5 and 6 or 7, 8 and 9 can be used to identify and rank areas for which remedy of problems should be developed as part of the long terms solution to flood problems in the City. Ihe areas with hip -hest flooding problem intensitv are in most urRent need of remedy, Comparison of the Exhibits 4 and 7, 5 and 8, and 6 and 9 shows considerable similarity between the two intensity results, though the use of the policy This comparison between the hot spot identification of Exhibits 4 and 7, 5 and 8, Mr. Ron Bottoms, City Manager October 1, 2008 Page 15 of 31 k I o t z associates and 6 and 9 suggests, however, that the weighting factors, in the approximate range of values used, is not critical to identify priority areas for remedy of flooding problems. Therefore only the hot spot identification provided by the policy -based weighting factors is used for actual prioritization for remedy of flooding problems. Based upon the maximum intensity within a hot spot zone, each high priority subdivision is given a rank, with highest ranking given to the area with greatest flood problem intensity. For convenience in analysis, the area with the greatest flood problem intensity is given a ranking of 10. The rankings given in Table I indicate the relative need for remedy of flooding problems in various subdivisions. This ranking can be used to identify an order for critical improvements, but as discussed in the following section, does not necessarily identify the most beneficial way to make improvements. I 1 111111 1 1 a . II I # I Te rei i Tieea or rein TTX prT #-= c T F7 aT MTTTMg"?TM that the importance of remedy of a flooding problem will depend upon the number of people beneficially affected by the remedy. Problem areas discussei, above and listed in Table I are associated with different subdivisions or different local areas. Different subdivisions or local areas about a hot spot area have different numbers of people, but when the drainage problems in the subdivision area are remedied, all, in broad general terms, of the people in the area benefit to some degree, not just the people with whom drainage problem reports are associated. That is, the value of a remedy of a drainage or flooding problem depends upon not only the severity of the problem but also the number of people -who will benefit by remedy of the problem. Drainage benefits in a subdivision area affect not only the residences which are reported to have flooded but also nearby residences because 1) nearby residences with flooding ma�,A e re-scorted the floodin 2�, street and VL� flooding, would have likely accompanied reported residential structure flooding and reduction of street flooding would benefit everyone living in the approximate area where residential structural flooding occurred; 3) reduction of street flooding would aid emergency access, which would benefit all people in the area if emergency access were required; and 4) less frequent or elimination of residential flooding contributes to improved property values (and consequent ad valorem evaluations). To capture these two aspects (severity of flooding and the number of people benefiting) for prioritizing CIP projects, we can plot for each subdivision or area identi bdivis 0- #$1 m I tj_qm Itaym argm I wma rA a It.] 4 U-141 FA IJ ly-M a t*111444 II_,M pi Mr. Ron Bottoms, City Manager October 1, 2008 Page 16 of 31 k I o t z associates problem rank) against the number of family units in the general area • where the flooding occurs which are estimated to benefit from remedy of the flooding problem. As a surrogate for the number of family units, we use the number of lots in the area estimated to be beneficially impacted; see Table 1. In identifying the number of lots beneficially impacted, professional judgment is used to identify the boundary of the area where the benefits would be significant in light of where the Egli concentration of flooding reports occurs. For convenience, the identified areas of benefit are referenced ► the subdivision in which they occur. The most critical and most beneficial problem areas to select for remedy are those which have high problem intensity (corresponding to a high rank number) and frugh numbers of people benefiting. By examining the pattern of points in the plot of problem rank and people benefiting, we can identify those subdivisions or local areas which have high •!• rank and high •` For the ranked subdivisions and local problem areas, Figure I plats the number of lots in the subdivisions or local area against the flooding problem rank. Based upon review of these plots, the recommended priority (all other factors, e.g., budget limitations, being equal) for implementing the long term drainage and flooding problem solutions are given in • 2. - I . UL '171111TJ Unclies-ITTU U1.70-ULICIS ALIU S111a11U1 ULURCS �SCC F-iMMUIL if Yor ynufary ditches and channels). When the capacity of a major channel ("drainage ditch") is insufficient to carry the flow delivered to it, two basic problems can arise: 1) flood waters can rise above the banks and overflow adjacent properties, and 2) flood waters can back up local runoff attempting to drain to the channel and contribute to local flooding problems These latter problems have been prioritized above. Hydraulic models can be used to assess channel overflow and flow obstructions (e.g., low bridges, small culverts). In terms of modeling, the major drainage ditches in the City fall into one of the following categories: • A drainage ditch for which a hydraulic model has been previously developed by FEMA or others; such models referred to as FEMA models. • A drainage ditch for which field survey has been made to define channel cross section at a limited number of locations. • A drainage ditch for which no model exists and no type of model is to be developed as part of this CWDS. Mr. Ron Bottoms, City Manager October 1, 2008 Page 17 of 31 k I o t z associates Table 3 summarizes the type of studied channels for the first two above categories (see also Exhibit 11). Major drainage ditches or channels evaluated for the CWDS are of two types: 1) those for which FEMA-based hydrologic and hydraulic models are available as a basis for model creation, update and revision, and 2) those for which survey has been done to approximately define the cross sectional shape of the channel. The FEMA-based models are for extensive lengths of channel and for relatively large drainage areas. The level of detail in the geometric description of these models is good as evidenced by the number of cross sections used in the model (see Exhibits 13, 14 and 15). Existing hydrologic and hydraulic models as given by FEMA models for these major drainage ditches and channels were reviewed and modified to reflect new conditions (e.g.,; change in channel. connectivity, crossing structures, or land use) and information about the waterway and the runoff to the waterway. These models provide a definitive description of flooding conditions along the ditch or channel, and detailed conclusions about flow containment or bank overflow along the length of the channel can be drawn. On the other hand, those ditches and channels for which no prior hydrologic and hydraulic models are available are generally short tributaries to the FEMA-based models. Only limited data on ditch or channel geometry are available for these ditches and channels; these data were obtained by field survey done for this study (see Exhibits 13, 14, and 15). Typically only 2 or 3 cross sections were surveyed along the ditch or channel. These survey data were collected for this study since no existing data describing channel geometry for the tributaries were available. In addition, information about structures (e.g., bridges) along the length of the channel is generally incomplete. Consequently it is difficult to draw reliable conclusions about the detailed behavior of water levels and consequent flow containment within channel banks. (Along F101-00-00 for which a HEC-RAS model was developed from hard copy data in the appendices of a master drainage plan done by HCFCD for La Porte in 1987, two channel sections between Hwy 146 and Sens Road were field surveyed as a check on the cross section data in the 1987 master drainage plan study. Comparison of the survey cross section data to the cross data provided in the 1987 showed the section data to be reasonable consistent. Bank elevations and bank to bank distances were quite similar, as were channel shape and side slopes except near the channel bottom. Flow line elevations in the surveyed sections were about 4 ft deeper than the sections taken from the 1987 study.) Mr. Ron Bottoms, City Manager k I o t z a s s o c i a t e s October 1, 2008 Page 18 of 31 Thus, two different methods were used to assess the capacity of major ditches and channels, the methods being selected to utilize the available data in a manner consistent with the level of detail about the ditch or channel. For major drainage ditches for which FEMA hydrologic and hydraulic models were available (with revision to reflect new information), the 10-, 50- and 100- year flood flows were simulated. Figures 2 through 16 show the flow profiles for various flood frequencies for the various modeled channels in the different major watersheds composing the study area. Note that the profiles sometimes combine separate FEMA models into a single model. The location and extent of the drainage ditches for which these flow profiles were determined are shown in Exhibit 11. Table 3 tabulates the models. When the model computed flood level rises above the bank elevation (compare the computed water surface profiles to the left and right bank elevation in these figures in Figures 2 through 16), channel capacity is at least locally deficient and im flood levels within bank for the frequency of interest. Potential measures to accomplish such lowering are discussed in section 5 below. Exhibits 16, 17 and 18 compare the locations where the computed flow depths exceed the channel banks for various flood frequencies to the locations of flooding reports (these reports have been discussed in section 3.2.1 above). It is seen that some areas with of out -of -bank conditions occur in the near vicinity of areas where flood reports are frequent, while in other situations out -of -bank locations do not closely correspond to the locations where flooding reports are numerous. Table 2 compares of the out -of -bank conditions to areas where flooding reports are numerous. Based on where out -of -bank conditions occur relative to flooding