construction and refurbishment of earthen irrigation channel banks

drop bars become difficult and time consuming to manually handle for depths ofwater in excess of a metre.The cost issues associated with structures include:• longer spans or additional spans on bridges with shallow piers verses shorter orfewer spans with deeper piers.• wider shallower headwalls verses narrower deeper headwalls.• larger number of regulator bays of shallow depth verses fewer bays of greaterdepthGeological conditions, slope stability, groundwater, limitations of construction andmaintenance equipment and the dimensions of structures generally governmaximum depth of cut for channels. Deep channels are more expensive toconstruct because the unit cost of excavation increases significantly with depth.In some channels, the desirable depths of flow may not be feasible because ofgeological conditions:• cuts that extend through impermeable strata into porous formations areundesirable because of increased seepage losses• cuts that extend through earthen deposits into rock strata are undesirable becauseof increased excavation costs• cuts that extend below the ground water table are undesirable because ofconstruction difficultiesA wider shallower channel provides more stable flow regulation conditions. Moreconstant water levels can be maintained in the channel because the higher width todepth ratio means that the levels are less affected by changes in flow rates. Anarrower, deeper channel will experience larger variations in water level withchanges in flow, and this can cause significant operational, service level and bankdeterioration problems.12.16.5.1 Emptying of ChannelDepths and structure inverts should be designed so that the down stream slope ofthe channel bed enables complete emptying of the channel (excluding siphons)via one practicable route during non-operational periods, so that maintenance andreplacement works can be carried out on the channel waterway and structures.12.16.6 Bed WidthThe adopted bed width is generally the most practical one for constructionpurposes.A wide bed width is an advantage for earth moving equipment, and the minimumpractical bed width for construction is about 1.5 to 2 metres. A bulldozer,excavator or grader can be used to construct beds of this width. While theminimum bed width for construction with a self-elevating scraper is 2.5 to 3 metres.For these reasons, a minimum bed width of 2 to 2.5 metres is frequently adopted.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-44