construction and refurbishment of earthen irrigation channel banks

Thus channels are usually designed to avoid erosion at maximum discharge, andprovision made to remove or exclude the sediment at the intakes or periodicallyremove it from the channel system.In earthen channels, sediments generally enter at the intakes, or are eroded from thebed and banks. Large particles eroded from the banks usually settle along thechannel bed, where they tend to prevent bed erosion. Small particles eroded frombanks are carried downstream, since scouring velocities are high enough tomaintain transportation.Fine sands and silts that can be carried in suspension are not normally as much of aproblem as the heavier materials. Coarse sand and gravel should be kept out ofchannels as much as possible by sediment-control works, either at the intake oralong the channel.Where channels flows carry silty water, minimum velocities should be high enough,wherever practicable, to keep the silt moving. The velocities required in particularchannels depends on quantities and qualities of silt transported. Velocities of 0.45to 0.6 metres per second are usually high enough to keep small quantities of finesand and silt in motion. Velocities of 0.8 metres per second or more are required toinhibit growth of aquatic plants. In areas where these velocities cannot be achieved,the minimum channel velocity should be kept above 0.3 metres per second whenever possible.Where the silt content of water is significant and velocities high enough to preventserious silting can not be maintained, silt-control works at the intakes would berequired. The design of such works is outside the scope of this Manual.12.12 Channel ControlThe two basic systems of channel control are defined by the location of the controlledwater surface relative to the control structure:• upstream – supply orientated operation• downstream – demand orientated operationUpstream Control refers to a system where the water level being controlled or maintainedconstant, is immediately upstream of the regulating point. Most manually controlled dropboard systems in Australia have been designed on this basis. Drop bars are removed orreplaced, or gates opened or closed to maintain a steady level upstream of a regulatingpoint.The opposite mode of operation, defined as Downstream Control, is where the control isoperated so the level on the downstream side of the regulating structure remains constant.Because it reacts to changes downstream, it is user-oriented. This control mode whichwas introduced to Australia in the 1970’s, can operate automatically since demand signalscan be passed up the system. If individual outlets turn on, the change in that pool levelcauses a change in the regulator to change the flow to re-establish the prior setpoint. Achange in any pool will cause a sequence of proper compensating actions to be made inthe upstream structures, all the way back to the source. A secondary benefit is theincreased distribution efficiency since the system is essentially closed at the downstreamend. Downstream control systems are always automated; upstream control systems maybe manual or automatic.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-20