02-18-08 Drainage and Flooding Committee Minutes

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CITY OF LA PORTE PLANNING DEPARTMENT 604 W. Fairmont Parkway, La Porte, TX 77571 www.iaportetx.gov Phone: (281) 471-5020 I Fax (281) 470-5005 July 31, 2007 RE: Drainage in Fairmont Park East Subdivision Dear Resident: This letter is in response to the concerns of the residents living in the Fairmont Park East subdivision about recent drainage issues. During the week of July 23, 2007, through July 27, 2007; and most notably during the rains on Thursday morning of that week, the Staffs of the Planning and Engineering Department and the Public Works Department went through your subdivision to assess the functioning of the drainage system. I, along with the City Engineer and the Public Infrastructure Coordinator noted the high water levels in many of the cul-de-sacs and intersections throughout the neighborhoods. We later met with several members of Public Works on site to compare ideas. STORM SEWER DESIGN In order to fully understand the issue, some background information is needed. Your storm sewer system is designed for what is known as a 3-year storm event. Ratings are based basically on two parameters, storm intensity (the amount of rainfall in a given time) and the total duration/volume of water conveyed by the system (measured in cubic feet per second or cft). As such; as-year, 10-year, 25-year, or 100-year storm system would be larger because those storms are expected to occur at those intervals and are of greater intensity and volume. It is common for the Houston region to design on the 3-year storm event. The inlets for your system are designed on the 3-year criteria. The capacity of these inlets, used for design purposes, is 5 cfs per inlet. If the drainage area discharging to anyone inlet is greater than its capacity, then a 5' (extension can be added to allow more water to get into the stann sewer system. Also, inlet spacing is very) important in roadway design. Most municipalities require an inlet spacing of less than } ,000 feet with La Porte now requiring an inlet spacing of 800' for resid~n1ial.2treets. Flooding in streets at the inlet fac~js allowed as,~ng as traffic can safely pass. ~..,. If the storm surpasses the 3-year event, streets are designed to convey water to the detention pond which is designed for the 1 DO-year event. The storm on Thursday of the week in question surpassed the 3-year event in the intensity (the amount of rainfall in a given time) and the storm surge backed up into the streets and intersections. Once the rain stopped, the drainage had a chance to catch up and most intersections were clear within 10 to 20 minutes. Unfortunately many cars were parked and not moved from the streets. Residents, knowing the function of the street in a storm event over the 3-year intensity, must make preparations to move cars from the street onto driveways. As far as I could tell, the cars in the driveways faired well.