Again because of the pandemic, we gathered virtually for a successful PhD defense by Ajimon Thomas. Most attendees joined online, and Ajimon shared his research with his fellow students and faculty committee. Congrats to Ajimon!
Author Archives: Casey Dietrich
Storm-Driven Erosion and Inundation of Barrier Islands from Dune- to Region-Scales
Barrier islands are susceptible to erosion, overwash, and breaching during intense storms. However, these processes are not represented typically in large-domain models for storm surge and coastal inundation. In this study, we explore the requirements for bridging the gap between dune-scale morphodynamic and region-scale flooding models. A high-resolution XBeach model is developed to represent the morphodynamics during Hurricane Isabel (2003) in the North Carolina (NC) Outer Banks. The model domain is extended to more than 30 km of Hatteras Island and is thus larger than in previous studies. The predicted dune erosion is in good agreement with post-storm observed topography, and an ‘‘excellent’’ Skill Score of 0.59 is obtained on this large domain. Sensitivity studies show the morphodynamic model accuracy is decreased as the mesh spacing is coarsened in the cross-shore direction, but the results are less sensitive to the alongshore resolution. A new metric to assess model skill, Water Overpassing Area (WOA), is introduced to account for the available flow pathway over the dune crest. Together, these findings allow for upscaled parameterizations of erosion in larger-domain models. The updated topography, obtained from XBeach prediction, is applied in a region-scale flooding model, thus allowing for enhanced flooding predictions in communities along the Outer Banks. It is found that, even using a fixed topography in region-scale model, the flooding predictions are improved significantly when post-storm topography from XBeach is implemented. These findings can be generalized to similar barrier island systems, which are common along the U.S. Gulf and Atlantic coasts.
Binary Building Attribute Imputation, Evaluation, and Comparison Approaches for Hurricane Damage Data Sets
Missing building attributes are problematic for development of data-based fragility models. Relative to other disciplines, the application of imputation techniques is limited in the field of engineering. Current imputation techniques to replace missing building attributes lack evaluations of imputation model performance, which ensure accuracy and validity of the imputed data. This paper presents two imputation approaches, along with imputation diagnostic and comparison approaches, for binary building attribute data with missing observations. Predictive mean matching (PMM) and multiple imputation (MI) are used to impute foundation type and number of stories attributes. The diagnostic approach, based on the logistic regression goodness-of-fit test, is used to evaluate the imputation model fit. The comparison approach, based on the percentage of correctly imputed observations, is used to evaluate the imputation model performance. A data set of single-family homes damaged by the 2005 Hurricane Katrina is used to demonstrate implementation of the methodology. Based on the comparison approach, PMM models showed 9% and 2% greater accuracy than MI models in imputing foundation type and number of stories, respectively.
Improving the Accuracy of a Real-Time ADCIRC Storm Surge Downscaling Model
During major storm events such as hurricanes, emergency managers rely on fast and accurate forecasting models to make important decisions concerning public safety. These models can be computationally costly and cannot quickly make predictions at the highest geospatial resolution. However, model output can be post-processed to mimic high-resolution results with minimal additional computational cost. This research proposes methods for improvement in the accuracy of downscaling (enhancing the resolution of) a real-time storm surge forecasting model. Such improvements to downscaling methods include 1) expansion in its spatial applicability, 2) adding physics using water surface slopes, and 3) adding physics using friction losses across the ground surface.
This research builds upon a process that uses maximum water elevation output from the Advanced Circulation (ADCIRC) model and downscales these results to a finer resolution by extrapolating the water levels to small-scale topography. This downscaling process is referred to as the static method. The method was originally designed for use in North Carolina (NC), where results from an ADCIRC model designed specifically for NC were downscaled to a set of NC topographical data. By joining the static method with an ADCIRC output visualization tool, the downscaling process is now able to run faster with the same level of accuracy and can run on any ADCIRC model with downscaling data from any geographical region or given resolution. This process is used to provide extra guidance to emergency managers and decision makers during hurricanes.
The downscaling process is also improved by adding physics using the slopes method and the head loss method. The slopes method incorporates the slopes of the water levels produced by ADCIRC, rather than only the value of the water level. By interpolating ADCIRC output water elevation points into a smooth surface, slopes of this surface can be used to influence the elevations of downscaled water levels. The head loss method adds friction loss due to variations in the ground surface based on land cover types and friction associated with each type. As water travels over any surface, head loss, or a loss in energy, occurs at different rates depending on the surface roughness. This rudimentary hydrologic principle is applied to increase the accuracy of the downscaling process at minimal cost. The downscaling methods are applied for results from an ADCIRC simulation used in real-time forecasting, and then compared with results from an ADCIRC simulation with 10 times more resolution in Carteret County, NC. The static method tends to over-estimate the flood extents, and the slopes method is similar. However, the head loss method generates a downscaled flooding extent that is a close match to the predictions from the higher-resolution, full-physics model.