reports, an assessment can be made as to whether inadequate channel capacity is likely a significant source of reported flooding problems in a particular area. Table 2 provides such assessment: For a storm frequency of interest, it is concluded that the overflow from the major channel is or is not a likely significant contributor to the reported flooding problems. If the drainage ditch or channel is judged to be a significant source of reported flooding problems, channel improvements should be given high priority in the type of solution proposed to address flooding problems along or near the channel. Mr. Ron Bottoms, City Manager October 1, 2008 Page 19 of 31 k I o t z t associates Major Drainage Ditch Assessment for Ditches without FEMA Models For major drainage ditches without a base FEMA model, a different approach for using the limited information on channel configuration was used to assess the capacity of the channel to contain flows of different frequencies. Since these drainage ditches, conveniently termed tributary channels (though in fact two of these channels drain directly to Galveston Bay) are generally short and drain to a channel (or in the two instances, Galveston Bay) for which a FEMA model has previously been developed, the tailwater level (either at the receiving channel or the bay) will dominate and largely determine the water level in the tributary channel. The potential out -of -bank conditions can be thus assessed for planning purposes by comparing tailwater levels (i.e., water level in the receiving channel for the storm frequency of interest) to the bank elevation at the available cross sections along the tributary channel. Table 3 summarizes whether or not the tailwater level is or is not above the top -of -bank at the available sections along the tributary. Based upon a review of these levels, an assessment can be made as to whether or not, for the storm event frequency of interest, the tributary size is likely sufficient to keep the flow within banks. These assessments are provided in Table bank overflow can be expected to be frequent, the tributary should be considered for improvement. If frequent out -of -bank conditions occur in the close vicinity of reported flooding, tributary capacity is likely a significant contributor to the flooding problems, and capacity improvements axe of high priority. Flooding problem reports have been described in section 3.2. The evaluation of drainage channel and ditch overflow discussed in sections 4.1.1. and 4.1.2 above can assist in identifying likely significant sources or causes of reported flooding problems. There are different likely significant sources or causes of reported flooding that can be reasonably distinguished by the frequency of channel overflow when there is a drainage charmel in the vicinity of an area of numerous flooding reports (i.e., near flooding "hot spot" areas). These sources or causes are categorized as follows: Insufficient Channel Capacity: The frequency at which channel overflow is so large (in a relatively sense) that it can be expected that an overflowing channel will be a ma or cause of the reported flooding if the area of flooding is reasonably close to the channel. For evaluation purposes, it is assumed if a channel is estimated to overflow its banks for 1 0-year and larger storm events that insufficient channel capacity is the primary or certainly a prime cause of the reported flooding. Note that the insufficient channel capacity could be due Mr. Ron Bottoms, City Manager October 1, 2008 Page 20 of 31 k I o t z associates to insufficient channel size and/or bridges and culverts which produced large hydraulic head changes at isolated locations. Inadequate Sewerage: In this situation, it is very seldom that channel overflow occurs and that therefore the source of reported flooding is unlikely due to insufficient channel capacity but rather due to the inadequacy of the sewerage (i.e., the pipes, inlets, manholes, etc.) that drains runoff waters from the problem area to the channel. Inadequacy of the sewerage could be attributed to insufficient sewer pipe sizes, numbers of storm water inlets, and/or lack of overland flow pathways. Infrequency of channel overflow is considered to occur if no channel overflow occurs for the 100-year storm eve.-tt. Mixture of Inadequate Sewerage and Insufficient Channel Capacity: This is an intermediate overflow condition for which both inadequate sewerage capacity .-and likely frequent high sewer - tailwaters occur, with both contributing to poor drainage. This condition is estimated to occur when overflow does not occur for the I 0-year storm event but does occur for the 5 0- and/or I 00-year storm event. Based upon these categories, the sources of flooding problems are identified in Table 3 in light of the estimated overflow conditions of the studied ditches and channels given in Table 2. This identification provides guidance for selection of likely needed drainage solutions to be identified in the CWDS report. Some short term flooding issues and conceptual solutions for addressing those issues have been discussed in Report 42. In addition, preliminary concepts for zddressing long term problems were identified in Reports 41 and Q. Conceptual solutions to address long term problems are described in the following: The concepts for remedy of drainage and flooding problems generally applicable to all drainage and flooding problem areas fall into the following categories: 11"Pgrade of storm sewer systems to 1) increase the capacity of a sewer line, 2) convert open ditches to sewers in streets with curb and gutter systems; and/or 3) increase the capacity of street inlets to lessen ponding in streets. To accomplish these types of upgrades, the following improvements may be done: Mr. Ron Bottoms, City Manager k I o t z a s s o c i a t e s October 1, 2008 Page 21 of 31 • Replacing an existing sewer pipe (or box culvert) with a larger pipe (or box) • Adding a new parallel sewer line near an existing pipe or culvert, the new line being of similar or larger size • Adding a diversion line which carries a portion of the total flow in an existing line to another location or outfall • Altering the slope or invert of existing sewer lines • Replacing existing manholes or junction boxes when in poor condition • Placing a new sewer line in an existing ditch with capacity similar to the ditch capacity • Placing a new sewer line along an existing ditch line and increasing the capacity of the original ditch with the new sewer line • Controlling the points of storm water entry into the sewer line more effectively • Altering the outfall configuration for the drainage system • Change the configuration of the inlet type or size to be used to one which has greater capture efficiency • Increase the size and/or number of inlets • Improve location of inlets so that runoff can be more effectively captured • Provide an overflow path to a ditch Note that proper and continuing maintenance of sewers, ditches and channels is an important requirement for maintaining design capacities. To reduce ponding of storm waters, provision of overland sheet flow paths to allow alternative ways to drain areas of ponding and excessive water accumulation can be provided. Provision of overland sheet flow paths have been discussed and proposed in Reports #1 and #2 as more immediate, short term remedies for some drainage problems because provision of sheet flow paths can be implemented rather quickly and often at relatively low cost. Provision of overland flow paths may not always be a comprehensive solution because if the outfall receiving point or water for the overland flow (e.g., a drainage channel) does not have sufficient capacity, then the overland flow path capacity may be reduced or even prevented from functioning. Thus overland flow path solutions Mr. Ron Bottoms, City Manager October 1, 2008 Page 22 of 31 k I o t z associates should be generally viewed as a complement to long term solutions dealing with channel capacity and sewer system capacity. Detention may be onsite or offlite, and usually consists of some type of pond with storage that is dry until a storm event. Wet bottom detention (i.e., the pond never complete drains) is also provided in several areas. Onsite detention is located at or near the source of runoff for which control is sought, while offsite detention is commonly a shared, or "regional detention," facility. Onsite detention is commonly built and maintained by the property owner of 2, single parcel of property, while ofEsite or regional detention is often owned and maintained by a government entity which sells or provides storage in the detention system for areas in need of detention storage. Onsite detention facilities can also be located relatively easily underground, though costs for underground detention will be typically much larger than a surface detention facility. IThile onsite detention is primarily used for mitigation of new development, regional detention offers an opportunity to address existing excess runoff and channel flow problems. The regional detention facility can be sized, if adequate space for the regional detention facility can be identified, to reduce downstream impacts of excessive runoff from already developed areas upstream of the detention facility. 