Drainage_and_Flooding_Report-dated_10_03_2008

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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.