Coastal Restoration on the North Sea

Figure 1. Sign introducing the coastal area and the restoration project. The mud flat here is a mile wide (estimated) because of about 20 feet of tidal range, twice a day.

Today’s post takes me to the North Sea coast of Germany, the city of Husum, and to one of the famous mud flats from the region. Rivers running from the Alps drain Germany, transporting mud (silt and clay) to the north coast, where it is transported along the coast and stirred around by strong tidal flows. We are going to look at efforts to stop dramatic erosion caused by a reduction of sediment input, because of dams and coastal construction, leading to a serious threat to the levee protecting Husum from the North Sea (Fig. 2).

Figure 2. Photo of levee that protects the city of Husum from the North Sea. (Right side of inset map of Fig. 1) The building to the right is an abandoned hotel inside the levee. The building to the left is a restaurant on pilings where people swim during high tide. The asphalt road is the path to the seashore.

The mud flats schematically shown in Fig. 1 are covered with fence-like structures designed to catch mud brought in the the high tide (Fig. 3).

Figure 3. Image of nearshore area (covered by grass), sediment retention fences, and reinforcing riprap where erosion occurs. Note that in this image, the fences do not appear to be collecting sediment on the landward side (to the right).

A quick look at the past. This area was covered by glaciers that filled the North Sea and transported rocks from Sweden to the north. These glacial erratics are rounded and scattered around the land in a random manner (thus the name). We found one used as street decoration in Husum (Fig. 4).

Figure 4. Close up of glacial erratic left along the coast. The boulder was about 3 feet in diameter. This close-up shows muscovite (shiny minerals), orthoclase feldspar (pink), amphibole and/or biotite (dark), and quartz (gray). This granitic rock was transported as much as hundreds of miles by ice, from Scandinavia.

In addition to boulders transported during the ice ages (less than a million years old), there are remnants of sandy sediment from the Quaternary, before the area was overwhelmed by mud (Fig. 5).

Figure 5. Image to the north in Fig.1, showing trees and a village on top of a low pile of quaternary sand, probably the erosional debris of a stream or coastal beach from the last ten-thousand years. This photo was taken to the east side of Fig. 1. Note the sporadic filling by grass, especially the sheep. This is interesting because sheep eat grass, so why they are loose in an area supposedly being reclaimed is confusing.

The result of the sediment retention project can be seen in Fig. 6.

Figure 6. The landward limit of the fencing project, less than 100 yards from the levee. Note erosion along the fence, leaving it standing 2 feet above the exposed mud. This could have been the result of long-term erosion, or a single storm.

This are represents an attempt to reconcile the problem of coastal development (the port of Husum ships out grain) and the protection from storm waves provided by a wide mud flat (which dissipates wave energy). Another issue is the encroachment of sheep grazing, which appears to be legal (there are fences and gates, etc). And then there is entertainment; this is a popular swimming location during high tide. Not to mention environmental degradation and fish hatcheries. Several attempts at mixing these applications can be seen in the hardened and dredged channel leading to the port (Fig.7), and buried groins which were apparently intended to keep the shipping channel open (Fig. 8).

Figure 7. Shipping channel to the port of Husum hardened by mortared rock.
Figure 8. Groin in mud flat. These coastal engineering structures are designed to prevent sediment being carried along the coast and blocking channels, as well as retaining sediment between adjacent groins. This is probably contributing to the erosion seen in Fig. 6.

It is difficult to reconcile the many uses the coastline is required to fulfill. This trip revealed that it is unreasonable to mix methods designed to preserve the status quo (Figs. 7 and 8), and those intended to change it (eg. Fig. 6), especially when these techniques are mixed (Fig. 3). A difficult decision will have to be made soon, or the levee protecting the bustling cit of Husum will be in danger of breach during a severe storm, which is becoming more common in the North Sea.

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