Drainage_and_Flooding_Report-dated_10_03_2008

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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