Location
States
CaliforniaEcosystem
River/streamIntroduction
California鈥檚 Los Angeles River was once a free-flowing river that frequently flooded. It was the primary source of freshwater for the City before the opening of the Los Angeles Aqueduct in 1913. Starting in the late 1930s, this urban corridor was channelized and lined with concrete to efficiently convey floods and minimize erosion. Today, high-velocity and uniform flow depths degrade habitat conditions for native aquatic species, and do not provide refugia during floods. This study examined how to redesign the channel bed to provide increased flow complexity and habitat heterogeneity within this confined urban stream and restore fish passage fish passage
Fish passage is the ability of fish or other aquatic species to move freely throughout their life to find food, reproduce, and complete their natural migration cycles. Millions of barriers to fish passage across the country are fragmenting habitat and leading to species declines. The U.S. Fish and Wildlife Service's National Fish Passage Program is working to reconnect watersheds to benefit both wildlife and people.
Learn more about fish passage by providing lower velocities suitable for migration. This study evaluated methods that could be implemented within confined urban channels to improve ecosystem function without significantly raising the flood stage at high flows. Additional goals include improving recreation and aesthetics, for example adding bike paths and improved access to the river.
Key Issues Addressed
Watersheds throughout the western United States have become increasingly urbanized over the last century. Degraded ecological conditions have resulted from increased water volume and intensity, decreased sediment yield, and constraints on channel meandering. Urban streams鈥� biological habitat values and species diversity have declined as rivers have been channelized and confined. Rivers that have been converted to urban flood control channels have also been negatively impacted by an economic disconnect between communities and their waterways. However, reliable base flows in many urban streams have led to opportunities for recreation and habitat features that can revitalize ecosystem services.
Providing fish passage through urban corridors can allow anadromous fishes to migrate to historical spawning grounds and improve connectivity for other aquatic species to access higher quality habitat areas upstream or downstream.
Project Goals
Evaluate concepts for ecosystem improvements at low to medium flows while maintaining high-flow capacity
Understand the benefits and effects of boulder clusters by testing a laboratory physical model
Identify and evaluate previous studies, develop partnerships, and refine the research strategy for a multi-year funding proposal
Project Highlights
Revitalized Urban Waterways: Through the Urban Waters Federal Partnerships, partners reconnect urban communities to their waterways, and revitalize the economic, environmental, and social benefits provided by urban streams.
- A Pilot Project: The Los Angeles River is being used to develop conceptual designs that increase the size and roughness of a low flow channel that fits within the larger concrete flood control channel. Design features include a meandering low flow channel with pools and riffles, flow deflectors, a multi-threaded channel, backwater areas, boulder clusters, and mid-channel islands with alternating bank-attached bars. Design concepts were tested and evaluated with a two-dimensional (2-D) numerical model and a physical model. Habitat suitability and effect on flood stage were assessed for each of the proposed ecological enhancement methods.
Purposeful Meetings: Meetings with stakeholder groups provided additional insight into some of the technical aspects that should be considered for research. The topic of access to the river by the public and City maintenance crews was discussed with regard to potential design constraints and opportunities for multi-purpose benefits. Data needs and availability were also addressed. Stakeholders were very enthusiastic about the research ideas and provided several helpful suggestions. They mentioned that it would be helpful to have scientific, data-driven studies to give more credibility to their ideas about improving habitat on the Los Angeles River.
Partnership through Collaborative Research: In addition to strengthening research proposals, partnerships provide benefits such as increasing the usefulness and relevance of the research projects. Therefore, the objectives for meetings and outreach were to:
Identify potential partners and inform them about the team鈥檚 research ideas
Identify potential roles for the partners and opportunities for collaboration
Identify in-kind data and potential funding opportunities
Learn more about technical and non-technical considerations that should be incorporated into the research
Visit river sites in the field to better visualize the challenges and opportunities for restoring urban streams
Lessons Learned
- It was beneficial to consider the different functions provided by the river at a variety of flows. At low to medium flows, the river is a multi-use system that can provide habitat, recreation, and water quality benefits. At high flows the river is a single use system with flood control being the primary concern. Urban rivers provide design challenges because the peak flows are increased, and the river is more constrained compared to historical conditions. Uniform channels do not provide places for fish to rest. Adding variability, roughness, and small flow obstructions to the channel bed improves aquatic habitat and fish passage by simulating a more natural system.
Engaging with stakeholders in person helped to connect with local communities to understand the social context for the research proposal. Engaging stakeholders early during the scoping phase provided tremendous benefits throughout the duration of the research project. Stakeholders provided additional expertise and review that improved the final product. For example, they helped define realistic biological objectives, such as fish passage, that could be achieved within the constraints of an urbanized channel. Furthermore, the stakeholders were approved for a grant to continue development of the research concepts through 60% design level. This increases the benefit, usefulness, and application of Reclamation鈥檚 initial investment in the research phase.
Aquatic species habitat is improved by creating areas of low velocity refugia and adding diversity and complexity to confined urban streams. Results from this study provide tools for transforming urban flood control channels to multi-function streams with increased ecological and aesthetic values. A relatively simple deepened and roughened low flow channel provides fish passage at low flows, but additional features are needed at high flows. Objects such as boulder clusters, islands, and flow deflectors provide low velocity patches for refuge as the average channel velocity increases at higher flows.
Next Steps
- Develop improved boulder cluster design guidelines using data from the physical and numerical model
Investigate combined effects of individual design features
Investigate changes in roughness from low to high flow and the corresponding effect on water surface elevation
Develop detailed designs, and eventually implement as a pilot construction project
Apply fish passage criteria to spatially map velocity patches that indicate where fish may be able to find refugia to wait out high flows during flood events
Funding Partners
Resources
Contact
Nathan Holste, Hydraulic Engineer, Bureau of Reclamation, [email protected]
Jennifer Bountry, Hydraulic Engineer, Bureau of Reclamation, [email protected]
Case Study Lead Author
Deanna Morrell, Program Analyst, Bureau of Reclamation, [email protected]
Suggested Citation
Morrell, D., P. (2020). 鈥淒esigning Low-Flow Channels in Confined Urban Rivers to Improve Aquatic Habitat Quality and Connectivity.鈥� CART. Retrieved from /project/low-flow-channels-confined-rivers.
