Bayview Sand Mine - Discharge Outfall Outlet Protection

Discharge Outfall Outlet Protection: Construction Details
Practice Standards and Specifications 6.41, 6.31 , 6.15
Bayview Sand Mine
Powells Point, NC
Currituck County
6.41 Outlet Stabilization Structure
6.31 Riprap and Paved Channels
6.15 Riprap

Riprap lined apron located at end of mine dewatering overflow pipes (3) to prevent scour and
undermining in existing ditch, and to absorb the impact of the water flow and reduce the velocity to
non-erosive levels, before flow reaches wetlands.
Riprap Apron Size: Overflow pipe discharges directly into well-defined channel. The apron will extend
across the channel bottom and up the channel banks to the top of the bank. Length of apron will be 20-
feet downstream to maintain velocity control of maximum 2.0 feet per second.
Grade: Apron will have zero grade and no overfill at end of apron.
Alignment: Apron will be straight throughout entire length if possible. If curve is necessary to align the
apron with the receiving channel, the curve will be located in the upstream section of the riprap.
Materials: Riprap will consist of a well graded mixture of NCDOT Class A (2” to 6”) stone or slag.
Thickness: Minimum thickness of riprap will be 1.5 times the maximum stone diameter.
Filter: Operator plans to install a layer of heavy duty silt fence filter cloth between the riprap and the
underlying soil to prevent soil movement into or through the riprap. Filter cloth will extend to the top of
both channel banks, and edges will overlap 12 inches. Upstream end will be buried 12 inches and
downstream end will be buried if necessary.
Maintenance: inspect riprap outlet structures weekly and after significant rainfall events to see if any
erosion around or below the riprap has taken place, or if stones have become dislodged. Repair all
damage immediately. Maintain all vegetation adjacent to the channel in healthy condition to prevent
scour or erosion during out-of-bank flow. Control weeds and brush growing in the riprap by cutting.

Ken Elliott, consulting
December 3, 2017