title: Quantifying the impacts of uncertainties in coastal hazard modelling
Creators: name: Lyddon, Charlotte
Publisher: name: University of Liverpool Repository
Year: 2020

description: This thesis applies coupled regional models to address coastal flood risk management needs in hyper-tidal estuaries. The project aims to understand how tide-surge-wind-waves combine to increase flood and wave hazard at the coast, using the Severn Estuary, southwest England as an extreme example. Little previous research has considered the impact of tide-surge-wind-wave interaction on total water level in a hyper-tidal estuary. Numerical modelling tools can be used to predict the individual contributions of physical factors to total water levels and forms a key component of flood hazard assessment. However uncertainty can be introduced into model predictions due to inaccurate boundary forcing or representation of the physical processes which control the volume and rate water moves through a model domain. Uncertainties in model predictions lead to a wide spread of results within which exposure or impacts could occur. Similarly, a range of possible values exist for a single parameter which may cause errors in the definition of critical thresholds or presents challenges to emergency response planners. Sources of uncertainty in flood hazard assessments should be identified and quantified as sustainable coastal management requires confidence in the knowledge of any possible future changes to flood and wave hazard. The thesis utilises wave, ocean and meteorological observation and model hindcast data to simulate total water level and significant wave height using the Delft3D-FLOW-WAVE modelling package. The validated Severn Estuary model domain is used to investigate the sensitivity of extreme water levels to changes in event severity, timing of the peak of a storm surge relative to tidal high water and the temporal distribution of the storm surge component, and wave heights to changes in wind-wave direction, model coupling and forcing processes. Model outputs from Delft3D-FLOW-WAVE are viewed in the context of the source-pathway-receptor-consequence model to better understand the influence of coastal hazard uncertainty on flood and wave hazard. Event severity is the most important control on flood hazard, and concurrence of the sources of flood hazard generate greatest water levels along the coastline of the estuary. Estuarine morphology acts as a pathway for flood hazard, as funnelling effects control the spatial variability of flood hazard and amplify surge magnitude up to 255% up-estuary. Surge predictions from forecasting systems at tide gauge locations could under-predict the magnitude and duration of surge contribution to up-estuary water levels. Wave height and wave period controls the response of wave generation and propagation to other factors. Wind speed generates greatest wave hazard, and uncertainty in wind and wave direction generate a large spread of results. Stronger, opposing winds steepen high amplitude, low period waves in the outer estuary and stronger, following winds enhance propagation of low amplitude high period waves up-estuary. The inclusion of locally generated winds is most important in regional models to continue to add momentum to the estuarine system, and model coupling processes (the representation of interaction between wave and currents) improve accuracy of flood and wave hazard predictions. Exclusion of locally generated winds can generate up to 1.45 m error in high water significant wave heights in the outer estuary, and 1.13 m error in the upper estuary. Coastal hazard uncertainty due to model coupling and forcing processes is propagated through the modelling chain to the two-dimensional inundation model LISFLOOD-FP to understand how changes in boundary condition and boundary position influences depth, extent and volume of inundation over a storm event. The exclusion of local atmospheric forcing increases coastal hazard uncertainty in the boundary forcing and under-predicts damage by up to £26.2 M at Oldbury-on-Severn. Once the threshold for flooding is exceeded, a few centimetres increase in coastal hazard conditions increases both the inundation and consequent damage costs for suburbia and arable land. The results of this thesis identify optimum model setups for simulating coastal flood hazard, which includes incorporating local atmospheric forcing and representing two-way interaction between waves and currents. Coastal hazard uncertainty can cause large variability in simulated total water level and wave heights, which has implications for flood damage assessments, shoreline management plans and emergency response plans. The research findings can aid long-term coastal defence and management strategies for improved public safety, and improve the timing and accuracy of early warning systems. Key sources of coastal hazard uncertainty have been identified here, e.g. the importance of storm surge timing relative to tidal high water and sensitivity of wave propagation to winds speeds, and these can be accounted for in future management plans. Utilising optimal model setups when predicting water level and wave height under current and future climate conditions can also help to increase confidence in results. Further to this, if the key sources of uncertainty which contribute to a large spread of results are known, e.g. exclusion of local atmospheric forcing, then this can be resolved in predictions which are used to inform early warning systems. The spread of model results can therefore be minimised to more accurately know who or what is in a flood or wave hazard zone.