W. R. Murphy | Geology Reports

USGS1984

Flooding in northwestern Hillsborough and southern Pasco counties, Florida, in 1979

Heavy rainfall in the late spring and summer of 1979 caused severe flooding in northwestern Hillsborough and southern Pasco counties, including north Tampa. May 7-9 rainfall for some stations had a recurrence interval in excess of 100 years; the August and September rainfall total had a recurrence interval in excess of 50 years. The frequency of flooding of streams and lakes in the study area for the May 7-9 storm ranged from less than 2 to 25 years; for the period August to October, recurrence intervals ranged from 3 to 25 years. Areas of maximum flooding for a 48-square-mile area of northwestern Hillsborough and southern Pasco Counties are indicated on 12 sheets of aerial photography. Drainage patterns, control structures, drainage basins, and existing and proposed urban developments are shown and related to flooding. Stage and streamflow hydrographs are shown for 12 lakes and 2 streams.

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USGS1978

Magnitude and frequency of flooding on the Myakka River, Southwest Florida

Myakka River is a coastal stream that drains approximately 550 square miles in southwest Florida. Under natural conditions, Tatum Sawgrass, a large depression, served as a flood overflow area for the upper part of the Myakka River basin. A recently constructed (1974) dike system across the lower part of Tatum Sawgrass has reduced previously available storage, thereby affecting downstream flooding. An evaluation of the effects of Tatum Sawgrass dikes on flooding indi-cates that flood-peak discharges and flood heights having recurrence inter-vals of up to 25 years are increased. As a result, about 1,200 additional acres along Myakka River main stem may be inundated during 2-year flood conditions. Maximum effects include a 19 percent increase in flood-peak discharge at State Road 780 and a 0.8 foot increase in flood height near the downstream end of the study reach. Floods having recurrence intervals of 50 years or greater will over-top dikes and the storage capacity of Tatum Sawgrass will revert to natural condition capacity. As part of the evaluation, diked condition flood profiles, having recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years were determined for the nontidal part of a 45-mile reach of Myakka River main stem, Clay Gully, and Blackburn Canal. These data indicate that Clay Gully flood water will overflow embankments at State Road 780 during the 2-year flood and will overflow the bridge on State Road 780 during the 25-year flood. Myakka River flood water will overflow embankments at State Road 72 and Border Road during 100-year flood conditions and at State Road 780 during 500-year conditions.

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