construction and refurbishment of earthen irrigation channel banks

To ensure that a channel bank has a long service life with minimum losses fromseepage and leakage, it is important that the channel is not operated well above itsdesign discharge level for extended periods of time.Channel design capacities are broadly based on the watering requirements of theshort periods of peak demand. Channel flow rates are normally held reasonablysteady at about the design figure during these peak periods, but at other times flowrates are considerably less than design and subject to change as the demandchanges. Careful operation is required to minimise water level fluctuations at thesetimes.However, open channel systems are inherently complex to operate and they affordlittle tolerance for error. Their non-linear flow behaviour and the long times neededfor flow changes to travel the length of a channel make them difficult to regulate.Consequently there are considerable fluctuations in a channel system above andbelow the design levels.Alternate wetting and drying, shrinkage and cracking, vegetation and animals actingon the top of a channel bank, reduces the compacted density and thus lessens thewater-tightness, slope stability and erosion resistance of the top section of bank.Where channels are operated well above their design level, with very littlefreeboard, this places increased pressure on any weaknesses in the channel bank andincreases rates of leakage, seepage and bank deterioration.The consequences of this are operational difficulties, reduced freeboard and reducedchannel safety, accelerated wear, increased scour and erosion, deterioration ofchannel capacity due to siltation from scour and erosion, increased leaks, seepageand overtopping. The increased seepage, leaks and overtopping reduces operationalefficiency, increases water losses, and can contribute to groundwater problems andwater logging.Limited encroachment on channel freeboard should only be planned during majorchannel system regulations or during short periods of maximum irrigation demand.Full freeboard should be maintained at all other times in order to ensure that thesafety of the channel is protected.While some degree of extra flow may be tolerated in most earthen channels,experience has shown that when channels are operating close to the top of thecompacted bank, the rates of erosion, slumping and leakage increases significantly.Reducing the channel running level reduces stresses on the top section of bank,reducing maintenance problems and the rate of deterioration, and extending the lifeof the bank. Running channel 100 to 150mm lower has shown significant benefitsin life extension and reduced maintenance.The water level that channels are operated at can affect the rate of seepage. Raisingthe channel water above the designed level will increase the head of water on thebank and bed and this will increase the rate of seepage through the bank and bed.This leads to increased losses of water from the channel system and increasedaccessions to the water table.23.3.3 Channel Level FluctuationsConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 23-7