construction and refurbishment of earthen irrigation channel banks

22.8.9.5 Cover MaterialGeomembranes have little resistance to the various field hazards and need a coverfor protection from damage. This is usually achieved by using a cover of:• Earth, or• Earth with Crushed Rock/Gravel upper layerThe second option has benefits of reducing maintenance costs by deterringaquatic animals and weeds, and reducing erosion of the cover material.However, it adds a significant cost.Figure 22.15 shows a typical profile of a Geomembrane-lined channel.WaterGravel CoverEarth CoverGeomembraneSubgradeFigure 22.15Typical Covered Geomembrane profileThe selection of the type and thickness of the cover material is a keyconsideration in reducing the life-cycle costs of membrane liners. Theexcavation for, supply and placement of the cover material is a significant costcomponent, and the cover material has a major influence on the life of the lining.For economy, the material should be locally available and placed as thin aspossible for adequate protection of the membrane.The cover material must remain stable on side slopes of the channel, resistingsloughing, erosion and wave action.The cover material needs to have the following physical properties:• It must be reasonably free draining so that it will remain stable duringdrawdown.• It must be coarse enough to resist erosion due to flow and wave action.• It must not be too coarse otherwise the geomembrane can be damaged duringcover placement. A well-rounded gravel placed close to the liner can beacceptable.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 22-63