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

The percent-of-flow reduction standards were developed using long-term
inundation analysis to assure that the number of days that flows sufficient to
inundate floodplain features are not reduced by 15% or more.
5.5 Prescribed Flow Reduction for Block 2
A prescribed flow reduction for Block 2 flows at the Braden River near Lorraine
gage site was based on review of limiting factors developed using PHABSIM to
model potential changes in habitat availability for several fish species and
macroinvertebrate diversity, and use of long-term inundation analyses to
specifically evaluate changes in inundation patterns of woody habitats. The
prescribed flow reductions were established by calculating the percent-of-flow
reduction, which would result in no more than a 15% loss of habitat availability
during Block 2, or no more than a 15% reduction in the number of days of
inundation of exposed root habitat over the entire year, after prescribed flow
reductions for Blocks 1 and 3 were applied. PHABSIM analyses yielded more
conservative percent-of-flow reductions than the long-term inundation analyses
for woody habitats. PHABSIM results were therefore used to establish a
prescribed flow reduction of 11% for the Braden River near Lorraine gage site.
5.5.1 PHABSIM Results for Block 2
Physical Habitat Simulation analyses were conducted for two representative sites
on the Braden River. The "upper" site was located downstream of the Braden
River near Lorraine gage, and the "lower" site was located approximately one
mile upstream from the Linger Lodge gage. For both sites, the Braden River
near Lorraine flow record was utilized in the PHABSIM time-series analyses. The
record was split into two benchmark time periods, 1988 through 1994 and 1995
through 2005, based on Atlantic Multidecadal Oscillation cycle changes.
Based on flow records from both benchmark periods, flow reductions that would
not be expected to result in more than a 15% reduction in available habitat were
identified for 2 to 10 species/life history stages at the upper site for the months
from October through April (Figure 5-9). Analyses for the lower site identified a
total of 1 to 9 species/life history stages that would be expected to be associated
with a 15% reduction in available habitat during the seven months of Block 2
(Figure 5-10).
Based on these results, November and December are the most restrictive
months in Block 2. An allowable percent-of-flow reduction of 11% based on
PHABSIM results was determined by averaging results from the two sites for
November and December.
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