Road crossing of a perennial or ephemeral stream at or near the existing cross-section gradient that is generally constructed of large rocks capable of bearing the weight of the vehicles and resisting transport by the stream.
Purpose: Armored crossings provide low-cost access across stream channels that are generally capable of handling large flows.
Relative Effectiveness: No interviewee rated this treatment.
Environmental/Implementation Factors: Large riprap is placed upstream and downstream of actual road crossing area. Armored crossings are often used for low traffic volume roads. Low water crossing were not used on one fire because they could attract an endangered toad species that would inhabit the crossing when wet and be killed by vehicle traffic.
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 even 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.