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J5~ characteristics. Typical subwatershed areas will not be less than 50 acres. Flows will not be developed for those waterways in the City for which FEMA has developed base flood elevations. Data developed for each subwatershed shall include: (1) drainage area; (2) land use breakdown; (3) percent imperviousness; -_. (4) average overland flow slope; (5) average roughness factor (Manning's) for overland flow; and (6) soils characteristics. 1.6.5 Compile existing and future land use data for each subwatershed for input to the stormwater hydrologic model. 1. 6.6 Assign imperviousness factors to the land use breakdown for each subwatershed. 1.6.7 Review information on soils in the watershed and determine hydrologic soil group and other pertinent soil infiltration parameters for each soil category. . 1. 6.8 Define design storms for master planning evaluations. At a minimum, the ENGINEER shall consider eight design storm years for the frequencies shown below for the existing and future land use conditions: (1) 2-Year Design Storm event typically recommended for streambank erosion controls; (2) 10-Year Design Storm event to control minor flooding; (3) 25-Year Design Storm event, for floodplain management and final screening of drainage improvement plan; and (4) 100-Year Design Storm event for base flood elevations, and for revisions to the CITY's Flood Insurance Rate Map. 1. 6.9 Apply the U.S EPA SWMM model to the study area to establish flow magnitudes for the design storm events, and use to determine water surface elevations. Where data is available, the models developed for this study will be calibrated against historical rainfall and streamflow records. The modeling results will be compared to storm flows and water surface elevations developed in previous drainage studies for the San Marcos area. The models will be applied to determine flooding based on current land use and anticipated future conditions based on ultimate land use projections- 1.6.10 Deliver a technical memorandum to the CITY which will include hydrologic and hydraulic model input/output (hard copies and magnetic copies on 3 1/2 inch diskettes), copies of SWMM model code and other spreadsheet tools used to develop the model input. 1.7 TASK 7 - IMPROVEMENT AL TERNA T IVE S The ENGINEER shall: ~ 1. 7.1 Identify flooding problems and corrective improvements using the results of the hydrologic and hydraulic modeling developed in Task 6 supplemented by existing complaint files, records and drainage studies. 1.7.2 Develop an evaluation system for prioritization which will assign points to each proposed improvement based on the reduction in flooding of roadways, homes and businesses. 1. 7.3 Develop an implementation schedule, in cooperation with the CITY, utilizing a system that will prioritize proj ects based on the most benefit to be received by the CITY for the least capital dollars expended. -4-