Identifying the Drivers of Chronic Coastal Flooding: A Community-Centric Approach

This project will address the problem of recurrent, shallow flooding in low-lying coastal communities. As local sea level rise (SLR), land subsidence, and heavy rainfall events increase, so does the frequency of flooding in low-lying coastal areas. The tidal cycle now takes place on higher average sea levels, resulting in “sunny-day” flooding of roadways during high tides. Sea water also infiltrates stormwater drainage systems at low tidal levels, such that ordinary rainstorms lead to flooding. While these minor floods draw less attention than catastrophic storms, their high frequency imposes a chronic stress on coastal communities and economies by disrupting critical infrastructure services. However, information on recurrent, minor floods is scarce because they are hyper-local – often affecting a city block or intersection at a time – and relatively brief in duration. Further, prediction of chronic flooding is hindered by a poor understanding of subterranean contributions, non-tidal hydrodynamic contributions, and their interactions with tides and SLR. This problem is particularly critical for (but not unique to) communities in North Carolina (NC), many of which experience chronic flooding during non-King tide events and are seeking engineered interventions for adaptation.

The proposed work integrates outreach and research activities over the two-year project period to improve our prediction and communication of chronic flood hazards to stakeholders in the Town of Carolina Beach, NC (CB), a community plagued by chronic flooding. We will couple an existing high-resolution hydrodynamic model with a stormwater management model to hindcast and test hypotheses on the drivers of chronic, and sometimes unexpected, flood events in CB. In parallel, we will deploy a real-time flood sensor network in CB to continuously measure the stormwater network capacity and fill data gaps on the incidence and causes of chronic flooding. In response to the expressed need from local officials, we will also use the in-situ data to develop an early-warning system and engage community members to co-develop flood-mitigation design scenarios for future testing using the new model framework.

K Anarde, M Hino, A Gold, JC Dietrich. “Identifying the drivers of chronic coastal flooding: a community-centric approach.National Oceanic and Oceanic Administration, North Carolina Sea Grant, 2022/02/01 to 2024/01/31, $119,411.

Ajimon’s Paper Selected as Editor’s Choice

Our recent paper, “Effects of Model Resolution and Coverage on Storm-Driven Coastal Flooding Predictions,” was selected as the Editor’s Choice by the Journal of Waterway, Port, Coastal and Ocean Engineering. The chief editor selects a paper from the current issue. The paper is made free with registration and featured on the journal home page for two months, after which it will continue to be featured in the Editor’s Choice Collection.

Congratulations to Ajimon!

Effects of Model Resolution and Coverage on Storm-Driven Coastal Flooding Predictions

Predictions of storm surge and flooding require models with higher resolution of coastal regions, to describe fine-scale bathymetric and topographic variations, natural and artificial channels, flow features, and barriers. However, models for real-time forecasting often use a lower resolution to improve efficiency. There is a need to understand how resolution of inland regions can translate to predictive accuracy, but previous studies have not considered differences between models that both represent conveyance into floodplains and are intended to be used in real time. In this study, the effects of model resolution and coverage are explored using comparisons between forecast-ready and production-grade models that both represent floodplains along the US southeast coast, but with typical resolutions in coastal regions of 400 and 50 m, respectively. For two storms that impacted the US southeast coast, it is shown that, although the overall error statistics are similar between simulations on the two meshes, the production-grade model allowed a greater conveyance into inland regions, which improved the tide and surge signals in small channels and increased the inundation volumes between 40% and 60%. Its extended coverage also removed water level errors of 20–40 cm associated with boundary effects in smaller regional models.

A Thomas, JC Dietrich, CN Dawson, RA Luettich (2022). “Effects of Model Resolution and Coverage on Storm-Driven Coastal Flooding Predictions.” Journal of Waterway, Port, Coastal, and Ocean Engineering, 148(1), 04021046, DOI: 10.1061/(ASCE)WW.1943-5460.0000687.

Impact of Storm Events on Density Stratification in the Pamlico and Albemarle Estuarine System

Tropical cyclones and other coastal storms have multiple effects on estuaries. They create storm surge, or the rise of water levels above the normal tides, which can cause flooding of coastal areas, including communities near estuaries. They can also alter ecosystems, including in estuaries with changes to nutrient loading and regeneration, abrupt changes in salinity, increases in the mixed-layer depth, decreases in sea-surface temperature, and breakdowns in water column stratification. The interactions between surge and estuarine circulation can enhance the storm effects. And with the increasing intensity of tropical cyclones, these effects will be further enhanced.

