Hurricane Gustav (2008) made landfall in southern Louisiana on 1 September 2008 with its eye never closer than 75 km to New Orleans, but its waves and storm surge threatened to flood the city. Easterly tropical-storm-strength winds impacted the region east of the Mississippi River for 12–15 h, allowing for early surge to develop up to 3.5 m there and enter the river and the city’s navigation canals. During landfall, winds shifted from easterly to southerly, resulting in late surge development and propagation over more than 70 km of marshes on the river’s west bank, over more than 40 km of Caernarvon marsh on the east bank, and into Lake Pontchartrain to the north. Wind waves with estimated significant heights of 15 m developed in the deep Gulf of Mexico but were reduced in size once they reached the continental shelf. The barrier islands further dissipated the waves, and locally generated seas existed behind these effective breaking zones. The hardening and innovative deployment of gauges since Hurricane Katrina (2005) resulted in a wealth of measured data for Gustav. A total of 39 wind wave time histories, 362 water level time histories, and 82 high water marks were available to describe the event. Computational models—including a structured-mesh deepwater wave model (WAM) and a nearshore steady-state wave (STWAVE) model, as well as an unstructured-mesh “simulating waves nearshore” (SWAN) wave model and an advanced circulation (ADCIRC) model—resolve the region with unprecedented levels of detail, with an unstructured mesh spacing of 100–200 m in the wave-breaking zones and 20–50 m in the small-scale channels. Data-assimilated winds were applied using NOAA’s Hurricane Research Division Wind Analysis System (H*Wind) and Interactive Objective Kinematic Analysis (IOKA) procedures. Wave and surge computations from these models are validated comprehensively at the measurement locations ranging from the deep Gulf of Mexico and along the coast to the rivers and floodplains of southern Louisiana and are described and quantified within the context of the evolution of the storm.

Traditional coastal flood hazard studies do not typically account for rainfall-runoff processes in quantifying flood hazard and related cascading risks. This study addresses the potential impacts of antecedent rainfall-runoff, tropical cyclone (TC)-driven rainfall, and TC-driven surge on total water levels and its influence in delineating a coastal flood transition zone for two distinct coastal basins in southeastern Louisiana (Barataria and Lake Maurepas watersheds). Rainfall-runoff from antecedent and TC-driven rainfall along with storm surge was simulated using a new rain-on-mesh module incorporated into the ADCIRC code. Antecedent rainfall conditions were obtained for 21 landfalling TC events spanning 1948–2008 via rain stations. A parametric, TC-driven, rainfall model was used for precipitation associated with the TC. Twelve synthetic storms of varying meteorological intensity (low, medium, and high) and total rainfall were utilized for each watershed and provided model forcing for coastal inundation simulations. First, it was found that antecedent rainfall (pre-TC landfall) is influential up to 3 days pre-landfall. Second, results show that antecedent and TC-driven rainfall increase simulated peak water levels within each basin, with antecedent rainfall dominating inundation across the basin's upper portions. Third, the delineated flood zones of coastal, transition, and hydrologic show stark differences between the two basins.

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Cobell, Z.; Zhao, H.; Roberts, H.J.; Clark, F.R.The goal of the study was to evaluate various coastal restoration and protection projects and the associated benefits for reductions in storm surge and wave height. Efforts in numerical modeling have been made to create a database of storm surge and wave responses to a set of hypothetical storms under current and various future conditions. The ADvanced CIRCulation (ADCIRC) and the Unstructured Simulating WAves Nearshore (UnSWAN) models were selected for this study. A coarser version of the state-of-the-art, southern Louisiana, unstructured mesh was developed to reduce computational overhead while maintaining critical hydraulic features. Model outputs were reviewed and analyzed from coastwide and onshore-transect points of view. The potential benefits of restoration and protection projects proposed in the Master Plan were examined by comparing Future without Action outputs to the Master Plan outputs. Hurricane protection projects, such as levees, provide remarkable protection at their leesides but increase and redistribute surge water at their front sides. Narrow, restored landscapes, such as barrier islands or ridges, may provide wave attenuation to some extent but, in many cases, provide minimal benefits in surge level reduction. Larger-scale restoration projects, such as sediment diversions, can result in land accretion and enhance vegetation coverage, thus resulting in notable benefits associated with damping waves and storm surge and ultimately reducing risk for inland communities. ADDITIONAL INDEX WORDS: Coastal Louisiana, storm surge, waves, restoration and protection, numerical modeling.

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