Shingle on the East Coasts of New Zealand. 75 



the shore or keeping it in the direction necessitated by the 

 slope of the bed of the sea. The shingle is at length forced 

 over into the water, and forms a bar where the waves break ; 

 over this the current from the river flows. 



53. The shingle being forced into the water would fall 

 to the bottom as soon as the forces acting on it failed to 

 keep it in suspension. Here it would accumulate till it 

 raised the bed or formed a bar across the channel just 

 sufficient for the current to pass over it without disturbing 

 it. Any shingle in excess of this w r ashed into the bed 

 would, with the current on the one side and the waves on 

 the other, be forced across the channel, when it would be 

 thrown up again on to the beach. It is impossible here to 

 analyse the action going on ; it must suffice to state the fact. 



54. The depth at which this action would take place 

 would vary according to circumstances. Thus, before the 

 channel leading to the inner harbour at Napier was con- 

 tracted by the present moles, the bar was at times not more 

 than 4 or 5 feet at low water. It will obviously be as 

 little as the current will permit. The important point 

 to ascertain is, what is the greatest depth at which the 

 action will go on ? — in other words, what depth can be 

 obtained and maintained over a bar where there is travelling 

 shingle ? Reverting to the case of the Napier channel, all 

 attempts to keep it clear have, except when the shingle 

 was trapped for a time behind the east mole, failed to 

 secure for any length of time a greater depth than 8 

 or 9 feet at low water. During westerly weather, in the 

 summer, a foot or two more may be obtained ; but the depth 

 is reduced by heavy easterly weather. There is a very 

 strong current through the channel — six to seven knots an 

 hour in mid-stream — and yet the above-mentioned depth 

 only is obtainable. A slightly weaker current might not 

 make much difference ; but if the current could be reduced 

 considerably in strength, the depth of water over the bar 

 would certainly decrease. A stronger current would probably 

 cause the bar to move further from the mouth without 

 improving the depth in any way; or it might increase the 

 depth, and at the same time form a long spit in a direction 

 between that of the beach and that of the stream, diverting 

 the latter and causing the channel to curve round. 



55. Mr. C. H. Weber, late engineer to the Napier Harbour 

 Board, who very carefully observed the action going on in 

 Hawke's Bay during many years, prepared a memorandum 



