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

flow, with the exception that withdrawals should not be allowed to reduce the flow
to less than 59 cfs at the Lithia gage site.
For the high flow season of the year (Block 3, which runs from June 25 to October
27), a prescribed flow reduction was based on review of limiting factors developed
using the HEC-RAS floodplain model and Regional and Long Term Positional
Hydrographic (RALPH) analyses to evaluate percent of flow reductions associated
with changes in the number of days of inundation of floodplain features. It was
determined that a stepped flow reduction of 13% and 8% of historic flows, with the
step occurring at the 25% exceedance flow (374 cfs), resulted in a decrease of
15% or more in the number of days that flows would inundate floodplain features
at the Lithia gage. Using these limiting factors, prescribed flow reductions
consistent with the stepped flow reductions described above were established,
with the exception that withdrawals should not be allowed to reduce the flow to
less than 59 cfs at the Lithia gage site.
For the medium flow period (Block 2, which runs from October 28 of one year to
April 19 of the next), PHABSIM analyses were used to model flows associated
with potential changes in habitat availability for several fish species and
macroinvertebrate diversity. In addition, flows associated with inundation of
instream woody habitats were evaluated using the HEC-RAS model and RALPH
analyses. Using the more conservative of the two resulting flows, it was
determined that woody habitat would define the percent flow reduction. It was
determined that a flow of 255 cfs at the USGS Lithia gage is required for
inundation to the mean elevation of exposed root habitat. Using these limiting
factors, the prescribed flow reduction during the medium flow period was defined
as a 15% reduction in flow at the Lithia gage site, with the exception that
withdrawals would not be allowed to reduce flow at the Lithia site below 59 cfs.
Because minimum flows are intended to protect the water resources or ecology of
an area and because climatic variation can influence river flow regimes, we
developed long-term compliance standard for the Alafia River gage site at the
Lithia gage. The standards are hydrologic statistics that represent flows that may
be expected to occur during long-term periods when short term-compliance
standards are being met. The long-term compliance standards were generated
using gage-specific historic flow records and the short-term compliance
standards. For the analyses, the entire flow record for each site was altered by
the maximum allowable flow reductions in accordance with the prescribed flow
reductions and the low flow threshold. Hydrologic statistics for the resulting
altered flow data sets, including five and ten-year mean and median flows were
determined and identified as long-term compliance standards. Because these
long-term compliance standards were developed using the short-term compliance
standards and the historic flow records, it may be expected that the long-term
standards will be met if compliance with short-term standards is achieved.
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