Investigating compound flooding in an estuary using hydrodynamic modelling: a case study from the Shoalhaven River, Australia

Abstract

Abstract. Many previous modelling studies have considered storm-tide and riverine flooding independently, even though joint-probability analysis highlighted significant dependence between extreme rainfall and extreme storm surges in estuarine environments. This study investigates compound flooding by quantifying horizontal and vertical differences in coastal flood risk estimates resulting from a separation of storm-tide and riverine flooding processes. We used an open-source version of the Delft3D model to simulate flood extent and inundation depth due to a storm event that occurred in June 2016 in the Shoalhaven Estuary, south-eastern Australia. Time series of observed water levels and discharge measurements are used to force model boundaries, whereas observational data such as satellite imagery, aerial photographs, tidal gauges and water level logger measurements are used to validate modelling results. The comparison of simulation results including and excluding riverine discharge demonstrated large differences in modelled flood extents and inundation depths. A flood risk assessment accounting only for storm-tide flooding would have underestimated the flood extent of the June 2016 storm event by 30 % (20.5 km 2 ). Furthermore, inundation depths would have been underestimated on average by 0.34 m and by up to 1.5 m locally. We recommend considering storm-tide and riverine flooding processes jointly in estuaries with large catchment areas, which are known to have a quick response time to extreme rainfall. In addition, comparison of different boundary set-ups at the intermittent entrance in Shoalhaven Heads indicated that a permanent opening, in order to reduce exposure to riverine flooding, would increase tidal range and exposure to both storm-tide flooding and wave action.

Impact of ENSO on dependence between extreme rainfall and storm surge

“…Coastal flood risk attributed to storm surge and rainfall (through pluvial or fluvial processes) has been reported in many regions around the world, including Asia (Lian et al 2013, Ikeuchi et al 2017, Xu et al 2018, Ghosh et al 2019, Europe (Svensson and Jones 2004, Klerk et al 2015, Van den Hurk et al 2015, Bevacqua et al 2017, Hu et al 2019, North America (Wahl et al 2015, Bass and Bedient 2018, Shao et al 2019, North Africa (Zellou and Rahali 2019) and Australia (Zheng et al 2013, Kumbier et al 2018, Wu et al 2018. There is strong evidence that the joint occurrence of extreme storm surge and heavy rainfall is related to large scale weather patterns, such as tropical cyclones, ex-tropical cyclones, frontal systems and East Coast Lows in Australia (McInnes et al 2002, Van den Hurk et al 2015, Wahl et al 2015, Kumbier et al 2018, Wu et al 2018. The weather patterns are a function of incipient climate conditions and are influenced by year-to-year climate variability from large scale climate phenomena (or climate drivers), such as the El Niño Southern Oscillation (ENSO) (Evans andBoyer-Souchet 2012, Santoso et al 2017).…”

Modelling Hydrodynamic Impacts of Sea-Level Rise on Wave-Dominated Australian Estuaries with Differing Geomorphology

“…At both sites, modelled and observed water levels correlated very well, whereas root mean square errors (rmse) were reasonably low, suggesting that the presented model setup is able to replicate inundation processes within the estuaries. This is further demonstrated by minor model underestimation of maximum current velocities (1 cm) at the Shoalhaven Estuary [17]. The reasonably good match between observed (satellite imagery and aerial photographs) and modelled inundation extents reinforces the accuracy of the modelling setup [17,18].…”

“…This is further demonstrated by minor model underestimation of maximum current velocities (1 cm) at the Shoalhaven Estuary [17]. The reasonably good match between observed (satellite imagery and aerial photographs) and modelled inundation extents reinforces the accuracy of the modelling setup [17,18]. However, the absence of suitable data to validate current velocities at Lake Illawarra limits the credibility of velocity modelling and at this site and in consequence those results should be interpreted with caution.…”

“…Therefore, observational data such as satellite imagery, aerial photographs, tidal gauges and water level logger measurements were used to compare against modelling results [17,18]. Table 1 summarises statistics derived from comparing modelled and observed water levels for several gauges shown in Figure 1.…”

“…Assessing these changes is of great importance as many people live along estuarine shorelines and likely experience changes such as more frequent inundation, so-called nuisance flooding [14], or water backwashing up storm water drains. In addition, some estuaries are likely to experience a coincidence of coastal flooding with riverine discharge, so called compound flooding [15][16][17].…”