9 MI MTN I m4- • Ability to mitigate excess runoff at far distance points • Ability to mitigate excess runoff from different points • Ability to achieve economy of scale in construction costs • Ability to transfer construction costs of detention facility to others not involved in the construction process • Possible dual use of the detention pond as an amenity facility • Reduction in the number of locations to be maintained • Need for the pond owner to construct the pond upfront before full construction cost recovery may be possible • Owner will typically be responsible for maintenance of the regional facility • Purchase of storage space by other parties requires proper legal transfers Mr. Ron Bottoms, City Manager k I o t z It a s s o c i a t e s October 1, 2008 Page 23 of 31 • If not properly designed, delivery of runoff waters to the regional detention facility from source areas may have adverse effects on upstream or downstream proper -ties. • The owner is liable for deficiencies in pond operation and maintenance NoTleme "84-me witegival"'ral 0114rA111$W11JJ6 21-1 5.1.4 Channel Modifications Channel modifications are generally made to increase the capacity of the channel. However, in addition to capacity increases, channel modifications can have to - following benefits and challenges: • Lowering water levels for similar flows, with resulting lowering tailwater levels for storm sewer systems discharging -to the channel. • Improvement in aesthetics • Improvements in habitat along the channel • Reduction in bank and/or bottom erosion • It may difficult to enlarge a channel in an already developed areas because of limited right-of-way • Access for enlarging a channel may be limited or virtually absent • A channel improvement may require coordination with or permission of an agency, such as HCFCD, U.S. Army Corps of Engineers, or TxDOT • Channel improvements may also require detention or other mitigation features Channel modifications for improving capacity can assume a variety of forms, as follows: • Widening of channel, with or without side slope modification • Channel flow line adjustment so that channel becomes steeper • Lateral shifting or realignment of channel to shorten its length and/or reduce loss producing curves or junctions • Increasing the steepness of side slopes (using concrete or similar material) to both increase the channel area and reduce roughness • Use smooth materials (e.g., concrete) to line the channel and reduce friction losses Any channel modification must recognize possible need for increased easement or right-of-way. When channel improvements are made, consideration should be given to storm water quality controls for the channel; such controls can include channel erosion Mr. Ron Bottoms, City Manager October 1, 2008 Page 24 of 31 k I o t z associates prevention measures, prevention of excessive siltation, and capture of floatables potentially entering the channel. Drainage channel cross or are crossed by numerous roads, railroads, and pipelines. Bridges and culverts are commonly used to make such crossings. Other miscellaneous hydraulic structures such as flow splitters, drops, and constructed bends can also be present in a channel. Crossings and miscellaneous structures, pipelines, and, in particular, culverts and smaller bridges, can significantly increase energy losses and reduce channel capacity. Improvements in structures can have conveyance improvement benefits and, as well, lower tailwaters. Structural modification to improve conveyance and capacity of a channel will commonly • one • the following: • • the size (diameter) • a culvert • the individual barrels • a culvert by adding barrels or replacing existing culvert barrels • Decreasing the length • culverts as •. • culvert replacement • Upgrade of a bridge by 1) raising the low chord of the bridge, 2) increasing the spacing between bridge piers, 3) reducing the bridge skew, or 4) increasing the size of the opening under the bridge • Adjustments around pipeline restrictions to reduce constrictions or limit blockage IZKIIRZ�i 1� • To remove a property subject to flood damage from the floodplain by property purchase, followed by removal of structures on the property • To allow for development of sheet flow pathways for safe discharge of ponded waters • To use the property for construction of critical flood control facilities, such as a detention pond • Allow adjustment of floodway boundaries to facilitate development in other areas Table 4 presents a summary of representative unit costs for estimating the potential costs of various conceptual projects for addressing particular drainage or flooding problems identified for the City Wide Drainage Study. Estimated costs Mr. Ron Bottoms, City Manager October 1, 2008 Page 25 of 31 k I o t z associates of specifically recommended solutions will be addressed in the CW`DS final report. Cities across Texas have reported a variety of capital project funding methods, including using ad valorem taxes to purchase bonds (the most common method), fee programs, and similar revenue generation measures. While ad valorem taxes are the most common form of revenue generation for construction, such taxes do have limits and municipalities often seek alternative funding sources. Use of these alternative funding sources can be uncertain because of often high competition for available funds, the often very limited amount of funds for distribution, and the specific purposes and priorities of the funding entities or sources for the use of available funds. Some ftmding sources, such as royalties from oil and gas drilling, are unique to the community. Many cities seek creative fariding tools and, many instances, with success. The following summarizes some alternative funding sources that the City of La Porte could consider as a potential part of their capital improvement planning A possible source of revenue is the creation of special Storm Water Utility Divisions that allows for the collection of funds dedicated to storm water management and quality issues. When the U.S. Environmental Protection Agency (EPA) required cities to obtain National Pollutant Discharge Elimination System (NPDES) permits for discharge of storm waters, cities have found they often need additional ftmds to respond to the requirements of these permits. The NPDES storm water permit (as implemented by the Texas Commission on Environmental Quality) requires cities to take responsibility for quality of storm water discharges. Many cities have created Storm Water Utility Divisions as a way to establish a new funding source that could pay for floodplain management, drainage costs, and/or water quality control costs, the latter arising from NPDES permit requirements, rather than using money from the general fund. The utility establishes a utility fee rate at a level needed to meet the drainage needs of the community. The fees usually appear in the water bill, and are commonly about I% of the water bill. Dallas was among the first cities in Texas to establish a Storm Water Utility Division, providing an organizational and process model for other cities to follow. San Antonio, Laredo, Tyler and Arlington are among those cities which have successfully implemented their own storm water utilities. Some cities, on the other hand, have after review of storm water utilities issues declined to establish such a utility. Mr. Ron Bottoms, City Manager October 1, 2008 Page 26 of 31 k I o t z ('41 associates L Another similar method for collecting funds for infrastructure improvements is to create a special taxing zone or tax increment financing zone. Tax increment financing (TIF) allows cities to capture increased property tax value and direct that money into capital projects serving the special taxing zone. For example, the City of Allen allows the developer to front the capital for infrastructure and the city pays back the developer, allowing the developer to keep the incremental increase in tax value. Many cities, including La Porte, establish tax increment reinvestment zones (TIRZ) in existing neighborhoods. The zone or neighborhood keeps the increase in tax value of their particular area by having the City can reinvest that money in capital projects serving the area in question. In some cases, cities may seek opportunities to leverage resources with government organizations with which they share interests. Some agencies that cities have worked with in the past include the Houston Galveston Area Council (HGAC), Texas Department of Parks and Wildlife, TxDOT, Texas Water Development Board (TVTDB), and the Federal Emergency Management Agency (FEMA). Of these agencies, TWDB is considered the most likely to provide additional funds for storm water drainage improvements. The agency administers two grant programs to mitigate flooding: the Federal Emergency Management Agency Flood Mitigation Assistance (FMA) Grant program, and the Flood Protection Planning Grant. The first grant covers acquiring property, relocating or demolishing structures, flood -proofing, elevating structures, minor localized structural projects, and beach nourishment (planting grass, etc.). The grant covers only insured property. The second grant, Flood Protection Planning, covers the evaluation of structural and nonstructural. flooding solutions in the context of the whole watersJQJ The TWDB also has three loan programs to which application can be made for municipal drainage projects. The State Loan Program (Development Fund 11) provides loans for storm water projects including the following: • Enlargement of stream channels • Modification or reconstruction of bridges • Acquisition of floodplain land for use in public open space • Acquisition and removal of buildings located in a floodplain • Relocation of residents of buildings removed from a floodplain • Public beach re -nourishment, control of coastal erosion • Flood warnings systems • Development of flood management plans Mr. Ron Bottoms, City Manager October 1, 2008 Page 27 of 31 0 t Z associates The second loan administered by the TWDB is the Clean Water State Revolving Fund (CWSRF) Loan Program which provides loans at interest rates lower than the market • can •. used for • water pollution • projects and non -point source pollution control projects. The third loan program is the Water Infrastructure Fund (WIF) that provides subsidized loans to fund projects already on the State Water Plan or TWDB- approved Regional Water Plan. In order to get a new project on the State or Regional Water Plan, the regional plan would have to be amended. In addition to the TWDB, FEMA also provides for federal • to • •-•• out of flood -prone areas. FEMA also has a grant program called the Repetitive Flood Claims Grant Program (RFC), which provides funding to acquire flood damaged structures and • the property to r• • The • is limited to properties that meet the following -• • One or more claim payments for flood damages • Property must have flood insurance • Located within a State or community that cannot meet the requirements of the FMA grant program. The City of La Porte may consider using this program to fund the creation of recommended overland flow paths in subdivisions with repetitive loss properties located near drainage channels. Another program, The Governor's Division of Emergency Management at the ird str 9-t 1, Y the Hazard Mitigation Grant Program (HMGP) and Pre -Disaster Mitigation Grant Program (PDM) in Texas. HMGP funds can be used for: • Acquisition/demolition/elevation of flood -prone structures • Community and individual storm shelter programs • Retrofitting facilities (flood proofing, high wind, etc.) • Small scale structural hazard control/protection projects • Limited funding of innovative projects such as public awareness, enhanced