Ryan P. Mulligan

Whitecapping and wave field evolution in a coastal bay

Evolution of the wave field in a coastal bay is investigated, by comparison between field observations and numerical simulations using a spectral wave model (Simulating WAves Nearshore (SWAN)). The simulations were conducted for the passage of an extratropical storm, during which surface elevation spectra were bimodal owing to local wind-sea generation and swell propagation into the bay. SWAN was run in stationary and nonstationary mode for two whitecapping source term formulations. The first was developed by Komen et al. (1984) and is dependent on spectrally averaged wave steepness, and thus includes swell in the calculation of whitecapping dissipation and typically overestimates wind sea in the presence of swell. The second, proposed by van der Westhuysen et al. (2007), estimates whitecapping of wind sea locally in the wave spectrum and is not coupled to swell energy. This formulation reproduced the magnitude and shape of the observed wind-sea spectral peak much better than the previous formulation. Whitecapping dissipation rates have been estimated from observations, using the equilibrium range theory developed by Phillips (1985), and are well correlated with both wind speed and acoustic backscatter observations. These rates agree with SWAN estimates using the spectrally local expression, and provide additional physical validation for the whitecapping source term.

Tsunamis generated by long and thin granular landslides in a large flume

In this experimental study, granular material is released down slope to investigate landslide-generated waves. Starting with a known volume and initial position of the landslide source, detailed data are obtained on the velocity and thickness of the granular flow, the shape and location of the submarine landslide deposit, the amplitude and shape of the near-field wave, the far-field wave evolution, and the wave runup elevation on a smooth impermeable slope. The experiments are performed on a 6.7 m long 30° slope on which gravity accelerates the landslides into a 2.1 m wide and 33.0 m long wave flume that terminates with a 27° runup ramp. For a fixed landslide volume of 0.34 m3, tests are conducted in a range of still water depths from 0.05 to 0.50 m. Observations from high-speed cameras and measurements from wave probes indicate that the granular landslide moves as a long and thin train of material, and that only a portion of the landslide (termed the “effective mass”) is engaged in activating the leading wave. The wave behavior is highly dependent on the water depth relative to the size of the landslide. In deeper water, the near-field wave behaves as a stable solitary-like wave, while in shallower water, the wave behaves as a breaking dissipative bore. Overall, the physical model observations are in good agreement with the results of existing empirical equations when the effective mass is used to predict the maximum near-field wave amplitude, the far-field amplitude, and the runup of tsunamis generated by granular landslides.

Performance of Nowcast and Forecast Wave Models for Lunenburg Bay, Nova Scotia

The results from a numerical modelling system are presented for wave prediction inside Lunenburg Bay. The Bay, typical of the coast in Atlantic Canada, is an environment where ocean swell enters only from selected directions; wind-sea dominates the wave spectrum from other directions, and shallow water physics are important. The modelling system consisted of wave models for both the present time (nowcasts) and forecasts using the Simulating Waves Nearshore (SWAN) model inside the Bay. Nowcasts (stationary computations of the wave field that ran every 30 minutes) were driven by real-time observations of the directional wave boundary conditions, winds and water levels. Forecasts (48 hourly non-stationary computations) were driven by boundary conditions from the WAVEWATCH III ocean wave model (implemented on a larger domain) and winds from the Global Environmental Multiscale (GEM) atmospheric model. The results were compared with wave observations inside the Bay and provided in real-time. Model performance was assessed for a storm event with 2.8 m significant wave heights that occurred in October 2007, by comparing nowcast predictions, forecast predictions and observations. The nowcasts provided the best correlation, R2 = 0.75, with observations inside the Bay, since they were driven by observations made at the model boundary. The forecasts tended to underpredict the significant wave height and peak period, but overall the model results compared well with the data over a wide range of wind and wave conditions.

Storm Surge and Surface Waves in a Shallow Lagoonal Estuary during the Crossing of a Hurricane

AbstractTropical cyclones deliver intense winds that can generate some of the most severe surface wave and storm surge conditions in the coastal ocean. Hurricane Irene (2011) crossed a large, shallow lagoonal estuarine system in North Carolina, causing flooding and erosion of the adjacent low-lying coastal plain and barrier islands. This event provided an opportunity to improve understanding of the estuarine response to strong and rotating wind forcing. Observations from acoustic sensors in subestuaries and water-level elevation measurements from a network of pressure sensors across the system are presented. Data are examined with two modeling techniques: (1) a simple numerical approach using a momentum balance between the wind stress, flow acceleration, pressure gradient, and bottom friction that gives insight into temporal variability in water levels through the passage of the storm; and (2) an advanced hydrodynamic model based on the full shallow water fluid momentum equations, coupled to a spectral su...

Barrier Island and Estuary Co-evolution in Response to Holocene Climate and Sea-Level Change: Pamlico Sound and the Outer Banks Barrier Islands, North Carolina, USA

Barrier islands and associated back-barrier estuaries and lagoons interact via hydrodynamic and sedimentary processes, affecting the evolution of both systems. Understanding coupled dynamic processes between both systems is vital to forecasts of future coastal morphologic and hydrodynamic changes in response to such factors as sea-level rise and storm patterns. The Pamlico Sound and the Outer Banks barrier islands of North Carolina, USA have co-evolved in response to Holocene climate and sea-level change, and autogenic processes. Recent data and models illustrate the dynamic response of this system to minor, but rapid, climate changes occurring throughout the Holocene, including the Medieval Climate Anomaly and Little Ice Age. Periods of extreme barrier segmentation occurred during times of rapid climate change, affecting tidal energy and salinity conditions within the Pamlico Sound. Hydrodynamic models aid in understanding the magnitude of changes, and the impact on barrier morphology. Future changes to coastal systems may be anticipated based upon changes that have occurred in the past.

