The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
There were some flow rates above 18 cfs that showed salinity observations greater than
3 ppt. These are shown as the individual data points above the 95 th percentile in Figure
5-5 (for flows less than 34 cfs). With the exception of the data for 26 and 30 cfs, these
observations were recorded during one of two periods during the year 2000; which were
February 12 through February 15 and April 27 through May 2. During these periods
there were major manipulations of water levels in the spring pool due to operation of the
outlet structure at the pool and withdrawals by the City. These manipulations typically
occurred during the middle of the day, so the average daily flows reported for the spring
included widely varying flows, which are not represented in the average daily flows
presented in Figure 5-5. The data reported for 30 cfs are similar in that the structure
was operated during that day and flows varied widely.
The high salinity observations for 26 cfs were a different situation, as there were no
manipulations of water levels in the pool and flows had been relatively stable for the
preceding seven days. Maximum salinity on the preceding days remained below 1.5
ppt, but on July 23, 2001, a peak tide stage of 4.95 feet was recorded in the
Hillsborough River near the spring. This is an unusually high tide that may have been
caused by prevailing southerly winds. The incursion of water over 2 ppt into the spring
run began when water levels in the river first exceeded 4.35 feet, and lasted for
approximately 7½ hours. There were 50 total days in which flows from the spring
equaled 26 cfs, but this was the only day among those during which a salinity incursion
occurred.
In summary, the data from the continuous recorder indicate the probability for salinity
incursions into the spring run increases as flows decline. However, it is not a linear
response and there appear to be breakpoints in this relationship. With the exception of
very high tides, 18 cfs appears to preclude salinity incursions into the run. Summary
statistics for salinity and tide stage from the continuous recorder are listed for all days
with flows between 5 and 20 cfs in Appendix I. There was a total of 11 days when flows
were 18 cfs. On each of these days, daily minimum and maximum values of salinity in
the run were nearly identical, indicating no incursion occurred.
It is reiterated that the recorder in the run is located about 100 feet upstream of the weir,
and small incursions could possibly occur that do not reach that far upstream or are
limited to deeper depths. An example of such an event is the experiment of Dec 13,
2001, when vertical salinity profiles (Figure 5-3c) showed a small incursion above the
weir that was not registered at the recorder. It is not known to what extent 18 cfs would
allow such small incursions, but it is expected such small incursions would not affect low
salinity zones in shallow and upstream areas, allowing sufficient refuge for salt-sensitive
species. Given this assumption, the data from the continuous recorder can be used to
evaluate the relationships of springflow with significant salinity incursions.
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