The Hamm local authority in Germany had a problem with its stormwater overflow lagoon near the village of Rhynern. During periods of prolonged rainfall, the lagoon frequently overflowed with the result that the lagoon could only guarantee a partial cleaning of the water by the physical process of settlement.

An alternative had to be found. The decision was made to construct a soil retention filtration system within the stormwater overflow lagoon. The objective was to limit the peak flows of surplus water discharging from the existing stormwater lagoon and storage channels. The new combined system would consist of an initial storage section and positive soil filtration. In contrast to a stormwater settlement lagoon a soil filtration area was involved with physical, chemical and biological purification equipment, with large enough storage capacity to ensure minimal overflow.

The construction of the soil filtration system could be broken down into a number of different stages.
The earthworks involved the construction of the filtration area and alterations to the base of the lagoon and the weir levels. Finally the discharge and connecting channels to the filtration plant were put in place.

However, there was another problem in stabilising the slopes and toes of the bank as well as the overflow areas against severe erosion. Vegetative bank protection was the specified stabilisation method for this site. These units, a form of cellular covering, had to be filled with armourstone class 0 and suitable graded granular material and installed over the complete prepared surface and then planted.

In the past this planting had proved difficult. Pre-cultivated vegetation mattresses, laid on bank protection to ensure a rapid and reliable growth of vegetation, often dried out and growth was uneven.

The landscaping contractor, Wilhelm Haddick GmbH & Co. KG, based in Borken / Westphalia, developed a system to overcome this difficulty, ensuring the cellular bank protection could be installed in one operation as a 'green' cover. The company developed a special device to fill the individual sections, almost eliminating soil voids. The individual tube-like sections were then enclosed within a special vegetation mattress, held together by strapping, and finally enclosed by a geotextile mesh. The fill material was a carefully balanced mixture of crushed stone and moist humus-rich topsoil. It was prepared in a barge and loaded by excavator bucket into the tube-filling device. As the tube elements were withdrawn, so the mineral mixture was placed in full contact with the vegetation mattress. The resulting tensile forces in the finished unit were carried by the supporting net enclosure.

HUESKER played an important part in this innovative solution. HaTe®, the well-known woven fabric, was used to form the erosion protection mattress in the cellular bank protection. This woven polyethylene monofilament geotextile was selected for its special filtration properties.

Unlike the earlier systems, the process developed by Haddick eliminated the risk of a void forming between the growing medium and the plants or seeds. The seed, incorporated in the vegetation mattress was specially selected for the site and the elimination of any void allowed nutrients and water to be taken up steadily and over an extended period from the growing medium. In frost-free weather, 100% vegetation cover could be guaranteed without the need for additional watering.

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