Beaches being denuded by destructive waves crashing on shore
Published 23/01/2016 | 00:00
'I've never seen the like of it before' said the man I got chatting to on the strand as we both viewed the impact of the recent storms.
There was a cliffed shelf more than a metre high on the upper beach marking the landward limit of the huge bite the sea had taken out of the strand. Considering the length of the beach, several tens of thousands of tonnes of valuable sand had been spirited away.
The lower beach was completely denuded of sand and consisted of exposed marl and a patch of bog with several branches exposed.
Waves breaking on the shore can be divided into two basic kinds: destructive waves and constructive ones.
Destructive waves destroy the beach by combing sand and other sediments off it. They are caused by wind and tend to be dominant in winter when atmospheric pressure is low and the weather is stormy. The waves themselves are generally tall, have a short wavelength, a steep profile and they break on the shore in plunging manner.
If you want to be sure you are dealing with destructive waves you need to measure their frequency by timing how many waves appear to pass a fixed point, or break on the shore, per minute. To be more accurate it is best to repeat the count a number of times and take an average. Destructive waves tend to have a frequency in the range 11-15 per minute.
Constructive waves, on the other hand, construct or build up the beach by bringing in sand and pushing it up the shore. They are caused by swells rather than by wind and they tend to dominate in summer when atmospheric pressure is high and the weather is calm. The waves themselves are generally low in height, have a long wavelength, a gentle profile and they break on the shore by spilling rather than by plunging. And they have a frequency in the range six to nine waves per minute.
When a wave breaks on the shore the water runs up the beach, stops and then retreats. The upward run of water is the swash and the reverse backward run is the backwash. The reason constructive waves build up the beach is that their swashes are stronger than their backwashes; destructive waves are the opposite.
In months to come, it remains to be seen if constructive waves will undo the damage wreaked by the recent winter storms.