Date of Award

2017

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

M. Reza Hashemi

Abstract

This thesis is comprised of two manuscripts, both of which involve investigating the sensitivity of storm surge in Rhode Island coastal waters. The first study details the effect of wave-induced enhanced bottom friction on surge for a simple case study. The second study sheds light on the impact of using different hurricane wind models to simulate storm surge and waves.

The interaction of waves and circulation (tide and surge) is characterized by the effect of storm surge and currents on waves, and the effect of waves on storm surge and currents. Quantifying this effect for a given area may be important for storm surge prediction purposes. As a result of wave-induced near-bed orbital velocities, the bed roughness will increase for storm surge propagation. Here, a sensitivity analysis was performed for Rhode Island coastal waters. A method developed by Soulsby (2006) was implemented to compute the increased bottom friction (i.e. drag coefficient) due to the effect of waves. Further, the interaction between waves and currents are incorporated in a coupled ADCIRC+SWAN model (which does not have this process). The storm surge was simulated with and without considering the impact of waves on the bottom roughness. Preliminary results indicate that enhanced bottom friction is largest in wave-dominant areas, compared with areas where currents are strong. In other words, if the wave induced shear stresses are higher than the current induced bed shear stresses, the bottom friction increases significantly. A case study for Hurricane Irene (2011) shows that although the effect is considerable on increasing the Manning coefficient, storm surge is not that sensitive to enhanced bottom friction.

The second study deals with the effect of wind models on storm surge. Storm surge and wave models are routinely used to assess the impact of hurricanes/cyclones for emergency preparedness. While these models are forced by wind fields, generated by meteorological models in hindcast or forecast mode, selecting a wind model which can accurately resolve the wind field, especially near the hurricane/cyclone core, is a challenging task. We use several wind hindcast models to force a coupled wave and storm surge model for selected hurricanes, including Bob (1991), Irene (2011) and Sandy (2012). The resulting simulated storm surge and wave parameters are compared to observations. The wind models include the European Center for Medium RangeWeather Forecasts (ECMWF), the Northeast Coastal Ocean Forecasting System (NECOFS) based on the Weather Research and Forecasting (WRF) model, and parametric wind based on National Hurricane Center (NHC) datasets. Storm surge and waves are best predicted using ECMWF wind for Hurricane Irene, parametric wind for Hurricane Bob, and NECOFS WRF winds for Hurricane Sandy. Our results show that a wind model, which has an error in peak wind speed of less than 20% when compared with observations, could lead to convincing storm surge of wave predictions. The impact of using a poor wind model can result in error as high as 50% in storm surge and wave predictions. There is no unique "best" wind model for all hindcast applications. This choice depends on the nature of the hurricane, in particular, the ability to adequately characterize the spatial structure of the wind forcing field. Therefore, storm track and storm scale should be considered in selecting a wind model.

Available for download on Sunday, January 28, 2018

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