The impact of willow brush mattresses on the mechanical filtration stability

Abstract

Geotextiles or grain filters are frequently used between technical revetment and subsoil at riverbanks on waterways as erosion control from hydraulic loads. Due to the EU Water Framework Directive of the year 2000 (Directive 2000/60/EC) and other EU Directives ecological and near nature hydraulic engineering measures are becoming ever more in demand. That's why more technical-biological bank protection under the use of plants should be applied, for example willow brush mattresses, plant mats, reed gabions.Based on the investigations of the filtration stability of geotextiles used at navigable waterways, a method, and a new test apparatus for verifying the soil retention capability and the mechanical filtration stability of willow brush mattresses and their dense, near-surface root system have been developed.Willow brush mattresses made of white willows (Salix alba L.) or basket willows (Salix viminalis L.) were planted in wooden sample boxes and placed upside down into the test apparatus after a pre-set growing time (0, 1, 3 and 6 months). By flooding and draining the test apparatus water level drawdown has been simulated.The eroded soil material was collected, weighed after drying until weight constancy, and compared with the dry mass of the retained soil material.The results of this new developed mechanical filtration stability tests for willow brush mattresses show that during the initial phase of six months the willow roots reduced soil erosion. Thus, in long-term plant roots can take over mechanical filter function in near nature bank protection. The basket willow led to significant reductions in soil erosion i.e., an increase of the soil retention capability after one month and the white willow after three months of growth.