Wave set-up in constricted estuaries: Fid-Bau Portal

Wave set-up in constricted estuaries

Rautenbach, Christo

Abstract Coastal inundation is a well-studied topic. Surprisingly, the extent and dynamics of wave-set-up in estuaries with constricted entrances has not been thoroughly investigated. The current literature mainly deals with the open coast and river mouths and the results vary significantly. The offshore wave height and period, entrance depth and local beach slope are predominantly considered as the main variables influencing wave set-up. Within estuaries the distance from the mouth is also considered. New Zealand estuaries (called harbours locally) are varied and represent a large range of hydro-systems (Hume, 2016). Here, wave set-up in estuaries with a constricted entrance will be investigated and compared to wave set-up reported in literature, with values ranging between 2 and 14% of the offshore significant wave height (Mohd Zaki, 2021). Estuaries with a large intertidal area and constricted entrances have not been investigated in combination with waves before (de Ruiter, Mullarney et al., 2017; de Ruiter, Mullarney et al., 2019). Ohiwa estuary (harbour), situated on the North Island of New Zealand in the Bay of Plenty district, will be the focus area of the present study. This study is a scenario based, numerical investigation, aimed at understanding the quantity and extent of wave set-up in the estuary during extreme events. The validated numerical model (estuarine water levels and inundation extents) found wave set-up to be significant in these types of estuaries, adding approximately 10 and 25% to the water levels within the estuary during an extreme event's high and low tide respectively (as compared to a spring tidal range of 1.7 m). The estuary entrance geomorphic configuration was found to make little difference to the total water level observed in the estuary but did influence the storm-tidal asymmetry and low tide level (estuary drainage). The wave set-up dissipated in the estuary and mostly contributed to elevated water levels in proximity of the entrance and deep tidal channels. This was consistent with previous studies focusing on open, river mouth, estuaries. Understanding prolonged high-water levels in these types of systems has implications for coastal infrastructure, the local ecology and farming (pasture inundation tolerance durations).

Highlights • Wave set-up in estuaries with constricted entrances and large intertidal areas has not been studied. • Ohiwa estuary, New Zealand is used as a numerical case study. • Large basin dilation factor estuaries trap high storm water levels, prolonging inundation. • The largest wave set-up was observed at low tide. • Entrance geometry mainly influenced water level asymmetry.