06-03-09 Drainage and Flooding Committee minutes

Browse Search

2.5.1 Design Frequency: Alternatives were evaluated based on a 5-year design storm for the main trunk line using a 25-year water surface in the receiving channel and adding additional inlets to accommodated increases in runoff. The 25-year starting water surface is at an elevation of about 15.8 (NA VD 88, 2001 Adj.) and was determined by plotting the 100-year water surface (EL=18.0) and 10-year water surface (EL=14.4) on Log-Normal paper. Evaluation of a 3-year design and 5-year design resulted in similar hydraulic results. The resulting cost differences between a 3-year and 5-year design is approximately twenty percent (20%). Fairmont Park East is located in an area that does not have overland relief for sheet flow for the extreme even; for this reason, the storm systems presented as Alternatives were developed based on a 5-year frequency for additional level of protection. 2.5.2 Hydraulic Grade Line Ponding along adjacent side streets, that cross Fleetwood Drive, is caused by surcharged pipes resulting in the hydraulic grade line (HGL) emerging above natural ground. When the HGL is kept below the pipe soffit or below natural ground then the storm sewer is considered adequately. Fleetwood Drive is approximately 1.0 feet to 1.5 feet above the sag locations of the crossing streets; therefore, to keep water from ponding in these areas the theoretical HGL should be at about 1.5 feet lower than the profile of Fleetwood Drive. Each of the alternatives presented here keep the HGL below the profile of Fleetwood Drive until Linwood or Spruce Dr. South. This is can is easily remedied based on additional capacity downstream. For the purpose of this report, the areas of repetitive losses were the focus. The added cost of increasing pipe sizes is negligible for the additional protection upstream. 2-6 Klotz Associates Project Noo 0127.009.000 May 2009 Fairmont Park East & Brookglen Drainage Study City of La Porte