April 25, 1901J 



NATURE 



623 



THE FORMATION OF WAVE SURFACES IN 



SAND.^ 



A TTENTION was first called to tidal sand ripples by Prof. 

 -^^ Osborne Reynolds,^ who found them submerged in chan- 

 nels between sand banks in estuaries. My observations were 



Fig. I. — Tidal sand ridges in the Dovey estuary. Photograph taken June 15. 1900. 

 Wave-lengths about 15 feet. 



on tidal sand ripples which I found above low water (Fig. i). 

 They are generally unsymmetrical in form, with the steep face 

 on the sheltered side, as are " current marks" and seolian sand 



-Fig. 2. — Current mark upon tidal sand ridges. ' Photograph taken June 15, 1900, in the Dovey estuary 



ripples. They apparently do not require for their formation any 

 co-operation between flood and ebb currents, although where 



' Abridged from a paper by Mr. Vaughan Cornish in the Scottish Geo- 

 graphical Magazine, ]z.n\i2^'cy igo\. 



' Reports of co nmittee appointed to investigate the action of waves and 

 currents on the beds and foreshores of estuaries by means of working • 

 models. — British Association Reports of meetings of 1889, 1S90 ^nd 1891. 



such co-operation occurs, the wave fronts probably become less 

 sinuous. They are to be seen, not only in tidal estuaries, but 

 also in some localities on the seashore ^ where the sands are 

 exposed to waves as well as to currents, but they face with the 

 current, not with the waves, and are thus readily distinguished 

 from the wave-formed ripple mark. The smallest tidal sand 

 tipples which I have found exposed at low 

 tide were 3 feet from crest to crest, and all 

 sizes from this to 20 feet wave-length are 

 common. On a sand bank in the Dovey 

 estuary (North Wales), opposite to the 

 town of Aberdovey, I marked out a plot 

 with stakes driven deeply into the sand, 

 and recorded by daily measurements the 

 increase, diminution and march of the sand 

 waves. At neap tides the sands were 

 nearly smooth, and as the tides increased 

 the tidal sand ripples appeared, short and 

 relatively steep. The amplitude increased 

 -Steadily, the average wave-length also in- 

 creased, apparently by elimination of some 

 of the ridges. When the highest spring 

 tide was passed the amplitude rapidly 

 diminished, the wave-length remaining 

 nearly, but not quite, constant, and the 

 mean sand level remaining practically un- 

 changed. Details of the measurements will 

 be given in the Geograthical Journal. The 

 circumstances favourable to the formation 

 and preservation of tidal sand ripples above 

 low-water mark are, gentle current at first 

 of the flood and last ot the ebb, and strong 

 current when the water is deep over the 

 sands. I have otten watched the course of 

 events when the last of the ebb has been 

 running over the ridges. The process is 

 then one of decay, not of growth, the sand 

 being swept from the crests into the 

 troughs. What goes on during the growth 

 of the ridges ? Let the depth of water be sufficient ; then, if the 

 velocityof the current be small, the sand grains roll and slide 

 along the bottom, but, as the speed increases, the water, almost 

 suddenly, becomes highly charged with 

 sand in "eddying suspension." A uniforna 

 current flowing swiftly over extensive 

 sands picks up as much sand as it drops, 

 thus causing a drift of sand which on the 

 whole is uniform, neither raising nor lower- 

 ing the bed.*^ My observations indicate, 

 however, that in detail the drift is not 

 uniform, but attended by alternate silt and 

 scour along lines at right angles to the 

 current and equidistant from one another, 

 the surface of the sand bank being thus 

 carved into transverse ridge and furrow 

 without change of mean level. The 

 slightest convexity of surface causes a con- 

 vergence of currents, a concavity a diver- 

 gence, and, under the conditions specified, 

 deposition occurs upon the convexities, 

 whilst the concavities are scoured. The 

 vertical inequalities are thus increased,* 

 and it is easy to see that the ridges will 

 extend themselves laterally.* Thus the 

 ridge and furrow inevitably form and grow. 

 What, then, are the limits of this growth ? 

 Obviously the depth of water is one limit. 

 When the cross section of the stream above 

 the sand ridge is reduced by a certain 

 -amount, the greater force of water there 



I And on the Goodwin Sands, where I found 



them, May 12, igoo. [Since the paper was writteo 



I have seen the same structures in a non-tidal part 



of the Eraser River, British Columbia,] 



'^ British Association Report, Manchester meeting (1887). Osborne 



Reynolds on " Certain Laws relating to the Regime of Rivers and Estuaries 



and on the Possibility of Experiments on a small scale." 



3 Compare G. H. Darwin's observation of the sand creeping up both sides 

 of a ridge when a current was caused to flow over it. — Proc. Roy. Sec, 

 vol. xxxvi. 1883 ; "On Ripple Mark. " _ 



4 Compare James Thompson on the "Winding 01 Rivers m AUuvoI 

 Plains." — Proc. Roy. Soc, 1876 and 1877. 



NO. 1643 VOL. 63] 



