Sea Technology

AUG 2012

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land, a barrier island at the mouth of Mobile Bay. Opened by Hurricane Ivan in 2004 and widened by Hurricane Katrina the following year, the breach left wetlands and oyster beds vul- nerable to oil from the spill. The Jag Ski, with the RiverSurveyor M9, was used to characterize the dynamics of the breach and study the impacts of BP's rubble mound structure. Surveys of bathymetry and tidal flows within the barrier is- land breach, known as Katrina Cut, revealed the presence of dynamic sand waves, a complex ebb tidal shoal and very strong currents in this shallow inlet that is 2 to 3 meters deep. The ADP surveys were conducted over a six-hour period near high tide. Tidal currents during the surveys were strongly ebb-directed with magnitudes more than 1 meter per second, and out of phase with the water surface elevation. This type of behavior is indicative of frictionally dominated systems where the tide propagates through the breach inlet as a standing wave. In Ka- trina Cut, the tidal flows lead the water surface elevation by about four to six hours on average. Tidal flows through Main Pass and Pass aux Herons exhibit similar behavior and flow magnitudes. Similar to ADP measurements at Main Pass, a strong recir- culation of equal magnitude and opposite sense was observed on the east side of the inlet. While there is a small deep chan- nel in this portion of the breach, the recirculation is likely an artifact of the local geomorphology, as there is no discernible lateral shear of the flows as in Main Pass. Construction of the breach closure structure commenced in July 2010 and concluded in April 2011. In the time since the breach was closed, sediment from the ebb tidal shoals has col- lapsed onto the face of the structure. When combined with sand transported through the local littoral drift, the gulf side of the structure is now almost completely covered with sand. Pe- riodic bathymetry surveys will be conducted to monitor the re- sponse of the island to the breach closure. Future Work The Jag Ski is expected to be used in future studies of bay dynamics, tracking harmful algal blooms, and measuring and analyzing water quality parameters for total maximum daily loads around the bay. The Jag Ski, along with land-based, real-time kinematic GPS equipment, will be used in a two-year monitoring plan to pro- file changes in beach sand volumes, and ebb and flood tidal shoals for the Alabama Department of Transportation's pro- posal to construct a new bridge over Little Lagoon Pass, as well as widen the inlet channel and lengthen the jetties. Data collected with the Portable SeaKeeper system will be used to develop water quality maps of the lagoon surface water. Steve Werblow is a freelance writer based in Ashland, Oregon. He cov- ers agriculture, resource industries and water issues. Examples of his published work can be found in Everything About Water, Industrial En- vironmental Technology, Brisbane Times, Australian Geographic, Pro - cess and Control Engineering, Hydraulics and Pneumatics, Industrial Water World, Marine Scientist and more. Bret M. Webb is an assistant professor of civil engineering at the Uni- versity of South Alabama, as well as a practicing engineer at South Coast Engineers LLC. His research interests include coastal and estuarine hydrodynamics, tidal processes, sediment transport and coastal mod- eling. www.sea-technology.com AUGUST 2012 / st 49

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