9 SURFACE AND SUBSURFACE FLOW CONSIDERATIONS FOR DESIGN OF CANAL STRUCTURES 9.1. GENERAL Canal structures include the following types of structures:
(i) Communication structures, such as roads and railways which have to be constructed across the channels. Such structures are in the form of bridges and are not included in this book.
(ii) Regulation structures which are meant for controlling discharges, velocity, and wa-ter levels in the channels. Canal falls, distributary head regulators, escapes, etc., are examples of regulation structures.
(iii) Cross-drainage structures which are required to pass natural drainage across chan-nels. Aqueducts, siphon aqueducts, siphons, and superpassages are examples of cross-drainage structures.
Canal structures listed at (ii) and (iii) above may fail on account of effects of either surface flow or subsurface flow. Water flowing over the structure causes hydrostatic forces, formation of hydraulic jumps, and scour upstream and downstream of the structure. Considerations of subsurface flow are important on all hydraulic structures which have foundations other than that of solid impervious rock. Subsurface flow endangers the stability of hydraulic structures in two ways—by piping and uplift pressure. Piping failure occurs when the seepage water is left with sufficient force to lift up soil particles at the downstream end of a hydraulic structure where it emerges. Uplift pressure is the pressure exerted by the seeping water on a hydraulic structure. If this pressure is not counterbalanced by the weight of concrete or masonry floor, the structure may fail because of rupture of the floor.
