Verification of the SBEACH Overwash Module on Santa Rosa Island, Florida

Overwash deposits are often observed along low-elevation barrier islands after significant storm surge and wave events. Waves overtop the peak elevation of the beach system, transporting and depositing sediment across the island in a process known as barrier island rollover. Overwash events result in millions of dollars of damage to coastal infrastructure, providing motivation to clarify the process of sediment transport across an island using numerical models. The macro-scale profile model SBEACH incorporates knowledge of storm surge, waves, and pre-storm coastal morphology to predict time-dependent profile evolution. This model utilizes an updated coastal overwash module, developing equations for sediment transport on the subaerial profile as a function of the velocity of the uprushing bore (Larson et al., 2004). Observations of coastal overwash on Santa Rosa Island, FL during Hurricane Ivan (2004) are utilized to calibrate and verify the overwash module employed by SBEACH. Santa Rosa Island is a low-elevation barrier island extending approximately 75-km along the Florida panhandle. Along most of the island, the peak elevation of the beach system rarely exceeds 4 m, with a mean peak elevation of 3.3 m. On September 16, 2004, Hurricane Ivan made landfall in Gulf Shores, AL as a Category 3 hurricane. Tide gauges operated by the USACE reported storm surge elevations exceeding 3 m in Pensacola Pass, directly to the west of the Santa Rosa Island. Lidar surveys performed before and after the storm show that the entire length of the island was subject to extensive overwash deposits, reaching one meter in thickness and extending hundreds of meters across the island. The updated SBEACH overwash module has been calibrated and verified on Assateague Island, NC by Larson et al. (2004). The current investigation utilizes the extensive Hurricane Ivan data set to apply previous model calibrations to a unique, low-lying barrier island setting characterized by continuous overwash, rather than isolated overwash fans. New calibration and verification data are presented for this set of conditions, which is common along the Gulf Coast islands of Alabama, Mississippi, and the Florida panhandle.