Case Study Contributors
Location
Humboldt Bay, Northern California, USA. MAP
Project Type
Pre-Project Conditions
Pre-Project Barrier
Hydrologic Characteristics
Ecological Value
Project Characteristics
Challenges and Lessons Learned
Project Contributors
Project Funding
California Department of Fish and Game
Completion Date
December 2004
Total Project Cost
| Design and Permitting | $ | 10,000 |
| Construction | $ | 16,000 |
| Total | $ | 26,000 |
Project Summary
The existing tidegate was a wooden vertical lifting door mounted onto a concrete box culvert. Both salmonids and estuarine fish species were only able to swim upstream through the gate during brief periods when it was open and water velocities were not excessive. During high flow, water velocities were excessive through the open gate, and during low flow the leaky gate failed to open.
The new tidegate is a 6 ft x 6 ft (1.8 m x 1.8 m), custom-fabricated, side-hinged aluminum gate mounted on the concrete wingwalls of the existing structure. The tidegate is designed to open and drain water stored in the upstream channel when the water level from the tide on the downstream side of the gate is lower than water level on the upstream side. When open, the side-hinged gate provides a larger, less constricted, opening than standard top-hinged gates, thus draining upstream slough channels at rates similar to unobstructed channels. The larger opening produces lower water velocities and improved fish passage inside the tidegate, but higher average water velocities in the upstream slough channels.
To create and manage a muted tidal system upstream of the gate, an adjustable 1 ft (0.3 m) wide auxiliary door was installed in the gate, which can be completely closed or opened up to 2 ft (0.6 m). When the tidegate is closed the auxiliary door allows a targeted amount of tidal water to flow through the tidegate and into the upstream channel. A hydraulic model was used to determine the optimum sized opening of 1 ft2 (930 cm2) for the auxiliary door to maintain existing tidal marsh upstream of the tidegate while protecting adjacent pasture from saltwater intrusion. The regular tidal flux helps maintain upstream channel capacity by flushing fine sediments during each ebb tide and suppress freshwater vegetation from growing in the channel. The muted tide mixed with freshwater from the stream creates a gradient from saltwater to brackish aquatic habitat available to marine fish and anadromous salmonids. The auxiliary door also provides fish passage when the tidegate is closed.
In addition to the tidegate modification, the project included restoration and habitat enhancement of 1 mile (1.6 km) of stream channel that traverses the pasture, reconstructed levees, and reduced cattle stream-crossing and grazing impacts on estuarine and riparian areas.
Post Project Observations and Lessons Learned
The tidegate fails to swing fully open, deflecting flow towards the opposite bank. This lead to substantial bank erosion, which was later mitigated by placing riprap along the bank. The fabricator reasoned that placing the auxiliary door in the gate reduced the forces on the gate, resulting in it not fully opening.
The summer after tidegate construction 32 young-of-year coho salmon were captured in a 1,200 ft (365 m) stream reach upstream of the tidegate, suggesting that during the previous winter adult coho successfully passed through the tidegate and spawned. Young-of-year densities were higher in summer 2006 (63 YOY in 700 ft) after channel restoration was completed.
Published 02/23/07