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

factors, the most conservative of which is used to develop a prescribed flow
reduction for Block 2.
4.7.1 PHABSIM – Application for Block 2
PHABSIM was used to evaluate potential changes in habitat associated with
variation in medium flows. For the analyses, historic time series data from the
Braden River near Lorraine gage site were used. The two benchmark periods,
utilized for PHABSIM, ran from 1988-1993 and from 1994-2005.
Simulations were conducted for various life-history stages of spotted sunfish,
largemouth bass, bluegill, and macroinvertebrate diversity at two representative
sites on the Braden River. Maximum flow reductions that resulted in no more
than a 15% reduction in habitat from historic conditions during Block 2, which
runs from October 25 of one year to May 6 of the following calendar year, were
determined to be limiting factors. These factors were used to derive prescribed
flow reductions, which identify acceptable flow requirements for the Braden River
near Lorraine gage site during Block 2, when flows exceed the low flow
thresholds.
4.7.2 Snag and Exposed Root Habitat Analyses – Application for
Block 2
Mean elevations of snag and exposed root habitats were determined for fourteen
instream habitat cross-section sites in the Braden River. Flows at the crosssection
sites and corresponding flows at the Braden River near Lorraine gage
that would result in inundation of the mean habitat elevations at each crosssection
were determined using the HEC-RAS model. Long-term inundation
analyses was used to determine the number of days that the mean elevations for
the snag or root habitat were inundated. Flow records from two benchmark
periods (1988 through 1994 and from 1995 through 2005) were examined to
identify percent-of-flow reductions that would result in no more than a 15% loss
of habitat defined as a reduction of no more than 15% of the number of days of
inundation from direct river flow for the entire year, after prescribed flow
reductions for Blocks 1 and 3 were applied. Although we acknowledge that a
15% change in habitat availability based on a reduction in spatial extent of
habitat may not be equivalent to a 15% change in habitat availability based on
number of days a particular habitat is inundated (Munson and Delfino 2007), the
peer review panel for the middle Peace River MFLs noted, “that the 15%
threshold selected for preventing significant harm is appropriate” (Shaw et al.
2005).
Loss of days of direct connection with river flows was evaluated for the entire
year since woody habitats in the river are expected to be inundated during
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