Development of a dredged material disposal, Germany

Investigations regarding the warranty of the stability of the southern threshold.

In the framework of the fairway adjustment of the river “Elbe” to the container shipping, the dredged material disposal (DMD) Krautsand-Süd was developed and completed in 2001. The northern threshold and the toe stabilization of the dredged material disposal (DMD) consist of a single-layer sand container revetment. The southern threshold was built of tailings with a coping of 60 to 80 cm made of stone facing of class II-III (see picture). According to the Water and Shipping Authority Hamburg a dredged quantity of about 750,000 m3 is stored on the dredged material disposal which was filled in April/May 2001 for the last time.

Subsequently, in the framework of bank nourishment and a dumping in front of the southern threshold a total of 243,000 m3 dredged material was dumped. Since the implementation of the dredged material disposal (DMD) the Water and Shipping Authority as authorized Federal Agency has observed the sole action of the dumping area.
In the last years a considerable scouring action has arisen in front of the southern threshold of the DMD with an extension of about 80 x 50 m and a maximum depth of about 8.4 m under sea level. In 2006 the position and the action of the scour in the surrounding of the southern threshold required an extensive investigation regarding the stability of the structure on the one hand and the causes of the scouring action on the other.

The following procedures were rendered:

• Implementation of flow measuring in the area of the dredged material disposal “Krautsand-Süd”
• Analysis of the flow measuring and a detailed representation in the explanatory report
• Analysis of the surveying of the underwater topography in the area of the southern threshold as of 1999 - 2006
• Analysis of causes for scouring action in the area of the southern threshold
• Investigation / recommendation of rehabilitation concepts comprising dimensioning, layouts and bill of quantities for the recommended solution

The analysis of the flow measuring and the scouring action reveals that the main scour has widely reached the final depth, but extends in the area. At present the direct toe area of the threshold is not at risk. Further surveying with high resolution in the area of the structure were recommended for observation. For the warranty of the threshold the installation of an erosion- and filter solid stone material was recommended which provides an efficient and economic solution and enables a safe connection to the toe area of the structure.