Functional Flow Rate Data

The thirteen Design Reaches were analyzed in order to quantify the hydraulics and flow characteristics at a range of potential Functional Flow Rate (FFR) conditions.  The FFR is a flow rate below which the River channel may be considered for multiple uses (i.e., uses other than flood risk management).  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 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.

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 as indicated in the following plots.  Plots are provided both within and outside the low-flow channel3 where appropriate.  Within each Design Reach there are variations in depths, velocities, and shear-stresses, which are illustrated by the error bars4 in the plots.  No results are provided for Reach K which is the Sepulveda Flood Control Basin that is not included in the 1-D HEC-RAS model.

 

Reaches B, C, E, L, and M each have a low-flow channel.  For certain flow rates the low-flow channel capacity may be exceeded over a certain portion of the reach length, as indicated in the figure below.  For example, for Reach L the median, 75th, and 90th percentile flows are fully contained within the low-flow channel.  For the 95th percentile flow there will be flow outside of the low-flow channel for approximately 30 percent of the length of Reach L, while for the 97th and 99th percentile flow there will be flow outside the low-flow channel for the entire Reach L.

1 The 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.

3 The low-flow channel is a deeper sub-channel within the main channel in specific Design Reaches that is designed to efficiently carry the low flows (i.e., non-storm flows).  Reaches B, C, E, L, and M each have low-flow channels.

4 Error bars within each Design Reach were calculated as the standard deviation of the results.