Chapter 1 Minimum Flows and Levels - Southwest Florida Water ...

Calculations for subcritical flow in the HEC-RAS model begin downstream where
a boundary condition is applied. For the Braden River, a known water-surface
elevation, calculated from a stage-discharge relationship at the Braden River at
Linger Lodge near Bradenton gage was used as a downstream boundary
condition. The energy equation is then solved between the first and second
(most downstream) cross sections. Once this is achieved, the model repeats this
process working its way upstream balancing the energy equation (or momentum
equation if appropriate) between adjacent cross-sections until the most upstream
cross-section is reached.
Model accuracy is evaluated by comparing calculated water-surface elevations at
any gage location with a stage-discharge relationship derived from historic data
for the location. The model is calibrated by adjusting factors in the model until
calculated results closely approximate the observed relationship between stage
and flow. While expansion and contraction coefficients can be altered, the major
parameter altered during the calibration process is typically Manning's roughness
coefficient (n), which describes the degree of flow resistance. Flow resistance is
a function of a variety of factors including sediment composition, channel
geometry, vegetation density, depth of flow and channel meandering. For the
Braden River HEC-RAS model, a rating curve at the most upstream gage site
(Braden River at Lorraine) was not available from USGS. Calibration measures
were made against the existing data for this site.
The Braden River HEC-RAS model calculates profiles for a total of 21 steadyflow
rates derived from historical flow data measured in the river. The boundary
conditions were specified with known water surface elevations for each flow rate
at the downstream boundaries. As was the case for the most upstream gage
site, a USGS rating curve was not available for the most downstream gage site
(Braden River at Linger Lodge). The lack of a rating curve for the site is due to
the influences of backwater effects from Bill Evers Reservoir, an impoundment
created by a broad–crested weir. Multiple regressions between the Braden River
near Lorraine gage and the gage at Linger Lodge were used to generate a series
of stage flow relationships at the Braden River at Linger Lodge near Bradenton
gage.
Accuracy of the step-backwater analysis for the Braden River was determined by
comparing the modeled water surface elevations with rated water-surface
elevations at the Braden River near Lorraine gage site. The HEC-RAS model
was considered calibrated when calculated water surface elevations were within
plus or minus 0.5 ft, in keeping with standard USGS practices where this range
of error is based on the potential error associated with using data collected to a
1-ft contour interval aerial mapping standard for model development (Lewelling
2004). The greatest error associated with the model is likely to be the accuracy
of the cross-sectional data.
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