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1 <br />excavations. <br />. The capacity of an interceptor drain is determined by calculating the maximum rate <br />of ground water flow to be intercepted. Therefore, it is good practice to make com- <br />plete subsurface investigations, including hydraulic conductivity of the soil, before <br />designing a subsurface drainage system. <br />• Size of drain - Size subsurface drains to carry the required capacity without pres- <br />sure flow. Minimum diameter for a subsurface drain is 4 inches. <br />• The minimum velocity required to prevent silting is 1.4 ft./sec. The line shall be <br />graded to achieve this velocity at a minimum. The maximum allowable velocity <br />using a sand -gravel filter or envelope is 9 ft/sec. <br />• Filter material and fabric shall be used around all drains for proper bedding and fil- <br />tration of fine materials. Envelopes and filters should surround the drain to a min- <br />imum of 3-inch thickness. <br />• The outlet of the subsurface drain shall empty into a sediment pond through a <br />catch basin. If free of sediment, it can then empty into a receiving channel, swale, <br />or stable vegetated area adequately protected from erosion and undermining. <br />. The trench shall be constructed on a continuous grade with no reverse grades or <br />low spots. <br />• Soft or yielding soils under the drain shall be stabilized with gravel or other suit- <br />able material. <br />. Backfilling shall be done immediately after placement of the pipe. No sections of <br />pipe shall remain uncovered overnight or during a rainstorm. Backfill material shall <br />be placed in the trench in such a manner that the drain pipe is not displaced or <br />damaged. <br />• Do not install permanent drains near trees to avoid the tree roots that tend to clog <br />the line. Use solid pipe with watertight connections where it is necessary to pass a <br />subsurface drainage system through a stand of trees. <br />• Outlet - Ensure that the outlet of a drain empties into a channel or other water- <br />course above the normal water level. <br />1 <br />• Secure an animal guard to the outlet end of the pipe to keep out rodents. <br />• Use outlet pipe of corrugated metal, cast iron, or heavy-duty plastic without per- <br />forations and at least 10 feet long. Do not use an envelope or filter material around <br />the outlet pipe, and bury at least two-thirds of the pipe length. <br />. When outlet velocities exceed those allowable for the receiving stream, outlet pro- <br />tection must be provided. <br />2014 Stormwater Management Manual for Westem Washington <br />Volume 11 - Chapter - Page 347 <br />