--- title: "Why a canal fall; site selection factors" subject: "Irrigation and Drainage Engineering" chapter: 7 key: "semester_6/irrigation_and_drainage_engineering:canal-fall-purpose-site-selection" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/canal-fall-purpose-site-selection site: BCT Engineering --- # Why a canal fall; site selection factors Irrigation and Drainage Engineering · Chapter 7: Regulating Structures - Asked in: 2079 Asoj, 5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/canal-fall-purpose-site-selection - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-7 ## Question Why fall is constructed in an irrigation canal? What factors do you consider while selecting the site for a canal fall? ## Short answer A fall lowers the canal bed to prevent excessive velocity and scour, avoid high embankments, and maintain command. Site selection considers: where filling would be excessive, near off-takes or bridges, good foundation, economical grouping of drops, straight approach, and proximity to materials. ## Answer #### Why a fall is constructed The ground level of a command often slopes more steeply than the permissible bed slope of the canal. If the canal were laid at the steeper ground slope, its velocity would exceed the non-scouring limit and the bed and banks would erode. If it were laid at the permissible (flatter) slope, it would rise above the ground downstream and run in high, costly embankment (filling). A **fall (drop)** lowers the canal bed at one point in a controlled way and dissipates the surplus energy safely. It therefore: 1. Prevents scour by keeping the bed slope within the permissible value. 2. Avoids excessive filling and keeps the canal near balancing depth, so earthwork is economical. 3. Keeps the d/s FSL just above the ground level, so the command is maintained. 4. Can be combined with other works (bridge, regulator, outlet, flume or power plant). #### Factors for the site selection of a fall 1. At the point where the canal would otherwise come into excessive filling, while the d/s FSL after the drop still stays above the ground level (so command is not lost). 2. Near a **distributary off-take** or a cross regulator, so that the structures can be combined. 3. Near a **bridge** or road crossing, so that one structure serves two purposes. 4. Good, stable foundation soil (rock or hard soil) with adequate bearing capacity; avoid loose or expansive soils. 5. Where the cost is lowest: a few moderate drops are usually more economical than many small ones. 6. At the head of a steep reach, so that the canal below has a flatter slope. 7. Away from bends or curves; the approach should be straight. 8. Close to the construction site and material sources, for lower cost. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a fall needed when ground slope is steeper than the permissible canal slope? A. Laying the canal at the ground slope would give velocity above the non-scouring limit and erode the bed B. Water would stop flowing C. The canal would carry too little silt D. The canal would become too wide Answer: A. A steeper bed slope gives excessive velocity and scour; a fall lowers the bed in one step and dissipates the surplus energy. (Key point: Why a fall is needed) 2. What is the problem if the canal is laid at the permissible slope on steep ground without a fall? A. It silts the outlets B. It goes into deep cutting and becomes costly C. It loses all water D. It becomes supercritical Answer: B. The ground drops away so the flat canal ends up in deep excavation. (Key point: Alternative without a fall) 3. Which site is favourable for a fall? A. Anywhere on a sharp bend B. On loose expansive soil C. Near a bridge or a distributary off-take so that structures can be combined D. On a gently rising reach Answer: C. Combining the fall with a bridge, regulator or off-take saves cost. (Key point: Site selection: combining structures) 4. What foundation and approach conditions are preferred? A. Loose soil and a curved approach B. Expansive soil and a bend C. Soft clay close to a bend D. Good stable soil with adequate bearing capacity and a straight approach Answer: D. A firm foundation and straight approach avoid settlement and uneven flow. 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