hazard information systems, enhanced warning capabilities, etc. • Limited funding of the development of state and local mitigation action plans (MAPs) Pre -Disaster Mitigation Grant Program (PDM) grants provide money for pre - disaster mitigation planning, and the implementation of mitigation projects •• natural hazards. Common mitigation projects include buy-outs, individual • • safe-rooms/storm shelters and retention ponds. :• grants require that the city or legal entity have a Mitigation Action Plan in place Mr. Ron Bottoms, City Manager October 1, 2008 Page 28 of 31 k I o t z associates prior to applying for the grant. Costs for implementation of a PDM program can be quite variable depending upon mitigation projects selected for the program. However, the planning component of a PDM program which identifies mitigation projects can be quite reasonable, comparable to the preparation of a master drainage plan. The benefits of such a program can far outweigh the costs of program preparation. Finally, the Texas Department of Parks and Wildlife also has a variety of grants available for municipalities, primarily for the construction and enhancement of recreation areas. The Regional Grant was created to assist local governments with the ac-ouisition and development of rnulti-J the state's metropolitan regions. The program provides 50% matching funds and reimbursement grants to eligible local governments for both recreation and conservation facilities. Znnff1 =� In some cases, a local agency such as HCFCD, will partner with local cities and community groups on selected flood prevention projects. Frequently, these Not all such projects can be accomplished, but all proposals are taken seriously, as evidenced in the District's current partnering with several area entities. Some particular projects include a study of the flooding mechanisms along a local tributary and identifying methods to lessen its flood risks. Cities which have teamed with HCFCD include Tomball, Humble, Pasadena, South Houston, Houston, Galena Park, and Katy. An alternative method of funding that may be used by the City is the collection of developer impact fees. In this situation, fees are collected based on the value of the new development compared to the tax credits. Developer impact fees have been an excellent source of infrastructure funding for some cities. In fiscal year 2004/2005 Sugar Land realized $1,303,000 in revenue from developer fees alone. However, estimating the actual developer fee a city will generate is not always calculated as simply as it is shown in the fon-nula legislated in Local Government Code Chapter 395.015, Developer Fee = (Cost of capital improvements — Property and utility tax credits) -- (Total number of projected service units necessitated by and attributable to new development within the service area based on the approved land use assumptions and calculated in accordance with generally accepted engineering or planning criteria) Mr. Ron Bottoms, City Manager October 1, 2008 Page 29 of 31 k I o t z associates because of the difficulty of determining the total number of projected service units necessitated by and attributable to new development. Impact fees are also subject to certain state regulations that may require advance funding by the City if impact fees are to be utilized. Overall, securing a grant, loan or partnership for a particular project requires significant effort. While grant money is limited, application deadlines and grant program criteria further limit options, and agencies must select from many partnership proposals to best use their limited resources. A targeted, organized, long-term effort to secure funding can improve a municipality's chances of getting the needed monies to fund critical projects. It is recommended that the City consider beginning applications for several of the funding methods discussed above, particularly for those programs offered by TWDB and FEMA. Partnering with HCFCD should also be pursued. A list of the websites for some of these agencies is also shown in the following section of this report. 6.5 References Related to Funding Websites for Grant and Loan Programs Harris Co" Fli Texas Parks and Wildlife: v.tnwd-state- I'll, !• - . Texas Water Development Board: www.twdb.state.tx.us/assistance/assistance—main.asi) Federal Emergency Management Agency: Repetitive Flood Claims Grant Program "Financial Assistance." 2006. Texas Water Development Board. March 14, 2007. <http://www.twdb.state.tx.us/assistance/financial/financial—main.asp> Mr. Ron Bottoms, City Manager k I o t z a s s o c i a t e s October 1, 2008 Page 30 of 31 Colley, Jack.. Letter to Emergency Management Colleagues. January 29, 2007. Repetitive Flood Claims Grant Program Guidance for FY 2007. Emailed to firm 3.1.07. Colley, Jack.. Letter to Emergency Management Colleagues. January 29, 2007. Pre -Disaster Mitigation (PDM) Grant Program Guidance for FY 2007. Downloaded from website I & 21 .2 V, _Psfsufs, k "Hazard Mitigation Grant Program (HMGP)." August 3 0, 2006. Texas Division of Emergency Management. March 14, 2007 g4ilt.a 44I Texas Statutes Local Government Code Chapter 395.015. Added by Acts 1989, 71st Leg., ch. 1, § 82(a), eff. Aug. 28, 1989. Amended by Acts 2001, 77th Leg., ch. 345, § 3, eff. Sept. 1, 2001. < http://tlo2.tic.state.tx.us/statutes/Ig.toc.htm> 7. Conclusions In this report, we have focused on several key issues in regard to long term solution of continuing, critical drainage problems. We have systematically identified areas of sinificant floodini and irovided a irioritization of those areas III d 111AULME TILLQ11 11191 *IIIJ I-OUVEIMLOS UIC �SCVUIILY 01 110OUIng DLL MR Denem to be derived by improved drainage to eliminate flooding and ponding in such areas. We have also examined potential overflow conditions in major drainage channels and ditches to generallv describe the severity of insufficient capacity in primary channels and ditches and assess whether channel capacity is a likely source of reported flooding for particular areas of reported flooding. We have also identified and reviewed the various conceptual approaches to address identified drainage and flooding problems. Implementation of such solutions will require funding. To assist the City in obtaining necessary ftmding, we have provided a general review of likely additional funding sources in addition to traditional tax -based sources. Critical to long-term prevention of increased floodmig and inadequate drainage in the City are the drainage criteria and standards to which new development and re- development must comply. While the City has a basically sound set of drainage criteria, we have identified a series of what we think would be potential improvements in the criteria to be applied to new development and redevelopment. Mr. Ron Bottoms, City Manager October 1, 2008 Page 31 of 31 k I o t z associates We hope that you will give serious consideration to our recommendations for potential changes and additions to the drainage criteria. Our next and final report will be the City Wide Drainage Study (CWDS). That report will include specific recommended solutions to drainage and flood control problems with emphasis upon higher priority problems, and estimates of costs of implementing identified problem solutions. Sincerely, Gary L ' SF tick, P.E., CFM L, Vice President CC: Mr. Steve Gillett, City of La Porte Mr. John Joems, City of La Porte Mr. Rodney Slaton, City of La Port* - October 1, 2008 List of Attachments k I o t z associates List of Figures 1. Prioritizing Drainage Problems 2. B106-00-00 Big Island Slough 10-YR Water Surface Profile 3. B106-00-00 Big Island Slough 50-YR Water Surface Profile 4. B106-00-00 Big Island Slough 100-YR Water Surface Profile 5. B109-00-00 Spring Gully 10-YR Water Surface Profile 6. B109-00-00 Spring Gully 50-YR Water Surface Profile 7. B 109-00-00 Spring Gully I 00-YR Water Surface Profile 8. B 112-00-00 Willow Spring Bayou I 0-YR Water Surface Profile 9. B 112-00-00 Willow Spring Bayou 50-YR Water Surface Profile 10. B 112-00-00 Willow Spring Bayou I 00-YR Water Surface Profile 112-02-00 Tributary to Willow Spring Bayou I 0-YR Water Surface Profile 12. B 112-02-00 Tributary to Willow Spring Bayou 5 0-YR Water Surface Profile 13. B 112-02-00 Tributary to Willow Spring Bayou I 00-YR Water Surface Profile 14. F216-00-00 Little Cedar Bayou I 0-YR Water Surface Profile 15. F216-00-00 Little Cedar Bayou 50-YR Water Surface Profile M-11 ME List of Tables 1. Prioritizing Drainage Proble 2. Rank of Flood Problems 3. Summary of Studied Channel] List of Exhibits 1. San Jacinto & Galveston Bay Floodplain Map 2. Armand Bayou Floodplain Map 3. Clear Creek Floodplain Map 4. San Jacinto & Galveston Bay Intensity Problem Area Map 5. and Bayou Intensity Problem Area Map 6. Clear Creek Intensity Problem Area Map 7. San Jacinto & Galveston Bay Intensity Problem Area Map With Weighting Factors 8. and Bayou Intensity Problem Area Map With Weighting Factors 9. Clear Creek Intensity Problem Area Map With Weighting Factors 10. Primary Channels 11. Inventory of Studied Channels 12. Examples of Inlet Capacity 13. San Jacinto & Galveston Bay FEMA Models & Survey Cross Sections 14. Armand Bayou FEMA Models & Survey Cross Sections 15. Clear Creek FEMA Models & Survey Cross Sections 16. San Jacinto & Galveston Bay Channel Capacity Assessment 17. Armand Bayou Channel Capacity Assessment 18. Clear Creek Channel Capacity Assessment M32= Maximum of Intensity in �J Maximum of Intensity in Rank (10 is Highest) Name of Flood -Affected Area Affected Area Assuming Affected Area Assuming Rank (10 is Highest) Assuming Flooding Number of Lots in Number of Lots Likely to (Also Show Name on Map) All Flooding Reports of Flooding Reports have Assuming All Flooding Reports have Different Subdivision Area Directly Benefit from Same Weight Different Weight Reports of Same Weight Weight Drainage Improvement Brookglen 11.84 35.37 10 10 835 600 Creekmont Section 1 10.99 15.11 10 10 338 110 Glen Meadows 10.17 13.43 9 9 744 160 La Porte 3.31 8.68 6 9 436 375 Spencer Highway Estates 2.76 8.61 6 8 381 100 Fairmont Park East 4.04 8.15 7 8 1318 500 Pinegrove Valley 4.92 7.64 8 8 275 220 Fairmonth Park West 5.44 6.79 8 7 1232 390 Bay Colony 2.40 6.69 5 7 128 128 Shady River 2.41 6.65 5 7 154 154 Creekmont Section 2 3.60 6.32 7 6 138 30 Fairmont Park 4.00 5.86 7 6 705 330 Lomax Garden 5.47 5.47 9 5 160 160 Meadow Park 4.18 5.32 8 5 91 91 Bayside Terrace 1.30 3.77 3 5 252 252 Old La Porte 1.62 3.36 4 4 1305 150 Battle Grounds Vista 1.10 2.75 3 4 