3101 CEDAR ST STORM DRAINAGE DESIGN 2025-07-25

MAGNUSSON KLEMENCIC ASSOCIATES ■ correlation could be made between the flooding of these lots and a lack of capacity of the combined system proposed to be used for this project. PRE- DEVELOPED HYDROLOGY Rainfall coefficients "m" and "n" are taken from Stormwater Management Manual Volume 1, Table 5.2 for purposes of creating an Intensity-Duration-Frequency curve for site storm event calculations for both the pre-developed and developed conditions. The pre-developed site is considered to be "light forested" land without a true drainage system for conveyance. Pre-developed hydrology analysis assu , ffj t of 0.15 for the entire impacted site (from Manual Volume 1,Table 5.3). area of 3.95 acre)orrelates to a total flow for a 2-year storm event of 0. 9 cf s. For the 50-year storm event,the Wow equates to 2. 1 cf s. In neither event does this runoff leave the site through a dedicated conveyance. The calculation of these flows is included in Figures 4 and 5. DEVELOPED HYDROLOGY The full drainage calculations for the developed hydrology are included in Figures 6 and 7. A runoff coefficient of 0.90 is used for areas of new impervious asphalt concrete paving, and 0.45 is used for areas to be vegetated (including the swale, rain garden, and ponds). For the 2-year event,the resultant flow is 3. 9 cf s,while for the 50-year event the resultant flow is 9. 2 cf s. As the pre-developed condition does not allow runoff to depart the site,the detention ponds serve to detain runoff with an orifice restriction to provide a slow release meeting the City requirements of matching existing flow (no flow leaving the site). In order to ensure that the proposed drainage system has adequate capacity for its 10-year design life and to avoid pond slope failures and combined system impacts,the proposed system is designed for the 100-year event,which generates a runoff of 10. 3 cf s. (vs. 2.3 cfs for the pre-developed condition). The pond and catch basin grates are designed such that the 100-year event will be held within the system. If an overflow does occur,the runoff would bubble out of the southerly-most catch basin grates and over pavement onto the Thirteen Street gutter line. STORMW\TER TREATMENT PLAN Stormwater treatment is not a requirement of the project since the project conveys runoff to the existing combined system. The vegetated ponds,swale, and rain garden will in fact provide some measure of treatment, but the pollutant load removals are not analyzed herein. STORMWXTER FLOW CONTROL PLAN The City Stormwater Manual requires that the proposed project match the pre-developed site's discharge durations from 50 percent of the 2-year recurrence interval peak flow up to the peak flow of the 50-year recurrence event. Calculations of these event storms are included in Figures 4 and 5 for the pre-developed conditions. Calculations of the developed conditions are included as Figures 6 and 7. Storm Drainage Design � . 6 Utility Plant Expanded Parking Facility, Everett,Washington •