Robert T. McCall

Deltares · Department of Marine and Coastal Systems

The ability of reefs to protect coastlines from storm-driven flooding hinges on their capacity to keep pace with sea-level rise. Here, we show how and whether coral restoration could achieve the often-cited goal of reversing the impacts of coral-reef degradation to preserve this essential function. We combined coral-growth measurements and carbonat...

Along much of the world’s coastline, coastal barriers serve as the first line of defense against oceanic and meteorological forces. Extreme storms cause large morphodynamic changes on coastal barriers through high sediment transport rates, which may degrade their defensive capabilities. The understanding of morphological impacts is therefore import...

Most numerical studies of sediment transport in the swash zone use depth-averaged models. However, such models still have difficulty predicting transport rates and morphodynamics. Depth-resolving models could give detailed insight in swash processes but have mostly been limited to hydrodynamic predictions. We present a depth-resolving numerical mod...

Spur-and-groove (SAG) morphology characterizes the fore reef of many coral reefs worldwide. Although the existence and geometrical properties of SAG have been well documented, an understanding of the hydrodynamics over them is limited. Here, the three-dimensional flow patterns over SAG formations, and a sensitivity of those patterns to waves, curre...

Intertidal bars are naturally occurring morphological features along the waterline of sandy beaches. Present quantitative knowledge on intertidal bar behavior is limited, due to the scarcity of data resources and the limitations of traditional survey techniques. To investigate and quantify the cross-shore morphologic behavior of intertidal bars, ho...

Many coral reef-lined coasts are low-lying with elevations <4 m above mean sea level. Climate-change-driven sea-level rise, coral reef degradation, and changes in storm wave climate will lead to greater occurrence and impacts of wave-driven flooding. This poses a significant threat to their coastal communities. While greatly at risk, the complex hy...

The importance of explicitly modelling sea-swell waves for runup was examined using a 2D XBeach short wave-averaged (surfbeat, “XB-SB”) and a wave-resolving (non-hydrostatic, “XB-NH”) model of Roi-Namur Island on Kwajalein Atoll in the Republic of Marshall Islands. Field observations on water levels, wave heights, and wave runup were used to drive...

Tropical coral reef-lined coasts are exposed to storm wave-driven flooding. In the future, flood events during storms are expected to occur more frequently and to be more severe due to sea-level rise, changes in wind and weather patterns, and the deterioration of coral reefs. Hence, disaster managers and coastal planners are in urgent need of decis...

Shore platforms are ubiquitous morphological features along rocky coastlines and display a spectrum of forms from gently-sloping to sub-horizontal with a low tide cliff. They generally front eroding coastal cliffs and play an important natural coastal protection role by dissipating wave energy, especially during energetic wave conditions. Sea-swell...

Overwash hydrodynamic datasets are mixed in quality and scope, being difficult to obtain due to fieldwork experimental limitations. Nevertheless, these measurements are crucial to develop reliable models to predict overwash. Aiming to overcome such limitations, this work presents accurate fieldwork data on overwash hydrodynamics, further exploring...

Wave transformation across reef platforms strongly controls sediment transport processes and coral reef island morphodynamics with infragravity (IG) waves playing an important contributing role. A small-scale (1:50) laboratory experiment and prototype numerical modeling are used to explore the characteristics of IG wave motion on coral reefs. The s...

Sea levels are rising, with the highest rates in the tropics, where thousands of low-lying coral atoll islands are located. Most studies on the resilience of these islands to sea-level rise have projected that they will experience minimal inundation impacts until at least the end of the 21st century. However, these have not taken into account the a...

Recent and historic low-frequency, high-impact events have demonstrated the flood risks faced by exposed coastal areas in Europe and beyond. These coastal zone risks are likely to increase in the future which requires a re-evaluation of coastal disaster risk reduction (DRR) strategies and a new mix of PMP (prevention, e.g., dike protection; mitigat...

In predicting storm impacts on sandy coasts, possibly with structures, accurate runup and overtopping simulation is an important aspect. Recent investigations (Stockdon et al., 2014; Palmsten and Splinter, 2016) show that despite accurate predictions of the morphodynamics of dissipative sandy beaches, the XBeach model (Roelvink et al., 2009) does n...

Overwash hydrodynamics datasets are mixed in quality and scope, being hard to obtain due to fieldwork experimental difficulties. Aiming to overcome such limitations, this work presents accurate fieldwork data on overwash hydrodynamics, further exploring it to model overwash on a low-lying barrier island. Fieldwork was performed on Barreta Island (P...

Incident-band and infragravity wave transformation on five rocky shore platforms is simulated using a numerical model. The model is calibrated using measurements of bed roughness and hydrodynamics under mild to energetic wave conditions. Validation of the model show that the model is capable of reproducing wave heights across the platform with litt...

