Project 4 – Acute dune erosion – Mike 21 IG model


 20.2.19

The aim of this project is to test and use the new MIKE 21 IG model to quantify the dune-strength against acute erosion and see how this correlates with coastal state indicators (CSI).

Coastal dunes are natural barriers protecting the hinterland from erosion and flooding under extreme events like storms. These natural barriers are often exposed to storm conditions where a combination of high wave energy and elevated water levels are capable of producing acute erosion of the dune system. Significant acute erosion occurred to the coastal dunes at Skodbjerge in January 2007. This erosion event is investigated through a morphological analysis of good profile data before and after the storm period and by numerical modelling of acute erosion with the MIKE 21 IG model.

DCA investigated acute dune erosion along the Danish North Sea coast for the period 1977 to 2011 where an applicable explanation was not found. However, the study showed a correlation between the nearshore bar morphology and dune erosion. This correlation was further investigated by numerical modeling of acute dune erosion at Skodbjerge in a 1D model – X-Beach. Simulation results showed overestimated dune erosion, which was explained by the lack of longshore sediment transport in the simulations.

This project under Building with Nature focuses on acute dune erosion at Skodbjerge and has investigated how this correlated with CSI by the use of the numerical model MIKE 21 IG. The results of these investigations are shown in a technical note there can be found under Filer.

CSI and morphological changes were analyzed from 8 pre- and post-storm coastal profiles. In January 2007 a period of high-energetic wave climates and elevated water levels caused local acute erosion to some of the profiles while others experienced no erosion. The eroded sand was typically accumulated at the upper shoreface or transported alongshore to the nearby profiles. Offshore bar migration and flattening of the bars were also typical morphological changes. The CSI analysis showed that the bar heights were generally higher at the northern and southern parts of the coastline than at the median part before the storm period. After the storm period, the bar heights were reduced at the northern and southern parts of the coastline while they remained constant or increased in height at the median stretch of the coastal laboratory.

Figure 1: Storm event at Skodbjerge where acute dune erosion was observed on 12-01-2007
Figure 1: Storm event at Skodbjerge where acute dune erosion was observed on 12-01-2007

The MIKE 21 IG model was used to simulate acute erosion under storm conditions. The model was run with wave parameters and sea-levels recorded from three storm events in January 2007. Real-time waves and infragravity waves were generated from the recorded dataset and applied to the model. The simulations were completed on 8 pre-storm profiles and directly compared with measured post-storm profiles.

This study showed that MIKE 21 IG can reproduce the magnitude of acute erosion under a storm impact close to measured morphological changes at Skodbjerge. The model predicts that the eroded sand from the dune accumulates in a sand deposit at the lower foreshore and upper shoreface, which is identical to what was observed at some of the profiles. At the other profiles this amount of sand this was not the case. The model shows some odd gradients in the modeled nearshore bathymetry. Further development of the model is encouraged.