31-03-2012, 03:20 PM
SURFACE AND SUBSURFACE FLOW CONSIDERATIONS FOR DESIGN OF CANAL STRUCTURES
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 water
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 channels.
Aqueducts, siphon aqueducts, siphons, and superpassages are examples of crossdrainage
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.
9.2. HYDRAULIC JUMP
Hydraulic jump occurs when, in the same reach of a channel, the upstream control causes
supercritical flow while the downstream control dictates subcritical flow. Hydraulic jump is
always accompanied by considerable turbulence and energy dissipation. The following are the
useful applications of hydraulic jump:
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SURFACE AND SUBSURFACE FLOW CONSIDERATIONS FOR DESIGN OF CANAL STRUCTURES
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318 IRRIGATION AND WATER RESOURCES ENGINEERING
(i) The dissipation of energy of flow downstream of hydraulic structures such as dams,
spillways, weirs, etc.
(ii) The reduction of net uplift pressures under hydraulic structures by raising the water
depth on the apron of the structure.
(iii) The maintenance of high water levels in channels for water distribution purposes.
(iv) The mixing of chemicals for water purification or other purposes (in chemical industries).
On applying Newton’s second law of motion to the control volume, shown in Fig. 9.1, one
obtains,
P1 – P2 + W sin θ – Ff = ρQ(β2 u2 – β1 u1) (9.1)
where P1 (= ρg z1A1) and P2 (= ρg z2 A2) are the pressure forces at sections 1 and 2, W the
weight of liquid between sections 1 and 2, Ff the component of unknown forces (along the
direction of flow) acting between sections 1 and 2, θ the longitudinal slope of the channel, β1
and β2 the momentum correction coefficients at sections 1 and 2, u1 and u2 are the average
velocities at sections 1 and 2 and z1 and z2 are the distances to centroids of respective flow
areas A1 and A2 from the free surface.