Project Summary
The pre-existing stream crossing consisted of an undersized culvert on a recently abandoned road. The culvert was set high in the road fill creating a 2 ft (0.6 m) vertical drop into a plunge pool at the outlet and a leap barrier for salmonids at low flows. The steep culvert was a depth barrier at low flows and a velocity barrier at high flows, but may have allowed some passage for adult salmon and steelhead at moderate flows. The preferred option was to remove the crossing and regrade the channel at the site.

The treatment consisted of removing the culvert and excavating 350 yd³ (270 m³) of earthen fill to reestablish the natural stream channel at the site. The channel bed through the former crossing was re-graded, connecting the upstream and downstream channel. It was constructed with a 19 ft (5.8 m) wide channel bottom to match the upstream channel width. The road fill at the stream crossing was excavated at a 2H:1V side slope, mulched and planted. Boulders and large wood unearthed during excavation were used to armor and roughen the stream banks, and excavated fill material was placed in a stable location near-by.

Prior to removal 3,800 ft (1,160 m) of unpaved road and inboard ditch discharged fine sediment directly into the stream at the crossing. During the road decommissioning the ditch was filled-in, the road was outsloped, cross-road drains (enlarged water bars) were built and the road-bed was decompacted, thereby eliminating this chronic source of fine sediment to the stream.

The difference in grade between the upstream and downstream channel posed a risk of downcutting in the upstream alluvial channel once the crossing was removed. Adding grade control was problematic due to expense and extreme difficulty in returning to the site with equipment if future maintenance became necessary. Regrading the channel over a long distance and removal of stored sediments was also considered, but the resulting disturbance to the channel and established riparian vegetation made this option undesirable. Adding grade control was eventually deemed unnecessary since wood buried in the stream bed would become exposed as the channel re-grades, minimizing the extent of headcutting. Also, mobilized streambed material would be predominately gravel suitable for spawning, rather than fine sediments.

Post Project Observations and Lessons Learned
Following construction, the new channel appeared to be providing free passage for fish and other aquatic organisms.  Photo monitoring of the site 15 months after removal found only minor adjustments to the channel at the former crossing location. Following large winter flows some upstream downcutting has occurred and deposition within the excavated crossing was observed.  The channel is expected to continue to adjust as it evolves to its stable grade and morphology.

The design and as-built survey of the channel and stream crossing was performed using a tape and clinometer. The accuracy of a clinometer is +/- 2% to 3%, which is ill-suited for surveying low gradient channels. More accurate equipment, such as a surveyors level, should be used for design and post-project surveys. The profile should also extend far enough upstream and downstream to assess the headcut potential and allow for monitoring post-project channel adjustments.

Surveying cross sections and a longitudinal profile through the culvert and adjacent channel is essential to avoid under-excavation during crossing removal.  If the excavation is not deep enough or the channel width is too narrow, the streambed and banks will continue to erode and deliver sediment to the channel.  Removal of a stream crossing can be a cost effective way to remediate barriers to aquatic organisms and should be considered when developing options for improving passage. More information on road-stream crossing decommissioning are presented in Part X of the California Salmonid Stream Habitat Restoration Manual

Published 05/01/07