11.8. UPLIFT PRESSURE ON TROUGH In case of siphon aqueducts, the roof of the culvert (or the underside of the trough) is subjected to uplift pressure because the downstream water level is higher than the lower surface of the culvert covering. This uplift pressure head at any point of the culvert covering is equal to the vertical distance between the hydraulic gradient line and the underside of the trough (or the roof of the culvert). The uplift pressure is maximum when the stream is carrying the highest flood. The worst condition for the design of roof covering of the culvert would occur when the highest flood condition coincide with no flow in the canal. Knowing the afflux and the downstream level, the hydraulic gradient line can be drawn as shown in Fig. 11.11. The maximum uplift pressure, obviously, occurs at the upstream end. The thickness of the culvert roof designed to support the load of canal water and its own dead weight would generally be adequate for uplift pressures too as part of the uplift pressure is counterbalanced by the weight of canal water.
