Ap van Dongeren

The dynamics of infragravity wave transformation over a fringing reef

A 3 week field study was conducted to investigate the dynamics of low-frequency (infragravity) wave motions over a fringing reef at Ningaloo Reef, Western Australia. Short-period wave motions (0.04–0.2 Hz) were observed to dissipate on the reef crest beyond which infragravity wave motions (0.004–0.04 Hz) gradually dominated toward the lagoon. However, both the short waves and the infragravity waves were relatively small (both <0.3 m) on the reef flat owing to the shallow water depth (<2 m). The results revealed that the surf zone generation of free infragravity wave motions on the steep (?1:20) fore-reef slope was dominated by breakpoint forcing (as opposed to shoaling bound waves), which was also supported by detailed numerical simulations of the generation process. This is consistent with theory suggesting the efficiency of the breakpoint forcing mechanism should be high in this steep-slope regime. Shoreward propagating infragravity waves traveled across the reef but were damped by bottom friction dissipation; however, this was at a rate much smaller than experienced by the residual short waves. With these rates of frictional dissipation also strongly dependent on the water depth over the reef, the infragravity wave heights increased at higher water levels and hence were strongly modulated by the tide. Due to the strong dissipation of infragravity waves over this wide and shallow reef that is hydraulically rough, any seaward propagating infragravity waves that reflected at the shoreline were small, leading to the dominance of progressive (shoreward propagating) infragravity wave motions throughout the reef and lagoon.

The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines

A numerical model, XBeach, calibrated and validated on field data collected at Roi-Namur Island on Kwajalein Atoll in the Republic of Marshall Islands, was used to examine the effects of different coral reef characteristics on potential coastal hazards caused by wave-driven flooding and how these effects may be altered by projected climate change. The results presented herein suggest that coasts fronted by relatively narrow reefs with steep fore reef slopes (~1:10 and steeper) and deeper, smoother reef flats are expected to experience the highest wave runup. Wave runup increases for higher water levels (sea level rise), higher waves, and lower bed roughness (coral degradation), which are all expected effects of climate change. Rising sea levels and climate change will therefore have a significant negative impact on the ability of coral reefs to mitigate the effects of coastal hazards in the future.

Title: MICORE: DUNE EROSION AND OVERWASH MODEL VALIDATION WITH DATA FROM NINE EUROPEAN FIELD SITES

In this paper we present the first results of beach profile hindcasting with XBeach using recently measured coastal data acquired under storm conditions at eight European sites, including a comparison to model results obtained with off-the- shelf models. The results show consistently that the XBeach has skill in predicting the coastal profile, albeit that in most cases the erosion around the mean water line is overpredicted and the depositions at the lower beach face are overpredicted. The causes for this model effect are under active investigation but not resolved yet. Likely candidates are the modeling of onshore (asymmetry) transports which reduces the offshore transports due to undertow (currents) or the modeling of sediment motion in the swash zone.

NUMERICAL SIMULATION OF LONG-PERIOD WAVES AND SHIP MOTIONS IN TOMAKOMAI PORT, JAPAN

Long waves can penetrate easily into ocean facing ports or even be amplified if the wave frequency is close to one of the natural frequencies of the harbor. Moreover, the moored ship can have its natural frequency close a natural frequency of the basin, so that large ship motions can occur at relatively low offshore wave heights. Correct prediction (using e.g. numerical models) of long wave penetration and amplification is important in the design of new ports or port expansions. In this work, numerical simulations are presented using a wave model that accounts for the generation and propagation of bound and free long waves. The forces on the moored ship are calculated using the time-series of wave elevations and fluid motions at the location of the ship applying a strip theory approach. The ship motions are simulated using the calculated exciting wave forces, considering the non-linear characteristics of fenders and mooring lines. The numerical results are compared with prototype measurements carried out during a typhoon event in the port of Tomakomai in Japan and show good agreement.

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.

Morphodynamic Response to Wave Group Forcing

A numerical model is used to compute the time-dependent behaviour of the nearshore bathymetry under wave group forcing using the depth-averaged shallow water equations in combination with an advection-diffusion equation for the sediment concentration. After verification with prototype flume measurements, a morphodynamic computation for a pocket beach has been carried out for two different conditions. The first case consideres wave groups made up by a bichromatic wave resulting in an alongshore rhythmic double-barred beach. The second case considers wave groups made up by a directionally spread short wave field resulting in a single barred beach intersected by irregularly spaced rip-channels.

