construction and refurbishment of earthen irrigation channel banks

10.1.5.4 Overtopping of banksIn adequate freeboard is a widespread problem in channel systems, and isprincipally caused by erosion of channel banks.If channels are run higher than the design operating level, the banks may beovertopped which will cause scouring of the crest and outside batter by theerosive effect of running water.Many older channels are required to carry flows to the absolute limit of theircapacity in an effort to provide irrigators with a satisfactory level of service. Thelimiting point is often a section of bank along the channel where no freeboard isavailable at maximum regulated flow. Any fluctuations that might occur abovethis maximum flow will cause water to spill over the banks. Resulting erosioncan weaken the bank and create a serious risk of breaching.10.1.5.5 High water velocitiesSome channel operators are under increasing pressure to supply higher flow ratesfor shorter durations and this can require operating channels of higher velocities.While the effects may not be immediately apparent, operating channels atvelocities above design levels for extended periods of time can cause scouring ofbanks and bed leading to loss of bank material and reduced channel bank life.Refer to Section 12.11.3 Maximum Velocities.scouringPhoto 10-8Scouring of channel bank caused by high flows (structure originallyflush with the bank)The rate of bank deterioration is dependent on the characteristics of the bed andbank material and the flow velocity in the channel. Loose sandy soils free ofcementing constituents are more easily eroded, where as, graded silt, sand andgravel materials containing cementing constituents are more resistant to erosion.10.1.5.6 Turbulence at structures and bendsChannel structures such as regulators, culverts etc can cause downstreamturbulence if not properly designed where substantial energy dissipation isinvolved. This turbulence can cause scouring downstream of structures, severelyundermining and weakening banks. Typical problems are bed scour with battercollapse downstream of regulators, and batter and bank erosion caused by reversecurrents downstream of culverts. In some cases this scouring stabilises to anequilibrium with a bell-shaped stilling basin being formed out of the banks andbed which reduces the erosive power of the water.Poor channel operating practices, particularly of multi-bay regulators and offtakescan also cause scour of the bed and banks. Flows should be evenly distributedacross the channel section and not concentrated in bays close to a bank.bell shapedPhoto 10-9Bell-shaped stilling basin eroded from banks and bed below regulatorConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 10-10