construction and refurbishment of earthen irrigation channel banks

In new channel construction, if the material is suitable, the earthworks are designedto balance the material required for banks, with the excavation of the channel crosssection. Should insufficient spoil be available from this approach, local wideningmay be used in combination with deepening to achieve a balance of earthworks.Since non-uniform flow conditions are then introduced, a backwater computationmust be carried out to determine the water surface profile.Where the bed width changes along a channel, the transition rate should be at 1metre width per 10 metre length (except through structures).The maximum bed width for large channels is usually based on the practical limitsof depth, the design flow rate and available grade.12.16.7 Top of Bank12.16.8 BermsPractical construction, operational and maintenance requirements usually dictate thewidth of the bank crest. In theory, the minimum crest width for a small channelcould be 2 metres if plant is available to work or trim such a narrow crest.However, to accommodate efficient construction equipment, minimum crest widthsare typically 2.5 to 3 metres. The minimum crest width should also meet thehydraulic gradient (phreatic grade) requirement from the maximum channel waterlevel to the outer toe of the bank.Where it is a major channel and vehicle access is located on the top of the bank, thecrest width may have been designed in the past to a 6 metre standard. Thesesituations need to be looked at more closely, as the crest width is a major costdriver. The wider the crest the greater the earthworks volumes and the larger theland acquisition. Rather than a set standard, each situation needs to be evaluated forthe lowest overall cost and the economics should prevail.Where vehicle access is required along the channel for operation and maintenance,the minimum crest width needs to be about 3.5 metres to allow the passage of largeplant, trucks and vehicles. If all-weather access is necessary, the crest can becovered with 75 to 100 mm of gravel.The crest of banks should be graded away from the channel section at a slope of notflatter than 1 (vertical):40 (horizontal), so that rainfall will drain down the outerslopes, and not down the less stable inner batter slopes that are more susceptible toerosion.Channels that have deep cuts through high land may need to have bermsconstructed on the inner batters between the water section and the top of the cut.These berms provide access to the channel for construction and maintenanceequipment, prevent sloughing of the batters caused by heavy loads too near the edgeof the channel, and reduce the erosion due to rilling that can be associated with longlengths of exposed batter draining into the channel. The erosive action of waterincreases as the slope length increases.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-45