819 N BROADWAY WASHINGTON STATE UNIVERSITY 2018-05-11

Design and Excavated Drop Inlet Protection -An excavated impoundment around the Installation storm drain. Sediment settles out of the stormwater prior to entering the Specifications storm drain. • Depth 1-2 ft as measured from the crest of the inlet structure. • Side Slopes of excavation no steeper than 2:1. • Minimum volume of excavation 35 cubic yards. • Shape basin to fit site with longest dimension oriented toward the longest inflow area. • Install provisions for draining to prevent standing water problems. • Clear the area of all debris. • Grade the approach to the inlet uniformly. • Drill weep holes into the side of the inlet. • Protect weep holes with screen wire and washed aggregate. • Seal weep holes when removing structure and stabilizing area. • It may be necessary to build a temporary dike to the down slope side of the structure to prevent bypass flow. Block and Gravel Filter- A barrier formed around the storm drain inlet with standard concrete blocks and gravel. See Figure 4.14. • Height 1 to 2 feet above inlet. • Recess the first row 2 inches into the ground for stability. • Support subsequent courses by placing a 2x4 through the block opening. • Do not use mortar. • Lay some blocks in the bottom row on their side for dewatering the pool. • Place hardware cloth or comparable wire mesh with '/2-inch openings over all block openings. • Place gravel just below the top of blocks on slopes of 2:1 or flatter. • An alternative design is a gravel donut. • Inlet slope of 3:1. • Outlet slope of 2:1. • 1-foot wide level stone area between the structure and the inlet. • Inlet slope stones 3 inches in diameter or larger. • Outlet slope use gravel Y2- to 3/4-inch at a minimum thickness of 1-foot. February 2005 Volume ll— Construction Stormwater Pollution Prevention 4-83