Numerical models can represent the coastal environment and its response to the combined effects of tides, river flows, and winds. It is especially challenging for numerical models to represent the response of estuaries to storms, due to the complex interactions of fresh and saline waters, and thus relatively few studies have used models to represent both storm- and density-driven circulation in estuaries. These few studies have shown that salinities and temperatures of estuaries can change significantly during storms and may require weeks to recover, depending on the amount of freshwater discharge. However, these studies have been limited in number and geographic coverage, relied on coupling to other models for baroclinic inputs, did not have the estuarine mixing and stratification as a focus, or were missing physics. Much is still uncertain about how estuarine circulation evolves during a storm event. How quickly do the horizontal salinities respond to the storm? How does the salinity transport vary through an estuary? How do freshwater discharges due to rainfall affect the mixing? Another uncertainty is the salinity response after the storm. How quickly does a system recover? Do the freshwater discharges interrupt the recovery? In this thesis, it is hypothesized that, for a large and shallow estuarine system with minimal connections to the open ocean, the storm forcing will cause large brackish and freshwater intrusions and recoveries that vary through the system.

To investigate this hypothesis, we developed a three-dimensional model of storm- and density-driven circulation in the Albemarle-Pamlico Estuarine System (APES) in North Carolina. Irene (2011) was used as the basis for storm event simulations to examine the evolution of the horizontal salinity distribution. Included in this model were hurricane-strength winds and pressures, tides, river discharges, and density circulation. Using this model, it was determined that during Irene, APES experienced movements of brackish water into the estuaries and saline water into the sounds. These movements were heavily dependent on the winds. After the stormsimulation, the large river discharges produced intrusions of fresher water into major areas of the sound, and after two weeks, the system was not fully regulated.

From this research, we have developed a better understanding of the horizontal salinity distribution of APES as well as how the system reacts to a single storm event. This research allows for future studies to consider different types of storms along with refinement of the river forcings, to understand better the full range of estuarine responses.

BA Rumbaugh (2021). “Impact of storm events on density stratification in the Pamlico and Albemarle Estuarine System,” North Carolina State University.

Improved Predictions with ST6 Physics and SWAN Version 41.31

These analyses were performed by Carter Day, an undergraduate researcher in our team.

Like other spectral wave models, SWAN uses parameterizations to represent sources (e.g. wind), sinks (e.g. whitecapping, bottom friction, depth-limited breaking), and resonance (e.g. quadruplet and triad wave-wave interactions). Each parameterization is based on laboratory and experimental data to represent the transfer of energy to, from, and between waves. It is necessary for each term to represent its physical process, but it is also necessary for the terms to be calibrated collectively to represent their combined effects on wave evolution.

SWAN release version 41.31 was modified in two main ways: derivative computation was changed to use the Green-Gauss formula, and a new ‘package’ of wave physics (the so-called ST6 physics) was introduced. This package includes new parameterizations of wind input, whitecapping, swell dissipation, wind speed scaling, and other processes. The ST6 physics have been adopted by other wave models (e.g. NOAA’s WaveWatch III), and it will likely become the preferred physics package for SWAN. However, because the ST6 physics package has changes to so many parameterizations, it is necessary to quantify its effects on wave predictions during recent storms.

In this study, we simulate two recent hurricanes, Gustav (2008) and Florence (2018), and we compare wave predictions with the new ST6 physics package. Do the ST6 physics improve the SWAN wave predictions?

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News: Jessica Gorski Featured in Lenovo Video

2021/11/15 – Intel + Lenovo
Coastal Computing

Our research into real-time erosion predictions using XBeach was featured in a recent video by Lenovo and CNN. Jessica Gorski describes how we are exploring the use of 1D transect models to predict erosion during storms.

Lenovo provides hardware and support for the HPC services at NC State. The video was produced as branded content for CNN, and it was featured on the CNN web site and social media.

The video required two days of shooting with a team of directors, photographers, audio specialists, and production assistants. Click below to see photos of the production.

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Formation of a Barrier Island Breach and Its Contributions to Lagoonal Circulation

Barrier islands are a primary coastal defense and often experience erosion during storms. When they fail due to storm-induced breaching, there can be significant changes to the small- and large-scale hydrodynamics and morphodynamics of the region. In this study, we explore the formation of a breach on Hatteras Island, North Carolina, during Isabel (2003) and the subsequent flooding into Pamlico Sound. Two-way coupling of high-fidelity, high-resolution numerical models for coastal erosion and flooding enables a better understanding of the formation of the breach, as well as scenarios of the breach’s effects on the circulation in the region. The breach connecting the ocean to the sound formed during the day of landfall. It is shown that, during the storm, overwash and inundation from the ocean led to deterioration of the beach and dunes, and then after the storm, the creation of channels through the island was sensitive to elevated water levels in the lagoon. Then flooding scenarios are considered in which the ground surface of the hydrodynamic model was (a) static, updated with the (b) pre- and post-storm observations, and updated dynamically with (c) erosion model predictions and (d) erosion model predictions with elevated lagoon-side water levels. The model results show that the breach has region-scale effects on flooding that extend 10 to 13 km into the lagoon, increasing the local water levels by as much as 1.5 m. These results have implications for similar island-lagoon systems threatened by storms.

A Gharagozlou, JC Dietrich, TC Massey, DL Anderson, JF Gorski, MF Overton (2021). “Formation of a barrier island breach and its contributions to lagoonal circulation.” Estuarine, Coastal and Shelf Science, 262, 107593, DOI: 10.1016/j.ecss.2021.107593.