Overflow Structures
What – Structures such as armored rolling dip, overside drain, or imbricated (overlapping) rock-level spreader.
Purpose – Used on roads to control runoff across the road prism and to protect the road fill. Armored rolling dips provide increased water flow capacity when hydrologic analysis indicates the current pipe size is too small for the short-term increased storm runoff created by fire. Dips prevent stream diversion by safely channeling increased flows back into the channel. Overside drains (berm drains and down drains) are placed in stream crossings where no culvert or armoring exists and in locations where the embankment (fill slope) needs protection. Imbricated rock-level spreaders have been used on high standard roads including highways and county roads. The imbricated rock-level spreader is a permanent structure that is built with large rock placed in a stairstep (shingled) design on excavated benches with either little or no grade along the revetment’s length. The spreader protects the road fill from overland flows.
Effectiveness – Armored rolling dips are effective low-cost treatments when properly designed and implemented. Qualitative monitoring data of armored rolling dips found erosion problems when the dip was too short and when insufficient riprap was used on the fill slope. Overside drains fail if not properly designed, installed, and maintained. Initial qualitative monitoring indicated imbricated rock-level spreaders (rock armored overflow) are effective when they discharge directly onto a vegetated/wooded zone.
Where
- Roads located below high-and moderate-burn severity areas.
- Road segments that have a long continuous grade and infrequent drainage.
- Roads that are insloped.
Costs – Treatment vary in cost. Cost estimates can be developed based on material and installation requirements.
How to Link – See USDA BAER Catalog page 115 (PDF page 123).