Channel restoration in highly urbanized systems: using ecological flow targets to optimize channel design under future management scenarios

The desire to restore urban rivers to improve habitat and provide social amenities can be complicated because flows are often highly altered and managed to accommodate water quality (e.g., stormwater capture), flood protection, wastewater discharge, and water reuse. A key challenge is often a lack of defined ecological flow targets to help inform restoration design. This study evaluates design options for restoration in the Los Angeles River that can offset the effects of flow augmentation or depletion associated with water reuse and management. Ecological flow targets associated with species of management concern (Willow, Steelhead, and Santa Ana Sucker) were computed from hydraulic-ecology models. Anticipated changes to flow were simulated using the EPA Stormwater Management Model (SWMM). These flows were routed through a 1-D HEC-RAS model of the Los Angeles River, used to simulate hydraulics and iterate potential channel cross-section configurations across the changes to flow. Hydraulic outputs were compared to eco-hydraulic targets for desirable aquatic species in the LA River (willow, steelhead trout, and Santa Ana sucker). For some reaches, a range of channel restoration designs may satisfy ecological targets under managed flows, but for others, trade-offs exist between water management goals and target species habitats. In particular, when future management actions deplete already low flows in tributary reaches, it may not be feasible to optimize restoration to meet targets for both fish and floodplain vegetation. Overall, this study illustrates the potential to use ecological flow targets combined with iterative hydraulic modeling to guide urban river restoration design and offers an approach to weighing trade-offs between sustainable water resource management and ecological conditions.