Case Study Contributors
Tributary to Ouachita River
Ouachita National Forest, Arkansas, USA MAP
Watershed and Channel Characteristics
Ouachita National Forest, Oden Ranger District
US Forest Service
Approximate Construction Cost: $70,000
More than a half dozen native fish are found in stream, including the orangebelly darter and the Caddo madtom. A threatened mussel that occurs downstream depends on several of the native fish, including darters, to disperse. Several species of turtles and crayfish; Ouachita dusky; spotted, tiger, and marbled salamanders; as well as frogs and toads also use the stream. Fish passage is required not only for spawning but also for thermal refuge, as fish seek out deep pools with cooler water in the heat of the summer. Aquatic species must also be able to recolonize areas after local extinctions due to droughts. The need for fish passage was one of the primary drivers for this replacement structure.
In 1964, an un-vented ford was installed at this site with 3.3 ft (1 m) gabions supporting the downstream edge of a gravel roadbed. The gabion structure was frequently damaged when high flows outflanked the approaches. By 1979, two 24 in (0.6 m) metal pipes had been placed under the roadbed, and the road was surfaced with concrete. The pipes plugged frequently, and many repairs were needed. When the current structure was built, several generations of concrete along with the gabions and pipes were removed
The new vented low water crossing, built in 2000, consists of three 6 ft wide x 3 ft high (1.8 x 0.9 m) concrete box culverts set at streambed elevation. The combined box culverts provide an opening that approximates the bankfull channel width and depth. A 10 ft (3 m) long outlet apron provides scour protection for overtopping flows, and a 6 in (15 cm) tall curb at the apron’s downstream edge is intended to prevent formation of a scour pool. Curbs under the roadway backwater flows in the side culverts and concentrate low-flows into the center box for fish passage. The curbs help to retain some streambed material along the floor of the side culverts. Boulders embedded into the center section of the concrete apron are intended to provide resting areas for smaller fish.
The approaches are concrete and the crossing has interrupted curbs on the roadway edges to keep vehicles on the roadway during overtopping conditions. Beyond the box culverts the downstream fillslope is concrete armored. Ditches along the toe of the fillslope direct the overtopping flow back into the main channel.
Post Project Observations and Lessons Learned
The splash apron, curbs, concrete fillslope, and ditches are working well to keep this structure and the roadway stable. There is no channel bed scour and bank scour appears minimal. Forest Service engineers use concrete box culverts instead of open bottom arches or bridges to minimize the thickness of the road deck. The culverts, or “vents”, are only intended to pass a 2 year return event or less. Minimizing the thickness of the road deck reduces channel constriction and resulting water surface drop across the structure during overtopping.
The vented low water crossing on rocky Creek has sustained numerous overtopping floods without requiring any maintenance. The box culverts, sized to the bankfull channel, have not experienced any plugging as of 5 years after construction. Traffic interruptions are brief enough to be tolerable even on this school bus route.
Fish are often observed in the crossing when it is wet. This particular crossing has been the subject of two fish passage studies but the stream is susceptible to drying and is quite remote so fish passage detection has been difficult. Nonetheless, 4 of the 8 to 13 species found above and below the crossing have been documented as passing the structure: grass pickerel, central stoneroller, orangebelly darter, and green sunfish. There is a thin veneer of fine sediment on the floor of the culverts but the bed lacks sufficient diversity to create areas of varying water velocities and depths, and may not provide passage for the non-fish species present in the stream.
While the flow restricting curbs are working, the designers recommend placing the structure deeper and allowing recruitment of natural streambed material instead of installing curbs. Having the project engineer on-site during excavation is critical for addressing issues arising concerning bedrock or other unexpected subsurface conditions.
This case study is featured in the 2006 US Forest Service publication:
Low-Water Crossings: Geomorphic, Biological, and Engineering Design Consideration.