MIGRATING SAND WAVES OR SAND HUMPS, WITH SPECIAL REFERENCE TO INVESTIGATIONS CARRIED OUT ON THE DANISH NORTH SEA COAST

Abstract

The transport of sediment by flowing water commands great interest in connection with the control of floods, land reclamation, and the construction of harbours and coast protection works. A distinction can be drawn between littoral drift in rivers and in the sea. The sediment transportation in rivers has been investigated by several authors, e.g. Shields, Meyer Peter, Kalinske, and Einstein, see (16) pp,, 769-83*+. Einstein's latest theories have given reliable results in practice (9). As pointed out by Einstein (7), there cannot be much difference, physically, between transportation of sediment in rivers and longshore drift at sea shores, apart from the littoral zone with its extremely complex conditions. In the attempt to understand the complex problem of sea shores the practice so far has been to split them up into several reaches and investigate them separately. This work has given a number of results of practical interest in connection with littoral drift and coastal protection technology, see (2), (3), (5)» (6), (11), (13), and (16). According to Einstein, Johnson and Chien (8) there exist two types of sediment load, one that bears a certain relationship with the discharge (bed-material load), and the other which does not (wash load). The result of flume study indicates that the transport rate of wash load, just as that of the bed-material load, can be calculated according to the Einstein bed-load function (9), if the instantaneous bed composition is known. On the other hand, the bedmaterial load is equally available'in the entire bed, but only the surface bed layer contains any significant amount of wash-load material. Any change of flow or of sediment supply may immediately change the composition of the wash-load material in the bed. The bed composition as determined from the instantaneous condition of the channel has no lasting significance so far as the wash load is concerned, and this makes the prediction of the wash-load rate from.the bed-load function impossible. The following deals with a mode of bed-load transportation which, as far as can be seen, takes place in large "waves" or humps. Introductorily are mentioned investigations made in the United States on migrating sand bars and sand waves in rivers, and investigations in Holland on migrating sand bars on the bottom of the sea. The major part of the paper deal with migrating sand humps along the North Sea coast of the peninsula of Jutland, Denmark, see Fig. 3.