931 BROADWAY TEUTSCH STUDENT HOUSING 2018-03-07

The pump system is an alternating duplex pump station. Each pump can individually convey events greater than the peak 25-year flow. This exceeds the requirement for conveyance system design of the Manual (minimum design event is the 25-year flow). The two pumps operating together in parallel can convey the 100-year peak flow (87 GPM). Refer to Appendix G for pump design, installation and maintenance details. The pumped drainage and the gravity flow drainage systems will combine at a new catch basin near the southeast property corner. From here, the drainage will flow via gravity through a new pipe, connecting to the existing inlet at the south end of the alley. The existing inlet will convey stormwater to new catch basin that will replace the existing 90 degree bend in the system. The _ existing 6"storm drain that outlets to the east from the existing inlet will be abandoned. The 8" storm drain line in 10th St discharges to the combined sewer in North Broadway. „ jlie emergency overflow conditions were evaluated. In the event the pump fails or th� discharge pipe from the site is clogged, stormwater would overflow through the grate of the catch basin on the east side of the building,just north of where the alley pavement ends. Once the rim of this catch basin is overtopped, stormwater would flow in to the pervious portion of the alley, which is graded to drain to the north. Any runoff that does not infiltrate in the pervious area of the alley would flow on to the adjacent property to the east, into a sizable landscaped area. Any runoff that does not infiltrate here would flow in to the parking lot, where it would be collected by parking lot collection and conveyance system and presumably discharge to the combined sewer system in 10`h St/Broadway. H.4 STORMWATER FLOW CONTROL PLAN This project is exempt from providing flow control facilities because it is proposing less than 5,000 square feet of net new effective impervious area per Table 2.2 of the Manual.This site is also subject to Public Works Interim Policy#2-11. That is, stormwater discharges shall match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50 percent of the 2-year recurrence interval peak flow up to the full 50-year peak flow.The predeveloped condition to be matched for sites within the combined sewer service area is the existing site conditions. As shown in Table 8-2,the developed discharge rates are less _ than the discharge rates from the existing site conditions. Refer to Appendix D for the full WWHM report, including the predeveloped and developed discharge duration curves. • 8.5 STORMWATER TREATMENT PLAN This project is proposing less than 5,000 SF of new and/or replaced pollution-generating impervious surfaces, and is therefore exempt from providing water quality treatment per Table 2.1 of the Manual.This project is resulting in a net reduction of 9,971 SF of pollution-generating impervious surfaces. 8.6 CONVEYANCE ANALYSIS To ensure that the conveyance system has adequate conveyance capacity, the peak 25-year flow rate from the developed site condition was compared to the conveyance capacity of the limiting EVCC STUDENT HOUSING-STORMWATER SITE PLAN-REVISED.DOCX 10 