1710 W MARINE VIEW DR 2024-01-03

Subsurface Drain Sizing and Placement . The quantity and quality of discharge needs to be accounted for in the receiving stream (addi- tional detention may be required). . The size of a subsurface drain is determined by first calculating the maximum rate of ground water flow to be intercepted, and then choosing a subsurface drain pipe (or pipes) with enough capacity to convey that flow. Therefore, it is good practice to make complete sub- surface investigations, including hydraulic conductivity of the soil, before designing a sub- surface drainage system. . Size subsurface drains to carry the required capacity without pressure flow. Minimum dia- meter for a subsurface drain is 4 inches. . The minimum velocity in the pipe required to prevent silting is 1.4 ft/sec. Grade the subsurface drain to achieve this velocity at a minimum. The maximum allowable velocity using a sand - gravel filter or envelope is 9 ft/sec. . Filter material and fabric shall be used around all drains for proper bedding and filtration of fine materials. Envelopes and filters should surround the drain to a minimum of 3-inch thickness. . The trench shall be constructed on a continuous grade with no reverse grades or low spots. • Soft or yielding soils under the subsurface drain shall be stabilized with gravel or other suitable material. . Baclkfilling shall be done immediately after placement of the pipe. No sections of pipe shall remain uncovered overnight or during a rainstorm. Backfill material shall be placed in the trench in such a manner that the drain pipe is not displaced or damaged. . Do not install permanent drains near trees to avoid the tree roots that tend to dog the line. Use solid pipe with watertight connections where it is necessary to pass a subsurface drainage sys- tem through a stand of trees. Subsurface Drain Outlets . An adequate outlet for the subsurface drain must be available either by gravity or by pumping. . The outlet of the subsurface drain shall empty into a sediment trapping BMP through a catch basin. If free of sediment, it can then empty into a receiving channel, swale, or stable veget- ated area adequately protected from erosion and undermining. . Ensure that the outlet of a subsurface drain empties into a channel or other watercourse above the normal water level. . Secure an animal guard to the outlet end of the pipe to keep out rodents. . Use outlet pipe of corrugated metal, cast iron, or heavy-duty plastic without perforations and at least 10 feet long. Do not use an envelope or filter material around the outlet pipe, and bury at least two-thirds of the pipe length. 2019 Stormwater Management Manual for Western Washington Volume // - Chapter 3 - Page 346