: : 10- '
/ ,,.- -.- ^ j. UNITED STATES
'..... DEPARTMENT OF THE INTERIOR
\ ,. .\ .^ /' GEOLOGICAL SURVEY
WATER RESOURCES DIVISION
'- ;| 903 WEST TENNESSEE STREET IN REPLY REFER TO
ro TALLAHASSEE, FLORIDA 32304
June 16, 1972
Dr. R. 0. Vernon, Director
Florida Division of Interior Resources
Department of Natural Resources
Tallahassee, Florida 32304
In response to your letter of May 5, 1972, we have reviewed the hydrolog-
ical aspects of the Brown and Root report on the feasibility of operating
a 1700 MW power plant on the Little Manatee River near Wimauma. Our
Tampa office also assisted with this review and we have included their
comments herein. We agree with most of the basic conclusions of the
report, but we feel strongly that the estimate of seepage through and
under the embankment must be more fully examined before the proposal is
accepted. Another problem that has arisen since the report was prepared
is that the 1965 Statute that defined the term "average minimum flow"
was repealed by the Florida Water Resources Act of 1972. This later act
does not provide a specific method for determining "minimum flow" but
states only that minimum flow shall be the limit at which
further withdrawals would be significantly harmful to the water resources
or ecology of the area." and that minimum flow shall be
calculated by the department and the governing board using the best infor-
The Little Manatee River will supply the water requirements listed in the
report and the cooling pond will provide enough storage to carry the
;1^^ ~ operation through a drought so that the river discharge need not be
i ;r ^reduced below "average minimum flow" as defined by Chapter 373, Florida
i,,' l" Statutes, 1965, which now is repealed. However, any future requirement
-+' ^: for additional diversion of water from the river will have to be re-
i , stricted.
!"::.. The water requirement for forced evaporation (18 cfs) is probably adequate
,for a plant heat rejection rate of 6.0 x 109 BTU/hr (about 80%
,J.|l! station capacity)." We call attention to this specified heat load because
,'.,' our Tampa office was furnished a copy of the report that contained a
S! i:l| different and much larger value for plant-heat rejection rate. The value
';:' in Tampa's copy may represent an error that was caught and rectified
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r before your copy of the report was mailed. The allowance of I cfs for
the net of natural evaporation in excess of rainfall also is adequate.
Under "HYDROLOGICAL ASPECTS", the following statement are made: 1) On
page 2, lines 10 and II, its states that .. the reservoir will be
operated at a level such that releases wi ll occur during periods of
excessive rainfall." 2) On page 5, lines 10 and 11, it states that "If
the reservoir had been full, no diversion would have been made." From
these remarks we conclude that releases will occur only as a result of
rainfall on the reservoir and that diversions from the river and releases
from the reservoir will not occur simultaneously. We suggest that these
points be spelled out positively and specifically in the proposal.
The report is also deficient in that it emphasizes that the power plant
should have least possible ecological effect on the stream and adjacent
land area but it does not describe what the limiting effects of changes
In temperature and chemical quality of the releases might be.
The report does not indicate the rate of flow of circulating water
through the steam condenser or the temperature drop across the condenser.
These data are needed to evaluate the probable temperature of the re-
leased water. On the assumtion that the temperature drop across the
condenser is about 150F, which appears to be a fairly realistic value,
the pond at the level at which spill can occur has sufficient area (4000
acres) to lower the temperature of the released water to less than 3F
above the temperature that would occur naturally without heat from the
power plant. Under conditions of excessive rainfall when releases occur
from the pond, the rate of natural discharge of the river will generally
be several times greater than the rate of the releases. Hence, after
mixing, the resultant rise in temperature of the river water will be
less than 1F which will not be significantly harmful to the ecology of
the downstream area.
On a long-term basis the concentration of dissolved solids in the water
released from the pond would average about twice the concentration of
dissolved solids in the river water. Concentration of dissolved solids
in the pond water will increase during a drought as the pond level de-
clines and as the total load of dissolved solids is increased by input
.l ~ of river water. However, water cannot spill from the pond until the
.. pond is completely refilled with relatively fresh water. Thus, the con-
1 ^^^centration of dissolved solids in the released water probably will never
ii ~ exceed 500 mg/1, compared to a concentration of something less than
..... 100 mg/1 for the river water. In view of the dilution that occurs as
*^^;|^ the released water is mixed with a greater quantity of river flow, the
.,-.. concentration of dissolved solids in the released water will not have
.... ^j ,a foreseeable harmful effect on the ecology of the downstream area.
