construction and refurbishment of earthen irrigation channel banks

The water section must be able to contain the maximum design flow below the top of thebanks by the amount of freeboard.The design of the longitudinal and cross sections of a given channel are usually carriedout simultaneously because the water surface slopes are tentatively selected before thecross sections can be determined to carry the design flow.The major design variables for the water cross section are the:• Bed width Section 12.16.6• Water depth Section 12.16.5• Freeboard Section 12.13• Batter slopes Section 12.16.4• Bed longitudinal slope Section 12.14Usually, the longitudinal bed slope is selected to be equal to the required water surfaceslope.Earthworks for new channel construction are usually designed as far as possible tobalance cut and fill. This involves selecting a channel section that provides withreasonable lead, sufficient suitable material for compacted banks. The objective is toachieve the most cost-effective solution overall by optimising channel size and theamount of borrow. To do this, the capabilities and limitations of available constructionequipment will need to be taken into consideration as part of the design process.As far as practicable, the cross sectional shape of a channel should be based on highhydraulic efficiency. However, while the shape that has the minimum wetted perimeterprovides the maximum hydraulic efficiency, achieving high hydraulic efficiency inearthen channels is usually much less important than economic considerations and certainpractical construction, operational and maintenance requirements.Channel sections based on hydraulic efficiency alone will have depths much greater thanwidths. These greater depths can be impractical to construct or very expensive, becausethe unit cost of excavation increases greatly with depth.Because of the small width-depth ratio, the channel will experience large variations inwater levels when flows are less than the full design capacity, and this can causesignificant operational, service level and bank deterioration problems.The cross section of channels should be designed so that available grades can maintainthe required flows at mean velocities between the maximum and minimum permissiblelimits discussed in Section 12.11.In the case of new channel construction, to produce enough bank material from thewaterway excavation, most low capacity channels will need to be over-excavated,resulting in low velocities and flat water surface grades.12.16.1 Hydraulic DesignIn Australia, the Mannings flow formula is the most familiar and widely usedchannel design approach for steady uniform flow conditions. Refer to Section12.11.1Mannings Formula V = R 2/3 S ½nConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-35