Erosion and Sediment Pollution Control Program Manual.pdf

DROP STRUCTURE
In many cases, significant energy reduction can be accomplished by using drop structures within a
piped outfall system (Figure 9.11). The resulting velocities at the discharge point are often within those
allowable for typical apron protection. Consideration should be given to incorporating drop structures
wherever anticipated velocities exceed 13.0 fps. They may also be desirable for lower velocities to
reduce the size of rock in an apron or its overall dimensions. To avoid potential problems with backup
of water within the manhole, the depth of the manhole should be at least 2 times the inflow pipe
diameter. The outlet pipe diameter should be larger than the inflow unless hydraulic calculations show
that a pipe of equal diameter is sufficient to convey the design discharge without backup.
Adapted from Westmoreland Conservation District
FIGURE 9.11
Typical Drop Structure
363-2134-008 / March 31, 2012 / Page 252