3101 CEDAR ST STORM DRAINAGE DESIGN 2025-07-25

MAGNUSSON I KLEMENCIC ASSOCIATES ■ The table below summarizes the results of the calculations: Table 2. Condition 2-Year Storm Event Discharge(cfs) 50-Year Storm Event Discharge(cfs) Pre-Developed 0.9 2.1 Developed 3.9 (0.9 al I oveabl e) 9.2 (2.1 al I ovabl e) It is critical to note that the pre-developed condition does not actually discharge flow into a conventional drainage system. Therefore,the proposed condition is designed to match the existing condition to the greatest extent feasible,which is to detain the runoff in the detention ponds and slowly release it back into the combined system. The detention pond design has been performed in accordance with Volume 4, Chapter 3 of the Stormwater Management Manual. C O N VFYA_N CF ANLAJ YSI S The drainage system's conveyance systems have been analyzed and included as Figures 6 and 7 of this Plan. Pipes are designed to be under pressure during large storm events to provide additional storage. A minimum time of concentration of 5 minutes is assumed. Riprap is utilized at the pipe outlets and inlets at the detention ponds, and also as the slope protection for the rain garden and swale leading down into the ponds. Riprap sizing is in accordance with the Open Conveyance Protection Table 6.3 of Volume 1, Chapter 6 of the Stormwater Management Manual. The highest concentrated flow velocity anticipated is 3.3 ft/s,which occurs on the 3:1 pond slope during the 100-year storm event. It is anticipated that rock cobble will be readily available in various sizes during the excavation of the pond. For uniformity and in order to re-use and avoid export of those available materials,the riprap for all locations will be designed to be 12 inches deep and relatively well-graded with stone size varying from 2 to 12 inches. MAINTENANCE AND OPERATIONS PLAN Maintenance and operations tasks and descriptions are included as Figure 8. SOURCE CONTROL PLAN As a temporary parking facility,the project does not propose process water or waste handling that would require source control BMPs. BMPs are implemented only for rain event runoff. The size of the temporary parking lot makes it cost-ineffective to cover with a roof or awning. Source control BMPs used include curbing along the south side of the parking facility and fine grading to ensure that runoff Storm Drainage Design 7 Utility Plant Expanded Parking Facility, Everett,Washington