Can Low-Flow Ecosystem Features be Sustainable in High Energy Urban Flood Channels
This scoping research considered how to rework the channel bed and banks, or add features such as flow deflectors and pools/riffles, to provide increased flow complexity and habitat heterogeneity. The primary constraints are the existing channel top width and flood stage at high flows. Ideally, increasing channel width, floodplain area, and stormwater detention would all be aspects of a comprehensive urban stream restoration project. However, these elements are often cost prohibitive or not feasible to implement in highly confined urban environments. Therefore, the scoping review focused on designing ecosystem features within the existing channel footprint.
Need and Benefit
In the community of river restoration, practitioners and scientists alike are drawn to rivers with opportunities to restore natural function through high profile projects such as dam removal, large scale road and levee removal or setbacks, restoration of riparian vegetation and large wood, and reconnection of floodplains. Yet in many locations river systems are highly altered leaving opportunities for restoration constrained by water availability and delivery requirements, local land use, recreation access, need for limited risk of erosion or flooding near private property and infrastructure, and desired high return on investment with sustainable features that require low maintenance. These challenges facilitate the need for thinking outside the box to develop feature concepts that can provide ecosystem function, while surviving higher energy intermittent floods in highly modified river areas. The Los Angeles River is a prime example of a hyper-urbanized watershed. Ecological values in this high capacity concrete flood control channel are currently minimal. The existing recycled water base flow may offer an opportunity to improve biological habitats and species diversity while maintaining the essential flood control function. A high-profile inter-agency effort is underway to restore this river (http://www.lariver.org/index.htm). The Los Angeles River was one of seven Urban Waters Pilot projects selected on June 24, 2011 when the Urban Waters Federal Partnership was announced with the goal of working closely with local partners to restore urban waterways. The Department of the Interior is one of 11 participating federal agencies. Identified goals specific to the Los Angeles River include: 1) restore ecosystem functions, 2) balance revitalization with flood avoidance to ensure public safety, 3) reduce reliance on imported water supply, and 4) foster sustainable stewardship. The watershed runoff characteristics are highly altered, including permeability and times of concentration. Fluctuations in the flow don't follow natural meteorological response. Peaks are altered in both timing and magnitude. Impervious surfaces in the highly developed urban environment result in increased peak flows of shorter duration and higher intensity. Base flow is not related to groundwater interactions. Very few rivers in southern California still flow perennially to the ocean. The Los Angeles River recycled water base flow may provide a unique opportunity. Recycled wastewater discharges make up the majority of the existing low-flow regime. This reliable year-round base flow offers an interesting and unique potential to support a suite of biological communities in the river. There is an opportunity to re-establish riverine processes - not all, but maybe an essential subset - with the recycled flows. The recycled water currently supports a non-native fishery, including carp, catfish, tilapia, mosquito fish, and minnow species. The low-flow channel lacks complexity, being primarily designed to transport water efficiently during high flows. Reclamation staff observed that restoring or creating stream flow complexity might substantially improve biological values, with negligible effect to flood water conveyance capacity. The Los Angeles River restoration is a specialized urban case of Natural Channel Design (NCD). The floor of the concrete lined channel could be modified to conform to or simulate a more natural channel. Essential features may include a guaranteed minimum base flow, sediment, cover elements, and room for flows to casually rearrange some of those elements. Some permanent elements are expected, such as a hard channel perimeter, populated with some permanent faux-large wood elements, and other cover elements such as boulder clusters. The natural sediment supply should be sufficient to 'fill-in' around the permanent elements to create a 'natural' channel supporting a targeted suite of biological communities.
Contact the Principal Investigator for information about partners.
Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Can Low-Flow Ecosystem Features be Sustainable in High Energy Urban Flood Channels (final, PDF,
By Nathan Holste
Publication completed on September 30, 2016