Coastal-Storm Model Development and Water-Level Validation for the North Atlantic Coast Comprehensive Study

The focus of this paper is on the storm surge modeling effort conducted as part of the North Atlantic Coast Comprehensive Study (NACCS). At the time of this study, the numerical modeling component of the NACCS was the largest civil works high-performance computational modeling effort conducted by the U.S. Army Corps of Engineers (USACE), requiring approximately 100 million CPU hours to complete. The NACCS modeling effort involved the application of the latest atmospheric, wave, and storm surge models and extreme value statistical analysis techniques. The methodology for computing winds, waves, and water levels involved the application of a suite of high-fidelity numerical models within the Coastal Storm Modeling System (CSTORM-MS), including an offshore wave model, WAM, and a nearshore wave model, STWAVE, coupled with the circulation and storm surge model ADCIRC. The high-fidelity modeling suite applied to the NACCS study required extensive grid development, implementation of bathymetric and topographic data, surge and wave model coupling and, because of the magnitude of the study, production script development for automation of model input development, execution, postprocessing, and archiving of model results. The ADCIRC model developed for this study included approximately 6.2 million computational elements and 3.1 million computational nodes, with ADCIRC mesh resolution concentrated in the approximately 1,450-km region from Virginia to Maine. In addition, 10 STWAVE model domains covered the nearshore zone from Virginia to Maine. The WAM model applied to the NACCS required three levels of refinement to adequately simulate offshore waves for this study. Prior to starting NACCS production simulations, the numerical models were validated for a set of historical tropical and extratropical storm events. Production model simulations (consisting of both synthetic and historical storms) were performed during an 8-month period, analyzed, and ultimately incorporated into the Coastal Hazards System (CHS). (The synthetic tropical storms were developed to populate the statistical parameter space as part of the project design process.) The suite of storms included 1,050 synthetic tropical storms and 100 historical extratropical events, each simulated for four conditions. Production storms were first simulated as surge only simulations with the application of ADCIRC in stand-alone mode and were then simulated as surge plus wave simulations by applying the CSTORM-MS to include the effect of wave radiation stress gradients on water levels. Next, the 1,150 surge plus wave storm suite was repeated with the inclusion of either a random tide (synthetic tropical storms) or a historical tide (historical extratropical events). Last, the surge plus wave and tide storm suite was simulated with the inclusion of a 1.0-m increase in sea-level adjustment. The storage requirement for the 4,600 NACCS production simulations and other project files was approximately 190 TB. This paper primarily focuses on the ADCIRC model development and validation, whereas other studies focus on other processes, such as the extreme value statistical analysis techniques and the characterization of storm surge response to the inclusion of waves, tide, sea-level change, and variation in storm forcing parameters.