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k PROJECT MEMO WHB L );;TO: File FROM: Craig Schoenberg DATE: 11/16/2010 PROJECT: Everett Community College Index Hall Replacement OUR FILE NO.: 209459.10 SUBJECT: Preliminary Downstream Conveyance Capacity Analysis Civil Engineers During the pre-application meeting with the City of Everett on Tuesday, October 12, 2010, City Structural Engineers Staff stated that the maximum rate of stormwater that may be discharged from the site is governed by the capacity of the downstream public system. To determine the capacity of the downstream storm system, the city requires that a quantitative analysis be performed for the Landscape Architects. downstream storm system proposed to accept the projects drainage to demonstrate that the public system will not overtop during a 25-year storm event using the Rational Method. Community Planners The following summarizes our findings and analysis for the capacity of the downstream storm systemproposed to accept drainage from the Everett Community College Index Hall Replacement Natural Resource Ecologists project site. Project Summary Land Surveyors ) The project proposes to discharge stormwater runoff from the site to the 18 inch diameter storm system within Tower Street where it intersects Waverly Street. In 2001, the College upsized an Neighbors existing storm pipe, located within Little Broadway Avenue,Tower Street and Waverly Avenue, to separate stormwater from the combined storm/sewer and achieve sufficient capacity. AHBL has reviewed the design drawings and Stormwater Hydraulic Report prepared by Reid Middleton for the storm drainage improvement. The Stormwater Hydraulic Report showed that their analysis of the stormwater improvement included a portion of the Index Hall Replacement site described as an"assumed future TACOMA expansion"area. The report assumed the future expansion area would include an area 016.1 2215North30thStreet acres(90impervious). When comparing the assumed expansion area to the proposed Index Hall Replacement project, it was determined that approximately 1.26 acres was not included in Suite 300 the 2001 analysis. The following analysis includes the missing 1.26 acres summing to a project Tacoma,WA 98403-3350 site area of 7.36 acres as shown below. 253.3832422 TEL 253.383.2572 FAX Flow Calculation To ensure consistency with previously calculated flow rates from the 2001 analysis and as SEATTLE approved by Jane Zimmerman (City of Everett Public Works),the methodology used for this 12006th Avenue study uses the SBUH methodology rather than the Rational Method. Suite 1620 Basin information summarized below shows the data from the 2001 Stormwater Hydraulic Report Seattle,WA 98101-3117 and new calculations for the larger tributary area for the Index Hall Replacement site. The new 206267.2425 TEL calculations use the 25-year, 24-hour design storm event for the City of Everett, and the 206.267.2429 FAX Stormshed Software which is consistent with the 2001 analysis. The calculated flow for 25-year storm event for the larger tributary basin for the Index Hall Replacement site was determined to s P O KA N E be 4.34 cfs. 827 West First Avenue Suite 301 Spokane,WA 99201-3912 509252.5019 TEL 509.315.8862 FAx Page 1 of 2 www.ahbl.com