Variation of Beach Profiles in Hell’s Mouth Bay, Llyn Peninsula for the Period 1979–1981

Abstract

An investigation was made into the changes in topography in Hell’s Mouth Bay in the Llyn Peninsula in North Wales. It has a very active beach and there has been considerable erosion over the past hundred years. Beach profiles were surveyed along three lines normal to the shoreline down to low springs level at fortnightly intervals at low spring tide times from February 1979 to April 1981.

Comparison of successive profiles showed considerable changes in topography. There was usually a substantial net gain or loss along each line. There were in addition substantial changes at various points along each line from one survey to the one before, the variations having a “wavy”appearance.

From the net results, it was possible to calculate the amount of build?up or erosion along the lines from February 1979 to the time of any survey. The results showed, as might be expected, a build-up during the summer months and an erosion during the winter months.

Some wave data were taken at the site and use was also made of wave data taken near Aberporth in the southern part of Cardigan Bay. It became clear that low amplitude waves with very little breaking caused the build?up whilst the erosion was caused by large breaking waves. As shown by Longuet-Higgins (1953), low non-breaking progressive waves produce a shoreward bottom current whilst the backwash of breaking waves flows along the bottom to give a seaward current. The direction of the current would thus depend on the height of the waves and the net effect of waves over a long period of time could be found by giving weight factors to each height value from 0.5 to over 2.5 metres and then summing up the weighted heights over the period required.By adjusting the weights good agreement could be found between the sum and the oberved net differnces. The weights given varied from 0.75 for 0.5 metres to −1.5 for over 2.5 metres. There is a conflict between the large breaking waves and the low waves, which have a smaller individual effect but are much more frequent.

Longuet-Higgins’s theory can also be applied to stationary waves. Here there is a variation of bottom current from seaward to shoreward in the space of a quarter wavelength. There is also a similar variation with vertical velocity, this being maximum downward at the node position and maximum upward at the antinode position. As the solutions for the progressive and stationary case are independent,the case of a partially reflected wave can be dealt with by taking the sum of the two solutions as the wave can be considered as a mixture of a progressive wave and a stationary wave. In the case of the stationary wave and the partially reflected wave, the suspended particles would tend to fall to the bottom with the downard velocity at the node while suspension would be enhanced by the upward velocity at the anti-node.

In view of these results, it was decided to calculate from the profile concerned, the reflection coefficient and hence the combined wave amplitude for a wide range of frequencies over all stages of the tide for a 14 day interval. The result for each frequency was multiplied by the power spectral density for that frequency for the day concerned. All these values were then summed up together. The sum showed that there was a net variation from point to point which was not unlike the observed changes.