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San Marcos Floodplain Management Plan: A Sustainable Future for Floodplain Management Application for Flood Protection Study Grant Cottonwood Creek, Willow Springs Creek, Purgatory Creek and Bypass Creek. Such a plan must consider the complex nature of flooding and its risk mitigation with both structural and non-structural solutions, while achieving the following goals: · Minimh'e the exposure of lives and property, · Serve multiple objectives (such as flood mitigation, erosion control, water quality control, and potentially water supply), · Recognize fundamental and unique ecosystem relationships, · Coordinate multiple jurisdictions, entities, and other planning efforts, · Enable the collection, sharing, and analysis of data for the future; and ultimately, · Share the risk of flooding, and the benefits of multi -objective mitigation efforts equitably across the community. Utilizing this framework, the community can best determine its priorities for effective stormwater and floodplain management. Furthermore, the hydrologic and hydraulic study effort associated with the existing watershed conditions will be compatible with potential future flood insurance studies that may be conducted by FEMA at some point in the future. The proposed planning is needed because despite these efforts to-date, the nature of the watersheds in the City's jurisdiction has changed dramatically since the last FIS study. Since 1989 (the date of the effective study), the population of San Marcos has grown by 56%, and the City's jurisdiction has grown by nearly 46% since 1995 (Somce: City of San Marcos population estimate, 1990 Census, 1995 San Marcos Horizons Plan, City of San Marcos GIS). Further, although new county-wide mapping efforts have been made through FEMA's Map Modernization Program, these maps still may not reflect the most current and accurate conditions. Currently, tlooding disproportionately affects the lowest income residents of the City. The City's median per capita income is $13,468. A map demonstrating the percent of median per capita income of the residents affected by various floodplain areas is provided as Attachment 4. There are several issues which affect the City's ability to be proactive with floodplain management. For one, the backwater effects of the Blanco and San Marcos Rivers, near their confluence, is not fully understood, and special modeling detail may be necessary to understand this backwater and its effect on secondary drainage systems in the area, as well as the hydrograph combination of these two watersheds. Secondly, the City's flood forecasting ability on the Blanco is limited to distant and sometimes unreliable discharge gauges and field measurement of flood stage. This significantly hinders the City's ability to make timely and sound decisions about evacuations, as was the case in November of this year as well as 2002, and 1998. M:lIJctiveI04P012 San Marcos M.D.P\Grant Applic-SMa.do~xhi1>it A, Page 8 December 16. 2004