Proeven over den invloed van golven op een strand. In verband met enkele waarnemingen aan de Nederlandsche kust

In order to throw more light on the formation of submarine bars and offshore bars or barrier beaches and the minor shore forms beach cusps and ripple marks a series of experiments was made in the new laboratory for experimental geology of the Geological Institution of the University at Leiden. The experiments were carried out in a tank, measuring six by four meters and half a meter in depth in which waves could be produced artificially varying in height from two to about twelf centimeters. The effects of waves on sandy beaches running parallel to or approaching the beach at an angle of fifteen degrees were traced, the beach itself having a slope of two, four or six degrees. Finally especially to detect possible rules for the formation of beach cusps in relation to the slope of the beach and the height and period of the waves some experiments were carried out with valves and valvegrit, for the principal part derived from Macta subtruncata Dac., and with fine pumice-stone-gravel the so-called „bims” Lapilli (original from the vulcanic Laacher-See country in Germany) ranging in diameter from 0.2 to 2 centimeters in which the beach also had a varying slope. Neither of those last two series of experiments gave the results hoped for with regard to the problem mentioned. Many of the results of the effect of waves during the experiments may be found in the numerous photographs and figures in chapter III. The development and the pushing backwards of the offshore bar by the waves are clearly visible in the figures 39, 40, and 41. In all cases the offshore bar was formed by a gradual lateral growing of a number of subaquatic accumulations of sand which were at the same time slowly pushed higher up the slope of the beach. The subaquatic bar and the channel at the back of it, the latter formed by the excavation through the breaking waves, both had a practically stationary position. An almost horizontal terrace was formed between the channel and the foot of the front slope of the offshore bar, the latter being then pushed backwards to its final position and this taking the slope of a beach ridge. When the waves were running parallel to the beach this terrace was covered with somewhat irregular symmetric oscillation-marks and in case of experiments with waves obligue to the beach with slightly asymmetric combined current- and oscillation-ripples. A number of profils through the beach were taken at the end of most experiments. A careful examination of these profiles gave the following results. 1°. The height of the submarine bar increases until the depth above it measured from the mean water-level is equal to half the height of the waves measured from trough to crest (diagram 1, fig. 85). 2°. Only in the experiments with the beach sloping four and six degrees the submarine bar grows beyond the original profile. 3°. The maximal depth of the channel at the back of the submarine bar is approximately equal to the height of the waves (diagram 3, fig. 87). 4°. The height of the offshore bar, driven back to its ultimate position, above the mean water-level is proportional to one third of the height of the waves (diagram 4, fig. 88). 5°. The actual height of the offshore bar, i.e. the height above the original slope of the beach, proved to be almost independent of the height of the waves (diagram 5, fig. 89). It thus turned out that the offshore bar is pushed back more or less according to the height of the waves without any real increase in height and volume. The number of profiles was not great enough to determine the influence of the slope of the beach though it may be established that indications tend to show a relatively small effect. The transportation of sand from the offshore zone to the beach and in the reverse direction and the transport along the beach by oblique wave attack were discussed at much length. In order to separate the zones in which the waves eroded the sea bottom from those in which sedimentation took place two neutral lines were introduced. The first, seaward one already introduced by Cornaglia where the land- and seaward components of the ground waves are supposed to be exactly balanced, inside of which the motion of debris is landward and out-side of which it is seaward. The second, nearer to the beach, where the eroding force of the waves is changed into a constructive agency. The position of this last line giving at the same time the depth at which the waves will break. The diagram 7 in fig. 91 proves this depth below mean water-level to be equal to three quarters of the height of the waves. The neutral line of Cornaglia was located at a maximal depth of two and a half times the height of the waves, a figure very well in accordance with those given by Cornaglia, Heiser and von Boschitsch for the Mediterranean, the Baltic and the Black Sea. It is generally supposed that the material in the offshore zone, deposited on the abrasion platform, cannot be considered as definitely lost for the formation of shore forms though the transport of sand up the sea-bottom slope is still a quite uncontrolable factor in the development of shores. This supposition however is not directly based on the experimental research but for the greater part on a study of the literature and reports of facts, observed in nature. It needs further experiments with coloured sand to prove the exactness of this opinion which are impossible to be carried out in nature on account of the big amounts of sand required. As to the transportation of sand along the beach it was stated that while in nature the litoral drift is divided into beach drifting- and offshore drifting sand, the former due to the oblique advance of the waves, the latter due to wind-generated and tidal currents, in the experiments the beach drifting only is of prime importance. The principal part of the longshore transportation consequently took place in the zone landward from the surfline. The deflection of the outlets, intersecting the full, by the gradual growing of the windward ridge could be seen in the experiments (exp. XIV). The experiments proved that all waves in the very first place tend to form a profile of equilibrium on the one hand by throwing much material on the beach and pushing this back on the