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

and Levels" found HEC-RAS to be an "appropriate tool" for assessing these
relationships and determined this to be a "scientifically reasonable approach"
(Gore et al. 2002).
4.3.2 Physical Habitat Simulation (PHABSIM) Modeling
In "A Review of 'Upper Peace River: An Analysis of Minimum Flows and Levels'"
Gore et al. (2002) suggests that the District consider use of procedures which
link biological preferences for hydraulic habitats with hydrological and physical
data. Specifically, the authors endorsed use of the Physical Habitat Simulation
(PHABSIM), a component of the Instream Flow Incremental Methodology (Bovee
et al. 1998) and its associated software for determining changes in habitat
availability associated with changes in flow. Following the recommendations of
the reviewers, the SWFWMD used the PHABSIM program for development of
minimum flows for the Alafia River and associated springs.
PHABSIM analysis requires acquisition of data concerning channel composition,
hydraulics, and habitat suitability or preferences. Required channel composition
data includes dimensional data, such as channel geometry and distance between
sampled cross-sections, and descriptive data concerning substrate composition
and cover characteristics. Hydraulic data requirements include measurement of
water surface elevations and discharge at each cross section. These data are
gathered under a range of flow conditions to provide for model calibration.
Habitat suitability criteria are required for each species of interest. Criteria may
be empirically derived for individual species/water bodies or developed using
published information.
Hydraulic and physical data are utilized in PHABSIM to predict changes in
velocity in individual cells of the channel cross section as water surface elevation
changes. Predictions are made through a short series of back-step calculations
using either Manning's equation or Chezy's equation. Predicted velocity values
are used in a second program routine (HABTAT) to determine cell-by-cell the
amount of weighted usable area (WUA) or habitat available for various
organisms at specific life stages (Figure 4-6). The WUA/discharge relationship
can then be used to evaluate modeled habitat gains and losses with changes in
discharge. Once the relationships between hydraulic conditions and WUA are
established they are examined in the context of historic flows, and altered flow
regimes. This process was accomplished by conducting a time series analysis
routine (TSLIB, Milhous et al. 1990) using historic flow records.
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