September 29, 1972
TO: Donald R. Feaster, Director, Wder Resources Division
FROM: James A. Mann, Chief, Permits Department
COMMENTS: Surface Water Hydrologic Aspect of EIS for Florida Power and Light
Company Manatee County Willow Site.
Flow in the Little Manatee River is adequate to supply the cooling water requirements of
a 1700 MW power plant utilizing re-circulation of cooling water via a cooling pond.
The long-term average flow in the river at the power plant diversion site is about 170 cubic feet per
second (cfs) -- estimated by applying a drainage area adjustment to recorded record at the
USGS streamflow measuring station of U.S. Highway 301 (02300500 Little Manatee River
near Wimauma, Fla.) about 3.5 miles downstream from the power plant diversion site.
The gross water withdrawal from the river will average 29 cfs. However, accounting for
the water returned to the river by cooling pond overflow during heavy rains results in a
net withdrawal of 22 cfs/or about 13 percent of the long term average (LTA) flow at the
diversion site. Minimum flow (as defined by Chapter 373.081 (8) Florida Statutes) at the
diversion site accounts for about 20 percent of the LTA flow.
Average monthly minimum flows at the diversion site are shown on Plate-6 attached. The
average of these monthly minimum is 35.6 cfs. The Department of Natural Resources (DNR)
has authorized diversion of all flow in excess of 40 cfs. For only three months during an
average year are the average minimum flows in excess of 40 cfs. This EIS was written b$ere
the DNR authorization of diversion and it was assumed that diversion would be possible on
those days during a particular month when flow exceeded the average monthly minimum flow
for that month. A streamflow hydrograph for the 1968 Water Year (WY) is shown on Plate-14
attached. This hydrograph depicts the amount of water that could have been diverted with
a pumping capacity of 135 cfs. However, with the 40 cfs cut-off in DNR's authorization no
water could be diverted when streamflow itwas below that amount and therefore pumping
capacity must be increased to permit diversion of an equal amount of water.
A more realistic comparison might be made using monthly mean flows rather than monthly
minimum flows. For an average of eight months each year (1940-71) the monthly mean flow was
greater than 40 cfs. Therefore, for an average of eight months each year during this period
all water above 40 cfs could have been diverted. Conversely, for an average of four months
each year during this period the month mean flow was less than 40 cfs and there could have been
Very limited diversion and only during these days when flow exceeded 40 cfs. A flow of 40 cfs
at the diversion site is equalled or exceeded 53 percent of the time.
The hydrology of the basin will probably not be greatly affected by the diversion of 13 percent
of the LTA flow of the river. However, when an additional 20 percent of the river's flow is
required to sustain minimum flows, together the two are quite substantial. This amount -
H 33 percent of the LTA flow can be placed in perspective by equating it to i50 per.'ct oi the
yearly mean flow for the minimum year of record I 956 WY on 15 percent of the yearly mean
flow for the maxim,- year of record 1959 WY.
* ,' /