Project Summary

Prior to 1997 a series of boulder jump-pool weirs were constructed below the outlet to improve passage for adult salmon and steelhead. These weirs were destroyed when the Mattole River migrated laterally during high flows and washed away the lower 160 ft (49 m) of Stansberry Creek. Subsequently, an 8 ft (2.4 m) vertical drop formed at the outlet of the 5 ft (1.5 m) diameter, 40 ft (12.2 m) long CMP creating a velocity and leap barrier to all salmonids at migration flows. Stansberry Creek received a priority ranking of 5th in the Humboldt County Inventory and Fish Passage Evaluation Report.

A corrugated aluminum box culvert mounted on a concrete base was selected as the replacement structure for the site. The culvert was designed to pass the 100-year flood without overtopping. Addressing the over 8 ft (2.4 m) difference in grade between the upstream and downstream channel was challenging in part due to private roads and structures located along the upstream banks. Stability of these banks could be compromised if the upstream channel bed was allowed to incise. The final design consisted of controlling the stream grade for 250 ft (76 m) of channel using a combination of concrete weirs located within the culvert and rock weirs outside the culvert. To construct the final design grade, the channel was regraded and 2,000 to 3,000 cy (1,529 to 2,294 m³) of gravel was removed from the stream bed. The Mattole River is only 30 ft (9.1 m) downstream of the crossing, limiting the available room for grade control structures below the culvert. Seven external rock weirs (six upstream, one downstream) were constructed with each having roughly 8 inch (20 cm) drops and spaced 20 to 30 ft (6 to 9 m) apart. The seven concrete weirs within the culvert have alternating tapered notches, spaced 10 ft (3.0 m) apart, with 8 inch (20 cm) drops, producing an overall grade of 6.7%. The final design was a refinement of a previous design used by the project engineer at South Fork Anker stream crossing.

Fifty cubic yards (38 m³) of ½ ton Rock Slope Protection (RSP) were used to armor the banks at the culvert inlet and outlet to prevent scour. During construction, traffic was detoured via a temporary flat car bridge. Upon completion, all disturbed areas were re-vegetated with a native grass seed mixture, and over 300 native trees and plants were planted to re-vegetate the site.

Post Project Observations and Lessons Learned

The stream crossing location was identified as vulnerable to backwatering by the Mattole River and resulting deposition, which can reduce the culvert capacity. Based on experience at the Clear Creek stream crossing, the culvert height was maximized to provide additional capacity.

Since completion, the Mattole River has continued to migrate laterally towards Stansberry Creek, undermining the downstream rock weir, which increased the drop over the weir to 2 ft (0.6 m). Additionally, the rock weir is no longer sealed, allowing water to flow between the rocks at low flow. The County is considering modifying the existing weir and constructing an additional weir below the culvert outlet to address these problems. In hindsight, with the river only 30 ft from the culvert outlet, it would have been better to avoid using rock weirs below the culvert. Instead, the concrete weirs within the culvert could have been constructed approximately 2 ft (0.6 m) lower and additional weirs, rock or concrete, could have been added on the upstream side.