Project Summary
The pre-existing low-water crossing was a low elevation concrete slab spanning the width of the Cosumnes River.  Downstream scour created a 3 ft (0.9 m) drop in the water surface between the upstream and downstream channel resulting in a velocity and depth barrier for upstream migrating Chinook salmon at flows under 100 cfs (2.8 cms).  The solution was to install a single 5 ft (1.5 m) wide box culvert in the middle of the road to facilitate salmonid passage at flows below 200 cfs (5.6 cms).  Culvert design was taken from the State of California, Department of Transportation (CalTrans) Design manual.  The culvert was sized to produce water velocities within the swimming abilities of adult Chinook salmon.  The designers relied on the “Alaska Curve” (Ziemer, 1965), which indicates adult salmon can swim a distance of 30 ft (9 m) against water velocities of 10 ft/s (3 m/s).

Post Project Observations and Lessons Learned
The culvert performed better than expected at low flows.  Depths greater than 6 inches (15.2 cm) were present in the culvert at flows above 5 cfs (0.1 cms).  Chinook were observed swimming through the culvert with ease at flows as low as 15 cfs (0.42 cms).  At flows up to 150 cfs (4.2 cms) velocities were less than 10 ft/s (3 m/s), which is within the accepted swimming abilities for adult Chinook salmon for the 30 ft (9 m) culvert length. During flows ranging between 100 to 120 cfs (2.8 to 3.4 cms) point velocities of 4 to 8 ft/s (1.2 to 2.4 m/s) were measured in the culvert.

Velocities were problematic at flows between 150 and 200 cfs (4.3 and 5.7 cms), when the culvert was near capacity.  Velocities of 10 to 13 ft/s (3 to 4 m/s) were recorded during these events.  Fortunately this flow range only occurs for short periods.  At flows above 200 cfs (5.7 cms) the tailwater rises sufficiently to backwater the crossing, filling the culvert and causing water to begin flowing over the road.  Measurements showed velocities decreasing in the culvert as overtopping flows increased, allowing Chinook salmon to swim through the culvert.

Lessons Learned
There were several unanticipated outcomes of the project detrimental to adult salmon and to native minnows upstream of the culvert.

Adding the culvert inadvertently lowered the hydraulic control of the upstream channel.  At flows less than 50 cfs (15.2 cms) this reach turned into a shallow braided channel that was difficult to pass.  Many fish were observed struggling through this reach, and several cases of stranding were observed at flows below 30 cfs (9.1 cms).  During subsequent years significant down-cutting of the sandstone upstream of the culvert was noted.

Prior to implementation the crossing acted as a low-flow barrier to non-native predatory fish (Striped Bass, Largemouth Bass, and American Shad).  These fish were rarely observed upstream of the structure in previous surveys, but observed frequently in May-July of the years following culvert construction.  Funding is currently being sought to construct a removable fish weir to be placed in the culvert during the late spring and early summer months.

Reference:
 Ziemer, G.L.  Culvert design.  Alaska Dept. of Fish and Game. 1965. 2 p.

Published 03/15/07