Overwash hydrodynamics were measured during a storm event. Data from literature and fieldwork were used to setup a numerical model, using XBeach in non-hydrostatic mode. Different simulations were conducted to assess the influence of several factors on overwash statistics. Nearshore morphology was found to greatly influence overwash. Sediment mean...

Final policy brief from the Resilience Increasing Strategies for Coasts - Toolkit (RISC-KIT) summarizing key recommendations for policy makers from the implementation of the toolkit in 10 European cities.

This chapter discusses the modeling of morphodynamical storm impacts on coasts. It gives an overview of classes of models, the physical processes that each class resolves, and the model class applicability on the different coastal environments such as sandy, gravel and coral/rock coasts, as well as coasts with hard structures and vegetation. With r...

This Synthesis Report provides an overview of the achievements, lessons learned and challenges identified through the RISC-KIT project activities, including the development and application of the tools at ten case study sites in a range of coastal regions across Europe. The lessons learned are then fed into a series of recommendations for improved...

Gravel barriers provide a natural form of coastal protection and flood defence for many sites around the UK and worldwide. Predicting their vulnerability to different storm impact regimes that cause overtopping and overwash is crucial as these processes can lead to hazardous consequences such as inundation of the back of the barrier or breaching. T...

Low frequency, high impact storm events can have large impacts on sandy coasts. The physical processes governing these impacts are complex because of the feedback between the hydrodynamics of surges and waves, sediment transport and morphological change. Predicting these coastal changes using a numerical model requires a large amount of computation...

Implementation of aquatic vegetation effects into XBeach, application to sea-swell waves, infragravity waves and wave setup.

Video derived runup statistics from ten separate deployments at six field sites have been used to develop a new parameterisation for the prediction of runup of runup on gravel beaches. These data were collected over a 2-year period under energetic storm conditions with significant wave heights of Hs=1–8m from gravel beaches and barriers composed of...

High-impact storm events have demonstrated the vulnerability of coastal zones in Europe and beyond. These impacts are likely to increase due to predicted climate change and ongoing coastal development. In order to reduce impacts, disaster risk reduction (DRR) measures need to be taken, which prevent or mitigate the effects of storm events. To drive...

Aquatic vegetation in the coastal zone attenuates wave energy and reduces the risk of coastal hazards, e.g. flooding. Besides the attenuation of sea-swell waves, vegetation may also affect infragravity-band (IG) waves and wave setup. To date, knowledge on the effect of vegetation on IG waves and wave setup is lacking, while they are potentially imp...

Storms across the globe and their associated consequences in coastal zones (flooding and erosion), combined with the long-term geomorphic evolution of our coastlines, are a threat to life and assets, both socioeconomic and environmental. In a changing climate, with a rising global sea level, potentially changing patterns in storm tracks and stormin...

This paper presents an extension of the XBeach-G numerical model with a sediment transport and morphology module, which includes the effect of groundwater ventilation and flow inertia on sediment transport, to simulate the morphodynamic response of pure gravel beaches and barriers to storms. The morphodynamic XBeach-G model is validated by simulati...

In 2012, Hurricane Sandy caused a breach at Fire Island (NY, USA), near Pelican Island. This paper aims at modelling dune erosion, overwash and breaching processes that occured during the hurricane event at this stretch of coast with the numerical model XBeach. By using the default settings, the erosion rates are substantially overestimated, which...

The southwest of the UK suffered a series of large storm events during the winter of 2014 resulting in significant damage to the coastline. Gravel beaches and barriers are widely regarded as effective forms of coastal defence and often protect low lying wetlands and communities. Here we present the significant morphological response recorded at thr...

Over the past decades the effect of vegetation (e.g. kelp, mangroves, sea grass) on nearshore coastal processes has received more and more attention. In recent years several numerical wave models have been extended to include this effect. In the current study, the numerical storm impact model XBeach is extended with formulations for damping of shor...

XBeach is an open-source numerical model which originally was developed to simulate hydrodynamic and morphodynamic processes and impacts on sandy coasts with a domain size of kilometers and on the time scale of storms. Since then, the model has been applied to other types of coasts and purposes. This reference guide provides an overview of the mode...

For additional information, contact: Pacific Coastal and Marine Science Center U.S. Geological Survey Pacific Science Center 400 Natural Bridges Drive Santa Cruz, CA 95060 http://walrus.wr.usgs.gov/ Abstract This study provides viable estimates of historical storm-induced water levels in the coastal communities of Gambell and Savoonga situated on...

Despite the clear societal importance of gravel beaches and barriers in protecting coastal areas from flooding, there are currently no reliable numerical models for predicting the morphological response of gravel beaches to storm events. This paper synthesises the results of a research project (NUPSIG) aimed at reducing this shortfall through an in...

