TrUtsacTioNs op section c. 733 



6. On Tidal Sand Hippies above Low-water MarJc. 

 By Vaughan Cornish, M.Sc, F.C.S., F.R.G.S. 



In the tliird Report of the Committee appohited to investigate the Action of 

 Waves and Currents on the Beds and Foreshores of Estuaries by means of Working 

 Models (Cardift'meeting, 1891), Professor Osborne Reynolds writes: — 'The large tidal 

 sand ripples below low water in the model estuaries, with the flood and ebb taking 

 the same course, constitute a feature which it is impossible to overlook, yet the 

 existence of corresponding ripples had been entirely overlooked in actual estuaries 

 until they were found to exist when they were looked for, having been first seen 

 in the models. The reason that they were overlooked before is, no doubt, ex- 

 plained by the fact that the bottom is not visible below low-water mark in actual 

 estuaries ; but this is not all. In the estuaries these ripples, where found, have been 

 confined to the bottoms and sides of the nari-ow channels between high sand-banks, 

 and they do not occur on the level sands below low water towards the mouths of 

 estuaries to anything like the same extent as in the models. ' 



In December 1899 the author observed that the extensive sand-banks which 

 are exposed at every tide in the Mawddach Estuary, opposite Barmouth, North 

 W^ales, were covered, even in their hiufhest parts, with remarkably regular series 

 of sand ripples, averaging about 16 feet from ridge to ridge. The identity of 

 origin of these sand ripples with the tidal sand ripples of Osborne Reynolds was 

 soon established, and it appeared that they afforded an opportunity for more de- 

 tailed study than is practicable in the case of structures beneath low-water mark. 

 The author therefore made observations, with measurements and photographs, 

 during the six mi mths January-June 1900, of the sands at Barmouth (N. Wales), 

 Grange (Lancashire), Findhorn (N.B.), Montrose (N.B.), Mundesley (Norfolk), the 

 Goodwins, Pegwell Bay (Kent), on the Severn between Newnham and Severn 

 Tunnel, and at Aberdovey (N. Wales). 



It appears that between certain limits of speed and depth the steady action 

 of a current can produce these ripples of regular wave-lenffth without the 

 agency of periodic quickenings and checkings such as operate in the formation of 

 the ordinary ripple-mark of the strand. The author has observed in the shallow 

 streams of sandy foreshores that a train of sand ridges of regular wave-length is 

 produced almost instantaneously when the velocity of the stream becomes 

 sufficient to render the water decidedly turbid with flying sand. On the other 

 hand, a current flowing over tidal sand ripples with clmr water can be seen 

 to lower them. It appears probable that their formation commences at that 

 critical velocity at which a great part of the moving sand is thrown into ' eddying 

 suspension,' and no longer merely rolls or slides along the bottom ; and that they 

 can be produced independently of co-operation between flood and ebb, although 

 where flood and ebb pursue the same course but in opposite directions the 

 ridges may become more regular. The mechanism of their formation seems to 

 be as follows : when a current, flowing over extensive sands, attains the velocity 

 at which the sand is largely thrown into eddying suspension, then a state is soon 

 reached in which the amount of sand dropped by the current is on thp ivhole equal 

 to the amount picked up by it, but any small excrescence causes a convergence of 

 current in the lowest layers of the water (' forced towards the centre of the curve ' 

 as at the bends of rivers). Under the specified conditions as to the charge of 

 sand held in suspension, the excrescence increases. In like manner any slight 

 depression is deepened. ' Scour ' in the troughs and ' fill ' on the ridges proceeds ; 

 and this goes on until the concentration of the stream over the ridge, and its 

 expansion above the trough, balance the effect. A considerable degree of 

 regularity in the size and form of the ridges is soon attained, and the dimen.sions 

 are limited by the depth and by the speed of the current. 



Since the amplitude of the sand ridge is limited by the depth of the water, it is 

 evident that a sand-bank in a tideway cannot dry in ridges if the final runnings of 

 shallow water be prolonged or violent. Similarly the rising tide produces the greater 

 effect in ridging when the first part of the flood is gentle. Thus the Dun Sands, 

 below the junction of the Wye and Severn, being protected from rapid shallow 



