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

Table 4-2. Beginning and ending dates for annual flow Blocks 1, 2, and 3 for the Alafia,
Hillsborough, Myakka and Peace Rivers for non-leap years.
Start Date (Julian day) End Date (Julian Day) Number of Days
Block 1 April 20 (110) June 25 (176) 66
Block 2 October 27 (300) April 19 (109) 175
Block 3 June 26 (177) October 26 (299) 123
4.5 Low Flow Threshold for the Alafia
As part of the protection of the flow regime, minimum flows and levels are
intended to protect aquatic resources associated with low flows. To accomplish
this goal, it is necessary to develop a low flow threshold, which identifies flows
that are to be protected in their entirety (i.e., flows that are not available for
consumptive-use). To determine this threshold, two low flow standards are
developed. One is based on the lowest wetted perimeter inflection point; the
other is based on maintaining fish passage along the river corridor. The low flow
threshold is established at the higher of the two flow standards. Although flows
less than the low flow threshold may be expected to occur throughout the year,
they are most likely to occur during Block 1 (Figure 4-10).
4.5.1 Wetted Perimeter Standard
Output from multiple runs of the HEC-RAS channel model were used to generate
a wetted perimeter versus flow plot for each HEC-RAS cross-section of the Alafia
River corridor (see Figure 4-11 as an example, Appendix WP). Plots were
visually examined for inflection points, which identify flow ranges that are
associated with relatively large changes in wetted perimeter. The Lowest Wetted
Perimeter Inflection Point (LWPIP) for flows up to 200 cfs was identified for each
cross-section. Inflection points for flows higher than 200 cfs were disregarded
since the goal was to identify the LWPIP for flows contained within the stream
channel. Many cross-section plots displayed no apparent inflection points
between the lowest modeled flow and 200 cfs. These cross-sections were
located in areas where the water surface elevation may exceed the lowest wetted
perimeter inflection point even during low flow periods. For these cross-sections,
the LWPIP was established at the lowest modeled flow. Flows associated with
the LWPIP at each cross-section were converted to flows at the USGS Lithia
Gage using empirical relationships from HEC-RAS channel model output. Flow
at the USGS Lithia gage that was sufficient to inundate the LWPIP at all sampled
cross-sections was used to define the wetted perimeter low flow standard for the
Alafia River corridor.
4-20