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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br /> , <br /> � 6. The capacity of an interceptor drain is detertnined by calcu(ating tlie maximum rate of <br /> groundwater flow to be intercepted. Therefore, it is good practice to make complete subsurface <br /> investigetions, including hydraulic conduclivity of the soil, before designing a subsurFace <br /> drainage systam. <br /> Size of drafn <br /> I. Siu subsurfacc drains to carry t6e required capacity without ptessure tlow. Minimum diameter <br /> for a subsurface drain is 4 inches. <br /> 2. The minimum velocity required to prevent silting is L4 CeeVsecond. T6e line shall be gtaded to <br /> achieve this velocity at a minimum. The maximum allowable velociry using a sand•gravel filter <br /> or envelope is 9 feet/second. <br /> 3. Filter materiel and fabnc shall be used around al!drains for proper bedding and filtration of fine <br /> materials. Envelopes and filters should surround thc drain to a minimum of 3-inch thiclmess. <br /> 4. The outlet of the subsurfa�e drain shall empry into a sediment pond through a catch basin. fF free <br /> of scdiment, it can then empty ioto a rcceiving channel,swalc,or staole vegetated arca adcquately <br /> protected from erosion and undemuniog. <br /> 5. The trench shall be consttucted on a continuous grade with no ceverse grades or low spots. <br /> 6. Soft or yielding soils under the drain shall be stabilized with gravel or othet suitable material. <br /> 7. BackfilGng shall be done immediately after placement af the pipe. No sections of pipe shall <br /> remain uncovered overnight or during a rainstortn. Backfill material shall be placed in the:tench <br /> in such a manner that the drain pipe is not displaced or demaged. <br /> ) 8. Do not install permanent dreins near trees to avoid the tree roots thet tend to clog the line. Use <br /> ' solid pipe wiUt watertight connections whcre it is necessary lo pass a subsurface drainage system <br /> ttuough a stand of hecs. <br /> Outlet <br /> 1. Ensure that the outlet of a drain empties into a channel or other watercourse above the normal <br /> water level. <br /> 2. Secure an animal guard to the outlet end of the pipe to keep out rodents. <br /> 3. Use outlet pipe of co[iugated metal,cast iron,or heary-duty plastic without pedorations and at <br /> least 10 feet long. Do not usc an envelope or filter material around the outlet pipe,and bury at <br /> least two-thirds of thc pipc lcngth. <br /> 4. When outlet velocitics excecd those allowable for the rcceiving stream,autlet protection must be <br /> provided. <br /> Maintenance Standards <br /> 1. SubsurCace drains shall 6c checked periodically to ensure that they arc Gee-tlowing and no1 <br /> clogged with sediment or roots. <br /> 2. The outlet shall be kept dean and free of debris. <br /> 3. Trees located too close to a subsurface drain oRen clog the system with their roots. If a drain <br /> becomes clogged,rolocate the drain or remove the trees as a lazt resort. Drain placement should <br /> be planned to minimize this problem. <br /> 4. Where drains are crossed by heavy vchicles use stecl plate or boards to prevent the lines fro.m <br /> being crushed. After work is complete the line shall be checked to ensure that it was not crushed. <br /> Volume II—Construcfion Sf ormwater Pollution Preventron <br /> 3•44 <br />