I-75 Widening Bridge Hydraulics

RS&H provided final design services to widen approximately ten miles of I-75 from north of River Road (CR 777) to north of SR 681 in Sarasota County, Florida. The four-lane, divided configuration was widened to six lanes with drainage facilities capable of accommodating the ultimate eight-lane corridor. As part of the proposed improvements, each of the four bridges along the corridor (three stream crossings and one controlled canal crossing) was widened or replaced, resulting in the need for corresponding bridge hydraulic reports.

At Cowpen Slough, a structurally controlled canal, one new bridge was added and one widened. A new bridge was also added and one widened at Salt Creek. The new bridges are the result of a partial realignment of the southbound lanes of I-75. The other two crossings are over Fox and Curry Creeks.

Fox Creek, Cowpen Slough, and Salt Creek all come together to form Shakett Creek, which flows directly into Dona Bay and is connected to the Gulf of Mexico by Venice Inlet. Due to the proximity of the I-75 to the ocean, all of the bridge crossings were considered tidally influenced. Additionally, Curry Creek included a FEMA Regulatory Floodway, which required FEMA documentation to be collected and updated for the new crossing. A No-Rise certification was approved by the Sarasota County FEMA liaison.

The modeling effort was completed using the HEC-RAS computer model, a one-dimensional hydraulic model for natural and constructed channels. Within the model, RS&H used the Steady Flow Analysis component to compute water surface profiles for the 50, 100 and 500-year frequency storm events incorporating the above discharge rates. The team also considered both riverine and tidally influenced flows. In addition to the hydraulic analysis, the team analyzed each conveyance and bridge system for scour associated with the 100 and 500-year frequency storm events.