Reso PC15-01 PC Reso (3051 ) to Adopt Initial Study-Mitigated Negative Declaration - La Palma

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3.0 Environmental Impact Checklist d) Substantially alter the existing drainage pattern of the site or area,including through the alteration of the course of a stream or river,or substantially increase the rate or amount of surface runoff in a manner that would result in flooding on or off site? e) Create or contribute runoff water that would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff? Less-than-Significant Impact with Mitigation Incorporated.The project would increase the amount of impervious surface on the project site, which would increase the amount of stormwater flows from the project site.Replacement of the three existing residential structures and driveways on-site with 12 single-family homes and walkway areas would increase the amount of impervious surface on the project site by approximately 13,409 square feet. Per Section 27A.8 of the City's Municipal Code, the City requires the limiting of post- development stormwater discharge to equal or less than pre-development discharge. The Hydrology Study (Appendix E)prepared for the project documents the post-development discharge effects relative to the pre-development discharge. In the event of a 10-year storm,the current runoff rate of the project site is 0.9 cubic feet per second (cfs) and the post-construction runoff rate of the project site would be 0.98 cfs,representing a nominal net change of 0.08 cfs. To reduce the runoff and to reduce potentially significant impacts,Mitigation Measure HYD-3 would be implemented.This measure includes the preparation of a SWCP that identifies several facilities to minimize stormwater runoff. These facilities would include an on-site detention system and metering device to minimize the increased runoff due to the additional impervious areas.The on-site storm drainage system would collect runoff from buildings, landscape, and hardscape areas to prevent localized ponding of storm water.Area drains would collect runoff and direct it to a new on-site,below grade detention system and would be discharged into a new off-site storm drain system under Kentfield Avenue, which would be connected to the City's existing storm drain system, approximately 500 feet from the project site. To ensure that the additional measures can reduce the runoff to less than or equal to pre-development levels, a Hydrology and Hydraulics Study would be prepared per Mitigation Measure HYD-4. Implementation of Mitigation Measure HYD-4 would reduce the project's stormwater drainage system impacts to a less-than-significant level. Mitigation Measure HYD-4:The applicant shall submit a Hydrology and Hydraulics Study for review and approval by Planning Services and Engineering and Transportation Services prior to issuance of Grading and Building Permits to confirm: 1. The permanent stormwater management facilities incorporated into the project would reduce post-development stormwater discharge to equal or less than pre- development discharge levels; and 2. The City's existing storm drainage system in Kentfield Avenue would have adequate capacity to accommodate the project. Additional features shall be incorporated into the SWCP as needed to ensure compliance with the City's Municipal Code. La Palma Subdivision Project Final Initial Study/Mitigated Negative Declaration 44