Quantification of the Uncertainty in Coastal Storm Hazard Predictions Due to Wave‐Current Interaction and Wind Forcing

Abstract

Coastal flood warning and design of coastal protection schemes rely on accurate estimations of water level and waves during hurricanes and violent storms. These estimations frequently use numerical models, which, for computational reasons, neglect the interaction between the hydrodynamic and wave fields. Here, we show that neglecting such interactions, or local effects of atmospheric forcing, causes large uncertainties, which could have financial and operational consequences because flood warnings are potentially missed or protection schemes underdesigned. Using the Severn Estuary, SW England, we show that exclusion of locally generated winds underestimates high water significant wave height by up to 90.1%, high water level by 1.5%, and hazard proxy (water level + 1/2 significant wave height) by 9.1%. The uncertainty in water level and waves is quantified using a system to model tide-surge-wave conditions, Delft3D-FLOW-WAVE in a series of eight model simulations for four historic storm events.Plain Language Summary Coastal zones worldwide are subject to combined effects of astronomical tides, meteorological storms surges, waves, and wind during storms and hurricanes, which can lead to flooding, property damage, and casualties. Coastal communities and critical infrastructure rely on accurate water level and wave forecasts to mitigate these combined hazards. Forecasts utilize hydrodynamic numerical models, which need to accurately represent these hazards and how they interact with, and feedback to, each other. This study uses a model, Delft3D-FLOW-WAVE, to calculate how tides and waves from four historic storm events combine to contribute to water level, wave height, and hazard proxy (water level + 1/2 wave height) in the Severn Estuary, southwest England. Additional simulations are run to show how local winds can further contribute to the hazard. Results show that including locally generating winds in simulations of water level, wave height, and hazard proxy is most important for accurate representation of physical processes that contribute to coastal hazards. Excluding locally generated winds from numerical model predictions could mean that flood alerts, warnings, and evacuation orders are missed, or coastal protection schemes are underdesigned, potentially leading to more flooding.

Sensitivity of Flood Hazard and Damage to Modelling Approaches

“…The most extreme event on record, 3 January 2014, generated a 0.48 m skew surge on a 10.01 m tide and had a return period of 20 years [39,40]. Large local fetch at high tide means southwest-west wind direction can generate and propagate locally generated waves up-estuary to Oldbury [26]. Historically, Oldbury and the low-lying area of Oldbury Naite has been susceptible to coastal flooding due to the combined effect of tide-surge-wind-wave-river hazards [41].…”

“…The Delft3D modeling system has been previously validated and successfully used to simulate coastal hazards in the Severn Estuary for two historic storm events; (i) 3 January 2014 (hereafter called January 14) which represents the maximum coastal hazard condition; and (ii) 16 December 2012 (hereafter called December 12) which represents the 90th percentile coastal hazard condition, represented by WL + 1 2 H s [26]. These storm events coincide with winds from a southwest-west direction at the time of high water, as wave hazard is amplified up-estuary with winds from this direction [26]. Flood warnings were issued along large stretches of the Oldbury coastline for the 3 January 2014 storm event, and flooding was reported 10 km further up-estuary locations in the Severn Estuary at Minsterworth as defences were overwashed [39].…”

“…All simulations were forced with river gauge data from the Environment Agency Sandhurst river gauge near Gloucester at the eastern open boundary. Run 8 represents a complete, 5-parameter multi-hazard simulation and outputs were graphically and statistically validated to tide gauges and wave buoys in the estuary [5,26]. The results of the Delft3D runs were used to force the boundary of the LISFLOOD-FP Oldbury model domains to represent coastal hazard uncertainty.…”

“…Inaccurate representation of the complex interactions between atmospheric, meteorological, fluvial and tidal processes in estuaries can propagate through the modelling chain and result in large uncertainties in the hydrodynamic forcing of process-based and shoreline response models [29], and have been shown to cause uncertainties in subsequent model outputs. The exclusion of locally generated winds underestimated HWH s by up to 90.1% in the upper regions of the Severn Estuary, southwest England, during simulations of the 3 January 2014 storm event [26]. Excluding the contribution of riverine discharge to total water level during simulations of a storm-tide, using hydrodynamic modeling package Delft3D, underestimates flood extent by 30% (20.5 km 2 ) in the Shoalhaven Estuary, south-eastern Australia [30].…”

“…Outputs from numerical modelling tools can be used to design crest levels of flood defence structures [17], or force the model boundary of process-based and shoreline response models to predict the pathway, maximum velocities and extent of floodwater, and timing of peak discharge, [18][19][20], wave overtopping [21], morphological change [22,23] and hazard rating [24] arising from the combined effect of these hazards. However, uncertainty can arise in modelled HWL and HWH s predictions due to bathymetric and topographic resolution [25] model coupling processes, local forcing processes and coastal geometry [26]. Therefore an additional key component of flood hazard assessment is to identify sources of uncertainty in HWL and HWH s predictions, and the main objective of this study is to quantify how these uncertainties can propagate through the modelling chain from regional models to generate uncertainty in the impacts of flooding events simulated by process-based and shoreline response models.…”