Agenda Packets - 2021/07/06 - Laserfiche WebLink

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1. Detain runoff from storms up through a 1-year or 2-year event for about 24 fours. 2. Excess flows from storms greater than a 1-year or ?-year event can be more quickly discharged from the basin in accordance with the city's stormwater management plan (SWMP). 3. The runoff V plume to be detained for an extended period of time (24 hours) should be at feast the runoff from a 2.5-inch rainfall in 24 hours. 4. In general the I 00-year, 24-hour SCS Type II storm outflow rate from the basin and the I00-year, IU-day snowmelt event outflow rate from the basin should not exceed the peak discharge rate designated in the city's SWMP. These parameters in turn dictate the tiers in a specially designed, multi -tiered outlet structure. Outlet Structure/Tier Description: ■ Tier-1: The lowest opening(s) is designed to force flows from a 1-year or 2-year storm event to pool in (lie basin for at least S hours and typically no more than 24 hours. The volume of a basin below the overflow elevation of the next tier (Tier-2) in the structure should be the runoff volume of the I -year or 2-year, 24-hour duration storm. This volume can be live storage (100% between the invert elevation and the Tier-2 overflow elevation) or a combination of live storage and dead storage. Dead storage is the volume of water below the Tier-1 invert. • Tier-2: The starting overflow elevation of Tier-2 is to be set at the elevation where 1 il0% of the runoff from the 1-yeas. 24-Hour or ?-year. 24-hour duration event is stored below this elevation. The wetted surface area of the basin at this elevation should be at least ?/i% to I of the total untreated tributary watershed to the basin. The size of the Tier-2 overflew is computed to meet the city's SWMP I00-year peak outflow requirement. In certain site - specific cases, Tier-2 may need to be a multi -tier overflow, but in most cases this requirement can be managed in a single second tier. • Tier-3: The overflow elevation of Tier-3 is to be set at the computed 100-year flood level of the basin, which is established by the i 00-year. 24-hour duration storm or the 100-year. 10-day snowntelt event, whichever is more severe. This could be a natural overflow, a structure overflow, or a combination thereof. P: MP 23 MN 622162I0A2 Silver view- F id NI se"Puent WorWles Report Silver V ica Pond Assessrneni-Fitie I dcwx 74