By improving the accuracy of downscaling methods at minimal computational cost and expanding the applicability of these downscaling methods, these methods can be used by emergency managers to provide a better estimation of flooding extents while simulating storm events.
Socially Distant Movie Party
The coastal engineering team at NC State enjoyed a Socially Distant Movie Party on Sat Apr 4. From our separate homes, we watched Sharknado and Cloudy with a Chance of Meatballs, which are mildly “coastal” and good distractions. It was fun to connect with each other, even from a distance. Stay healthy!
Carter Rucker defends MS Thesis
Despite a challenging situation in the world, we gathered for a successful MS thesis defense by Carter Rucker. With only a few people in the room and a larger online audience, Carter presented his research and answered questions with great skill. Congrats to Carter!
Wind and Tide Effects on the Choctawhatchee Bay Plume and Implications for Surface Transport at Destin Inlet
Multiple river-dominated estuaries line the northern Gulf coast and introduce substantial density variations. Their plumes have been shown to be highly sensitive to wind and tide effects, but in studies with limited observations and idealized wind forcing. This study explores these effects with a dynamic model that can represent the full behavior from river through estuary to shelf, and for a period with extensive observations. The inner shelf adjacent to Choctawhatchee Bay, a micro tidal estuary situated along the Florida Panhandle, is subject to buoyant, brackish outflows during the ebb-phase of the tidal cycle.
In December 2013, experiments were performed in this region to study mechanisms that influence near-shore surface transport. Satellite imagery showed a visible brackish surface plume at Destin during low tide. The goal of the present study is to quantify variability in the plume signature due to changes in tidal and wind forcing. Density-driven flows near Destin Inlet are modeled with the recently-enhanced, three-dimensional, baroclinic capabilities of the ADvanced CIRCulation (ADCIRC) model. Modeled tides, salinities and plume signature are validated against in-situ observations and satellite imagery. Model results reveal substantial changes in the length, width and orientation of the plume as the wind direction varied on consecutive days due to winter cold fronts. During a period of near-constant winds and variability in tidal amplitude, the model predicted a larger plume during spring tides than during neap conditions. Coriolis effects on the plume are minimized due to its small scale nature. Therefore, when the wind forcing is weak, the plume signature spreads radially from the inlet with slight preference to the down-shelf. The Choctawhatchee Bay plume is representative of other small-scale plumes formed in river-dominated and micro-tidal environments, and this work demonstrates the sensitivity of these plumes to changing environmental conditions.
Improving Predictions of Estuarine Flooding and Circulation during Storms
This project will address the problem of storm-driven circulation and flooding in estuaries. Our motivation is the recent Hurricane Florence (2018), which pushed surge and mixed saline waters into the estuaries of North Carolina (NC). There are remaining questions about how storm surge can interact with winds, riverine flows, and friction in estuarine systems, as well as how stratification is removed during and then re-established after storms.
The research plan will have two components. First, the existing modeling system will be enhanced for the NC estuaries, and numerical experiments will explore the sensitivities of estuarine flooding to the main drivers during storms. By varying systematically the atmospheric forcing, bottom friction, incoming river flows, and other parameters, we will improve our understanding of how storm surge is developed in these regions. Second, the modeling system will be extended to consider density-driven circulation and salinity transport, by leveraging earlier work for estuarine circulation in the northern Gulf. It is known that horizontal salinity transport during storms can threaten marine life and vegetation, but there is not currently a modeling system that can predict both transport and overland flooding. This project will combine those processes and explore questions about stratification during storms. While these interactions are important in estuaries along the U.S. Gulf and Atlantic coasts, they are especially important for the NC estuaries and their nearby communities, which have been devastated by storms in recent years.
The project will also have an extensive education component. Via collaboration with the Coastal Studies Institute, we will develop and implement lesson plans for storm surge and coastal flooding. It is expected that this new program will engage with more than 300 students in northeastern NC. The research team is well-positioned to contribute to these outreach activities, thus benefiting coastal communities in NC.
JC Dietrich, RJ McCord. “Improving Predictions of Estuarine Flooding and Circulation during Storms.” National Oceanic and Atmospheric Administration, North Carolina Sea Grant, 2020/02/01 to 2022/01/31, $119,370 (Dietrich: $99,610).