(A) Computations to support all drainage designs shall be submitted to the appropriate city departments for review. The computations shall be in such form as to allow for timely and consistent review and also to be made a part of the permanent city record for future reference. Computations shall include the impact of the proposed development to the downstream properties and existing structures adjacent to the drainage resulting from the 100-year event. All computations submitted shall be certified by a Professional Engineer licensed in the State of Texas. The Engineer shall provide the report to the city in both hard copy and a scanned electronic .pdf file with the proper seal, signature and date.
(B) Determination of runoff. Numerous methods of rainfall-runoff computation are available on which the design of storm drainage and flood control systems may be based. The Rational Method shall be an acceptable means of computing runoff for drainage areas of 200 acres or less when designing streets, storm drainage systems, channels and culverts. When the drainage area exceeds 200 acres in size, the National Resource Conservation Service (NRCS) (formerly the Soil Conservation Service) hydrologic methods (available in TR-20, TR-55 or HEC) should be used.
(C) Detention pond storage determination. A flow-routing analysis using detailed hydrographs must be applied for all detention pond designs. The NRCS hydrologic methods (available in TR-20, TR-55, HEC-1, HEC RAS and the Hydrologic Engineering Center (HEC)) hydrologic methods may be used for areas of 200 acres or more. Use of the Modified Rational Method is limited to drainage areas less than 200 acres. If additional methods become available, they must be approved by the city prior to submittal of plats.
(Ord. 2007-18, passed 6-26-07)