National Network of Regional Coastal Monitoring Programmes

Bimodal Seas around the English Coastline

Introduction

A sea state is typically composed of either wind-sea, swell-sea or a combination of the two. Wind-seas are generated by local winds, while swell-seas generally have been generated in a distant storm and dispersed out of the generating area. A combination of sea and swell can produce bimodal sea conditions.

Data from Datawell directional Waverider buoys deployed by the National Network of Regional Coastal Monitoring Programmes of England, plus data from 3 industry buoys kindly provided by RWE Innogy and Wave Hub Limited, together provide an opportunity to assess the extent of bimodal seas both spatially and temporally.

Monthly percentage of occurrence of bimodal sea conditions are given in the following spreadsheet, together with summary statistics for monthly, seasonal and annual occurrence.

Download Bimodal Seas Data

The method for deriving statistics for bimodal seas and an assessment of their occurrence around the English coastline are described in:

Mason, T. & Dhoop, T. (2018) Occurrence of Bimodal Seas around the English Coastline TN02.

Results

Figure 1: Annual occurrence of bimodal seas.
Figure 2: Seasonal occurrence of bimodal seas.
Figure 3: Monthly occurrence of bimodal seas.
Figure 4: Occurrence of bimodal seas during storms (> 1 in 25 year return period).

Storm events were cross-referenced with occurrences of bimodal sea states in order to determine the proportion of storms which had a bimodal component (Figure 4). For each storm, the individual spectra were examined for the presence of bimodal sea conditions 3 hours before or after the peak of the storm (since the Beach Management Manual [Rogers et al., 2010] suggests a typical 3-hour storm peak).

Those wishing to calculate their own statistics can download raw spectral files (*.spt) directly from the website’s real-time data pages.