construction and refurbishment of earthen irrigation channel banks

Unless the inside batters are protected by a cover of crushed rock or gravel material,rapid fluctuations of the water levels in earthen channels should be minimised toavoid the slumping of the bank material.Under the less stable saturated soil conditions, the high pore water pressure in thebank can induce slumping of the batters into the channel. Drawdowns in excess of150 mm in 24 hours should be minimised, and severe drawdowns of more than 300mm in 24 hours should be avoided wherever possible.Experience has shown that rapid fluctuations in water level speed up the bankdeterioration processes considerably, with the steeper the batter slope, the greaterthe impact. Where large water level fluctuations are eliminated, it has beenobserved that the rates of bank deterioration can be slowed significantly.Close control of water levels is particularly important to protect newly constructedearthen banks while they are undergoing the initial aging and stabilising process.23.3.4 Discharging Drainage Water into ChannelsThe practice of discharging drainage water into earthen channels to relieve floodingof adjoining properties after rainfall can speed up the bank deterioration processes.It is recommended that this practice not be allowed, other than in specialcircumstances and only then after due consideration of the risks and long termimplications.In accepting the drainage water, conveying it through the channel system andoutfalling it, the following adverse impacts can occur:• scour of the channel banks and bed at the discharge point• slumping of the channel banks due to the rapid rise and fall in water levels• erosion of the channel section from higher water velocities• overtopping of the downstream sections as the channel capacity reduces• deterioration in water quality• chemical contamination of water• spread of diseases23.3.5 Draining Channels during Non-Operational PeriodIrrigation channels are filled and drained many times over in their life. For manyirrigation schemes that do not supply water to irrigators for the whole year, drainingthe channels during the non-operational period is standard practice. However, someirrigation schemes prefer to not drain their channel systems. The advantages anddisadvantages of draining channels are outlined below:Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 23-8