other hand by pulling it down to greater depth. Whilst this is happening the transversal transportation of sand predominates strongly the alongshore transportation, the latter getting more importance when the profile of equilibrium approaches its completion. Much attention was paid to the formation of the low and ball of a sandy shore. The conditions during the experiments resemble closely those of a shore line of emergency along which the formation of submarine- and offshore bars is typical. The coast, the sea-bottom offshore over a varying distance from the coastline and the neighbouring coasts must be considered the sources of the material required for their upbuilding. The results of the experiments and a study of the facts brought forth by other investigators lead the author to the conclusion that the submarine bars, parallel to the shore, are formed by the loss of transporting power of the waves at the surfline. The undertow being only a factor in the regulation of the position of this line and not a determinative factor in the formation of the bars. The socalled „Zuwachsriff” of Braun (lit. 3), a transitional form between the submarine bars and the offshore bar, must be formed by the uprush of the waves after the final breaking. The channel at the back of the submarine bar is in the experiments merely due to the excavating action of the surf but in reality tidal- and wind-generated currents will also have some influence on its formation. The changes in form and position of the three or four, sometimes even more parallel bars are believed to be greatest in the zones nearest to and farthest from the beach. The latter, exposed to the effect of all changes in the height of waves, protects the inner zones to a large extent against a stronger attack by an increasing height of the waves. The former being exposed to the relatively greatest changes in depth and receiving much material eroded from the higher beach and the coast. Any submarine bar, build up to its maximum height by one series of waves will suffer erosion from a higher series and probably be driven landwards more or less. The complex of bars in general is a stabile phenomenon with regard to the external form as to the material it is a dynamic and by no means a static equilibrium. The submarine bars being considered fore runners of the offshore bars only a few remarks need be made about the formation of the latter. I wholly accept de Beaumont’s theory according to which the longshore transportation of debris is a factor completely superfluous for the formation of the offshore bars. Even without the slightest litoral drift bars will be build up consisting of material eroded from sea-bottom farther offshore, the slope of the offshore itself and during periods of rough weather from the coast also. The formation of offshore bars as the „Flugelriffe” after von Cholnoky’s theory is strongly rejected. „Flügelriffe” could be called „delta wing-bars”, a name expressing the connection with delta’s to which the bars are attaced and of which they derive their material after the theory mentioned above. The fifth chapter deals with the formation of beach cusps. Observations in the laboratory and along the Dutch shore show that conditions for their formation only are favourable when regular waves advance and retreat strikingly parallel to the beach. It is thought that the backwash plays an important part in the formation of this minor shore form. Starting its course without any momentum it is more apt to be captured and divided into a number of small streams, running down the slope, by small irregularities and depressions in the surface of the beach. The uprush or swash, which swirls over the slope, will try to erase those irregularities as often could be observed in the experiments. Beach cusps are only formed when the eroding power of the backwash slightly exceeds the constructive power of the uprush. In this case an equilibrium will be attained after a small partial erosion of the slope of the beach concentrated in the bays intervening the beach cusps. Whenever the difference between the two agencies becomes greater the whole slope will be eroded. Along the Dutch shore the beach cusps are buid up out of sand, valves and valve grit of Mactra subtruncata Dac. and the little tubes of Pectinaria belgica Pall. and Terebella conchilega Pall. Those consisting of sand are only found in connection with beach ridges, the seaward slope of which is a subject to erosion in the manner described above. The formation of ripple marks during the experiments did not have the authors special interest. It may only be stated that oscillationripples as well as current-ripples were formed. During the experiments with waves parallel to the beach, all oscillation-ripples formed were parallel to the beach. In the experiments with waves running oblique to the beach, the oscillation-ripples in the deeper zones trended parallel to the waves, in the higher zones parallel to the beach while on the terrace at the back of the channel combined current- and oscillation-ripples were formed at a considerable angle with the beach. True current-ripples appeared in the outlets where they were parallel to the beach and with a general trend at right angles to the beach at the back of the offshore bar. The latter are quietly buried by the transgression of the offshore bar in a manner strictly comparable to the fosilization of ripple marks in delta deposits and by the wandering of banks. The author hopes to have found a new ripple-mark, the „backwash sand-wave” which being always parallel to the beach in a fossil state will give fairly good evidence of the position of the ancient beach. And if these sand-waves are found over some distance in a direction at right angles to their trend they clearly show changes in the water-level, which may be due to the pushing of water against a coast, but in sediments with a marine facies is the result of the tides. These ripples, formed by the back wash, show a slightly arched form measuring some fourty to fifty centimeters in length and not more then one or two centimeters in height. They may cover the whole width of the beach after a period of stormy weather and high tide but under normal circumstances, they are formed on the seaward slope of the ridges lying at the front of the beach near to the low-tide line.