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

section sites and corresponding flows at the USGS Lithia gage that would result
in inundation of the mean habitat elevations at each cross-section were
determined using the HEC-RAS channel model. RALPH plots were used to
determine the number of days that the mean elevations for the snag or root
habitat were inundated. Flow records between 1980 and 1999 were examined to
identify percent-of-change 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. Ordinarily, the dry period associated with the multidecadal
trends identified would be used. However, low flow augmentation between 1970
and 1980 inflate the flow values found and should not be expected in the future.
These percent-of-flow reductions were considered to be limiting factors and used
for development of a prescribed flow reduction (PFR2) for Block 2.
4.8 Prescribed Flow Reduction for Block 3
Junk et al. (1989) note that the “driving force responsible for the existence,
productivity, and interactions of the major river-floodplain systems is the flood
pulse”. Floodplain vegetation development and persistence does not, however,
necessarily depend wholly on inundation from the river channel. Groundwater
seepage, hyporheic inputs, local tributaries, and precipitation can also lead to
floodplain inundation (Mertes 1997). However, because river channel-floodplain
connections are important, can be influenced by water use, and may be a
function of out-of-bank flows, it is valuable to characterize this connectivity for
development of minimum flows and levels.
Highest flows, including out-of-bank flows, are most likely to occur during Block 3
(Figure 4-10), which for the Alafia River extends from June 26 to October 26.
Minimum flows developed for this period are intended to protect ecological
resources and values associated with floodplain by maintaining hydrologic
connections between the river channel and flood plain and maintaining the
natural variability of the flow regime. This goal is accomplished through the
HEC-RAS modeling and use of RALPH analysis to evaluate floodplain feature
inundation patterns associated with channel-floodplain connectivity. Based on
these analyses, a prescribed flow reduction for Block 3 (PFR3) can be
developed.
4.8.1 Floodplain Connection Analyses for Block 3
HEC-RAS model output and RALPH analysis were used to evaluate floodplain
inundation patterns associated with river flows at the eight floodplain vegetation
cross-sections. Inundation of elevations associated with floodplain features,
including vegetation classes and soils, were evaluated to establish percent-of-
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