NCG020925_James Cahoon Mine - Cahoon_Rock Dam in Ditch Construction Details_20180220

Rock Dam in Ditch Construction Details
Practice Standards & Specifications 6.63
James Cahoon Mine
US 64 East, Columbia, NC
Tyrrell County

A rock embankment located to capture sediment in a naturally formed drainage feature. The purpose of a rock dam is
to trap sediment on the construction site, and prevent off-site sedimentation in streams, lakes and drainageways.
Planning: Used where it is desirable to have the top of the structure serve as the overflow outlet. A long weir crest is
designed to keep flow depth shallow and discharge velocity low. Inside face of the dam will be covered with gravel to
reduce rate of seepage so that a sediment pool will form during runoff, draining slowly through the dam.
Design considerations:
 Maximum height – weir elevation 6 feet above ditch grade.
 Embankment – top width 2 feet minimum, side slopes maximum 2:1 upstream slope, maximum 3:1 downstream
slope. Rock abutments will extend to an elevation at least 1 foot above the spillway.
 Outlet protection – rock apron minimum 1.5 feet thick will extend downstream from the toe of the dam a
distance equal to the height of the dam to prevent channel erosion.
 Rock fill – rock will be well graded erosion resistant stone with a minimum d50 size of 12 inches, Class II
downstream layer of riprap and Class I upstream layer of riprap.
Maintenance: Check sediment basin after each rainfall, remove sediment when sediment accumulates to ½ the height
of the dam. Check structure for erosion or rock displacement weekly or after each significant (1/2 inch or greater)
rainstorm and repair immediately.

Ken Elliott, consulting
January 28, 2018