Permanent flow control and conveyance measures

Subsurface Drains

A perforated pipe or conduit placed beneath the surface of the ground at a designated depth and grade.

Used to drain saturated soils by lowering the water table.

Used in areas free from tree roots where the soil later depth is deep enough to allow proper installation.

Not recommended in areas where the drain will pass under heavy vehicle crossings.

Should be designed for required capacity. The minimum diameter should be 6 inches. The line should be graded to achieve at least a velocity of 2.4 ft./sec. To prevent silting.

Level Spreader

Structure that converts sediment-free channel flow into sheet flow and uniformly releases it over a stabilized area.

Provides stable outlets for diversion channels, basins, dikes and other structures.

Used where sediment free storm runoff can be released as sheet flow over a stable area.

Install in areas where the level lip can be constructed without using fill material. The discharge area should have a maximum 10% slope and be well-vegetated and stable for design flow conditions. There should be no traffic across a level spreader.

Capacity determined by peak flow from the 10-year storm. The maximum desirable flow into the spreader should not exceed 30 cfs. Spreader length, width and depth are designed based on flow. There should be a 20-ft. long transition section between the channel entrance and the design width of the spreader. The grade of the spreader invert and lip must be 0.