Matt J. Lewis

Quantifying the Uncertainty in Future Coastal Flood Risk Estimates for the U.K.

Abstract Future sea-level rise will increase coastal flood risk in the U.K., yet the hazard uncertainties associated with such future risk estimates have not been fully explored. The sensitivity of coastal flood-risk mapping to future uncertainties was investigated by propagating ranges of plausible parameters through a LISFLOOD inundation model of a significant historic flood event to the North Somerset (U.K.) coast. Mean sea-level rise (including land movement) was found to have the greatest effect on the extent of flood inundation. Analysis of the latest research into the future storm-surge climate of the U.K. indicates no change above natural variability, thus, future, extreme water-level estimates (for the U.K.) should be based on observations and not Regional Circulation Models until research indicates otherwise. Evidence suggests that the current approach of forcing the inundation model with an extreme water level of a constant return period is incorrect. This uncertainty of the peak storm tide height along the coastline had a significant effect on our results. We present a new boundary-forcing technique to force the inundation model with (method C), based on the spatial characteristics of real events, which can account for the natural storm-surge variability. Indeed, if sea-level rise is included with method C, a great deal of the uncertainty surrounding such a future flood-hazard estimate can be quantified and communicated clearly and effectively.

Assessing the temporal variability in extreme storm‐tide time series for coastal flood risk assessment

The probability of extreme storm-tide events has been extensively studied; however, the variability within the duration of such events and implications to flood risk is less well understood. This research quantifies such variability during extreme storm-tide events (the combined elevation of the tide, surge, and their interactions) at 44 national tide gauges around the UK. Extreme storm-tide events were sampled from water level measurements taken every 15 min between 1993 and 2012. At each site, the variability in elevation at each time step, relative to a given event peak, was quantified. The magnitude of this time series variability was influenced both by gauge location (and hence the tidal and nontidal residual characteristics) and the time relative to high water. The potential influence of this variability on coastal inundation was assessed across all UK gauge sites, followed by a detailed case study of Portsmouth. A two-dimensional hydrodynamic model of the Portsmouth region was used to demonstrate that given a current 1 in 200 year storm-tide event, the predicted number of buildings inundated differed by more than 30% when contrasting simulations forced with the upper and lower bounds of the observed time series variability. The results indicate that variability in the time series of the storm-tide event can have considerable influence upon overflow volumes, hence with implications for coastal flood risk assessments. Therefore, further evaluating and representing this uncertainty in future flood risk assessments is vital, while the envelopes of variability defined in this research provides a valuable tool for coastal flood modelers.

Predicting suitability of cockle Austrovenus stutchburyi restoration sites using hydrodynamic models of larval dispersal

Abstract An important aspect of shellfish restoration projects is to evaluate whether potential sites are likely to be recolonised naturally once disturbances are removed and habitats are restored. Similarly, if active translocation or reseeding of juveniles or adults is undertaken, it is important to understand whether these introduced populations will be self‐maintaining in future, and whether any seed they produce are likely to be retained within the restored area. We used a combined hydrodynamic and particle tracking model to predict larval dispersion patterns for the common cockle Austrovenus stutchburyi under different hydrodynamic scenarios for seven release locations in Whangarei Harbour, New Zealand. Our results implied that sites varied substantially in their potential for self‐seeding and for exporting seed to other locations. For sites with more restricted dispersal, the model predicted that most larvae originating at these sites would settle inside the release region (68–94% for passive particle simulations), whereas relatively few larvae originating from the other release sites settled at these sites. In contrast, model larvae released from sites exhibiting high connectivity dispersed to all sub‐regions in the harbour, and export outside of the model region was high. Forthcoming field validation of these model predictions will result in better integration of hydrodynamic connectivity in whole estuary restoration programs.

Investigating River–Surge Interaction in Idealised Estuaries

ABSTRACT Maskell, J.; Horsburgh, K.; Lewis, M., and Bates, P., 2014. Investigating river–surge interaction in idealised estuaries. A finite volume model (FVCOM) was used to investigate the combined influence of storm surge and river flow on floodplain inundation on the basis of idealized estuary test cases. The combined influence of storm surge and river discharge typical of extremes in estuary systems in Britain (up to 2 m and 1500 m3 s−1) was found to induce interactions that lead to increases in the nontidal residual elevation of up to 0.35 m. However, the extent of the inundation was found to be mainly controlled by the surge elevation. Exceeding the threshold of the up-estuary channel capacity was found to cause a nonlinear increase in the area of the nontidal inundation for any given peak river discharge, after which the rate of increase in inundation area as the surge height increases declines and is determined by the slope of the floodplain. This threshold is determined by the surge elevation with exception of the highest peak river discharges, where the surge elevation threshold is lowered. It was also found that the extent of the interactions and inundation were highly dependent on the geometry of the estuary and the timing of the surge with respect to peak river discharge, in particular the slope of the floodplain and at such times where the river discharge was similar in magnitude to that of the surge and the tide. After calibration an idealized estuary based in the LISFLOOD-FP code, using a simplified form of the two-dimensional (2D) shallow-water equations, was found to simulate the area of maximum inundation to a similar extent as the FVCOM model (based on the full 2D shallow-water equations) with a much reduced computation time. This paper highlights the potential advantages that simplified 2D inundation models may have for simulating estuarine flooding due to combined surge and river discharge, where surge–river interaction due to momentum exchange is insignificant in determining the flood extent and simplified equations capture the dominant hydrological drivers of coastal inundation.

Wave-Tide Interactions in Ocean Renewable Energy

Some regions of the world concurrently experience a high wave and a high tidal energy resource. These regions include the seas of the northwest European continental shelf, the Gulf of Alaska, New Zealand, northwest Australia, and the Atlantic seaboard of Argentina. Due to the interaction of waves and tides, special consideration needs to be given to resource characterization of marine renewable energy schemes developed in such regions. Waves have been shown to reduce the tidal current, which, because tidal-stream power is proportional to the cube of velocity, reduce the available energy resource. Further, waves can reduce the tidal-stream energy resource during extreme wave periods when ocean renewable devices may not operate. Waves should be also considered in the design and resilience of tidal-stream energy devices. Hence, waves can have a critical effect on the planning, operation, maintenance, and resource assessment of tidal energy sites. Conversely, tides can significantly alter wave properties through various wave-current interaction mechanisms. For example, tidal currents can alter wave steepness which is an important consideration in the design of marine energy mooring. Wave power, in general, is proportional to the wave group velocity and the wave height squared, both of which change in presence of tidal currents. Therefore, resource assessments of such regions should account for the way that one marine energy resource affects another at a variety of timescales from semidiurnal, spring-neap, to seasonal. Finally, wave-current interaction processes affect turbulence, and the dynamics of sediment transport; therefore, they should be considered when the impact of an energy device, or an array of such devices, on the environment is studied. This chapter introduces the basic concepts of wave-tide interaction in relation to the ocean renewable energy resource assessment. Various aspects of the marine renewable energy industry that are affected by wave-tide interactions , such as resource assessment and the influence of wave-tide interactions when characterizing the oceanographic site conditions, are discussed. Methods ranging from simplified analytical techniques to complex fully coupled wave-tide models are explained.