1600 121ST ST SE 2016-01-01 MF Import

��..� �� z To: Mary Cunningham From: �tephen Stanley Memorandum �� Re: Casper Creek Hydrological Analysis The project applicants have not clearly demonstrated with calculations that the predevelopment hydrology of the onsite wetlands (wetlands 1 and 2) will be maintained. Therefore. it is recommended that:: 1) The project's civil engineer calculate the amount of runoff that would enter these two wetlands from the project site in its present undeveloped condition. A suitable model such as the Santa Barbara Hydrograph or the Snohomish County Runoff model should be used. The post development runoff into the wetlands should also be calculated. If a deficit in the predevelopment runoff is calculated, then it should be made up by conveying roof top drainage to these wetlands. 2) The calculations demonstrate that the hydrolofly will be sufficient to maintain a water level in the wetland which will support both the existing vegetation and future mitigation plantings. Submitted topographic maps indicate that both wetlands are supplied by a small westem portion of the sife; the balance of the site appears to drain towards Silver Lake Creek. However, berms will be placed around the wetlands to prevent paved surtace runoff from entering and contaminating the wetlands. This will probably result in a reduclion of the annual runoff into these two wetlands which could affect their hydrology.