construction and refurbishment of earthen irrigation channel banks

The type of rock beaching (sized grading), its extent and shaping, depends on localconditions and the flow pattern produced at each type of structure. The localconditions include the cost of beaching, risk to structure, type of soil and velocity ofwater, and consideration of these issues are outside the scope of these Guidelines.Depending on the velocities involved, the size grading of the beaching and thecharacteristics of the bank material, the beaching may need to be placed over ageotextile filter fabric to prevent washing out by the channel flow of the subgradesoil, and hence, the slumping and collapse of the beaching protection.In cases, where the soils are unable to withstand the erosive forces, caused byflowing water or wave actions, a protective layer of gravel or crushed rock can beprovided over the soil.The cost of reducing the velocity with a larger section, as compared with the cost ofa smaller section with its higher velocity and protection with a gravel or crushedrock cover, should be considered if suitable material is available from localdeposits.To ensure effective long-term protection, the size, grading and thickness of thegravel or crushed rock layer, as well as the slope of the batters, needs to be carefullyconsideration. Refer to Section 18, Inside Batter Treatment.Open channel flow behaviour can be difficult to predict at times, and withborderline bank materials, rather than taking a conservative approach and usingextensive and possibly excessive lengths of protective material, it may be morecost-effective to adopt a policy of wait and see. Periodic inspections of the channelwill identify localised erosion sites at an early stage of development, whereadditional spot protection can be easily provided.12.11.5 Minimum VelocitiesIdeally a channel should have sufficient velocity at low flows to prevent siltdeposition and growth of aquatic plants. Channel designs in Australia are generallybased on the capacity required for short periods of peak demand. This results inchannels being operated for most of the time at below design capacity. Because ofthis, any sediments above colloidal size in the supply water will have to beexcluded at the intakes or periodically removed from the system.Where the silt content of the water is high and the silting of channels is a significantproblem, the prevention of silting during long periods of partial-capacity operationmay be more important than the prevention of a limited amount of erosion duringthe short periods of full-capacity operation.Generally, the silt content of water in Australian rivers and channels iscomparatively low and is mainly limited to suspended clay. The silting of irrigationchannels is therefore not a significant problem. There are times after heavy rain inthe catchments or immediately after channels has been built, when the silt contentrises considerably. However, it has been found that the thin layers of silt depositedat these times are helpful in reducing channel seepage and erosion. It is thereforenot necessary in most Australian situations to adopt any particular measures toavoid silting. The low silt loads permit adoption of low velocities, and permissiblemaximum velocities are generally the primary design consideration.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-19