The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Salinity incursions above the weir were not apparent at any of the stations in the upper
run during experiments 1, 2 and 4 (Figures 5-3 a, b, and d). The daily flow values
corresponding to the second sampling event for these experiments ranged from 15 cfs
to 23 cfs (Table 5-1). Salinity in the spring run, however, can vary with tide stage during
low flows, and the sampling event could have missed the time of salinity incursion. To
check the representiveness of the experimental results, time series plots of 15-minute
values for salinity and water levels at the continuous recorder in the spring run are
presented in Appendix H for each experimental period.
Efforts were made to sample the spring run near slack high tide, when the potential for
incursions of river water is the greatest. Plots of water levels at the continuous recorder
in the upper run show that this was largely achieved for experiments 1 and 4, but
sampling for experiment 2 occurred after high tide and was actually near the time of
minimum low water. Salinity remained stable at the recorder throughout experiments 1, 2
and 4, however, indicating that salinity incursions did not occur over the tidal cycle. In
contrast, salinity values did vary with tide at the recorder at the spring mouth (also
shown in Appendix H). Elevated salinity was more prolonged during experiments 2 and
4 than during preceding or following days when higher flows occurred.
The box and whisker plots show that slight salinity incursions occurred during
experiments 3 and 6 (Figures 5-3 c and g). Slightly higher salinity values were observed
at station 20 near the opening of the weir during experiment 3, when the flow was 19 cfs.
This sample was taken near the secondary high tide peak for the day. Salinity at the
recorder in the upper run did not show evidence of any salinity incursion at that location.
This recorder is located about 100 feet upstream of station 20, indicating that the salinity
incursion in the upper run during experiment 3 was restricted to near the weir opening.
Experiment 6 (June 20, 2002) showed a similar result, in that sampling was conducted
near high tide and comparison of the box and whisker plot with the recorder data
indicates that salinity increases were restricted to the most downstream stations in the
upper run. This test produced unusual results in that the flow from the spring was only
2.5 cfs. There was no flow from the Hillsborough River dam on this day, which generally
results in high salinity in the river near the spring outfall. However, the summer rainy
season had commenced by mid-June, and reductions in salinity were observed at the
nearby continuous recorders in the river. The lack of a strong salinity incursion during
experiment 6 may have been related to low salinity in the river and the effects of
localized rainfall on salinity in and near the spring run.
The most striking result among the six test flows was the result for test 5 on June 12,
2002 (Figure 5-3e). Flow on this day was 13 cfs and was maintained at that rate for the
previous 9 days (Appendix G). This represented the only successful attempt to maintain
a constant, intermediate flow rate during the experimental period. Boat failure restricted
sampling on that day to stations 16 through 24, but the data indicated a substantial
salinity incursion. Mean water column values were between 6.7 and 10.6 ppt at stations
16 through 21 in the upper pool, with bottom values ranging from 11.5 to 15.9 ppt.
5 - 4