3214 BROADWAY BROADWAY KINECT APARTMENTS 2021-08-11

• CITY OF EVERETT STORMWATER MANAGEMENT MANUAL • • 6. The capacity of an interceptor drain is determined by calculating the maximum rate of groundwater flow to be intercepted. Therefore, it is good practice to make complete subsurface • investigations, including hydraulic conductivity of the soil, before designing a subsurface • drainage system. • Size of drain • 1. Size subsurface drains to carry the required capacity without pressure flow. Minimum diameter • for a subsurface drain is 4 inches. • 2. The minimum velocity required to prevent silting is 1.4 feet/second. The line shall be graded to achieve this velocity at a minimum. The maximum allowable velocity using a sand-gravel filter • or envelope is 9 feet/second. • 3. 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. • 4. The outlet of the subsurface drain shall empty into a sediment pond through a catch basin. If free • of sediment, it can then empty into a receiving channel, swale,or stable vegetated area adequately protected from erosion and undermining. • 5. The trench shall be constructed on a continuous grade with no reverse grades or low spots. • 6. Soft or yielding soils under the drain shall be stabilized with gravel or other suitable material. • 7. Backfilling 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. • 8. Do not install permanent drains near trees to avoid the tree roots that tend to clog the line. Use • solid pipe with watertight connections where it is necessary to pass a subsurface drainage system through a stand of trees. • Outlet r 1. Ensure that the outlet of a drain empties into a channel or other watercourse above the normal water level. • 2. Secure an animal guard to the outlet end of the pipe to keep out rodents. • 3. 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. • 4. When outlet velocities exceed those allowable for the receiving stream, outlet protection must be provided. • Maintenance Standards • 1. Subsurface drains shall be checked periodically to ensure that they are free-flowing and not clogged with sediment or roots. • 2. The outlet shall be kept clean and free of debris. • 3. Trees located too close to a subsurface drain often clog the system with their roots. If a drain • becomes clogged, relocate the drain or remove the trees as a last resort. Drain placement should • be planned to minimize this problem. • 4. Where drains are crossed by heavy vehicles use steel plate or boards to prevent the lines from being crushed. After work is complete the line shall be checked to ensure that it was not crushed. • • Volume II—Construction Storm water Pollution Prevention 3-44