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Sand mats are composite materials for hydraulic applications. They consist of a geotextile top and base layer with a fill of natural sand or aggregates to increase the mass per unit area. The two geotextile sheets are needle punched or sewn together to hold the fill in place and to provide shear strength in the plane of the mat. An advantage is the easier installation under water. Concerning the application as a filter layer, two types of sand mats must be distinguished. Type A are sand mats where only the top layer must be considered for the filtration behaviour. Type B are products where the entire composite is relevant for the hydraulic properties. Concerning the functions filtration and separation EN 13253 requires only the characteristic opening size and water permeability under laminar conditions. For the present study samples were selected, which are representative for the product types and have been currently installed to a great extent at navigable waterways. The paper shows hydraulic properties of the single layers and of the entire sand mats. An important design criteria for use in erosion control works, coastal protection or bank revetments can be the water permeability of the geotextile filter after testing the amount of soil passing (ISO 10772). Sand mats in erosion control works are often exposed to turbulent external water flow conditions. Therefore the paper shows also the determination of the filtration behaviour under turbulent water flow conditions and the hydraulic filtration stability of the single layers as well as the entire sand mats.