.- .:Under "HYDROLOGICAL ASPECTS", page 5, lines 16-20, the statement is made,
":'; ( "The average minimum flows in the river represent approximately 20% of
"i;' the river flow. Therefore, on a drainage area and stream flow ratio
basis, no water would be diverted from the stream when the flow into the
bay was less than 20% of the average flow in the river." A similar remark
-'^ ~~s made at the top of page 6 in reference to the fresh-water salt-water
Interface in the river. These statements are not correct because the
average flow into the bay is given as 262 cfs and 20% of this value would
be 52 cfs. Plate 4 shows that "scalping" is anticipated for a number of
months when the daily flow is less than 52 cfs but still greater than
"average minimum flow". The statements referring to "20% of the average
river flow" conflict with the other analysis of the data and are not
With respect to the position of the fresh-water salt-water interface in
the river, the most inland position occurs when the flow of the river
Is at its lowest. Low-flow conditions often last for several weeks.
Diversion will not be made when the daily flow is equal to or less than
the "average minimum flow". Hence, the most inland position of the
salt-water fresh-water interface will not be changed by the diversion
of water from the river. Because of the "scalping" of daily flows that
are only slightly greater in magnitude than the "average minimum flow,"
periods of sustained low flow will be lengthened to some extent. Hence,
the salt-water fresh-water interface will remian inland at our near its
point of maximum intrusion for a longer period of time than would occur
without such diversion. In view of the length of time that the natural
flow of the river falls below "average minimum flow" it is doubtful that
the general increase in salinity of water downstream from the salt-water
fresh-water interface in the stream would be significantly harmful to
the ecology of the downstream area.
Our chief point of disagreement with the report is in relation to the
estimate of seepage. Section V on "WATER REQUIREMENTS" states as follows:
"The seepage rate was assumed to be 2 cfs. The seepage rate may be
greater but rates greater than this would most likely return to the river.
Seepage from the reservoir will supply water to the shallow ground water
In the shallow sand deposits in the vicinity of the facility. Soil
borings made to date indicate that the perched water table is some 3 to 5
"':j --feet below the ground surface."
We think the report dismisses too lightly the recognized possibility
i, that seepage might be greater than 2 cfs for reasons as follows:
I1) We were not furnished a representative cross section of the embankment;
however, the embankment presumably will have a core of relatively imper-
meable clay with a keying trench that extends about 5 feet below ground
;';;i surface. The surficial materials consisting of silty sand, clayey sand,
i.., and sandy clays are in the order of 30 feet thick. Depending on the pond
-,' ~ range from zero to 46 feet. We can conceive of seepage through and under
.. the embankment totaling as much as 15 to 20 cfs.
S -| 2) Whether seepage is large or small, a substantial part of the seepage
will not return to Little Manatee River. Seepage through and under the
%i-- southern embankment that does not evaporate will flow to the Manatee River.
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Seepage through and under the western embankment will flow downslope west-
ward toward what appears to be a naturally swampy area. Only the seepage
that passes through and under the northern embankment will return naturally
to the Little Manatee River. From what is known of the thickness and dis-
tribution of the surficial materials, seepage through the northern embankment
will not necessarily represent a disproportionately large part of the total
3) Part of the seepage through each embankment wi ll emerge at the land
-%^^tt.+ surface and be lost by evapotranspiration without any appreciable benefit
4) We see no benefit that would accrue from the addition of water to the
shallow aquifer in the "vicinity of the facility". The shallow aquifer
cannot continually store water. Water will move through the shallow aquifer
and emerge as described in item 2 above. The same objection applies to
the remark about the "perched water table."
5) Page 3 of the SOILS INVESTIGATION REPORT notes the presence of
numerous surface depressions ranging in size from 200 to 1000 feet." The
report suggests that the depressions are indicative of solution activity
but that the activity is limited to the surficial materials and that it
does not extend through the Hawthorn formation. The significance of the
sink holes is questioned in respect to foundations problems but no mention
is made to the significance with respect to leakage. We think the signi-
ficane of the sinkholes should be examined with respect to leakage through
the bottom of the reservoir.
6) Page 3 of the SOILS INVESTIGATION REPORT mentions that borrow for the
embankment can be obtained from immediate adjacent areas. We feel that
excavation of a deep borrow ditch immediately next to the embankment on
4 the inside of the reservoir may cause seepage under the embankment to be
greater than it would be otherwise.
In short, we feel that the possibility and consequences of seepage through
and under the embankment being substantially greater than the assumed 2 cfs
|1 should be examined in much greater detail.
We appreciate the opportunity to review and comment on the subject report
i^ i which is returned herewith.
^^S1 Sincerely yours,
r'^,,tPF^ ,Clyde S. Conover
".,1 'I: District Chief
.,s'i+:5:, iale Twachtmann, Executive Director, Southwest Florida Water Management
,6::-'. Di district
i;^ Subdistrict Chief, WRD, Tampa
;"","l ,: G. H. Hughes, WRD, Tallahassee
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