Robert McCall

RISC-KIT: Resilience-Increasing Strategies for Coasts - toolKIT

ABSTRACT Van Dongeren, A., Ciavola, P., Viavattene, C., De Kleermaeker, S., Martinez, G., Ferreira, O., Costa C., R. McCall, 2014. RISC-KIT: Resilience-Increasing Strategies for Coasts–toolkit. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 366–371, ISSN 0749-0208. 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 Disaster Risk Reduction formulated the Hyogo Framework for Action, and the EU has issued the Floods Directive. By their nature, neither is specific about the methods to be used to assess coastal risks, particularly those risks resulting from dune and structure overtopping, the non-stationarity of surge and flash flood events, and coastal morphodynamic response. This paper describes a set of open-source and open-access methods, tools and management approaches to fill this gap. A Coastal Risk Assessment Framework will assess coastal risk at a regional scale. Thus critical hotspots can be identified for which an impact-oriented Early Warning System/Decision Support System is developed. This can be applied in dual mode: as a forecast and warning system and as an ex-ante planning tool to evaluate the vulnerability. The tools are demonstrated on case study sites on a range of EU coasts with diverse geomorphic settings, land use, forcing, hazard types and socio-economic, cultural and environmental characteristics. Specific DRR plans will be developed for all sites. A management guide of PMP measures and management approaches is to be developed. The toolkit will benefit forecasting and civil protection agencies, coastal managers, local government, community members, NGOs, the general public and scientists.

Predicting overwash on gravel barriers

ABSTRACT McCall, R.T., Masselink, G., Poate, T.G., Bradbury, A.P., Russell, P.E. and M.A. Davidson, 2013. Predicting overwash on gravel barriers. 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 range of the empirical model. When applied to 25 historical storm impacts, the process-based model shows improvement over the empirical model in predicting overwash. The model is applied to study the sensitivity of overwash to input parameters outside the validity range of the empirical Barrier Inertia model. This analysis shows that two parameters currently missing in the Barrier Inertia Model, the depth of the gravel beach toe and the gravel beach slope, greatly affect the threshold criteria for overwash.

Modeling the effect of wave‐vegetation interaction on wave setup

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 important parameters for coastal risk assessment. In this study, the storm impact model XBeach is extended with formulations for attenuation of sea-swell and IG waves, and wave setup effects in two modes: the sea-swell wave phase-resolving (nonhydrostatic) and the phase-averaged (surfbeat) mode. In surfbeat mode, a wave shape model is implemented to capture the effect of nonlinear wave-vegetation interaction processes on wave setup. Both modeling modes are verified using data from two flume experiments with mimic vegetation and show good skill in computing the sea-swell and IG wave transformation, and wave setup. In surfbeat mode, the wave setup prediction greatly improves when using the wave shape model, while in nonhydrostatic mode (nonlinear) intrawave effects are directly accounted for. Subsequently, the model is used for a range of coastal geomorphological configurations by varying bed slope and vegetation extent. The results indicate that the effect of wave-vegetation interaction on wave setup may be relevant for a range of typical coastal geomorphological configurations (e.g., relatively steep to gentle slope coasts fronted by vegetation).

Modelling dune erosion, overwash and breaching at Fire Island (NY) during hurricane Sandy

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 was also concluded in several previous case studies. If the discretization of bed roughness along with wave skewness and asymmetry are improved in the model, XBeach is capable of simulating the various morphological changes within the chosen model domain.

Most atolls will be uninhabitable by the mid-21st century because of sea-level rise exacerbating wave-driven flooding

Sea-level rise and wave-driven flooding will damage freshwater resources of most atolls and soon render them uninhabitable. 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 additional hazard of wave-driven overwash or its impact on freshwater availability. We project the impact of sea-level rise and wave-driven flooding on atoll infrastructure and freshwater availability under a variety of climate change scenarios. We show that, on the basis of current greenhouse gas emission rates, the nonlinear interactions between sea-level rise and wave dynamics over reefs will lead to the annual wave-driven overwash of most atoll islands by the mid-21st century. This annual flooding will result in the islands becoming uninhabitable because of frequent damage to infrastructure and the inability of their freshwater aquifers to recover between overwash events. This study provides critical information for understanding the timing and magnitude of climate change impacts on atoll islands that will result in significant, unavoidable geopolitical issues if it becomes necessary to abandon and relocate low-lying island states.

