Q. Show the filter below the revetment?

A filter placed between the revetment / protection works at the surface and the soil bed below the revetment / protection works. Main function of a filter below revetment is to allow the ground water to pass through the filter layer but arrest the movement of soil particles of the bed material. Hydraulic conditions of the river, characteristics of bed / bank material, etc are studied before deciding the properties of filter. Some of the important variables can be listed as, location of filter i.e. whether on slope or on bed, hydraulic gradient through the filter and its direction, characteristic properties of sub-soil, water permeability of sub-soil and soil tightness, suspended sediment load in the flow and its characteristics, etc.