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

To further investigate limiting factors associated with the Braden River floodplain,
percent-of-flow reductions that would result in a 15% loss of the number of days
river flows reached a range of flows were identified for the Braden River near
Lorraine gage, using flow records for the period of record (Figure 5-8). The low
end of the flow range examined reflects the approximate 50% exceedance flow
for the period of record (6 cfs), a flow which defines the beginning of Block 3.
The high end of the plotted flow range was selected to exclude rare flow events
(approximately the 1% exceedance) that would be expected to occur for
relatively short durations; durations for which 15% changes would be difficult to
evaluate.
Figure 5-8 indicates that for flows of approximately 100 cfs or greater, flow
reductions that result in a 15% reduction in the number of days the flow is
achieved, tend to stabilize around 10% for Braden River near Lorraine gage site.
This percent-of-flow reduction is comparable to the flow reduction values derived
for mean flows that would inundate dominant wetland vegetation classes, mucky
soils, and top of bank elevations (Table 5-3). Collectively, these data indicate
that up to a 10% reduction in the flows necessary to inundate floodplain features
of the Braden River, including those we have not identified, will result in a 15% or
less reduction in the number of days the features are inundated. However,
Figure 5-8 also shows that there is a range of flows that occur during Block 3
which do not require flow reductions to be limited to 10% to avoid a 15%
reduction in the number of days the flows are achieved. Using the period of
record 15% exceedance flow of approximately 54 cfs at the Braden River near
Lorraine gage as a cutoff for this range of flows, we can apply a stepped
prescription, which allows a 10% reduction in flows when flow exceeds 54 cfs,
and a 19% reduction in flows when the flow is below 54 cfs (Figure 5-8). While
additional flow reduction steps or percentages could be identified, or an algorithm
applied to determine allowable percent-of-flow reductions, the single step
approach provides a conservative means for assuring that unidentified factors
are likely to be protected and that flows not necessary for prevention of
significant harm are available for consumptive use. Unidentified factors could
include vegetative classes or species that we did not examine, or inundation of
vegetative classes to specified depths.
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