Canal lining

Unlined canals in permeable soils can lose a large share of their flow by seepage before water reaches the fields. Seepage also raises the water table nearby, leading to waterlogging and salinity, and weakens banks. Lining the canal with an impermeable layer reduces these losses, allows higher velocities and thus smaller sections, controls weeds and reduces maintenance. The saved water can irrigate more land.

Common linings include cement concrete cast in place or as precast slabs, burnt clay tiles or bricks laid in mortar, stone masonry, shotcrete, soil-cement, and flexible membranes of low-density polyethylene or geomembrane protected by a cover of concrete or soil. Choice depends on cost, available materials, canal size, ground conditions and expected life. Concrete is durable and hydraulically smooth, with a Manning n around 0.015 to 0.018; membranes are very watertight but need protection from damage.

Linings fail mostly from what happens behind them. Rapid drawdown leaves water pressure behind the lining with nothing in front, causing uplift and cracking; pressure relief valves and under-drains are provided where the water table is high. Expansive soils heave and crack rigid linings, so a cushion of non-expansive soil is placed under them. Joints must be sealed to stay watertight. Seepage losses are estimated by ponding or inflow-outflow tests before and after lining to measure the benefit.

Manning formula: Q = (1/n) A R^(2/3) S^(1/2), with n about 0.015 to 0.018 for good concrete linings, so a lined section can carry the same discharge at a smaller cross-section than an unlined earthen one.