Bimodal Summary

Before beach management at Eastoke, beach lowering in front of the seawall was leading to significant over washing, annual flooding and the requirement for several seawall repairs. The consequences of this can be seen in the images below (Figure 1 and 2). Given a well sorted beach is one of the most effective defences against wave attack and overtopping, over 500,000 cubic metres of gravel was placed at Eastoke in 1985 to restore the beach and form the primary defence. Since then, the beach has been maintained through various Beach Management Plans, with the current South Hayling Beach Management Plan (BMP) running from 2017 – 2022 (with an extension to 2024). The BMP is funded by the Environment Agency at a cost of £3.3m and reduces the risk of flooding and erosion to over 1500 properties. The funding allocated includes several studies to better understand the coastal processes along the south coast of Hayling Island, one of which considers so called ‘Bimodal Waves’ which are known to be more damaging than standalone swell or wind waves.

Background

A bimodal sea state exists where high energy swell waves generated in the Atlantic Ocean occur alongside locally generated wind waves (shorter period but higher wave height). The image below explains how the two interact (Figure 3). The result is a high energy environment that can do a lot of damage through increased overtopping of our beaches and sea defences. In recent years, the effect of bimodal seas has been more widely recognised and several research studies have been carried out to investigate the phenomenon as traditional design methods do not fully consider these conditions (e.g. Mason et al., 2009, Bradbury et al., 2007).

Figure 3: An illustration of how bimodal waves manifest © Uwe Dornbusch Figure 3: An illustration of how bimodal waves manifest © Uwe Dornbusch
Figure 3: An illustration of how bimodal waves manifest © Uwe Dornbusch

Direct observations of beach response at Hayling Island and the measurements from the Hayling Island wave buoy indicate that certain storm events, typically bimodal and extreme long period swell events, also erode the beach more rapidly than would be expected, putting homes at increased risk from flooding. Figure 4 and Figure 5 show the effect that bimodal seas can have on the beach – the crest rolls back and is flattened, increasing the potential for overtopping and flooding.

Since the record 2013-14 winter, we have experienced the highest significant wave height recorded at the Hayling wave buoy, an increase in long period swell events, as well as the highest percentage of bimodal seas recorded in a month and the stormiest season since recording began in 2003 (see SCOPAC Storm Analysis for more information). We are concerned as this shows that the hydrodynamic regime may be changing, towards a trend that is more damaging for the beach.

In response to a lack of design tools for assessing the impact of bimodal waves on shingle beaches, a model called Shingle-B was developed in 2016 by consultant HR Wallingford (Polidoro et al., 2018).

Figure 4: 3rd November 2005 – through significant crest flattening, this image illustrates how the potential for overtopping is increased Figure 4: 3rd November 2005 – through significant crest flattening, this image illustrates how the potential for overtopping is increased
Figure 4: 3/11/2005 – through significant crest flattening, image shows how the potential for overtopping is increased
Figure 5: 10th February 2020 – the beach crest has narrowed, allowing water to pool in the prom behind Figure 5: 10th February 2020 – the beach crest has narrowed, allowing water to pool in the prom behind
Figure 5: 10/2/2020 – the beach crest has narrowed, allowing water to pool in the prom behind

Development Of Bi-Modal Extremes For Hayling

To have the SHINGLE-B model available was a big step forward. Our first step was to apply the latest wave buoy records to refresh our extreme wave estimates, and to extend these to include bi-modal waves. Figure 6 shows how return periods are normally extracted from a combined water level and wave height threshold. However when wave period and % swell are added, establishing return periods becomes more complicated. Therefore, in 2020, HR Wallingford were commissioned to produce a new joint probability dataset for Eastoke that incorporated bimodality, resulting in a multivariate analysis, one of the few sites in the UK to have done this.

Figure 6: An Example Of Bivariate Joint Probability For Water Level And Wave Height Figure 6: An Example Of Bivariate Joint Probability For Water Level And Wave Height
Figure 6: An Example Of Bivariate Joint Probability For Water Level And Wave Height

The new bi-modal extremes were run in SHINGLE-B as a comparison with the older uni-modal extremes at Hayling. As experienced in the field, increased erosion was evident for bi-modal sea states when comparing the same return period for a uni-modal condition. With respect to flooding the picture is less clear, as the peak wave condition for beach erosion might not be the peak condition for wave overtopping and flooding. It is a complex problem that will be investigated further through the South Hayling BMP.

What Does This Mean?

The 1985 beach recharge and subsequent management have successfully reduced the risk of flooding at Eastoke. Recent monitoring has shown that our wave conditions are becoming more onerous, in particular when bi-modal sea conditions occur. Through our studies, we are now able to better understand the response of Eastoke to these extreme bi-modal seas and therefore improve our management of flood risk under these conditions. The research confirms increased erosion of the beach as a result of these extreme bi-modal events, which requires further testing to optimise beach performance. Alongside further testing as part of the South Hayling BMP, the Hayling Island Coastal Management Strategy will confirm the preferred future management approach for Eastoke.

References

  • Polidoro, A., Pullen, T., Eade, J., Mason, T, Blanco, B., and Wyncoll, D., 2018, Gravel beach profile response allowing sea-states, Published in the Institution of Civil Engineers
  • Bradbury, A.P., Mason, T.E. and Poate, T. (2007).  Implications of the spectral shape of wave conditions for engineering design and coastal hazard assessment – evidence from the English Channel
  • Bradbury, A. (2010). A Review of regional wave climate and implications for shoreline management – extremes, swell, bimodal conditions
  • Mason T, Bradbury A, Poate T, Newman R (2009). Nearshore wave climate of the English Channel evidence for bimodal seas. In: Proceedings of the 31st International Conference on Coastal Engineering, USA. American Society of 832 Civil Engineers. New York. 605–616.
  • SCOPAC (2020). SCOPAC Storm Analysis Study https://southerncoastalgroup-scopac.org.uk/scopac-research/scopac-storm-analysis-study/