construction and refurbishment of earthen irrigation channel banks

10.1.2.5 BendsA bend in the channel which has too small a radius will cause the erosion of theoutside bank of the channel. The outside bank of a bend can be beached or anappropriate design radius is incorporated before construction.bendPhoto 10-390 degree bend in channel10.1.2.6 Channel reach lengthWave damage can occur on long channel reaches depending upon the orientationto the prevailing winds and how open the country is.If a long straight reach of channel is aligned in the direction of the prevailingwind, large waves can develop which can overtop banks. A greater freeboardallowance and/or rock beaching may be required to protect the bank.10.1.2.7 Orientation of channel in relation to prevailing windsIf a channel is planned in the same direction as the prevailing wind, waves candevelop which can overtop banks. This problem is exacerbated if on a longchannel reach as described above.10.1.2.8 Meander formationSome older channels form meanders which may replicate what happens in naturalstreams. This is due to siltation and local erosion of banks - it is a natural processof reducing the erosivity of the channel water by reducing its hydraulic head.10.1.3 Bank materialSelection of appropriate bank material is critical to allowing a bank to reach itsdesign life without excess erosion. The material must perform adequately underdesign conditions to ensure the design life of the bank.Channels are commonly constructed in the higher lands of an irrigation area whichnormally corresponds with lighter more permeable soils. In older irrigation areas,channel banks were constructed with the available material on the channelalignment, regardless of the soil quality. The resulting leakage and seepage throughthe banks and bed of the channel leads to bank deterioration.10.1.3.1 Soil typeThe following physical and chemical properties of a potential bank-buildingmaterial all effect the integrity of the bank:• the physical characteristics of soils; particle size distribution, clay content,tendency to disperse (in irrigation supply water and/or rainwater), plasticity,hydraulic conductivity• whether bank material is susceptible to shrinking and cracking during hot dryweather.• the chemical characteristics of soils; pH, EC, Sodium adsorption ratioConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 10-6