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.
