The Determination of Minimum Flows for Sulphur Springs, Tampa

CHAPTER 5
RESULTS OF THE MINIMUM FLOWS ANALYSES
DRAFT
5.1 Introduction
This chapter presents the technical findings that support the recommended minimum
flow for Sulphur Springs. The determination of the minimum flow was based on the
degree that it met the three management criteria that were described in the previous
chapter, which are: (1) maintain low salinity habitats in the spring run; (2) maintain low
salinity habitats in the lower Hillsborough River, and (3) maintain a thermal refuge for
manatees. The findings related to these three criteria are presented sequentially below.
5.2 Criterion 1 – Maintain low salinity habitats in the upper spring run
As discussed in Section 3.4, the background salinity in the upper spring run in the
absence of spring water withdrawals is determined primarily by the salinity of the
discharge from Sulphur Springs pool. Under these normal flows, the background low
salinity habitat of the upper spring typically ranges from 1 to 3 ppt (Figure 3-5). By
reducing flow from the pool, withdrawals can raise salinity values in the spring run by
allowing the movement of brackish water from the lower river into the upper spring run.
Depending on the salinity tolerances of the species occurring in the spring run, such
salinity incursions may, or may not, affect the species composition and ecology of the
spring run.
The most conservative way to maintain the low salinity characteristics of the upper
spring run would be to keep spring flow above a flow rate that hydraulically prevents
salinity incursions. However, if salinity incursions were infrequent and of short duration,
they may not result in adverse impacts to the biota of the spring run. The determination
of the criterion 1 minimum flow thus focused on the probability that various rates of flow
from the spring would result in salinity incursions and the level and duration of such
incursions.
5.2.1 Criterion 1 data sources
Two types of measured data provided an empirical basis for identifying the Criterion 1
minimum flow. These were a set of controlled spring release experiments conducted by
the District during 2001 and 2002, and the multi-year continuous recorder data
measured in the spring and river system by the USGS. The time periods of the
experiments and continuous recorder data represent conservative periods of unusually
low rainfall and were thus ideally suited for these analyses.
5.2.2 Controlled release experiments
The District measured salinity at 21 stations in the upper spring run during a set of
controlled spring release experiments conducted between November 2001 and June
2002. During these experiments, the City of Tampa diverted variable amounts of water
from Sulphur Springs pool to either into the Hillsborough River Reservoir or to the base
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