This paper presents adaptations to the XBeach model aimed at including the relevant processes for the generation of scour holes at the toe of a revetment. Dutch assessment rules for the safety of sea defenses need to be adjusted to cope with a combination of sandy dunes and hard elements. To that end, the XBeach model is prepared to be incorporated...

In this paper we present a process-based numerical model for the prediction of storm hydrodynamics and hydrology on gravel beaches. The model comprises an extension of an existing open-source storm-impact model for sandy coasts (XBeach), through the application of (1) a non-hydrostatic pressure correction term that allows wave-by-wave modelling of...

A 2D laser-scanner was deployed on four different coarse-grained beaches (Chesil, Loe Bar, Hayling Island and Seascale – all in UK) to measure the swash morphodynamics during energetic wave conditions (offshore Hs > 2m). Field observations performed with the laser-scanner showed that different types of coarse-grained beaches present contrasting mor...

Recent and historic high-impact events have demonstrated the flood risks faced by exposed coastal areas. These risks will increase due to climate change and economic development. This requires a re-evaluation of coastal disaster risk reduction DRR strategies and prevention, mitigation and preparedness PMP measures. To this end, the UN Office for Di...

A comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level chang...

This paper presents novel data collected over four weeks at Loe Bar, a gravel barrier, during energetic wave conditions. Combined remote and in-situ measurements were used to identify 3D morphological response, profile change and runup behaviour during successive energetic periods (Hs = 2.5 m). Gravel beaches provide a highly effective form of coas...

The collection of detailed field measurements from the swash zone during storms is an extremely challenging task which is difficult to execute with traditional in-situ deployments (e.g., scaffold rigs with instruments). Levels of difficulty increase for gravel beaches where the wave energy reaches the beach face with almost no loss of energy, leadi...

A process-based non-hydrostatic flow model, which includes the effect of infiltration and exfiltration, but no morphology, is applied to simulate overwash events on gravel barriers. After calibration, the model is shown to produce similar predictions for overwash as the empirical Barrier Inertia Model for parameter combinations within the validity...

Between the Bou Ghrara lagoon (South of Jerba island, Tunisia) and the Mediterranean Sea, the lack of water renewal, especially through the Bine El Ouediane eastern passes, is a real threat to the sustainability of tidal channels. In 2006, the opening in the causeway, that separates the Bou Ghrara lagoon from the Bine El Ouediane lagoon, was widene...

A quasi-3D process-based and time dependent groundwater model is developed and coupled to a hydrodynamic storm impact model to simulate the effect of infiltration on overwash on a gravel barrier. The coupled model is shown to accurately reproduce groundwater variations, runup properties and overwash time series measured in the gravel barrier during...

A one-dimensional hydrostatic version of the XBeach model (Roelvink et al., 2009) is applied to hindcast swash morphodynamics measured during an accretive, and an erosive tide at Le Truc Vert beach (France) in early spring 2008 (Masselink et. al, 2009; Blenkinsopp et al., 2011). Swash hydrodynamics are solved by applying the nonlinear shallow water...

Topographic surveys are used to assess response characteristics of eight gravel beaches to storm conditions across the south of England. Using profile volumes, pre- and post-storm, from surveys undertaken within a short temporal interval a quantitative assessment of beach response to corresponding wave conditions has been undertaken. Wide disparity...

A comprehensive study of swash-zone sediment transport was conducted on a macro-tidal beach in Perranporth, UK. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations and near bed velocity on a natural beach. Data collected during the study will be used to address the importance of sheet fl...

XBeach, a process-based numerical model capable of computing nearshore circulation and morphodynamics, including overwash and breaching, has been used in 1D mode to simulate erosion occurring on a gravel barrier beach (D50 = 11 mm) subjected to wave and tidal forcing during selected tests undertaken during the BARDEX experiments. The model demonstr...

Field experiments were conducted on two coastal gravel barriers backed by freshwater lagoons to examine the groundwater dynamics and to investigate the potential for saline intrusion. At Slapton Sands, groundwater, lagoon and ocean water level data were collected over a one year period; at Low Bar, data were collected over a two week period. The gr...

An advective-deterministic approach (ADA) to model wave energy dissipation through breaking is presented. This is calibrated against experimental irregular wave data with a high spatial density of surface elevation records using a wave-group-forced surf-beat model, XBeach. In the ADA breaker model, wave breaking is turned on and off by specifying u...

In this paper we present a series of physical model experiments with a mobile bed in a wave flume showing the temporal morphological response of a barrier island to the collision, overwash and inundation regime. The hydrodynamic conditions and morphology are monitored a combination of wave height meters, velocity meters, resistance wires and camera...

A 2DH numerical, model which is capable of computing nearshore circulation and morphodynamics, including dune erosion, breaching and overwash, is used to simulate overwash caused by Hurricane Ivan (2004) on a barrier island. The model is forced using parametric wave and surge time series based on field data and large-scale numerical model results....