Coastal flooding: impacts of coupled wave–surge–tide models

Abstract

Wind waves and elevated water levels together can cause flooding in low-lying coastal areas, where the water level may be a combination of mean sea level, tides and surges generated by storm events. In areas with a wide continental shelf a travelling external surge may combine with the locally generated surge and waves and there can be significant interaction between the propagation of the tide and surge. Wave height at the coast is controlled largely by water depth so the effect of tides and surges on waves must be also be considered, while waves contribute to the total water level by means of wave setup through radiation stress. These processes are well understood and accurately predicted by models, assuming good bathymetry and wind forcing is available. Other interactions between surges and waves include the processes of surface wind-stress and bottom friction as well as depth and current refraction of waves by surge water levels and currents, and some of the details of these processes are still not well understood. The recent coastal flooding in Myanmar (May 2008) in the Irrawaddy River Delta is an example of the severity of such events, with a surge of over 3m exacerbated by heavy precipitation. Here we review the existing capability for combined modelling of tides, surges and waves, their interactions and the development of coupled models.Keywords: tides, storm surges, wind waves, coastal flooding, wave-current interaction, numerical modelling 1 Coastal Flooding -Impacts of coupled wave-surge-tide models Judith Wolf AbstractWind waves and elevated water levels together can cause flooding in low-lying coastal areas, where the water level may be a combination of mean sea level, tides and surges with storm surges and waves often being generated by the same storm event. In areas with a wide continental shelf a travelling external surge may combine with the locally generated surge and waves and there can be significant interaction between the propagation of the tide and surge. Wave height at the coast is controlled largely by water depth so the effect of tides and surges on waves must be also be considered, while waves contribute to the total water level by means of wave setup through radiation stress. These processes are well understood and accurately predicted by models, assuming good bathymetry and wind forcing is available. Other interactions between surges and waves include the processes of surface wind-stress and bottom friction as well as depth and current refraction of waves by surge water levels and currents, and some of the details of these processes are still not well understood. The recent coastal flooding in Myanmar (May 2008) in the Irrawaddy River Delta is an example of the severity of such events, with a surge of over 3m exacerbated by heavy precipitation. Here we review the existing capability for combined modelling of tides, surges and waves, their interactions and the development of coupled models.

Impact of Cyclone Track Features and Tidal Phase Shift upon Surge Characteristics in the Bay of Bengal along the Bangladesh Coast

“…On the other hand, at locations away from landfall, especially outside the RMW, surge waves tend to propagate as a long wave and maximum surge tends to accumulate towards the high water areas, which is referred to as "surge clustering". Such phenomena was also observed by Horsburgh and Wilson [19] and Wolf [49] in the case of North Sea and later by Hussain and Tajima [10] at Moheshkhali for Cyclone 1991 at Sitakunda and Chittagong for Cyclone Sidr of 2007. In the present paper, the combined effect of cyclone translation speed and tidal phase upon surge arrival time at Sitakunda, which is away from the landfall location and also outside the RMW, has been investigated in case of Cyclone Sidr.…”

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

“…However, the true storm-tide time series is likely to vary spatially (along a section of coastline) and temporally (between subsequent events), even for a given event magnitude, due to variability in tide and surge characteristics, and the complexity of their interactions [Horsburgh and Wilson, 2007;Wolf, 2009;Brown et al, 2011;Lewis et al, 2013]. No two storm-tide events are the same, because different combinations of tide and surge (superimposed on different mean sea levels) combine to give the total water levels observed during events.…”

Flood Hazard Assessment for a Hyper-Tidal Estuary as a Function of Tide-Surge-Morphology Interaction

“…This is particularly evident in the upper estuary; when the peak of the surge occurs 2-6 h after high water, there is a greater percentage change in maximum water level. Increased water depth at the time of tidal high water could induce surge propagation through the estuary (Wolf 2009) to increase extreme water levels up-estuary. Increased water depths would limit shallow water and quadratic friction effects on the tidal amplitude.…”

“…However, sources of coastal flood hazard are not just limited to the contributions of astronomical tide and storm surges to water level, but also wave run-up and overwashing or overtopping, driven by coincidental sea state (Prime et al 2016). Locally-generated wind waves and propagating swell waves, generated by an offshore storm, which coincide with an extreme water level can increase flood hazard at the coast (Wolf 2009;Pye and Blott 2010). Maximum wave height at time of tidal high water can be significant for coastal flooding (Fairley et al 2014), and runup associated with direct wind setup or breaking offshore waves can influence defence overtopping and breaching (Cheng et al 2015).…”