Waves in Shallow Water: A new approach based on high-frequency remote sensing and wave-by-wave analysis

The storms experienced in the UK during the winter of 2013/2014 highlighted the vulnerability of the coast to structural damage, flooding and coastal erosion due to extreme waves and water levels, and the economic and societal costs of such events. Predictions indicate that these events will become increasingly common due to rising sea levels and greater storminess, presenting a significant challenge for the sustainable management of the coast, approximately 30% of which is already protected by hard engineering structures on the UK mainland. As nearshore waves are a key cause of coastal flooding and erosion, the goal of this research project is to increase our understanding of their behaviour, providing the basis for improved predictions of wave processes and their effect on our engineered and natural coasts.

Sandy beaches account for approximately 75% of the World's ice-free coastlines. As ocean waves approach beaches they undergo rapid transformation as they break in shallow water, propagate through the surf zone as white-water bores and then drive wave runup in the swash zone. Shallow water waves cause forces on coastal structures, drive sediment transport, lead to overtopping of coastal defences and dunes, and cause beach and cliff erosion. Consequently, an understanding of wave transformation is of critical importance for coastal engineers. However, due to the complex nature of waves in the nearshore, theoretical descriptions are limited and numerical modelling approaches typically rely on empirical approximations to describe breaking and broken waves based mainly on laboratory results which are subject to scale effects and do not necessarily reproduce the variability of waves in nature. Shallow water wave processes are key to the numerical models of shoreline change and coastal flooding which are used by engineers to inform coastal management decisions. To improve the predictive capability of these models, high quality field data are required, but existing measurements fail to fully capture the variability and highly non-linear shape of shallow water waves due to their limited coverage across the surf zone and low spatial resolution.

To address this internationally relevant research gap, the proposed study will apply a newly developed remote sensing approach using a network of jetty-mounted Lidar at two typical beach sites to obtain measurements of rapidly evolving waves across the complete surf and swash zone at high frequency and a spatial resolution an order of magnitude higher than previously achieved. Uniquely, this capability allows individual waves to be tracked from the break point to the limit of maximum wave runup on the beach, enabling an analysis of wave characteristics on a wave-by-wave basis. The two sites have been selected to obtain a wide range of wave conditions at locations with jetty infrastructure, but the results will inform our fundamental understanding of waves and so be relevant to the majority of sandy coastlines.

The new data, combined with beach topography information and measurements of flow velocities will form a valuable field-dataset, which will be analysed to answer fundamental unresolved questions related to wave transformation in the nearshore, and improve the representations of breaking and broken waves used in predictive wave models. Additionally the new dataset will be made available to the research community following project completion. Improvements in our ability to model nearshore waves will reduce uncertainty in predictions of wave forcing on engineered structures and the natural coastal environment. This would enable better assessment of shoreline erosion and coastal hazards, providing the opportunity for more efficient coastal planning and design of coastal defence schemes which would directly or indirectly impact a range of academic, public and industry stakeholders including coastal engineers and scientists, coastal communities, insurers and coastal managers.

The risks to the natural and built environment along the UK coastline from extreme storms, and the likely exacerbation of these hazards in the light of expected rising sea levels and increased storminess represent a key economic and societal challenge. These risks were highlighted during the winter of 2013/2014 when a series of major storms led to extensive coastal flooding and damage to coastal defences.

The fundamental goal of this research project is to increase our understanding and provide the basis for improvements in the predictive modelling of nearshore waves. The most immediate beneficiaries of the project are the related academic community identified elsewhere. Looking forward, any improvements in the prediction of waves at the coast would enhance our ability to estimate design forces on coastal defence structures, predict erosion rates on beaches, cliffs and dunes and forecast coastal flooding. This would enable more sensitive, economic and sustainable coastal planning, and benefit stakeholders including the residents and local councils of coastal towns, tourism operators, insurers and harbours. Identified communities that would be impacted by the direct and indirect advances leading from this study include:

COASTAL ENGINEERING INDUSTRY
An advanced understanding of nearshore wave processes is essential for practicing coastal engineers who undertake coastal vulnerability assessments, develop coastal management plans and design coastal structures such as jetties, seawalls, groynes and breakwaters.

It is anticipated that the improved understanding of nearshore waves and parameterisations of wave transformation leading from the study will enable more accurate calculations of wave forces on coastal structures, wave heights in the surf zone, wave runup on beaches and other critical design values. In the longer term, it is expected that the capability of numerical modelling tools used by engineers to predict wave transformation will be further developed based on the benchmark dataset and verified parameterisations provided by the study. This enhanced capability would enable greater confidence in predictions of nearshore waves and their effects at the coast, leading to more efficient coastal engineering design.

FLOOD AND COASTAL EROSION RISK MANAGEMENT (FCERM)
The Environment Agency, along with relevant local authorities are responsible for flooding and coastal risk management in England. This role involves avoiding inappropriate development in areas at risk of coastal flooding or erosion, maintaining existing coastal defences, reducing the coastal flooding threat and lowering the risk of coastal erosion. To ensure optimum levels of protection, economy and sustainability, any decisions related to coastal management must be informed by the best available predictions of coastal change and most accurate estimates of engineering design conditions.

Additional organisations that play an important role or have a direct interest in flood and coastal risk management in England include Natural England, English Heritage, The Met Office, transport and utilities providers, the National Trust, Association of British Insurers, Royal National Lifeboat Association, National Voice for Coastal Communities and many more.

Note that while the discussion above focusses on coastal management in England, comparable management strategies are in place in Scotland, Wales and Northern Ireland. Furthermore, any fundamental improvements in understanding and modelling capability leading from the proposed study would benefit flood and coastal risk management internationally.