Erosion and Sediment Pollution Control Program Manual.pdf

CHAPTER 9 - OUTLET PROTECTION
As a rule, outlet protection has little, if any, sediment removal potential. Therefore, outlet protection
by itself is not an ABACT. However, it should be used in all watersheds to prevent erosion due
to concentrated discharges.
General - Discharge velocities from pipe outfalls, outlet channels, or similar structures should not cause
erosion in downstream channels. The conditions in the downstream channel should be analyzed to
determine if existing conditions will be stable or if a protective lining is necessary. When designing
outlet protection for a channel or swale, use the guidance for a pipe having an equivalent crosssectional
area and design discharge to the channel or swale.
When a pipe is flowing full, the anticipated velocity can be calculated from the continuity equation:
V = Q/A
where V = Velocity (fps)
Q = Design Discharge (cfs)
A = Cross-sectional Area of the Pipe (ft 2 )
When a pipe is not flowing full, a different procedure should be used to determine the velocity. Due to
the increased friction that occurs along the wetted perimeter, full flow does not represent the greatest
flow capacity of a pipe. For circular pipes, peak flow actually occurs at 93% of the total inside diameter.
Likewise, the average velocity of a pipe flowing ½ full is equal to that for one flowing full. Therefore, the
anticipated velocity should be determined either by use of Manning’s equation for circular channels,
standard design nomographs, or by use of Figure 9.1 — limited to pipes with gradients