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t MOUNTAIN HOUSE NEIGHBORHOODS KAND L INITIAL STUDY S.ENVIRONMENTAL CHECKLIST 8.HYDROLOGY AND WATER QUALITY i development of Neighborhoods K and L would substantially alter the existing 9 drainage patterns of the Neighborhoods K and L project site after construction. Once the project is completed,storm water runoff generated on the _[ Neighborhood K and L project site would be conveyed via a series of street gutters, local drainage swales,catch basins, and storm drain pipes to the respective lake features. The lake features would help to settle out sediment and particulates in storm water runoff. Storm water would be detained in each lake until water elevations in the lake rise to a critical elevation below the streets and building pads at which time the storm water would enter an inlet structure and be discharged to Mountain House Creek via a new outfall structure.The proposed outfall for the Neighborhood K lake would be located approximately 1,200 feet south of the confluence with Old River;the proposed outfall for the Neighborhood t L lake would be located approximately 600 feet south of the confluence. Each outfall structure would be comprised of a retaining headwall with flap gates, concrete apron,and rip rap slope protection.The concrete apron and rip rap Fhl would dissipate the energy of the discharges and protect the creek from erosion. The outfall structures are designed to release the full volume of the 100-year storm at a constant rate over a 48-hour period (C8G,2010). As a result of the improvements described above,the alterations in drainage patterns associated Flo with the proposed project would not result in substantial soil erosion or sedimentation on-or off-site, and the impact would be less than significant. r, Issues related to drainage and potential erosion/siltation are also addressed in the following: Master Plan Policies 15.3(Implementation Measure[a]) (Off-Site Watersheds); 15.4(Policies[b], [c],and[h]) (Primary Storm Drain Collection System); 15.6(Policy[d], and Implementation Measure [a](Mountain House Creek Improvements); 4.2.2(Policies[a]and [d]) (Grading Standards); and 6.8.3 (Objective [b]and Policy[b] (Soils, Geologic and Seismic Hazards). I No additional mitigation measures are required to reduce potential project impacts related to erosion and siltation. i d) Would the project substantially alter the existing drainage pattern of the site or area, including alteration of the course of a stream or river, or substantially increase the rate or amount of surface runoff in a manner which would result in flooding on-or off-site? As discussed above under Item (a),the removal of the existing irrigation ditches and associated infrastructure, changes in site topography, creation of new impervious surfaces, and construction of the lake features and storm water drainage infrastructure associated with the proposed development of Neighborhoods K and L would substantially alter the existing drainage patterns of —� the project site after construction.The conversion of the existing agricultural fields to suburban development would result in an increase in impervious surfaces on the Neighborhoods K and L project site.Without implementation of t� measures to reduce post-construction runoff, an increase in impervious surface i � f (sans) 5-115