The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Similar to the coldest period simulations, the thermal refuge simulations also found little
difference in bottom temperatures between any of the springflow scenarios. Again, there
were differences in surface water temperatures, with these differences increasing with
the amount of springflow removed from the system. Differences in mean temperatures
between the baseline and the other scenarios for the entire thermal refuge period
ranged from 0.53 o C degrees for scenario A to –1.35 o C degrees for scenario C It is
reiterated that during these conditions temperatures were above 20 o C were in bottom
waters.
5.5.9 Summary and application of thermal modeling results to a minimum flow
for Sulphur Springs
The combined results for the coldest period and thermal refuge period scenarios
indicate that that flows from Sulphur Springs have an effect on water temperatures in
the lower Hillsborough River. This thermal effect is largely manifested in the surface
layers above the pycnocline, or the depth at which there is strong vertical gradient in
water density. The thermal refuge scenarios indicated that all of the springflow
scenarios would provide a suitable thermal refuge when surface water temperatures
fluctuated between 17 and 20 o C. The coldest period scenarios, however, indicated that
only scenario A would not exceed the 2 o C change suggested by the FRMI. To prevent
unacceptable changes to a thermal refuge in the lower river, scenario A (18 cfs flow to
the spring run) should be required when water temperatures in the river fall to low
values. As described in Section 5.4, scenario A is the recommended minimum flow for
routine operation based on salinity criteria. Requiring scenario A to be in effect during
very cold periods would be in keeping with the desirability of this flow rate based on
other criteria.
Water temperature measurements are now collected at a number of sites in the Lower
Hillsborough River and one or more of these sites could be continued with real time
data availability to track the occurrence of low temperatures in the river. Based on the
results of the thermal modeling simulations, it is recommended that if water
temperatures in either surface or bottom waters fall below 15 o C, then a minimum flow of
18 cfs be required at Sulphur Springs with no adjustment for low tide stages. Using
either surface or bottom as the trigger for the determination of low temperatures would
ensure that this minimum flow would be implemented before the entire water column
went below 15 o C.
Such a low temperature stipulation would likely be in effect for only short periods of
time. A plot of daily surface and bottom temperatures for a continuous recorder located
in the river about 100 meters upstream from Sulphur Springs was presented in Figure 3-
31. Of the four winter seasons covered by this graphic, water temperatures were below
15 o C in either surface or bottom waters for a total of 11 days. Low reservoir levels and
impending water shortages are most acute for the City of Tampa in the late spring
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