Functional Flow Rate
The major shift in thinking of this study compared to previous studies attempts to change the performance metric for evaluating the usability of the river. Other studies have used the criteria of slowing down the speed of the water to sustain plant and animal species to determine the usability of the river channel. This study, understanding that it is very challenging to modify the speed of water without impacting flood control, evaluated the percentage of time the river channel was servicing storm water. The team quickly realized that 99% of the time, the majority of the river channel was being underutilized with only a few feet of water—even during rain events.
After further research we found that the Los Angeles River has been looked at primarily in two conditions:
Dry Flow– the condition when the only water in the river channel is supplied by storm drain run-off, ground water up swelling and discharges from the water reclamation facilities.
Wet Flow– the condition when it is raining.
The design team is proposing the wet flow can be subdivided into multi-use and single use conditions, which allow parts of the river channel to be used while not impact flood control or public safety. The team identified the dividing line as the Functional Flow Rate (FFR).
The thirteen Design Reaches were analyzed in order to quantify the hydraulics and flow characteristics at a range of potential FFR conditions. The most suitable FFR will vary by location along the River and the intended design intervention, based upon a trade-off between how often the FFR is exceeded (in which case the River will be not be available for multiple uses) and the difficulty and cost of designing and constructing an intervention to be functional at higher flow rates. Rather than prescribe specific FFR for each Design Reach, a range of potential FFR were analyzed in order to provide relevant hydraulic and flow information (flow depth, velocity, and shear-stress) for designers and planners to consider.
The median, 75th, 90th, 95th, 97th, and 99th percentile flow rates for each Design Reach were determined by analyzing output from the LA County LSPC model1 and are shown in the figure below. As an example, the 97th percentile flow rate for Reach D is approximately 2,000 cubic feet per second (cfs), which means that 97 percent of the time the flow rate in Reach D is less than 2,000 cfs. Thus, if an intervention in Reach D is designed for a FFR of 2,000 cfs, then the intervention would be useable 97 percent of the time (i.e., 354 days per year on average). By contrast, the 90th percentile flow rate in Reach D is approximately 500 cfs. If an intervention is designed for a FFR of 500 cfs then it would only be useable 90 percent of the time (i.e., 329 days per year on average), but this lower usability may be off-set by lower costs to design, construct, and maintain the intervention.
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Hydraulic analyses using the 1-D HEC-RAS model2 was conducted for each of the potential FFR in order to calculate Reach-averaged flow depths, velocities, and shear-stresses. These model results are summarized in the Functional Flow Rate Data section.
1The Loading Simulation Program C (LSPC) is a watershed hydrology model developed and calibrated by LA County over a 25-year period (1987 through 2011).
2 The model was modified to enable flow characteristics to be calculated separately within and outside the low-flow channel as appropriate.