45 



SEA WAVES AND THE DESTRUCTION OF 

 CRAB POTS. 



By II. MORRIS AIREY, M.Sc. 



The crab fishing industry on the Northumberland coast 

 attracts a sufficient number of fishermen to make the conditions 

 under which their work is carried on a matter of general 

 interest. Not the least interesting amongst the many problems 

 which their calling gives rise to, is the question of the frequent 

 damage to their gear by the North Sea storms. While it is quite 

 obvious that the heavy seas are responsible for the wholesale 

 destruction that sometimes occurs, it is by no means clear 

 exactly how the mischief is done. It seems therefore desirable 

 to draw attention to some of the factors which probably are of 

 importance. 



The destroying powers of breakers on a beach are of course 

 evident enough, but the destruction of crab pots rarely takes 

 place through this cause. The damage is mostly done in deep 

 water, often in a depth of 20 fathoms or more. As will be 

 pointed out later, waA^es may become breakers even in such deep 

 water but in this case it is no longer obvious that their violence 

 extends sufficiently far down to cause damage. The motion of 

 the under water when a wave breaks in deep water has not been 

 investigated theoretically and little experimental data are avail- 

 able. It is certain however from the experiences of divers that a 

 long wave breaking in 5 or 6 fathoms causes a do-^vnward rush 

 of water extending to the sea floor, which is violent enough to 

 cause considerable damage to structures mucii less fragile than a 

 crab pot. There is reason for thinking that this downward 

 current extends to the bottom in still deeper water, but only in 

 exceptional cases would this compare with the under water 

 motion which accompanies all unbroken waves. It is the latter 

 motion which will be considered in the following. 



