FLORIDA GEOLOGICAL SURVEY


 Mean daily discharges for Withlacoochee-Hillsborough overflow
were subtracted from the mean daily discharges for the Tampa
station two days later to synthesize the effective discharge that
would have occurred at the Tampa station if all the flow from
Green Swamp to the Hillsborough River had been impounded. This
adjusted discharge was used to compute the inferred stage
hydrograph for 22nd Street shown by the broken line in figure 56.
The difference between the two hydrographs on March 21 was 1.2
feet which indicates the maximum reduction in crest stage at 22nd
Street that would have occurred if the total flow from Green
Swamp to the Hillsborough River had been impounded. Similar
computations of the theoretical flood reduction at 22nd Street by
complete impoundment of the flow from Green Swamp were made
for the September 1960 flood. This flood crest would have been re-
duced by about 1 foot, approximately the same as that of the
March flood.
 Other reservoirs and channel changes proposed for the
Hillsborough River basin (Corps of Engineers, 1961) would further
reduce the flood peaks at Tampa.

 REDUCTION OF FLOOD PEAKS IN THE
 WITHLACOOCHEE RIVER

 The effect on the lower Withlacoochee River by storage of flood
discharge in the proposed reservoirs in Green Swamp was also
estimated on the basis of the March 1960 flood. Table A-1 of the
Corps of Engineers Comprehensive Report (1961) shows that the
total drainage area of the Withlacoochee River above Green Swamp
Reservoir is 328 square miles, and that above the Upper
Hillsborough Reservoir is 66 square miles or a total of 394 square
miles above the Trilby gaging station. Figure 57 shows the relation
between runoff in acre-feet and drainage area for the Withlacoochee
River basin for the flood period March 16 to April 20, 1960. From
this relation, the indicated flood runoff from a drainage area of 394
square miles was about 200,000 acre-feet during the March flood.
The allocated flood storage capacity of Green Swamp Reservoir is
326,000 acre-feet. Assuming that the flood-control pool would be
empty at the beginning of the flood period, the entire volume of
runoff from the basin above the reservoir could be impounded for
a flood greater than that of March 1960. This would be equivalent
to reducing the effective drainage area for uncontrolled flood runoff
above the Trilby gaging station to about 186 square miles (580


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