Hydraulic Design of Highway Culverts - DOT On-Line Publications

passage of the peak flow. Methods for predicting scour hole dimensions are found in Chapter 5
of HEC No. 14, "Hydraulic Design of Energy Dissipaters for Culverts and Channels" (40). The
second type of scour is classified as general stream degradation. This phenomenon is
independent of culvert performance. Natural causes produce a lowering of the stream bed over
time (Figure VI-13). The identification of a degrading stream is an essential part of the original
site investigation. Both types of scour can occur simultaneously at a culvert outlet.
Figure VI-12--Scour Hole at Culvert Outlet
Since prediction of scour at culvert outlets is difficult, and protection is expensive, a prudent
approach involves providing a minimum amount of protection followed by periodic site
inspection. As part of the field investigation, scour and outlet protection at similar culverts in the
vicinity will provide guidance. The initial level of protection should be sufficient to withstand
extensive damage from one storm event. Once the initial minimum outlet protection is
constructed, an assessment of its performance after a number of storm events should be
evaluated and reviewed. If the outlet protection is insufficient, additional protection should be
provided. If the outlet protection is sufficient, inspection is required only after larger storm
events.
Protection against scour at culvert outlets varies from limited riprap placement to complex and
expensive energy dissipation devices (Figure VI-14). At some locations, use of a rougher
culvert material or a flatter slope alleviates the need for a special outlet protection device.
Preformed scour holes, approximating the configuration of naturally formed holes, dissipate
energy while providing a protective lining to the stream bed. Riprapped channel expansions
and concrete aprons protect the channel and redistribute or spread the flow. Barrel outlet
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