Contents preface (VII) introduction 1—37

G = (H / d)	1	= 5.0 ×	1	= 0.128 = 1 in 7.8
E		π	λ	7.0	3.14 3.15

348 IRRIGATION AND WATER RESOURCES ENGINEERING
9.4. UPLIFT FORCE ON THE FLOOR OF CANAL STRUCTURE
Seepage below the canal structure causes uplift force on the floor of the structure. This force is likely to be maximum when the water is ponded up to the highest level on the upstream side with no discharge on the downstream side. The subsoil hydraulic gradient line for this case has been shown in Fig. 9.27 and the net uplift pressure head at any section on the downstream
U/S Tel

A
B

Subsoil HGL (With flow) ^E D/S Tel

D

Pond level

E

C

0₂

0₁

F
G
Fig. 9.27 Determination of uplift forces
of the barrier is the difference between the hydraulic gradient line and the floor level. This has been shown as o₁ in the figure. When there is flow, hydraulic jump forms and corresponding subsoil hydraulic gradient line for this case is also shown in Fig. 9.27. The net uplift pressure head for this condition will be obtained by measuring the distance of the water surface from the subsoil hydraulic gradient line at the desired section. This has been shown as o₂ in the figure. For most of the downstream sections, o₂ will be smaller than o₁. However, in the vicinity of the jump trough, o₂ may be greater than o ₁. The floor thickness on the downstream side should, obviously, be based on the larger of the two values of o₁ and o₂.
For determining the uplift pressures at any section upstream of the jump trough the water surface profile needs to be determined. For this purpose, measure the difference between the levels of the total energy line and the floor at the section. This difference E is the value of specific energy at that section. Using Montague’s curves (Fig. 9.28) or the specific energy equation, E = h + (q²/2gh²), one can determine the supercritical depth of flow h for known E and q. In this way, one can obtain the water surface profile upstream of the jump. The jump profile can be obtained as explained in Sec. 9.2.4. The downstream sub-critical depth can also be obtained by solving the specific energy equation or using Montague’s curves.

SURFACE AND SUBSURFACE FLOW CONSIDERATIONS FOR DESIGN OF CANAL STRUCTURES

349

	6.5
	6.0
	5.5
	5.0
	4.5
	4.0
													2
									depth			12	m	/s
									q	=
								critical
(Meters)	3.5						of
					Locus	10
					9
E	3.0
							8
							7
	2.5
						6
	2.0				5
					4.5
					4
					3.5
	1.5			3
				2.5
				2
	1.0		1.8
			1.6
			1.4
			1.2
			1.0
	0.5
			line
			h
		E	=
	0
	0		0.5	1.0	1.5	2.0			2.5					3.0	3.5	4.0	4.5

h (Metres)