The Limfjord is an estuary in Denmark between The North Sea and Kattegat. The Thyboroen Channel connects the fjord with The North Sea. The water levels in the western part of the Limfjord are strongly correlated with the water level in the sea at Thyboroen. Analyses revealed a close to threefold increase in the channel cross section over the last 100 years. This has led to a detailed investigation into the effects of the channel cross section on the extreme high water levels in the Limfjord now and in the future. For Lemvig in the western part of the Limfjord the water level with a return period of 100 years would have been 1.73 m with the 1958 channel bathymetry throughout the whole period and 1.99 m with the 2005 channel bathymetry. With the extrapolated 2060-bathymetry the 100 years water level will be 2.38 m. A number of measures to counteract the consequences of this development have been evaluated.

The overall coastline management strategy for the central section of the Danish North Sea coast is to halt the process of coastal retreat and maintain a certain safety level against flooding. The sudden erosion of 10-25 m of a dune during a storm or a gale is thus a cause for concern. Such local dune erosion is often observed where an inward coastline undulation is situated off a section of the coast where the dune is not in equilibrium with the rest of the profile. During a storm this latent dune erosion is then released. By analysing the migration of coastline undulations and detecting the profiles with latent dune erosion the expectation is that the majority of large-scale dune erosion can be foreseen and avoided by adaptation of the nourishment programme. Keywords: nourishment planning; coastline undulation; latent dune erosion INTRODUCTIONIn January 2005 the Danish North Sea coast was hit by a storm with mean wind velocities of up to 36 m/s. After the storm, the level of dune foot retreat was registered along the 110 km long central section of the coast. It appeared that the average level of dune foot retreat was 3.3 m, but at certain points the dunes had retreated by up to 25 m. Long stretches of coastline were also observed, however, in which the dunes had not retreated.The water level during the storm has a return period of about 100 years. However, significant dune erosion is often observed at some local sections of the coastline after more moderate storms with lower water levels. It has been very difficult to explain why erosion is concentrated in these particular areas.On a coastline where the general objective is to halt the process of coastal retreat and maintain a certain safety level against flooding, large-scale local dune erosion is obviously a cause for concern. It is also difficult to explain to the wider public how large scale dune erosion can take place on a coastline where erosion is supposed to be under control. Revetments have often been built to address these issues although nourishment is now generally considered to be the most appropriate solution.Our hypothesis is that large-scale local dune erosion results from a combination of inward coastline undulation and latent or unreleased dune retreat. By monitoring these two phenomena continuously we expect to be able to predict which sites are likely to be at enhanced risk of large-scale dune erosion in the event of a storm. By using beach nourishment in advance the assumption is that dune erosion can be prevented or at least reduced in scale. DESCRIPTION OF THE COASTThe following is a description of the central section of the Danish North Sea coast, see Fig. 1. The 110 km section is mainly a sandy coastline dominated by westerly winds and waves. On average there are about 75 days per year on which significant wave heights are above 2.5 m. The tidal range is small, and the high water level within a 100 year return period is about 2.5 m to the north and 3.5 m to the south.The area's first sea groynes were built in the 1880s and around 1960 a...