The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
The 15-minute data from the USGS recorders are also useful for examining the salinity
characteristics of the lower river. Time series plots of surface and bottom salinity values
for four recorders in the river are presented in Figure 3-15. Salinity is averaged over the
preceding 29-day period to aid visual interpretation of the plots. The longest period of
record is for the Rowlett Park recorder, which dates back to 1997. The unusually high
values that occurred during the 2000 –2001 drought are apparent from this plot. It is
also notable there is very little vertical stratification at this site, as evidence by the
similarity of surface and bottom values. The period of record at Hanna's Whirl is much
shorter, as it started in the spring of 2001 as part of the re-evaluation of the minimum
flow for the Lower Hillsborough River. This station has remained fresh since the
summer of 2002, when an extended period of flow began at the Hillsborough River dam.
During the dry season of 2001-2002, salinity at this site varied between 5 and 12 ppt.
The site at Hillsborough River near Sulphur Springs (HILLSULPH) is especially
informative for it is located only about 100 meters upstream from the mouth of the
spring run. Both surface and bottom waters can go fresh during the wet season, but
bottom salinity at this site fluctuates above 15 ppt during no-flow conditions. An
important characteristic of this site is the large difference between surface and bottom
salinity values in the dry season. This is largely due to the effect of Sulphur Springs, as
the low salinity water from the spring run layers over the higher salinity water in the
river. This results in pronounced density stratification, which as will be discussed later,
contributes to frequent hypoxia in this reach of the river.
3.9.2.2 Hydrodynamic salt transport model of the river
An effective tool for examining the effect of freshwater inflows on the salinity regime of
the Lower Hillsborough River is a laterally averaged two-dimensional hydrodynamic
model of the Lower River developed by District staff (Chen et al. 2001). This model was
used in the establishment of 10 cfs minimum flow (SWFWMD 1999), and its further use
was recommended by the scientific review panel (Montagna et al. 1999). Since the
model's initial use in the late 1990s, the model has been recalibrated using new data
from the expanded array of continuous recorders in the river.
Graphs of salinity distributions in the river for a series of different inflows from the dam
and Sulphur Springs are presented in Figures 3-16 through 3-21. The results were
generated by assigning inflow values to initial conditions as measured on January 16,
2002 and running the model for 20 days. This no-flow period was chosen because it is
more representative of the long-term no-flow conditions in the river than the much saltier
conditions that occurred during the 2000-2001 drought.
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