55 10 Bay Shore Park 1.10 2.74 3 3 217 50 Beach Park 1,06 167 2 3 73 50 Pine Bluff 0.64 2.20 1 3 189 189 Villa Del Rancho 2.13 2.13 5 2 21 21 Woods On The Bay 0.73 1.84 2 2 76 76 Spencer Landings 0.71 1.78 2 2 216 10 Meadowcrest 1.52 1.52 4 1 351 50 Battleground Estate 0.55 0.55 1 1 197 20 Bay Front To La Porte 0.55 0.55 0 0 436 40 San, Jacinto Homes 0.55 0.55 0 0 281 10 Klotz Associates City Wide Drainage Study Project No.: 0127.008.000 City of La Porte Name of Flood -Affected Area Rank (10 is Highest) Number of Lots Likely (See Exhibit A, 8, 9 for Assuming Flooding to Directly Benefit Likely Significant Source/Cause of location) reports have from Drainage Flooding Fifferent Weight Improvement Brookglen 10 600 Mixture of Inadequate Sewerage B112-00-00 Creekmont Section 1 10 110 Inadequate Sewerage La Porte 9 375 Insufficient Channel Capacity F216-00-00 Glen Meadows 9 160 Mixture of Inadequate Sewerage B106-00-00 Fairmont Park East 8 500 Inadequate Sewerage Pinegrove Valley 8 220 Mixture of Inadequate Sewerage F101-00-00 Spencer Highway Estates 8 100 Inadequate Sewerage Fairmont Park West 7 390 Inadequate Sewerage Shady River 7 154 Mixture of Inadequate Sewerage Al04-12-00 Bay Colony 7 128 Inadequate Sewerage Fairmont Park 6 330 Inadequate Sewerage Creekinont Section 2 6 30 Inadequate Sewerage Bayside Terrace 5 252 Inadequate Sewerage Lomax Garden 5 160 Mixture of Inadequate Sewerage F101-03-00 Meadow Park 5 91 Mixture of Inadequate Sewerage B106-05-00 Old La Porte 4 150 Mixture of Inadequate Sewerage F216-00-00 Battle Grounds Vista 4 10 Inadequate Sewerage Pine Bluff 3 189 Mixture of Inadequate Sewerage Al04-12-00 Bay Shore Park 3 50 Data Insufficient Beach Park 3 50 Inadequate Sewerage Woods On The Bay 2 76 Mixture of Inadequate Sewerage A104-12-00 Villa Del Rancho 2 21 Mixture of Inadequate Sewerage B106-05-00 Spencer Landings 2 10 Inadequate Sewerage Meadowcrest 1 50 Inadequate Sewerage Battleground Estate 1 20 Inadequate Sewerage Bay Front To La Porte 0 40 Data Insufficient San Jacinto Homes 0 10 Data Insufficient Klotz Associates City Wide Drainage Study Project No.: 0127.008.000 City of La Porte Table 3 - Summany of Studied Channels Type ®utfall Description Clear Creek Watershed A104e (Taylor el ou)ban wit, rt the city ts (Taylor ) S ve ed Sectoons Y CI_a Creek n year, sea s wi hin its channel banks for the 10 year, 50 mays within year 100 Year s4o m event 00 (Tributary to 7aylDr Bayou) with in the (Tributary AI04-0® 00 TnbuBaw 3.93 to Surveyed Sections Taylo Bayou aty limds stays wahn its channel . banks for the 10 .Taylor Bayou) ' c.itsr, 50 §ea-, and 100 +e, rstorm.event. --�-� Al04-12-01 stays within its channel banks fat the A 104-12-01 Surveyed Sectonsl Taylor Bayou 10 year storm event. The flow is not contained for g the 50 year and 100 year storm event Armand Bayou Watershed -- -- B106-00-00 (Big Is and Slough) stays within its Revised FEMA channel banks fo. the 10 year and 50 yea, storm B 106-00-DO (Big Island Slough) Model Armand Bayou event. The flow is not contained within bank for the 100 year storm event. .,,.,,,i-e,-„ 3106-02-00 stays within its channe. banks for the B106-02-00 Surveyed Sections Big island Slough 10 ve�nd 100 Year storm event.,,,,,, �,... , '.8106-05-00 stays within its channel banks {or the B 106-05-00 Surveyed Sections s Big Island Slough 10 year and 50 year storm event The flow is not contained fo-the 100 Year B109-00 00 (Spring Gully) stays within "ts channel 1 B I09-00-00 (Spring Gully) f Revised FEMA Armand Bayou banks for the 10 year, 50 year, and 100 year storm Model event. B109-03-00-00 stays with its channel banks for the B 109-03-00 (BI 12-02-00 Revised FEIVL4 Spring Guly 10 year storm event The flow is not contained for 'Intercoimect) Model the 50 year and 100 yea, storm event. I 12-02-00 (Connected to B Y09- B 12 B Revised FEMA Willow Sp-`ing Bayou B 10 9-03-00-00 stays with its channel banks for the 10 year storm event. The flow is not contained for Model I lthe 50 yea- and 100 year sto,m event ) . B112-00-00 (Wil`ow Spring Bayou) stays within its B112-00-00 (Willow Spirmigsli evised FEMA (channel for the 10 year storm event. The fow is nol. 'Bayou) Model Armand Bayou y contained for the 50 year and 100 year storm event �.k... 6112-02-00 (Tributary 1 78 to W Ilow Spring B112-02-00 (Tributary 1.78 to Revised FEMA Willow Sprng Bayou Bayou) stays within its channel banks for the 10 Willow Spring Bayou) Model year, 50 year, and 100 year storm event San Jacinto/Galveston Bay Watershed F101-00-00 is not within its channel banks for the FI01-00-00 3 Surveyed Sections Lsan JacintolGalveston Bay 100 '!ear star-n event FI01-03-00 Surveyed Sections FtDt -DD-OO F101-03-00 is not within its channel banks for the 100yearstormevent F2I2-00 00�Deel Creek) _ Surveyed Sections i. San Jacinto/Galveston Bay Data insufficient___ Revised FEMA F216-00 00 (Little Cedar Bayou) flow is not F216-00-00 (Little Cedar Bayou) I San Jacinto/Galveston Bay contained for the 10 year, 50 year, and 100 year Model storm event. 216-01-00 stays within banks for the 10 year 50 F216-01-00 Surveyed sections Lifter Cedar Bayou year, a d 100 year storm event City Wide Drainage Studv Klotz Associates City of La Pone Project No.: 0127.008. 000 Table 4 - Summary ®f Unit Costs Detention Pond - aboveground ( I ae-ft storage) Pond structure $109,594 Inlet -outlet -emergency overflow works $33,158 SWPPP (storm water pollution prevention plan) $2,803 Debris trapping devices $118,856 Geotechnical (I borings per ac-ft of storage) $4,110 Total $268,521 Estimated Unit Cost $269,000 per ac-ft of storage Detention Pond - below ground (I ac-ft of storage) Storage pipes with excavation (from sewer costs-6162 It total) $1,101,285 Inlet -outlet -emergency overflow works $33,158 SWPPP (storm water pollution prevention plan) $2,803 Debris trapping devices $118,856 Geotechnical (I borings per ac-ft of storage) $4,110 Total $1,260,212 Estimated Unit Cost $1,260,000 per ac-ft of storage In -line Detention (providing I ac-ft of additional storage) Channel length required (ft): Estimated Unit Cost (based uLon channel widening) $ 122,000 per ac-ft of additional storage Channel Widening (500 ft long, with 4 ft of widening) Widening with headwall improvements $16,136 SWPPP $1,600 Geotechnical (3 borings per 500 11) $4,645 Total Cost $22,381 Estimated Unit Cost $11.00 per It of channel lengLLer ft of widening New Channel (for diversion or bypassing-500 it long, 12 tt bottom width) Excavation, headwalls and related $45,658 SWPPP $1,600 Geotechnical (3 borings per 500 ft) $4,645 Total Cost $51,903 Estimated Unit Cost $8.67 per It of channel length per ft of bottom width Klotz Associates City Wide Drainage Study Project No.: 0127.008.000 City of La Porte Sewers - replacement or new 36" pipe installation $150 per If of installation 48" pipe installation $210 per If of installation 60" pipe installation $220 per If of installation 72" pipe installation $330 per If of installation Inlets $2,000 per inlet Concrete Pavement $40 per square yard Utility Relocation $30 per If of relocation Remove existing, placemenet, material $80,607 Roadway repair $6,400 Geotechnical (equivalent to 3 borings per culvert) $4,645 Total Cost $11,525 Estimated Unit Cost $24.00 per It of culvert length per sq-ft of flow area per barrel Other Unit Costs Excavation $20.00 per cy Property Buy Outs $135,000.00 per house Concrete Flume $40.00 per square yard Environmental Improvements and Enhancements Floatable trapping devicies for ponds or channels Representative cost $118,856 Estimated Unit Cost $119,000 for each pond Extra sediment trapping for water quality - aboveground pond Increase in storage volume - 10% Estimated Unit Cost $26,900 per ac-ft of Extra sediment trapping for water quality - belowround pond Increase in storage volume - 10% Estimated Unit Cost $126,000 per ac-ft of storaRe Other Costs (as % ofconstruction cost) Survey 5% Typical Engineering and related costs 20% Representative mobilization costs 5% Klotz Associates CiProjectty nvvvmenmmooenmuv Figure 1 - Priorzing Drainage Problems 11 -- 10 9 8 7 S 6 5 4 3 2 1 San Jacintodom I Klotz Associates City Wide Drainage Study Project No,: 0127.008.000 City of La Porte B106-00-00 Big Island Slough 10-YID WATER SURFACE PROFILES 30 2y _ r c s - C Legend 25 � 7 l o-YR } ... �. .,,. a _ ,.... I .... .. � Leh Bank 15 10 - Right Bank Existing Flowline a ®1.. _ Number of Flooded ® -5 _ -- _ Homes cd 210 -15 -20 -25 -30 _ (Outside City Limits) -35------ —_ --- --- — -- ----- a soao roaoo ls000 20000 25000 30000 ,associate° 06-00-00 FRin Strew Distance (ft) nn,l Rimirih 1 6-00-00 Big Island Slough 50-'YR2 WATER SURFACE PROFILES 35.00 30.00 Legend 25.00 — _- _ _ s_ v C n z 50-YR 6 20.00 7n 3 Left Bank 15.00 10.00 Right Bank z 5.00 - - - - - - - - - - — �. ,. Existing Flowline o.00 Number of Flooded -5.00 __ __ _ - Homes �10.00 . -15.00 ------ I -20.0a -25.00 i -30.00 (Outside City Limits) -35.00 0 5000 10000 15000 20000 25000 30000 k(otz associate B 106-00-00 Stream Distance (Rt) Big Island Slough REVISED EXISTING 50 YEAR CONDITION 'aecP No 0927 006 000 S ale NTS Date OC72608 B109-00-00 Spring Gully, 13109-03-00, and B112-02-00 10-YR WATER SURFACE PROFILES 30 7 ! c 3 — _ o Q Legend nd 25 i. p _ _ N l0-YR 13 0 � fi w i 20 _ - - - s - - - - - Left Bank 1 - Right Bank 15 l Existing Flowline �10 Number of Flooded 0 w -Homes i eC 5 -------- . ..... ....... _ W 0 _ _ __ _ I � _5 (inside Cly Units) -10 . _ ---- ------------ - ®. 0 5000 10000 15000 20000 25000 kiotz>;F:a ssocia 2es Stream Distance (ft) 00, an B109-00-81012E-02 00 REVISED EXISTING 10 YEAR CONDITION Prolec[ No.. 0127 0e8.00 Scale. NTS 5 mm _ date: OCT 2008 Legend mmmm 10-YR Left Hank Right Bank Existing Flowline Number of Flooded ®Homes — --- — ----------------- BI 12-00-00 Willow Spring Bayou 50-YR WATER SURFACE PROFILES 35.00 30.00 E 75 f4 L�qend 25.00 50-YR 20.00 Left Bank 15.00 10.00 Right Bank 5.00 . .... - - - - - - -------- — ------- —Existing Howline 0.00 Number of Flooded .2-5.00 --l-Honies 210.00 --------- -- -15.00 -20.00 -25.00 -30.00 (liside City Limits) -35.00 --- --- ---- ---- — — --- — -- — -- — -------------- — ---- — --------- 0.00 5000.00 10000.00 15000.00 Stream Distance (ft) B1 12-00-00 Willow Spring Bayou REVISED EXISTING 50 YEA'i CONDITION Pmect No. 0127.0 B000 9 — — ------------- .. .. .. .. .. -at'e-.