Culvert Inlet/Outlet Armoring/Risers

Protective covering, such as rocks, vegetation or engineering materials used to protect drainage structure outflows from flowing water.

Purpose: These treatments reduce scouring around the culvert entrance and exit. They allow heavy particles to settle out of sediment laden water and reduce the chance of debris plugging the culvert.

Relative Effectiveness: Not enough interviewees rated this treatment to make any statements about its effectiveness.

Environmental/Implementation Factors: Sometimes culvert risers can clog and may be difficult to clean.

Structure Implementation


Major structures - To provide direct protection to life and property

Armoring - To protect structures, the road and downstream values

Trash racks - To prevent floating debris from plugging culverts


Major Structures - Generally, this type of treatment has limited applicability.  Use only when all other treatments have been evaluated and shown to be insufficient to reduce the emergency to an acceptable level.

Armoring - Used to protect bridge abutments, and culverts from increased flows. Look for evidence from previous winter runoff where poor channel alignment has threatened abutments, or where there are signs that scouring eddies have been working around a culvert entrance.

Trash Racks - Applicable in drainages where (larger) debris is present and the possibility for mobilization is high. Use where flows are not predicted to exceed design capacity (because of the burn) but where plugging is a concern.


A bridge or large culvert may be appropriate in a case where the emergency is defined as being a threat to human life because traffic cannot be interrupted on a road with a crossing not expected to survive a flood flow. In this example, a 100yr event culvert may be installed or in a large drainage, perhaps a bailey bridge.

Installation of trash racks may be more often one of a combination of treatments designed to protect a crossing.


The design of major structures must be prepared by certified professional engineers.


Use engineering judgment to insure that simplifying assumptions in hydraulic analysis do not yield inaccurate results.

Use National Cooperative Highway Research Program Report No. 108, entitled "Tentative Design Procedure for Riprap-Lined Channels", to obtain armoring specifications for uniform channels.

Use U.S. Department of Transportation, Federal Highway Administration (FHWA). Hydraulic Design of Energy Dissipaters for Culvert and Channels, Hydraulic Engineering Circular No. 14 (HEC-14), FHWA-EPD-86-110. 1983.


Use U.S. Department of Transportation, Federal Highway Administration (FHWA). Debris Control Structures, Hydraulic Engineering Circular No. 9 (HEC-9), FHWA-EPD-106. 1971.