February 2, 1972
TO: Donald R. Feaster, District Engineer
? FROM: James A. Mann, HydrologiLs
RE: Report/Water Supply Study Little Manatee River
Flow in the river is adequate for their needs. Plant can operate for 18 months on water in
cooling reservoir without replenishment from the river (Communication, Brown Root). This
should carry them through the dry years.
This proposed withdrawal will amount to approximately 40% of the river's long term average
flow. Minimum river flow is about 20% of the long term average flow. Therefore, upon
approval of the withdrawal, 60% of the river's flow will be allocated.
A pumping capability of 300 cfs will "scalp" over the long term 75-77 cfs as shown by the
mass curve and indicated in the text Plate 8. However, the report associates a pumping
capability of 300 cfs with a long term scalped flow of 60 cfs. A pumping capability of 200
i cfs would "scalp" 60 cfs over the long term but would supply only 46 cfs during the critical
period (1939-45) of the analysis.
The minimum flow computations seem OK. I checked only the figures for May since the
Department of Natural Resources will establish minimum flow at the point of diversion.
Figures for minimum flows on Plates 2, 3-5 are for the gaging station not the diversion
site. At the diversion site, minimum flow would be approximately 10 percent greater
as shown by penciled figures on Plate 2.
No mention was made in the report of a cooling water reservoir. Although the reservoir
doesn't concern us directly, since it will be in Manatee County, some pertinent facts
are as follows. The reservoir size (surface area) will be 6,400 acres. Capacity will be
98,000 acre feet -- average depth 15'. Estimated reservoir losses are, E=55 cfs=40,000
acre feet/year=6.2 ft./year, and seepage =5 cfs=3,600 acre feet/year = 0.8 ft./year. Use
of cooling towers would reduce the reservoir size by 1/3 but consumptive use would be
about the same. High maintenance costs and the necessity for continual pumping are
detrimental aspects of cooling towers. Statistics on the reservoir were supplied by Carl
Fick Brown and Root, Inc.
Brown and Root has taken a thorough look at the river's flow and can't detect any incorrect
analysis. Oliver and I have discussed comments and satisfied most of his questions. Hydrology
might need to take a quick look to make certain something hasn't been overlooked.