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

The flows required for inundating snag habitats at most of the instream crosssections
are less than the low flow threshold (LFT) of 59 cfs. Snag habitat may,
therefore, be expected to be inundated under all but the lowest flow conditions.
Because the LFT is protective of these low flows, we did not further evaluate use
of snag habitat for developing a Prescribed Flow Reduction for Block 2.
A flow of 255 cfs at the USGS Lithia gage is required for inundation of the mean
elevation of exposed root habitat. Based on the historic gage record, inundation
of this habitat is expected during Block 2, and would therefore also occur during
Block 3 when flows are higher. Flows sufficient to inundate the habitat may also
occur in Block 1 during some years. Because this important habitat may be
inundated during all three seasonal blocks, we determined a percent-of-change
flow reduction for inundation of the habitat during Block 2 using prescribed flow
reductions developed for Blocks 1 and 3. Percent-of-flow reduction during Block
2 was derived by calculating the flow reduction, which results in no more than a
15% loss of days of inundation of the habitat, over the entire year, after the flow
reductions for Block 1 and Block 3 were applied. Using RALPH analysis and flow
records from 1970 through 1999, we decreased the flows in Blocks 1 and 3 by
10% and 13% respectively, and evaluated percent-of-flow reductions for Block 2,
which combined with these prescribed flow reductions would not violate the
habitat availability criterion. Because the flow requirement at the Lithia Gage to
inundate mean exposed root elevation is 255 cfs, which is below the Block 3 step
of 374 cfs, a flow reduction of 13% was used for Block 3 rather than higher flow
step reduction of 8%. The same method was applied to the 1940 to 1969
benchmark. The 1970 through 1999 period resulted in a more restrictive criterion
and are thus is utilized as the more conservative approach. Based on this
criterion, a percent-of-flow reduction of 15% was identified for exposed root
habitat.
5.5.4 Selection of the Prescribed Flow Reductions for Block 2
Percent-of-flow reduction associated with PHABSIM modeling and RALPH
analyses associated with inundation of woody habitats were compared for
identification of a prescribed flow reduction. A prescribed flow reduction was
established for the USGS Lithia gage site based on percent-of-flow reductions
derived from woody habitat. These analyses indicated that up to 15% reduction
in flows would be acceptable for the Lithia gage site, while PHABSIM analysis
yielded percent-of-flow reductions around 34%.
5.5.5 Short-Term Compliance Standards for Block 2
Short-Term Compliance Standards represent a flow prescription that can be
utilized for evaluating minimum flow compliance on a short-term basis, for
example, based on measured daily flows. During Block 2, which for the Alafia
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