Alafia River Minimum Flows and Levels - Southwest Florida Water ...

modeling was used to characterize potential changes in the availability of fish
habitat and macroinvertebrate diversity. Recent and Long-term Positional
Hydrograph (RALPH) analysis was used to examine inundation durations for
specific habitats or floodplain elevations and to also examine changes in
inundation patterns that could be expected with changes to the flow regime.
4.3.1 HEC-RAS Modeling
The HEC-RAS model is a one-dimensional hydraulic model that can be used to
analyze river flows. Version 3.1.1 of the HEC-RAS model was released by the
U.S. Army Corps of Engineers Hydrologic Engineering Center in November 2002
and supports water surface profile calculations for steady and unsteady flows,
including subcritical, supercritical, or mixed flows. Profile computations begin at
a cross-section with known or assumed starting condition and proceed upstream
for subcritical flow or downstream for supercritical flow. The model solves the
one-dimensional energy equation. Energy losses between two neighboring cross
sections are computed by Manning's equation in the case of frictional losses and
derived from a coefficient multiplied by the change in velocity head for
contraction/expansion losses. For areas where the water surface profile changes
rapidly (e.g., hydraulic jump, bridges, river confluences), the momentum equation
is used (US Army Corps of Engineers 2002).
We used the HEC-RAS model and available flow records for the USGS Lithia
gage to simulate flows at cross-section sites within the Alafia River corridor.
Data required for performing HEC-RAS simulations included geometric data and
steady flow data. Geometric data consisted of connectivity data for the river
system, cross-section elevation data, reach length, energy loss coefficients due
to friction and channel contraction/expansion, stream junction information, and
hydraulic structure data, including information for bridges, culverts, etc. Required
steady-flow data included the flow regime and boundary conditions.
Calculations for subcritical flow begin downstream where a boundary condition is
applied. For the Alafia River corridor, a known water-surface elevation,
calculated from a stage-discharge relationship at the USGS Lithia 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 locations with a stage-discharge relationship derived from historic data
at that location. The model is calibrated by adjusting factors in the model until
the calculated results closely approximate the observed relationship between
stage and flow. While expansion and contraction coefficients can be altered, the
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