Quantifying the impacts of uncertainties in coastal hazard modelling

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

Sensitivity of flood hazard and damage to modelling approaches

Combination of uncertainties in water level and wave height predictions for extreme storms can result in unacceptable levels of error, rendering flood hazard assessment frameworks less useful. A 2D inundation model, LISFLOOD-FP, was used to quantify sensitivity of flooding to uncertainty in coastal hazard conditions and method used to force the coastal boundary of the model. It is shown that flood inundation is more sensitive to small changes in coastal hazard conditions due to the setup of the regional model, than the approach used to apply these conditions as boundary forcing. Once the threshold for flooding is exceeded, a few centimetres increase in combined water level and wave height increases both the inundation and consequent damage costs. Improved quantification of uncertainty in inundation assessments can aid long-term coastal flood hazard mitigation and adaptation strategies, to increase confidence in knowledge of how coastlines will respond to future changes in sea-level

Compound flooding: Dependence at sub-daily scales between extreme storm surge and fluvial flow

Estuaries are potentially exposed to compound flooding where weather-driven extreme sea levels can occur synchronously with extreme fluvial discharge to amplify the hazard. The likelihood of compound flooding is difficult to determine due to multiple interacting physical processes operating at sub-daily scales, and poor observation records within estuaries with which to determine potential future probabilistic scenarios. We hypothesize that fluvial extremes can occur within the peak of the surge in small/steep catchments because of rapid runoff times, whilst the length-scale in larger/flatter catchments will result in fluvial and marine extremes being out-of-phase. Data (15 min river flow and hourly sea level) spanning 40 years were analyzed to assesses the behaviour and timings of fluvial and sea level extremes in two contrasting estuaries: Humber and Dyfi (United Kingdom). Compound events were common in the Dyfi, a small/steep catchment on Britain’s west coast with fast fluvial response times. Almost half of the 937 skew-surge events (95th-percentile) occurred within a few hours of an extreme fluvial peak, suggesting that flood risk is sensitive to the storm timing relative to high tide—especially since flows persisted above the 95th-percentile typically for less than 12 h. Compound events were more frequent during autumn/winter than spring/summer. For the Humber, a larger/flatter catchment on the east coast with slower fluvial response times, extreme fluvial and skew-surge peaks were less frequent (half as many as the Dyfi) and compound events were less common (15% of events co-occurred). Although flows in the Humber persisted above the 95th-percentile for typically between one and 4 days, hence overlapping several high tides and possibly other surges. Analysis of 56 flooding events across both estuaries revealed: 1) flooding is more common in the Dyfi than Humber; 2) Dyfi flooding is driven by 99th-percentile flows lasting hours and co-occurring with a 95th percentile skew-surge; 3) Humber flooding was driven by 95th-percentile flows lasting days, or surge-driven—but rarely co-occurring. Our results suggest that compound flooding studies require at least hourly data (previous analyses have often used daily-means), especially for smaller systems and considering the potential intensification of rainfall patterns into the future

Quantification of the uncertainty in coastal storm hazard predictions due to wave‐current interaction and wind forcing

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

Flood hazard assessment for a hyper-tidal estuary as a function of tide-surge-morphology interaction

Astronomical high tides and meteorological storm surges present a combined flood hazard to communities and infrastructure. There is a need to incorporate the impact of tide-surge interaction and the spatial and temporal variability of the combined flood hazard in flood risk assessments, especially in hyper-tidal estuaries where the consequences of tide and storm surge concurrence can be catastrophic. Delft3D-FLOW is used to assess up-estuary variability in extreme water levels for a range of historical events of different severity within the Severn Estuary, southwest England as an example. The influence of the following on flood hazard is investigated: (i) event severity, (ii) timing of the peak of a storm surge relative to tidal high water and (iii) the temporal distribution of the storm surge component (here in termed the surge skewness). Results show when modelling a local area event severity is most important control on flood hazard. Tide-surge concurrence increases flood hazard throughout the estuary. Positive surge skewness can result in a greater variability of extreme water levels and residual surge component, the effects of which are magnified up-estuary by estuarine geometry to exacerbate flood hazard. The concepts and methodology shown here can be applied to other estuaries worldwide

Uncertainty in estuarine extreme water level predictions due to surge-tide interaction