-'ECT —20G8 Legend 1 OQ-YR Left Bank Right Bank Existing Flowline Number of Flooded Homes B112-02-00 Tributary to Willow Spring Bayou 10-YR WATER SURFACE PROFILES 25 1 Legend b 10-YR ( ® - - - * _.... _ ( Left Bank a .. ..r . i ....._ - Right Batilc Existing Ftowl ne Ss Number of Flooded • w Homes C� 0 10 _ (inside City Limits) 5 _ klotz"tassociates B112-02-00 Stream Distance (ft) Tributary to Willow Sprin s REVISED EXISTING i 10 YEAR CONDITION { 9 ,Pro ect Na: 012706860�. ...._-.-.....-- F216-00-00 Little Cedar Bayou 50-YR WATER SURFACE PROFILES 25.00 .... _ _ .... 20.00 _ __ _ _ _ ,. s ,w _ _ - ,� Legend o 15.00 _ _ _ .. v � E — � � � �_ _ -- ��. 'o y � z 1 50-YR 10.00 �. --- Left Bank 5.00 _ _..... ....... _..... Right Sank 0.00 v Existing Flowline r. -5 00 1 ----- _ _ -- _ C Number of Flooded = 10.00 Homes) ee -15.00 L_ 20.00------------ -25.00 -30.00--� _ I -35.00 — 0 5000 10000 assoP 15000 klot c,at �00 ._ Fz Stream Distance (ft) Little Cedar Bayou REVISED EXISTING 50 YEAR CONDITION 15 Scale NTS — , — - - — — _ H xxxxx x F216-00-00 Little Cedar Bayou 100-YR WATER SURFACE PROFILES ,--M — _ _ —.__ . --- --- (( 2, -_..._.........................................................._.................................____._ 5 __---...._.. ___ —__ . II r A v Legend 9 _ e s I00 YR _ 15 - ro — j Lett Bank I i 5 �f _.. Right Bank 0 . I Existing Flowline -5 C j Number of Flooded .010 __ _ _ _ Homes) 15 ._..... -20 -25 -30 0 5000 10000 15000 klotz associate_ F216-oa-oo Stream Distance (ft) i Little Cedar Bayou REVISED EXISTING a 100 YEAR CONDITION 16 ff2�T�- -.,.._ ...,. .. ®. .....e....... .......-.-.-,-,..-.-.... _.�. � -.. Date. OC 6 I� I CapacityInlet For Brookglen ` I F 0-00 0 La o o�F101-0 -0 W �915-00.0 P 0.00 8113-00-00 Legend t4 TSE Locations TSA Locations Severely Damaged Homes Streets Open Channel Drainage B110-00.0 100 Year Flood Plain City Inlet Capacity " r! For Fleetwood 500 Feet of Road Per Inlet { f f M MIMI Inlet Capacity For Glen Meadow Area 420 Feet Road Per Inlet <IotzQ associates EXAMPLES OF INLET CAPACITY REPORT # 3 C•TY OF LA PORTE C?TY-WIDE DRAINAGE STUDY vio�z €ea ro oiz�cos ooa EXHIBIT i DATE OCTDEER - 112 CHAPTER 5 • 70 Drainage Criteria for development within the City of La Porte, and where applicable within La Port Extraterritorial Jurisdiction (E.T.J.) is dependent upon the size and type of development, the conditico the receiving stream--bEI channel, roadsil-- swale, culvert, • roadway. I The basic objective of this policy is to minimize the threat of flooding to areas within the City and its E.T.J. and to minimize the effect of continued development on individual watersheds. 1. The City believes that the best long-term means of accomplishing its objective is a continued program of improvement and extension of the Harris County Flood Control District's system of ••- channels. This statement recognizes the technical reality that an essential prerequisite to an effective flood control program is a system of open channels capable of carrying storm runoff of any type in Harris County to Galveston Bay without adversely impacting existing urban areas adjacent • the channels. F— The City recognizes that both District's and City's existing open channel system is, in many iristances, inadequate to accomplish the goal of eliminating existing flooding conditions for existing levels • •. development • for ultim ate development in the watershed. 3. The City • recommends that where required, certain additional flood control facilities be utilized to supplement the open channel system. Such flood control facilities shall be designed to preclude flooding in areas that do not presently flood and not micrease flood levels where flooding now occurs. Specifically, the City supports the use of storm water detention to supplement the open channel system until long-term channel improvements can be completed, or as permanent facilities where additional open channel improvements are not feasible. The result will be that new development will limit or restrict the impact downstream. 5.1.1 All the • plans and construction shall meet ♦ exceed the requirements • the City of Porte, Han -is County Flood Control, Harris County, TxDOT, or any other entities ha jurisdiction •' a facty (i.e. roadway, channel, etc.). I Unless otherwise provided for in these policies, development shall follow the Harris County Flocm Control District Criteria Manual for the desim of Flood Control and Drainage Facilities in Hal County, Texas. 5.1.3 If application of the policies and criteria contained in this document conflict with the City's dutiM under the Flood Hazard Prevention Ordinance, the regulations of the Flood Hazard Preventi Ordinance shall apply. i 5.1.4 Drainage structures shall be constructed in such locations and of such size and dimensions adequately serve the development and the contributing drainage area. Iri new developments, developer shall provide all the necessary easements and rights -of -way required for cdirai;al structures, including storm sewer and open or lined channels. 5.2 CONSTRUCTION PLAN REQUIREMENTS 5.2.1 A drainage map shall be included in the construction plans. The drainage area map shall include: 1111� F ii 11 "i �� I Stormwater Criteria Page 1 of 10 B. Design storm runoff, based on the type of facility and storm frequency listed in Section 5.4. C. 100 — year runoff D. Route of overland flow including the overflow to a drainage channel or detention facility E. Water surface profiles for the 25-year and/or 100-year storms in the outfall channel. All available information will be considered when malting this determination- F. Flow per inlet G. Maximum ponding elevation 5.2.2 Detailed drainage calculations shall be submitted with the constriction plans. 5.2.3 A lot grading plan should demonstrate that the finished grading plan will drain to approved collection and discharge points and that the overland flow of water from adjacent properties will not be impacted 5.2.4 The hydraulic gradient for the design storm may be shown on the construction drawings. Calculations for the elevation for the by ._gradigl shall be_provided with the design storm drainage calculations. The hydraulic gradient must be below the gutter jive for the design sto m The tailwater elevations ased on a 25-y�y~in the outfall channel shall be— used fo> calculations of the hydraulic gradient, - 5.3 USE OF PREVIOUSLY DESIGNED AND INSTALLED INFRASTRUCTURE Situations where previously installed infrastructure is in place but not yet utilized to its design capacity will be considered on a case by case basis. The developers engineer shall after consultation with the City, prepare a report that: a. outline the original design criteria b. evaluates the impact of the original design on the receiving stream, adjoining properties and/or the 100 year Floodplaim Based on the report, the City Engineer may allow full or partial use of the previously installed infrastructure and may require it to be supplemented with detention or other facilities. 5.4 STORM FREQUENCY, RUNOFF AND DATUM A Storm Frequency All drainage improvements shall be designed for the following storm frequencies. Type of Faciti Road Side Ditches and Culverts 3 year Storm Sewers 3 year Ditches & Culverts Drainage 100 acres and more 25 year Bridges 100 year Creeks 100 year Detention Facilities Refer to Section 5.5 B. Storm Runoff Design flow of storm water rmoff is to be calculated using the Rational Method - The Rational Method is based on the direct relationship between rainfall and runoff and the method is expressed by the following equation: Stormwater Criteria Page 2 of 10 Q = CIA, where Q is the storm flow at a given point in cubic feet per second (c.£s). C is a coefficient of runoff (see Table 1). I is the average intensity of rainfall in inches per hour for a period equal to the time of flow from the farthermost -point of a area to the point under consideration. (See`f e 1, I D-F Curves and Figure 2, etermination of Time UJ-k," of Concentration) A is the drainage area in acres The size and shape of the watershed must be determined for each installation The area of each watershed may be determined through the use of planimetric-topographic maps of the area, supplemented by field surveys in areas where topographic data has changed or where the contour interval is insufficient to adequately determine the direction of flow. The outline of the drainage area contributing to the system being designed and outline of the sub -drainage area contributing to each inlet point shall be determined. When calculating the peak flow rate of storm runoff; rainfall intensity will be determined from the rainfall intensity, duration and frequency curves, shown in Figure 1. The storm frequency used for this determination will be according to the facility to be designed as listed in Section A Runoff Coefficients and Time of Concentration Runoff coefficients, as shown in Table 1, shall be the minimum used, based on total development under existing land zoning regulations. Where land uses other than those listed in Table 1 are plarmed, a coefficient shall be developed utilizing values comparable to those shown. Larger coefficients may be used if considered appropriate to the project by the City Engineer. The time of concentration is defined as the longest time, without unreasonable delay, that will be required for a drop of water to flow from the upper limit of a drainage area to the point of concentration. The time of concentration to any point in a storm drainage system is a combination of the "inlet time" and the time of flow in the drain. The inlet time is the time for water to flow over the surface of the ground to the storm drain inlet_ Because the area tributary to most storm sewer inlets is relatively small, it is customary in practice to determine the inlet time on the basis of experience under similar conditions. Inlet time decreases as the slope and the imperviousness of the surface increases, and it increases as the distance over which the water has to travel and retention by the contact surfaces. Time of concentration shall be computed fromi —e 2 'and in no case shall the inlet time be more that the time shown in Tablel . , I Stormwater Criteria Page 3 of 10 Table 1 Zone Zoning District Name Runoff Coefficient "C" Maximum Het Time in Minutes R_1 Low Density Residential 0.50 15 R-2 Mid Density Residential 0.60 15 R,_3 MH High Density Residential Manufactured Housing District 0.80 0.55 10 15 NC Neighborhood Commercial District 0.80 10 GC BI General Commercial District Business Industrial Park District 0.85 0.85. 