Visualising uncertainties in coastal flooding

Low-lying coastal and estuarine regions can be susceptible to flooding. Modelling coastal flooding is complex and is often the result of a combination of physical phenomena that need to be simulated to capture the extent of the inland inundation. Three case studies are considered in the UK, where uncertainty and drivers of coastal flood risk are explored through modelling and visualisations. At New Brighton, a town on the coast of the Eastern Irish Sea, an investigation of flood risk due to the combination of high-water levels and waves was carried out. A long-term Monte-Carlo simulation was used to simulate correlated samples of combined water levels and wave heights. Coastal flood simulations were then carried out for 8 samples of combined water levels and wave heights with a joint probability return period of 100 years. An offshore model system (TELEMAC-2D) was used in combination with a high-resolution nearshore modelling system (TELEMAC-2D coupled with TOMAWAC) to couple the tide and surge with the waves and simulate water levels and wave heights right up to the defence line. At the defence line overflow and wave overtopping rates were used as boundary conditions to an inundation model. The extent of coastal flooding in New Brighton varies significantly for wave-surge events with a joint probability of 100 years. Waves are an important flooding mechanism but are dependent on high water levels reducing the effective freeboard of the coastal defence. In a second case study uncertainty in coastal flooding was visualised at Hornsea due to the range of uncertainty in the 100-year return period coastal water elevation and the overtopping due to 3 m waves at the defences. In addition to the uncertainty, the wave overtopping is dependent on the water level that determines the freeboard at the defences. Considering the range of uncertainty at this location decreases or increases the simulated flood extent by 58% and 82% respectively. On December 5th-6th 2013, Cyclone Xaver generated storm surge levels along the coastal regions of the southern North Sea that were the highest on record at some tide gauge locations on the UK East Coast. Close to Boston dike failure led to the inundation of a recycling plant, a number of businesses and the surrounding crop fields. TELEMAC-2D was used to simulate the dike breach as it allows you to simulate dike breaching for a range of scenarios and growth mechanisms. Visualisations of the flooding due to the breach were created. Including dike breaching in coastal flood risk simulations is an important mechanism in determining the range of potential flood extents.