Storm surge is often the greatest threat to life and critical infrastructures during hurricanes and violent storms. Millions of people living in low-lying coastal zones and critical infrastructure within this zone rely on accurate storm surge forecast for disaster prevention and flood hazard mitigation. However, variability in residual sea level up-estuary, defined here as observed sea level minus predicted tide, can enhance total water levels; variability in the surge thus needs to be captured accurately to reduce uncertainty in site specific hazard assessment. Delft3D-FLOW is used to investigate surge variability, and the influence of storm surge timing on barotropic tide-surge propagation in a tide-dominant estuary using the Severn Estuary, south-west England, as an example. Model results show maximum surge elevation increases exponentially up-estuary and, for a range of surge timings consistently occurs on the flood tide. In the Severn Estuary, over a distance of 40 km from the most upstream tide gauge at Oldbury, the maximum surge elevation increases by 255%. Up-estuary locations experience short duration, high magnitude surge elevations and greater variability due to shallow-water effects and channel convergence. The results show that surge predictions from forecasting systems at tide gauge locations could under-predict the magnitude and duration of surge contribution to up-estuary water levels. Due to the large tidal range and dynamic nature of hyper-tidal estuaries, local forecasting systems should consider changes in surge elevation and shape with distance up-estuary from nearby tide gauge sites to minimize uncertainties in flood hazard assessment

Flood hazard sensitivity to storm surge-high water concurrence in a hyper-tidal estuary

A web-based, geospatial decision support tool (DST) is described here as a means to assess potential flooding for low-lying coastal and estuarine areas. These areas are at risk from inundation in the future as a result of sea-level rise, coastal storms and high river flow. Flooding assessments use a 2D inundation model, LISFLOOD-FP, and utilize open source GIS. The DST enables users to explore the depth and extent of potential flooding from combinations of sea-level rise and storm surges at sites of nuclear energy assets in the Severn Estuary, South West England. LISFLOOD-FP uses a standard model setup and is forced at the boundaries using tide gauge data. Delft3D is used to assess spatial variability in extreme water levels in the Severn Estuary, with the aim of reducing uncertainty associated with the timing a storm relative to tidal high water in the boundary conditions for the DST

Increased coastal wave hazard generated by differential wind and wave direction in hyper-tidal estuaries

Wave overtopping and subsequent coastal flood hazard is strongly controlled by wind and water levels, and is especially critical in hyper-tidal estuaries where even small changes in wave heights can be catastrophic if they are concurrent with high spring tide. Wave hazard in estuaries is largely attributed to high amplitude shorter period, locally generated wind waves; while low amplitude longer period waves rarely impact low-lying coastal zones up-estuary. Here, the effect of wind and wave properties on up-estuary wave propagation and the sensitivity of significant wave height are investigated numerically along the shoreline of the Severn Estuary, southwest England, as an example. Representative values for wind speed and direction, wave height, period and direction are used to identify key combinations of factors that define the wave hazard generation. High amplitude, short period wind waves are sensitive to opposing winds, with a steepening effect that varies along the estuary shoreline, highlighting the effect of estuarine geometry on wave hazard. Low amplitude, long period wind waves respond with maximum variability in significant wave height to strong winds resulting in their propagation further up-estuary. Our results advance current understanding of the compound interaction between wind and waves, and identify critical conditions maximizing the hazard and hazard variability along the shoreline. The outcomes from this research can help to avoid economic losses from operational downtime in ports and harbors, inform sustainable coastal sea defense design and understand how wave hazard may vary under future climate due to changing storm tracks. Results can also be applied to the design of coastal infrastructure and facilitation of emergency response planning

Thresholds for estuarine compound flooding using a combined hydrodynamic-statistical modelling approach

Estuarine compound flooding can happen when extreme sea level and river discharges occur concurrently, or in close succession, inundating low-lying coastal regions. Such events are hard to predict and amplify the hazard. Recent UK storms, including Storm Desmond (2015) and Ciara (2020), have highlighted the vulnerability of mountainous Atlantic-facing catchments to the impacts of compound flooding including risk to life and short- and long-term socio-economic damages. To improve prediction and early warning of compound flooding, combined sea and river thresholds need to be established. In this study, observational data and numerical modelling were used to reconstruct the historic flood record of an estuary particularly vulnerable to compound flooding (Conwy, North Wales). The record was used to develop a method for identifying combined sea level and river discharge thresholds for flooding using idealised simulations and joint-probability analyses. The results show how flooding extent responds to increasing total water level and river discharge, with notable amplification in flood extent due to the compounding drivers in some circumstances, and sensitivity (∼7%) due to a 3h time lag between the drivers. The influence of storm surge magnitude (as a component of total water level) on the flooding extent was only important for scenarios with minor flooding. There was variability as to when and where compound flooding occurred; it was most likely under moderate sea and river conditions (e.g. 60th-70th and 30th-50th percentiles) and only in the middle-estuary zone. For such cases, joint-probability analysis is important for establishing compound flood risk behaviour. Elsewhere in the estuary, either the sea state (lower estuary) or river flow (upper estuary) dominated the hazard, and single-value probability analysis is sufficient. These methods can be applied to estuaries worldwide to identify site-specific thresholds for flooding to support emergency response and long-term coastal management plans