The Role of Bed Roughness in Wave Transformation Across Sloping Rock Shore Platforms

We present for the first time observations and model simulations of wave transformation across sloping (Type A) rock shore platforms. Pressure measurements of the water surface elevation using up to 15 sensors across five rock platforms with contrasting roughness, gradient, and wave climate represent the most extensive collected, both in terms of the range of environmental conditions, and the temporal and spatial resolution. Platforms are shown to dissipate both incident and infragravity wave energy as skewness and asymmetry develop and, in line with previous studies, surf zone wave heights are saturated and strongly tidally modulated. Overall, the observed properties of the waves and formulations derived from sandy beaches do not highlight any systematic interplatform variation, in spite of significant differences in platform roughness, suggesting that friction can be neglected when studying short wave transformation. Optimization of a numerical wave transformation model shows that the wave breaker criterion falls between the range of values reported for flat sandy beaches and those of steep coral fore reefs. However, the optimized drag coefficient shows significant scatter for the roughest sites and an alternative empirical drag model, based on the platform roughness, does not improve model performance. Thus, model results indicate that the parameterization of frictional drag using the bottom roughness length-scale may be inappropriate for the roughest platforms. Based on these results, we examine the balance of wave breaking to frictional dissipation for rock platforms and find that friction is only significant for very rough, flat platforms during small wave conditions outside the surf zone.

The effect of tides and storms on the sediment transport across a Dutch barrier island

Under natural conditions, barrier islands might grow vertically and migrate onshore under the influence of long-term sea level rise. Sediment is transported onshore during storm-induced overwash and inundation. However, on many Dutch Wadden Islands, dune openings are closed off by artificial sand-drift dikes that prevent the influx of sediment during storms. It has been argued that creating openings in the dune row to allow regular flooding on barrier islands can have a positive effect on the sediment budget, but the dominant hydrodynamic processes and their influence on sediment transport during overwash and inundation are unknown. Here, we present an XBeach model study to investigate how sediment transport during overwash and inundation across the beach of a typical mesotidal Wadden Sea barrier island is influenced by wave, tide and storm surge conditions. Firstly, we validated the model XBeach with field data on waves and currents during island inundation. In general, the XBeach modelperformed well. Secondly, we studied the long-term sediment transport across the barrier island. We distinguished six representative inundation classes, ranging from frequently occurring, low-energy events to infrequent, high-energy events, and simulated the hydrodynamics and sediment transport during these events. An analysis of the model simulations shows that larger storm events cause larger cross-shore sediment transport, but the net sediment exchange during a storm levels off or even becomes smaller for the largest inundation classes because it is counteracted by larger mean water levels in the Wadden Sea that oppose or even reverse sediment transport during inundation. When taking into account the frequency of occurrence of storms we concludethat the cumulative effect of relatively mild storms on long-term cross-shore sediment transport is much larger than that of the large storm events.

PHYSICAL MODELING OF STORM-INDUCED EROSION AND OVERWASH

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 cameras. Three-dimensional stereo photo reconstruction data of the barrier island at various stages during the experiment show the morphology of the back barrier to be highly non-uniform in longshore direction during overwash. The entire measurement dataset is made publicly available online via the OpenEarth initiative.

Hindcast storm events in the Bering Sea for the St. Lawrence Island and Unalakleet Regions, Alaska

This study provides viable estimates of historical storm-induced water levels in the coastal communities of Gambell and Savoonga situated on St. Lawrence Island in the Bering Sea, as well as Unalakleet located at the head of Norton Sound on the western coast of Alaska. Gambell, Savoonga, and Unalakleet are small Native Villages that are regularly impacted by coastal storms but where little quantitative information about these storms exists. The closest continuous water-level gauge is at Nome, located more than 200 kilometers from both St. Lawrence Island and Unalakleet. In this study, storms are identified and quantified using historical atmospheric and sea-ice data and then used as boundary conditions for a suite of numerical models. The work includes storm-surge (temporary rise in water levels due to persistent strong winds and low atmospheric pressures) modeling in the Bering Strait region, as well as modeling of wave runup along specified sections of the coast in Gambell and Unalakleet. Modeled historical water levels are used to develop return periods of storm surge and storm surge plus wave runup at key locations in each community. It is anticipated that the results will fill some of the data void regarding coastal flood data in western Alaska and be used for production of coastal vulnerability maps and community planning efforts.

MODELLING SCOUR IN FRONT OF DUNE REVETMENTS IN A SURF-BEAT MODEL

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 the assessment rules. Until now, XBeach did not model scour hole development in front of dune revetments accurately. We suggest to include the advection of turbulence as well as the effect of backwash of short waves that creates additional turbulence in the model. Verification with three physical model experiments shows that with the suggested adaptations of the model a scour hole with significant depth can be modeled.