5. SEDIMENT CONTROL.
Typical sediment control practices are included in
a. Traps. Traps are small and temporary, usually created by excavating
and/or diking to a maximum height of five feet. Traps should be cleaned
(1) Size the outlet structure to accept the design storm.
(2) Size the pond length, width and depth to remove the desired
percentage of sediment. See Figure 13 (modified after Reference 15, Trap
Efficiency of Reservoirs, by Brune). For design criteria see Reference 16,
Reservoir Sedimentation, by Gottschalk.
(3) If pond is permanent, compute volume of anticipated average
annual sedimentation by the Universal Soil Loss Equation. Multiply by the
number of years between pond cleaning and by a factor of safety. This
equals minimum required volume below water level. Dimensions of the pond can
then be calculated based on the available area. The design depth of the
pond should be approximately three to five feet greater than the calculated
depth of sediment at the time of clearing.
6. RIPRAP PROTECTION. Frequently coarse rock is placed on embankments
where erodible soils must be protected from fast currents and wave action.
When coarse rock is used, currents and waves may wash soil out from under
the rock and lead to undermining and failure. Soil loss under rock slopes
can be prevented by the use of filter fabrics or by the placement of a
filter layer of intermediate sized material between the soil and rock. In
some cases soil loss can be prevented by the use of well-graded rock
containing suitable fines which work to the bottom during placement. For
further guidance see Reference 17, Tentative Design Procedure for Rip Rap
Lined Channels, by the Highway Research Board.
For determining rock sizes and filter requirements use Figure 14
(Reference 18, Design of Small Dams, by the Bureau of Reclamation).