Large-Scale Coastal Morphology
A sandy coast is a highly variable environment due to the transport, erosion and deposition of sediment. The shifting sand changes the morphology over a large range of scales in time and space. On the smallest scale we often observe wave ripples with a wave length of the order of 10 cm, which can form or change on a time scale of a few minutes. On an intermediate scale the profile of the beach changes continuously. Longshore bars are formed during storms and change their position and dimensions under more gentle wave conditions.
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The profile variation takes place on a time scale from a single storm event up to seasons or a few years. On larger scales, coastal features such as barrier islands, tidal inlets or spits develop over decades or centuries.
On the larger scales the dominant sediment transport mechanism is related to wave-driven currents. Water waves are associated with a "thrust" or momentum flux, the radiation stress. When the waves break in the surf zone the thrust is exerted on the water, thereby driving a current. The wave-driven current can be in the form of horizontal or vertical circulations (the latter known as undertow), and obliquely incident waves will drive a current along the coast. The longshore current causes a sediment transport, the littoral drift, which in many cases plays a very important part in the sediment budget for a stretch of coastline.
Research on large- and medium- scale coastal morphology has been carried out in the section for Coastal and River Engineering, focusing on the formation of spits and on longshore bars and rip channels (in Danish: krumodder, revler og hestehuller). The morphology of spits has been studied by a combination of mathematical models and laboratory experiments aiming at finding the mechanisms governing the dimensions and growth rate of a spit, and at determining the shape of a spit growing with constant form.
A growing spit is built up by deposition of sediment supplied by the littoral drift from the up-drift coastline. Sediment accumulates along the coast and the littoral drift decreases gradually to become zero at the tip of the spit. The variation in the littoral drift is due to the variation in the orientation of the coastline relative to the incoming waves. Practically no wave energy comes to the tip of the spit, because the waves are deflected by refraction at the up-drift part of the spit. The detailed distribution of wave energy along the spit determines the shape and the growth rate. The growth rate varies with the width of the spit and attains a maximum for a width of approximately 8-10 times the width of the breaker zone where the littoral transport takes place.
The plane shape of longshore bars has been studied by application and development of numerical models. Different descriptions of the processes forming the profile have been introduced, giving a very detailed representation of the dynamics of a bar on a shorter time scale and a more integrated long-term prediction of the bar geometry.
The work has been carried out in combination with several externally financed research programmes: "Intermediate Scale Coastal Behaviour: Measurement, Modelling and Prediction" under the NICOP programme financed by the U.S. Navy; "HUMOR: Human Intervention with Large Scale Coastal Morphological Evolution", project under the EU framework programme 5; the frame research programme "Coasts and Tidal Inlets" financed by the Danish Technical Research Council.
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Two spits formed at Sdr. Aaby, Denmark. An island southwest of the site shelters for waves from this direction (insert), so the two spits are formed by waves coming predominantly from west and south respectively. Aerial photo from DDOland, with permission from COWI A/S
Spit formed in MEK’s laboratory. The blue string indicates the water line. Note theoblique wave incidence, and the small wave ripples that are parallel with the fronts of the waves that have formed them.
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