Wave setup over a fringing reef with large bottom roughness

AbstractThe effect of bottom roughness on setup dynamics was investigated using high-resolution observations across a laboratory fringing reef profile with roughness elements scaled to mimic the frictional wave dissipation of a coral reef. Results with roughness were compared with smooth bottom runs across 16 offshore wave and still water level conditions. The time-averaged and depth-integrated force balance was evaluated from observations collected at 17 locations along the flume and consisted of cross-shore pressure and radiation stress gradients whose sum was balanced by quadratic mean bottom stresses. The introduction of roughness had two primary effects. First, for runs with roughness, frictional wave dissipation occurred on the reef slope offshore of the breakpoint, reducing wave heights prior to wave breaking. Second, offshore-directed mean bottom stresses were generated by the interaction of the combined wave–current velocity field with the roughness elements. These two mechanisms acted counter to...

Identification and classification of very low frequency waves on a coral reef flat

Very-low frequency (VLF, 0.001-0.005 Hz) waves are important drivers of flooding of low-lying coral reef-islands. In particular, VLF wave resonance is known to drive large wave runup and subsequent overwash. Using a five-month dataset of water levels and waves collected along a cross-reef transect on Roi-Namur Island in the Republic of the Marshall Islands, the observed VLF motions were categorized into four different classes: (1) resonant, (2) (non-resonant) standing, (3) progressive-growing and (4) progressive-dissipative waves. Each VLF class is set by the reef flat water depth and, in the case of resonance, the incident-band offshore wave period. Using an improved method to identify VLF wave resonance, we find that VLF wave resonance caused prolonged (∼0.5 – 6.0 hr), large-amplitude water surface oscillations at the inner reef flat ranging in wave height from 0.14 to 0.83 m. It was induced by relatively long-period, grouped, incident-band waves, and occurred under both storm and non-storm conditions. Moreover, observed resonant VLF waves had non-linear, bore-like wave shapes, which likely have a larger impact on the shoreline than regular, sinusoidal waveforms. As an alternative technique to the commonly used Fast Fourier Transformation, we propose the Hilbert-Huang Transformation that is more computationally expensive but can capture the wave shape more accurately. This research demonstrates that understanding VLF waves on reef flats is important for evaluating coastal flooding hazards. This article is protected by copyright. All rights reserved.

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.

Dynamic Modelling of Rip Currents for Swimmer Safety on a Wind-Sea–Dominated Mesotidal Beach

ABSTRACT Sembiring, L.; van Dongeren, A.; Winter, G., and Roelvink, D., 2016. Dynamic modelling of rip currents for swimmer safety on a wind-sea–dominated mesotidal beach. This study applied the process-based model XBeach to predict bathymetrically controlled rip current flow occurrence, velocities and duration in the wind-sea–dominated environment of Egmond aan Zee, The Netherlands. The model is validated against data obtained during a 5-day field experiment with GPS-equipped floating drifters. Numerical model results show good agreement with data and simulate flow patterns that resemble the offshore flow through the rip channel and subsequent alongshore drift of the drifters. The average relative error of the drifter position is 0.15 and 0.19 for the cross-shore and the alongshore position, respectively. Results from the numerical experiments suggest that the rip current intensity is tidally modulated. For the period of analysis, the rips are initiated approximately 5 hours before low tide, reaching a maximum speed during low tide, and becoming inactive 3 hours after low tide. Initiation and duration of the rip current is correlated to a ratio of ∼0.55 between offshore wave height and water depth over the bar. Further experiments with the numerical model show that at Egmond, drifters that are transported offshore by the rip current are unlikely to circulate back to the shore (with only a 14% return rate). Instead, they are ejected from the surf zone and advected alongshore, dependent on the direction of the tidal current. Without wave group forcing in the model (i.e. with a stationary wave resolving approach), the models predictability decreases, which mostly affects the prediction of the spatial flow patterns. The rip current durations can still be predicted very well, whereas the rip current magnitude is underestimated. With wind stress forcing, the models predictive skill increases for the alongshore flow when the wind speed is relatively high, but this is not the case for low wind conditions.