Case Study Contributors
Tributary to Hayfork Creek, Trinity River Basin, Shasta-Trinity National Forest, Northern California, USA
US Forest Service
Total Project Cost
|Planning / Permitting||$||4,000|
|Construction / Materials||$||280,000|
The pre-existing 6 ft (1.8 m) diameter x 115 ft (35.1 m) long multi-plate steel culvert was installed in the 1950s. The culvert was sized for the 25-year peak flow and had a fill volume of about 2,000 yd3 (1,530 m3). There was a notable amount of sediment aggradation upstream of the culvert inlet, potentially due to frequent backwatering caused by the undersized culvert. The crossing was identified as a velocity barrier for adult rainbow trout at high flows and a leap barrier for juvenile trout due to the perched apron at the outlet. Three methods were used to estimate design flows for the site: slope area, unit discharge, and rational.
The replacement structure was a 64 ft (19.5 m) long x 16 ft (4.9 m) wide precast concrete bridge designed to pass the 100-year peak flow. The bridge deck consisted of four pre-stressed concrete beams supported by cast-in-place abutments set back from the top of bank. Geotextile fabric was placed on the new embankments under the bridge and then armored with class 4 riprap.
Due to concerns about scour and incision of the channel under the bridge, a series of four “vortex rock weirs” were constructed through the project reach; two downstream and two upstream of the bridge. Additional riprap was added to the channel between the weirs. Following construction, exposed soil was mulched with straw as an erosion control measure.
Post Project Observations
Instream turbidity and suspended sediment data collected during implementation show that construction activities measurably increased instream fine sediment. Downstream measurements show that levels returned to background within ¼ mile of the site. No turbidity or suspended sediment increases were measured three miles below the site during project activities.
Measurable incision of the stored sediment upstream of the road-stream crossing occurred, but was considered minor compared to the amount predicted to mobilize without grade control structures.
It is unclear if preventing incision of upstream stored sediments and use of grade control was necessary for this project. A common alternative is to remove aggraded sediment stored upstream of a replaced culvert as part of the project. This can prevent delivery of the fine sediment to downstream habitat, while allowing for a natural streambed through the crossing. Installing grade control to retain the stored sediments results in a channel that is steeper than the adjacent natural channel. This can increase water velocities and turbulence and potentially lead to problems with inadequate depth during low flows. As a result, grade control has the potential to hinder upstream passage of weaker swimming fish, such a juvenile trout and speckled dace, found in the downstream channel.
Depth of scour between the bridge embankments due to a hydraulic constriction during large flows can often be assessed with a standard scour analysis (Evaluating Scour At Bridges, FHWA 2001). If excessive scour depths are predicted, the problem can often be addressed by placing rock armoring sufficiently below the predicted stable channel bed. This allows for a natural channel bed to persist through the crossing while preventing excessive scour during large, infrequently occurring, flood events.
Recommendations For Future Projects