10 10 LI Light Industrial 0.85 10 HI Heavy Industrial 0.85 10 PUD Planned Unit Development District variable 10 to 15 Nliscellaneous Land Uses Land Use Runoff Coefficient "C" Church 0.70 to 090 School 0.50 to 0.90 Park 0.30 to 0.70 C. Datum All drainage plans shall be prepared based on United States Geodetic Survey datum, 1978 adjustment, consistent with National Flood Insurance Program, Flood Insurance Study for the City of La Porte. 5.5 REQUIRED DETENTION Detention Basins — Unless otherwise provided for in this Section, Detention Basins will be required for developments within the following watersheds. Little Deer Creek — F212 • Upstream of Main Street Little Cedar Bayou — F216 • Upstream of State Highway 146 Big Island Slough— B 106 • All segments Spring Gully — B 109 • All segments Willow Spring Bayou — B 112 • All segments Taylor Bayou — Al04 • All segments The listed watersheds are shown oAFigure 3.i Stormwater Criteria Page 4 of 10 5.5.1 Design Standards for Detention Basins Detention requirements for developments less than 50 acres shall be according to the following table. For developed areas of 10 acres or less, the required volume equals the total development area times the appropriate storage coefficient. For areas greater than 10 acres and less than 50 acres the volume is determined by applying Harris County Flood Control District criteria for small watersheds. Table 2 Devel ed Area Stara e Coefficient 0 to 3 acres 0.20 acre ft. / acre 3 to 10 acres 0.45 acre ft. / acre 10 to 50 acres per HCFCD criteria For developments larger than 50 acres, Harris county Flood Control District and the City Engineer shall approve the detention facility criteria. 5.2.2 Outlet Sizing The outlet structure shall be designed using the orifice equation as follows: Q = CA x (2ghr Where, C=0.8 A= cross sectional area g = 32.2 feet / see2 h = head differential For head differential use 2' or the 100-year water surface in pond minus th 25-year water surface in receiving channel, if available. 2. Minimum restrictor shall be 6' diameter. 5.5.3 Additional Standards for Detention Basins The detention facility shall be designed for easy maintenance. For smaller developments the designer is encouraged to use parking lots, underground piping, swales, green spaces, etc. to achieve the volume required. For larger developments every consideration shall be given to designing of the facility for multipurpose use, such as playgrounds, miniparks, required green spaces, etc. to assure that maintenance will be accomplished. The design shall include the following: 1. an earthen detention basin shall have minimum side slopes of 4:1 and a minimum bottom width of ten feet; 2. the bottom of the detention basin shall have a minimum 0.50% cross slope to facilitate quick drainage. 3. a v-shaped trickle channel a minimum of 5' wide, six-inch thick, reinforced concrete shall be constructed through the detention basin at a longitudinal slope of 0.20% to accommodate low flow and facilitate rapid drainage. For developments less than 3 acres, the trickle channel may be 2 feet wide and 4" thick. 4. a minimum 12-foot wide maintenance berm shall be provided around the perimeter of the detention facility. 5. ingress and egress for maintenance including a dedicated right-of-way if required, shall be provided to the detention basin and clearly shown on the construction drawings or site plan subdivision plat. Stormwater Criteria Page 5 of 10 6. the detention basins, slopes, bottom, maintenance berm, and other associated right -of --way shall be final graded with a minimum of 6" top soil the hydro -mulch or drill -seeded and watered to facilitate full grass coverage. 7. parties responsible for maintenance of the detention facility must be shown on the plat or plans. 8. pumped detention systems may be allowed with specific approval from the City Engineer. 5.5.4 Ownership and Maintenance of Facilities The City will not accept maintenance of on site facilities that serve only one tract or development, unless it is determined to have other public benefits, is recommended by staff and approved by the PIanning and Zoning Commission. Harris County Flood Control District may, at their discretion, accept maintenance of facilities, provided they are designed in accordance with the District's criteria manual. Requests for Harris County Flood Control District to assume maintenance of any facility should be coordinated with the City prior to any development approvals. 5.6 ADDITIONAL DESIGN REQUIREMENTS 5.6.1 Discharge Points The developer shall terminate all drainage improvements at a discharge point approved by the City. The developer shall design and construct such discharge point, or outlet, to prevent damage to or overflowing into adjacent property. The City may require creek improvement, channel lining, energy dissipaters or other improvements for such outlet to prevent erosion or increase the flow capacity. Finished elevations of new pavement, parking areas, or other improvements shall be designed so that each succeeding high point is lower when moving in a downstream direction. This ensures the 100-year discharge has an unobstructed path to the discharge point whether discharging to a channel or detention pond 5.6.2 Public Streets as Drainage Facilities 1 1. Maximum depth of water to be allowed in local streets during design flow shall be at the top f crown, or top of curb, whichever is less_ 2. Maximum spread of water in collector streets during design flow shall allow for one clear lane of traffic (12 feet wide)_ 3. Maximum spread of water in arterial streets during design flow shall allow for two clear lanes of traffic (24 feet wide). 5.6.3 Drainage Channels and Structures 1. The developer shall install an underground storm drain on curb and gutter streets beginning at the point where calculated storm water runoff is of such quantity that it exceeds the height specified above. The developer shall construct the storm drain system from the point to an approved outlet. 2. For non -curb and gutter streets, the developer may use open channel (channel or ditch) methods -�o dispose of storm water specified above. Such channels may be in dedicated draining easements outside the standard street right-of-way upon City approval of the location and alignment of such easements. Alternatively, the developer may widen the street right-of-way to accommodate an open channel of greater capacity than the standard street/ditch section. 3. If the developer locates the channel in a widened street right-of-way, the City shall approve the right-of-way width and channel configuration. the depth of flow in the channel shall not exceed one (1) foot as measured from the ditch flowline to the point on the roadway established as the high water level in this section. Storrnwater Criteria Page 6 of 10 4. The developer shall design and construct all channels to terminate at an approved outlet. 5.6.4 Habitable Structures The developer shall provide adequate means for storm water run off in excess of the "design storm" capacity (i.e., 3, 10-year storm) to flow around habitable structures. a.) The developer shall provide a grading/drainage plan which shows that all building sites can provide a finished floor elevation: (1) At least one foot(l') above the top of the curb using the highest point along the portion of such curb fronting the building site, or (2) At least eighteen inches (18') above the crown of the road elevation, using the highest point along the portion of such road fronting the building site. (3) At least on foot above the ground elevation along all sides of the building site. b.) In addition to paragraph (a) above the d I=1 er shall proyide a grading/drFp' ye pl which meets or exceeds the provisions of�ha�ter 94, Code of Ordinances, F ood Hazard Reduction. c.) The developer shall design and constnict all streets to minir tze any fill required to bring building pads into compliance with this code. d) Alternate methods of building protection may be accepted by the City upon submittal of detailed information, review and approval by the City Engineer. 5.6.5 Drainage System Criteria If an underground drainage system is required, and a 60-inch or smaller pipe will handle the design slow, pipe sball be used. If a 60-inch pipe is not adequate, the developer has the option to use concrete pipe or natural and/or a lined drainage channel_ If pipe is selected, the maximum allowable velocity shall be 8fps in the pipe. Lining materials, if used, shall be approved by the City. 5.6.6 Public storm sewers are defined as sewers and appurtenances that provide drainage for a public right-of-way, or more than one private tract, and are located in public right-of-way or easement, private storm sewers provide internal drainage for a reserve or other tract Private storm or sewer connections to public storm sewers shall occur at a manhole or at the back of an inlet as approved by the City Engineer. All private storm sewers within the public right-of-way shall be constructed in conformance with the Standards. 5.6.7 All construction shall conform with the City of La Porte Construction Details. 5.6.8 All storm sewers shall meet or exceed the requirements of the "Drainage Criteria Manual fors Harris County, Texas" and the requirements of the City of La Porte. 5.7 LOCATION OF STORM SEWER 5.7.1 Public storm sewers shall be located within a public street right-of-way or storm sewer easement, dedicated to the public and adjoining a public street right-of-way. Storm sewers through side lot drainage easements are highly discouraged. Limited use may be approved at the discretion of the City Engineer. If approved, a minimum twenty -foot (20') wide easement is required (10' on each lot). 5.7.2 Recommended alignment within a public street right-of-way. A Boulevard pavement section with median — along centerline of the right-of-way. stomr=er Criteria Page 7 of 10 B. Undivided pavement section five- feet (5') inside the right-of-way. For storm sewer located in a public street right-of-way, a minimum of two -foot (2') shall be maintained inside the right-of-way line to the outside edge of the storm sewer unless otherwise accompanied by an adjacent easement C. Alternate locations for a storm sewer will be permitted by the City Engineer. 5.7.3 Recommend alignment within an exclusive storm sewer easement. A Stone sewers placed in easements shall conform to the requirements o Section 2.4.5 B. Storm sewers within easements shall be placed no closer than five feet ) measured from the outside edge of the pipe to the edge of an easement, except when adjoining another easement or public right-of-way where the distance may be reduced to two feet (2'). The storm sewer shall be placed in the center of the easement. When the storm sewer easement adjoins a public right-of-way, the easement may be reduced to a minimum of ten feet (10') and the storm sewer may be aligned close to the right-of-way line, as long as required clearances are met, with specific approval of the City Engineer. 