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<br />San Marcos Floodplain Management Plan: A Sustainable Future for Floodplain Management <br />Application for Flood Protection Study Grant <br /> <br />City of San Marcos engineering records. Times of Concentration and the <br />corresponding lag times will be computed using the TR-55 method. <br /> <br />Particular attention will be devoted to rainfall intensity and storm. frequency. <br />Coordination with GBRA, the USGS, and the National Weather Service may <br />provide additional insight into developing the most appropriate and accurate <br />:frequency scenarios. A frequency analysis based on historical record of discharge <br />will be performed for appropriate gauges on the Blanco. These frequencies will <br />be compared to the recently-developed USGS regression equations and the <br />current FIS flowrates. <br /> <br />7) Hvdraulic Model Development - Using existing model data from FEMA, the <br />collected field SUlVey data, information from design plans, and recent LIDAR <br />topographic data, the existing conditions hydraulic models will be updated and <br />converted to HEC-RAS format. The HEC-RAS model will be geo.referenced for <br />correlation with the City GIS data, using the HEC-GeoRAS extension in ArcView <br />3.2. Flood profiles for the 2-year, to-year, 25-year, and tOO-year frequency storm <br />events will be developed for the existing watershed conditions. A flood profile <br />for fully developed tOO-year watershed conditions will also be developed. The <br />hydraulic model may also include a floodway run for existing conditions. <br /> <br />8) Evaluation of Flood Protection Criteria - A review of existing design flood <br />criteria (2-yr, lO-yr, 25-yr, tOO-yr, 100-yr ultimate) will be performed for the <br />problem areas and a determination of a desirable or acceptable level of protection <br />within each problem area will be made. <br /> <br />9) Review of Flood Protection Measures and Alternatives to be Evaluated - Flood <br />protection measures to be considered may include the following structural and <br />non-structural measures as independent or combination solutions. For the Blanco <br />River reach (a known problem area), an upstream detention reservoir, a bypass <br />channel, or a pump and levee system may be evaluated, among others. For other <br />areas, channel improvements and culvert upgrades may be viable structural <br />alternatives. Buy-out and flood-proofing measures may also be evaluated as <br />alternatives, particularly for repetitive loss structures and public infrastructure and <br />utilities. With input from the advisory committee, several scenarios will be <br />evaluated. Depending on their complexity, it is anticipated that no more than six <br />scenarios will be identified and evaluated under existing and future conditions. <br /> <br />10) Hvdrologic and Hvdraulic Analysis of Alternatives - An analysis of the effects of <br />each alternative scenario and resulting level of flood protection with respect each <br />flood events. A detailed report that summarizes the technical aspects of the study <br />and modeling results will be prepared, which will include a comprehensive <br />presentation of the methods of analysis, summary of results, exhibits, and model <br />output. <br /> <br />M:lactrve\04POI2 San Marcos M.D.P\Grant Applic-SMa.liKhibft5A, Page 19 <br /> <br />December 16. 2004 <br />