MORPHOLOGICAL CHANGES IN A FINE SAND TIDAL ESTUARY AFTER MEASURES OF RIVER IMPROVEMENT

摘要

Artificial shipping channels in fine sand tidal estuaries are difficult to maintain. The attempt to stabilize the river beds and thus the fairway in the upper reaches of the Outer Weser by means of systems of river structures has been a success. The further deepening of the fairway necessitated the artificial narrowing of the river bed. The successes of improvement achieved over a prolonged period have up to now been satisfactory. Some explanations concerning the effects of the river structures on the morphology are given in the present report. In the improved river bed a continuous shifting of sediments is taking place, which is mainly due to the incessant action of the tidal currents. It is inevitable in this process that sand is drifted into the deep excavated channel. If one has a good knowledge of the natural shifting processes, it is possible nowadays, by using suitable Hopper suction dredgers, to employ to a certain extent methods of dynamic river engineering by means of the proper artificial shifting of the excavated soil within the cross sections of the river. The capability of improvement of tidal rivers and their estuaries is of great importance for the sea ports that are situated on them. This is particularly true in a time when the dimensions of sea-going ships are still increasing and the countries are confronted with the problem of permanently having to improve the approaches to their sea ports, if they want to participate in overseas trade also in the future. The reason why the improvement of fine sand tidal estuaries is so difficult and so expensive, depending on the local hydrographical and morphological conditions, is that the sediments are continually shifted on account of the tidal currents which are constantly changing in place and time and varying in their force as well as on account of the motion of the sea and the wash of the waves. This renders the improvement, the maintenance, and the marking of artificial fairways for ocean shipping rather difficult.