5.8 STORM SEWER MATERIALS 5.8.1 Storm sewer and culvert pipe shall be precast reinforced concrete pipe, unless specifically approved by the City Engineer. Concrete pipe shall be manufactured in conformance with the requirements of ASTM C 76, " Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe," current revision. Reinforced concrete pipe shall be.Class III or stronger. The design engineer shall provide for increased pipe strength when conditions of the proposed installation exceed the allowable load for Class III pipe. All concrete pipe constructed in water -bearing soil or forty-two inched (42") in diameter or larger, shall have rubber gasket joints meeting the requirements of ANSI/ASTM C 443, "Joints for Circular Concrete Sewer and Culvert pipe, Using Rubber Gaskets", current revision. Concrete pipe with diameter of less than forty-two inches (42") may be installed using pipe with tongue and groove type joint and Ram-nek, or approved equal, as a joint filler. When specifically approved by the City Engineer, reinforced concrete arch and elliptical pipe conforming to ASTM C506 and ASTM C507, respectively, current revision, may be installed in lieu of circular pipe. Reinforced concrete box culverts shall meet the .minimum requirements of ASTM C789, "Precast Reinforced Concrete Box Sections for Culverts, Storm Drains, and Sewers", current revision. pipe joints for arch and elliptical pipe and box culverts shall be sealed using Ram nek or approved equal. 5.8.2 Storm sewer outfalls into open channels shall be constructed using corrugated steel pipe_ Corrugated steel pipe shall be manufactured in conformance with the requirements of AASHTO Designation M 36-82, current revision. Pipe material shall be Aluminized Steel Type 2, meeting the requirements of AASHTO Designation M-27-79L current revision, or Pre -coated Galvanized Steel, AASHTO M-246, 10 mil coating on both sides. All pipe shall have a full double coating, Type A, in accordance with AASHTO Designation M-190, current revision. Pipe joints and fittings shall meet the minimum requirements of these specifications and shall have an 0-Ting gasket seal meeting the requirements of AASHTO C-361, current revision. (See City of La Porte Construction Details). 5.8.3 Stone sewer outfalls shall have a slope protection to prevent erosion. Slope protection may be constructed of slope paving or rip rap. Slope paving shall be four -inch (4") five (5) sack concrete with six-inch by six-inch (6" x 6") welded wire mesh (W14 x W14) or three eighths inch (3/8") steel rebar on twenty -four -inch (24") centers, each way. Rip rap shall be a minimum of six-inch (6") broken concrete rubble with no exposed steel or well-rounded stone and shall be a minimum of eighteen inched (I8'� thick Slope protection texturing shall be required where public access likely. Refer to the Construction Details for minimum dimensions. Stormwater Criteria Page 8 of 10 5.9 ADDITIONAL REQUIREMENTS 5.9.1 Minimum depth of storm sewer (measured to the top of pipe) shall be twenty-four (24") below the top of curb or finished grade, whichever is lower. Minimum size storm sewer for main, land and inlet lead shall be eighteen inch (18'). 5.9.2 Storm sewers shall be bedded using cement stabilized sand (See specification in Section 4.2.3) as shown in the City of La Porte Construction Details. 5.9.3 Pipe requirements. A Reinforced concrete pipe installed at a depth greater that thirty feet (30') shall be designed by the engineer for the specific installation and approved by the City Engineer, Reinforced concrete pipe shall be designed in accordance with the American. Concrete Pipe Association, "Concrete Pipe Design Manual", Maximum cover on the pipe shall be measured from the top of pipe to the ultimate finished grade or natural ground, whichever is greater. B. Corrugated steel pipe shall have a minimum thickness as follows: PIPE SIZE MINDvfUM THICKNESS (Inches) Corrugations (inches 242-2/3"X 1/2" 0.052 30- 482-2/3" X 1/21 0.064 54- 723'tX 1" or 511 X 1" 0.064 78- 102311 X 1" or 5" X 1" 0.079 Bedding for corrugated steel pipe shall be cement -stabilized sand. Corrugated steel pipe less than or equal to fifty-four inches (54') in diameter and less than thirty feet (30') deep shall have the minimum thickness given above. C. Design storm flow in a street shall not exceed the capacity of the street, for the water surface equal to the top or curb and shall not exceed the inlet capacity. Design storm flow shall meet Harris h County criteria. C,Mj. D. (All bridges must be a minimum of eighteen inches (18") above the 100-year water surface s elevation or in accordance with the Federal Emergency Management Agency (FEMA) $ f regulations, latest revisions, or HCFCD requirements, whichever is greater. _ 5.9.4 Storm sewers less that forty-two inches (42") in diameter shall be constructed on a straight horizontal and vertical alignment between manholes. Storm sewers greater than or equal to forty- two inches (42') in diameter may be laid along a curve using manufactured bends of less than or equal to I 1°/a'. 5.10 APPEARANCES 5.10.1 Manholes A Manholes shall be placed at all changes in alignment, grade and size of the storm sewers; at the intersection of two or more storm sewers; at all inlet leads; and at the end of all storm sewers. B. Maximum spacing between manholes shall be six hundred feet (600') C. Manhole covers shall be cast iron, traffic bearing, type ring and cover with the words "storm sewer" cast into the cover. Starmwater Criteria Page 9 of 10 5.10.2 Inlets A Curb inlets shall be spaced and sized to intercept the calculated runoff for the design storm. The water surface elevation at the inlet shall be Iess than or equal to the top of curb for the design storm flow. S. Maximum travel distance of water in the street to a curb inlet shall be three hundred feet (300') on a major thoroughfare and in a commercial area. The maximum travel distance of water in the street permitted in a single-family residential area shall be four hundred feet (400'). C. No Valley Gutter without prior approval. D. Curb inlets should be on the intersecting side street at intersections with a major thoroughfare. The City Engineer shall specifically approve locations at intersections. E. Grated inlets will not be permitted in an open ditch- F. 13ackslope swale interceptors shall be placed in accordance with the requirements of Harris County. G. Curb inlets shall have ho d inlet ids Grate or Curb and Grate inlets shall not be allowed`=� for residential subdivisions. Curb inlets shall be recessed, unless otherwise directed by the City Engineer. H. Backfrll around inlets d to top of first stage inlet with cement stabilized sand. Stormwater Criteria Page I0 of 10 City of La Porte. A � ff1 � IE; intensity vs. Time of Concentration vs. Rainfall Frequency (IDF Curves) Source: Hydro 35rrP-40 ���5����■�MON����i�■ HE ME MEN 1 =_ IntensityP. b FIGURE I (d +TC)e Rainfall Frequency b d . e 2-year 75.01 _ 16.2 0.8315 3-year 77.27 17.1 0.8075 5-year 84.14 17.8 0.7881 10-year 93.53 18.9 0.7742 25-year 115.9 21.2 0.7808 100-year 125.4 21.8 0.7500 s zy • �� Lam. o t � 7CC! - 5Gi?,(r-•c�C3 25 25- Sca �� 400 Cy,��.t7ar: f ZO 2 1 tai 1 t iU Ci - 3'JO � wr•r z 24Q sif !` 2` l,DQ 15 L5 COB IFGCR GROG) 15:3 =04 W; y1Z, + p 10^ h 0 c 0{ u;z G cAsS i� � so IU; 20.0. ]a. LO u: 4r� EOM Q!Q O as 70 to SG o` 0-6 OVERLAND r Laa GUTTER ELMY L = 200 L = -SOO' • - .: = 0.40 i= WAss; n = a-Z.75 - + _ J. TC-srA: TEME OF GONG RTRAT:Q?: = 20.o + 2.L = 2Z.0 —5 0.5 --4 0 4 3 p5'" FIGURE 2 TIME OF CONCENTRATION FOR SURFACE FLOW As 1 � CURVE FOR DETERMINING � STORAGE COEFFICIENT i I I I I I I 0.60 0.50 v rw 0.40 z Li 0.30 L ;7, a U 0.20 w 0 0.10 --- ---- No Fig W10■��i�.'�MiENNEMNINIMBILIN Ell ....■ o■gyp..■■..■..W.■.... � .� �.:■ ■■■■■ ■■.�V� .■N' OXIMN ■ �� . ■.H.■ ..■■a .■■■..H■ WORSE; So MEN RON ■ ERN pONES IS MAXIME no ■H .... ...■■■■..■ NOW .■■. ■N.■ NOW ■■■ ■.H MEN SON. � j.jH■�■H .■S 0 ■ o© .'. .. ...n WERE ■. ��■n t��'� u m ��i ■iiRQm 0 ■..■.■a �■ .■ ■=.UMBER 0 � I man KNEIR mom .. . M■■ B..B....la.■.■ 0 3 10 20 30 40 DRAINAGE AREA (ACRES) S = CSA. FIGURE 4 APPENDIX B RAINFALL PARAMETERS AND AMOUNTS FOR LA PORTE COMPARISON TABLE RAINFALL PARAMETERS AND AMOUNTS FOR LA PORTE COMPARISON TABLE city of La Porte note 1 TxDOT Rainfall Frequency ears b d e b d e 2-year 75.01 16.2 0.8315 68 7.9 0.800 3-year 77.27 17.1 0.8075 --- -- --- 5-year 84.14 17.8 0.7881 70 7.7 0.749 10-year 93.53 1 &9 0.7742 81 7.7 0.753 25-year 115.90 21.2 0.7808 81 7.7 0.724 50-year --- --- --- 91 7.7 0.728 100- ear 125.40 21.8 1 0.7500 91 1 7.9 0.706 City of La Porte Inches/hour for various times of concentration minutes Rainfall Frequency ears 10 20 30 60 120 1440 2-year 5.0 3.8 3.1 2.0 1.3 0.2 3-year 5A 4.2 3.4 2.3 1.5 0.2 5-year 6.1 4.8 4.0 2.7 1.7 0.3 1 0-year 6.9 5.5 4.6 3.2 2.1 0.3 25-year 7.9 6.4 5.4 3.7 2.4 0.4 50-year --- --- --- --- --- --- 100- ear 9.4 7.6 6.5 4.6 1 3.1 0.5 TxDOT Incheslhour for various times of concentration minutes Rainfall Frequency ears 10 20 30 60 120 1440 2-year 6.8 4.7 3.7 2.3 1.4 0.2 3-year --- --- --- --- --- --- 5-year 8.1 5.8 4.6 3.0 1.9 0.3 10-year 9.3 6.6 5.3 3.4 2.1 0.3 25-year 10.1 7.3 5.9 3.8 2.4 0.4 50-year 11.2 8.1 6.5 4.2 2.7 0.5 1 00-ear 1 11.9 1 8.7 1 7.0 1 4.6 3.0 0.5 TSARP Technical Paper Recommendations (Region 3, which includes Average inches/hour for various times of concentration minutes Rainfall Frequency ears 10 20 30 60 120 1440 2-year no recommendation 5-year 10-year 25-year 50-year 1 00-ear -- --- 3.0 2.0 1.2 0.2 --- --- --- -- --- --- -w --- 3.8 2.5 1.6 0.3 -- -- 4.2 2.9 1.9 0.3 -- --- 4.8 3.4 2.2 0.4 --- - I 5.4 3.8 1.3 0.5 --- - 6.0 4.3 2.9 0.6 City of La Porte Inches of rain for various times of storm durations(minutes Rainfall Frequency ears 10 20 30 60 120 1440 1 00-ear 1.6 2.5 3.2 4.6 6.1 12.7 TxDOT Inches of rain for various times of storm durations minutes Rainfall Frequency ears 10 20 30 60 120 1440 1 00-ear 2.0 2.9 3.5 4.6 5.9 E 12.8 Note 1. Same Coefficients as City of Houston, Harris County. Note 2. HCFCD 24 hour 100-year rainfall for Galveston Say, as based upon TSARP data, is 13. 5 inches. Compare to the rainfall for the 100-year event for 1440 minutes = 24 hours Klotz Associates Project No. 0127.008.000 City Wide Drainage Study October 2008 City of La Porte