Examining Material Transport in Dynamic Coastal Environments: An Integrated Approach Using Field Data, Remote Sensing and Numerical Modeling

Coastal environments are critical ecological systems and offer vital resources and functions to societies worldwide. As a major interface between terrestrial and ocean environments, coastal water bodies (rivers, estuaries, bays and coastal margins) provide key ecological services and are the major conduit and processors of terrestrially derived particulate and dissolved material as they are transported to the ocean. Consequently, coastal environments have been shown to play a major role in global bio-geochemical cycles and provide critical habitat for a host of marine species. Globally, these important environments are under considerable pressure from high population densities, increasing growth rates and are particularly vulnerable from the effects of projected climate change such as sea level rise and increased storm events. Despite their importance, significant gaps remain in our understanding of how these environments will respond to climate change, increasing human population, land use changes, and over exploitation of natural resources. This lack of understanding is due in part to the difficulties in developing effective monitoring and analysis programs using only a single measurement approach that is limited in its spatial and temporal coverage.

Modelling Coastal Sediment Transport for Harbour Planning: Selected Case Studies

During the planning phase of coastal development projects, it is often necessary to determine potential sedimentation and erosion rates. This is particularly relevant at harbours where dredged channels are proposed, and accurate dredging projections are crucial for economic feasibility analyses. In addition, new structures that interfere with the natural processes may have major impacts on the adjacent shoreline. In this chapter we consider a range of approaches for evaluating sediment transport for harbour planning studies (section 2), and present two detailed cases from Atlantic Canada. The sites described are representative of very different coastal environments. They include Saint John Harbour (section 3), a uniquely dynamic estuary on the Bay of Fundy with huge tides, a very large river outflow and significant sedimentation of silt and clay presenting various navigation and dredging challenges. The other site described is located on the sandy North coast of Prince Edward Island at Darnley Inlet, an exposed area where tides, storms and sea level rise are continuously reshaping the shoreline and navigation channels (section 4).

Transport and transformation of dissolved organic matter in the Neuse River estuarine system, NC, USA, following Hurricane Irene (2011)

Rivers are major conduits for the transport of allochthonous dissolved organic matter (DOM) to the ocean in coupled land–coastal systems. DOM can regulate biogeochemical processes and affect water quality, depending on the concentration and quality of DOM. By using spectral parameters calculated from chromophoric dissolved organic matter (CDOM) ultraviolet-visible absorption spectra, along with dissolved organic carbon (DOC) concentrations, we examined the input and change in the amount and quality of DOM in surface waters of the lower Neuse River and upper–middle regions of the Neuse Estuary following a major rainfall (30cm in 1 day) associated with Hurricane Irene (2011). CDOM and DOC nearly tripled in the 3 days following the storm. Although a strong linear relationship was observed between DOC and CDOM absorption coefficient at 350nm (R2=0.85), a higher fraction of non-chromophoric DOC to CDOM was observed during the rising river discharge. The spectral slope at 275–295nm and the slope ratio (275–295:350–400nm) indicated a shift from higher to lower molecular-weight DOM as it transited through the estuary, probably as a result of photodegradation. The present study demonstrated the utility of using CDOM spectral parameters for examining the flux and transformation of DOM in coastal waters following major rain events.

Alongshore momentum transfer to the nearshore zone from energetic ocean waves generated by passing hurricanes

Wave and current measurements from a cross-shore array of nearshore sensors in Duck, NC, are used to elucidate the balance of alongshore momentum under energetic wave conditions with wide surf zones, generated by passing hurricanes that are close to and far from to the coast. The observations indicate that a distant storm (Hurricane Bill, 2009) with large waves has low variability in directional wave characteristics resulting in alongshore currents that are driven mainly by the changes in wave energy. A storm close to the coast (Hurricane Earl, 2010), with strong local wind stress and combined sea and swell components in wave energy spectra, has high variability in wave direction and wave period that influence wave breaking and nearshore circulation as the storm passes. During both large wave events, the horizontal current shear is strong and radiation stress gradients, bottom stress, wind stress, horizontal mixing, and cross-shore advection contribute to alongshore momentum at different spatial locations across the nearshore region. Horizontal mixing during Hurricane Earl, estimated from rotational velocities, was particularly strong suggesting that intense eddies were generated by the high horizontal shear from opposing wind-driven and wave-driven currents. The results provide insight into the cross-shore distribution of the alongshore current and the connection between flows inside and outside the surf zone during major storms, indicating that the current shear and mixing at the interface between the surf zone and shallow inner shelf is strongly dependent on the distance from the storm center to the coast.

SEDIMENT DYNAMICS AND MORPHOLOGICAL EVOLUTION OF A LARGE BACK-BARRIER ESTUARY

A numerical model was used to simulate water levels, currents, waves, suspended sediment and salinity distributions in Pamlico Sound, a large and shallow back-barrier estuary in eastern North Carolina, for four distinct time slices during its geomorphic evolution over the late Holocene. Present-day bathymetry was obtained from a high resolution digital elevation model of Pamlico Sound, and paleobathymetric model grids were created for 500, 1000 and 4000 calibrated years before present (cal yr BP) using age-depth relationships developed from sediment core and time-constrained seismic observations. Hydrodynamic and sediment model results for a one-month simulation at the 0 and 4000 cal yr BP time slices are compared to assess the impacts of varying degrees of barrier island segmentation, long-term changes in basin geomorphology, and sea-level rise on the flow and transport response in Pamlico Sound.