Location
Klamath River Basin, Northern California, USA. MAP
Project Type
Pre-Project Conditions
Pre-Project Barrier
Watershed Characteristics
Ecological Value
Project Characteristics
Challenges
Project Contributors
Project Funding
Completion Date
October 2003
Project Cost
| Construction | $ | 261,236 |
| Materials | $ | 139,111 |
| Engineering | $ | 40,704 |
| Planning | $ | 715 |
| Total | $ | 441,766 |
Project Description
The existing double box concrete culvert and outlet apron had a wide flat bottom that created both a water velocity and depth barrier to all adult and juvenile salmonids. The perched outlet apron also presented a leap barrier at migration flows. The undersized culvert, possibly combined with historic realignment and straightening of the channel, caused the upstream channel to aggrade more than 3 ft (0.9 m). This channel aggradation buried storm drains, decreased channel conveyance, and increased flooding of surrounding pastures and a local elementary school. This site received a priority ranking of 5th in the Del Norte County Inventory and Fish Passage Evaluation Report.
A 1,600 ft (490 m) longitudinal profile of the channel was surveyed to identify the stable channel grade and estimate the amount of stored sediment upstream of the culvert. Analysis of the profile indicated localized channel aggradation, as deep as 3 ft (0.9 m), present at the culvert inlet and extending approximately 800 ft (244 m) upstream. The aggradation is partially attributed to frequent ponding of floodwaters at the undersized culvert inlet. In this reach the channel grade was uncharacteristically low and severely lacked complexity, such as pools and woody debris.
Removing the culvert would lead to flushing of aggraded sediment and temporarily impact the downstream channel, which recently received fisheries habitat enhancement structures. To accelerate recovery and avoid downstream impacts, 1,100 ft (335 m) of upstream channel was regraded at the identified stable grade (1% slope) and 1,200 cy (920 m3) of sediment was removed.
The barrier was replaced with a 28 ft (8.5 m) wide pre-cast bottomless arch culvert placed on concrete strip footings. The top of the formed-in-place strip footings were constructed level with the channel design grade. The arch-culvert is wider than the active channel and of sufficient size to accommodate the passage of the 100-yr flow and associated substrate and debris. Two boulder weirs were constructed above the crossing to maintain channel grade, reduce additional sediment release, and improve fish habitat. Additionally, four boulder wing deflectors were installed to improve fish habitat. Upon completion, disturbed areas were revegetated with a native grass seed mixture, ferns, and over 250 native conifer and decidious trees to expand and diversify the riparian area.
An excellent public education opportunity presented itself with this fish passage project. Students from Margaret Keating Elementary, located next to Mynot Creek, participated in riparian planting efforts and made an educational display on salmon and stream restoration that was placed at the new crossing.
Post-Project Observations and Lessons Learned
Removing the aggraded sediment with heavy equipment appears to be an appropriate solution for this site for several reasons: (1) Mynot Creek was dry during construction, thereby avoiding the challenges and biological impacts associated with dewatering, (2) the channel completely lacked large wood and other elements that typically store and meter out sediment, and (3) the sediment was predominately fines and small gravel that would have temporarily impacted downstream habitat if released. These factors were considered when determining whether to remove the aggraded sediment, meter it out slowly using temporary grade control, or let it naturally regrade and transport the sediment downstream.
There continues to be post-project channel adjustment. After removing the sediment additional bedload mobilized during the first winter, resulting in aggradation within the culvert. Further post-project monitoring is needed to determine the sources of the mobilized sediment and if the channel has reached its new equilibrium. Currently there are plans to lower the two boulder grade control weirs to allow the channel bed to continue lowering.
Spawner surveys identified a coho salmon carcass upstream of the culvert the first winter after construction (2003). A spawning adult steelhead and a redd were observed just above the project area in 2004.