Flood Channels: Developing Fish Resting Pools in a Concrete Channel Using Fish Energetics and Hydrodynamic Modeling

Corte Madera Creek in Marin County, California flows through an urbanized corridor before entering San Francisco Bay. It historically supported runs of coho salmon and continues to maintain a run of ESA listed steelhead trout.  The lower 6.2 km of Corte Madera Creek is contained within a flood control channel designed by the US Army Corps of Engineers.  Most of the flood control channel is tidally influence and provides no impediments to migrating adult steelhead.  However, the upper 730 m of channel is above mean lower-low tide, consists of concrete vertical walls and V-shaped floor, and designed for supercritical flow.  Prior to floodway construction in 1972, the potential of creating a steelhead depth and exhaustion barrier was recognized.  As mitigation, 28 rectangular pools spaced 19.5 m apart were included in the channel. Each pool is 1.2 m long, 4.0 m wide, and has a flat bottom placed 2.5 cm below the channel invert. The outside edges of the pools are 38 cm deep due to the V-shaped bottom.  Examination of the flow patterns within the pools combined with observations of steelhead swimming within the channel suggests the existing pools are undersized and provide minimal resting habitat during typical migration flows. 

As part of a project lead by the Friends of Corte Madera, steelhead passage conditions within the present channel were assessed and conceptual designs were developed for providing fish passage over a wide range of stream flow through improved resting pools.  The project team used several innovative approaches to address the problem, including (1) using volunteers to record observations of flow conditions and fish movement, (2) development of a 2-D hydrodynamic model to analyze existing hydraulic conditions and evaluate hydraulic performance of different resting pool alternatives, (3) development and application of a population-based fish routing and energetics model to evaluate passage conditions and determine preferred pool spacing, and (4) use of a 1-D HEC-RAS model to evaluate impacts of the preferred pool geometry and spacing on water levels during the capacity discharge of 153 cms.

Project issues addressed include attempting to provide upstream passage at migration flows for an acceptable proportion of the steelhead population, minimizing pool sedimentation, minimizing flooding impacts, and ensuring proposed alternatives are constructible.  This presentation will outline the alternatives development process and focus on how combining 2-D hydrodynamic modeling and population based fish passage modeling can provide valuable insight during the design process.