80 Shingle on the East Coasts of New Zealand* 



APPENDIX. 



(See par. 42.) 



(1.) Result of Sir John Coode's Observations of 

 Wave-Action on Shingle. 



(a.) Extracts from Proceedings of the Institution of Civil 

 Engineers (Vol. Xll. pp. 539-541) :— 



" An examination at low- water with the wind off-shore, or 

 just along the shore, will show that, as a rule, the largest 

 shingle to be found on any particular beach, at that par- 

 ticular time, is just about the level of the previous high- 

 water, or so far above it as the wash of the previous tide 

 may have extended ; and the size decreases from this down 

 to low-water." 



" After the prevalence of heavy on-shore winds, or a 

 ' ground-swell,' the large shingle will be found to be 

 entirely scoured away from the beach." " Shingle accumu- 

 lates upon any beach with off-shore winds, whilst it is carried 

 off, or scoured away, during on-shore winds, and more especi- 

 ally by the ground-swell which follows." 



" Seven, or any less number of waves per minute, indicate 

 the destructive action, and nine, or any greater number, the 

 accumulative action ; but no very precise rule can be framed 

 upon this basis." " A more certain indication is found by 

 watching the course of the water as it falls from the crest of 

 the wave after breaking. If it falls upon the water which 

 may be returning down the slope from the wave immediately 

 preceding — as it will do when the waves follow in rapid 

 succession — this may be taken as an evidence that the 

 accumulative action is going on. If, on the other hand, the 

 water descends directly upon the pebbles — as is the case 

 when the waves break at comparatively long intervals — it 

 carries down with it a portion of shingle, and is, in fact,, a 

 case of destructive action." 



The cross-section or profile of the Chesil Bank, near the 

 west end, fallen after a gale, was very nearly a " parabolic 

 curve from a point more than 25 feet above high -water level 

 down to a little below high- water level." The slope between 

 high- water and low- water levels was, after the same gale, 

 " slightly flatter than 1 in 9 ; and precisely the same incli- 

 nation has been observed, within the same limits, after a 

 heavy ground -swell. ... After a continuance of off- 



