Contents preface (VII) introduction 1—37

Table 13.2 Likely extent of scour along a weir (2)

Location	Range		Mean
Upstream of impervious floor	1.25 R to 1.75 R	1.50	R
Downstream of impervious floor	1.75 R to 2.25 R	2.00	R
Noses of guide banks and divide wall	2.00 R to 2.50 R	2.25	R
Transition from nose to straight part	1.25 R to 1.75	R	1.50	R
Straight reaches of guide banks	1.00 R to 1.50	R	1.25	R

If the sub-stratum contains any continuous layer of clay in the vicinity of the downstream cutoff, the depth of the upstream and downstream cutoffs should be suitably adjusted to avoid increase of pressure under the floor.

13.7.2. Weir Crest, Glacis, and Impervious Floor
The weir crest is provided flat at the computed level with a width of about 2 m (2). If the weir is to behave as a broad-crested weir, the width should be more than 2.5 times the head over the weir. The upstream slope of the weir is fixed between 2(H):1(V) to 3(H):1(V).
The downstream slope of the weir crest and downstream horizontal floor (i.e., stilling basin) should be such that they result in the maximum dissipation of energy through stable hydraulic jump besides being economic. The slope of the downstream glacis should be around 3(H):1(V). The level of the downstream horizontal floor is fixed in such a manner that the hydraulic jump starts at the end of the glacis or upstream for all discharges. The location of the hydraulic jump is determined (for high flood and pond level discharges) by using the method of Sec. 9.2.8. The floor level is kept at or below the lower of the required floor levels for these two conditions. A concentration factor of 1.2 is usually adequate for the design of the stilling basin.
The total floor length of impervious floor includes the downstream basin length, glacis, weir crest, and upstream floor. The impervious floor in conjunction with the downstream cutoff

446 IRRIGATION AND WATER RESOURCES ENGINEERING
should result in safe exit gradient. Besides, the hydraulic jump must remain confined within the downstream floor. It should also satisfy the requirements of uplift pressures.
The length of the downstream horizontal floor should be such that the entire jump is confined only to the floor. This will ensure that the filter and the stone protection provided on the downstream of the floor are not affected adversely by the jump. Hence, the length of the downstream horizontal floor is kept equal to the length of the jump which is equal to five to six times the height of the jump [(i.e., 5 to 6 (h₂ – h₁)]. Here, h₁ and h₂ are pre-jump and post-jump depths of flow. Obviously, the maximum height of the jump should be considered for these calculations.
The length of the upstream horizontal floor, provided at the river bed level, is decided in such a manner that the resulting exit gradient, G_E is less than the safe exit gradient for the soil under consideration. In Eq. (9.66), the depth of the downstream cutoff is measured below the bed (or the scoured bed, if scour is anticipated). Theoretically, the safe exit gradient should be equal to the critical gradient which is unity for average soils as given by Eq. (9.50). However, in practice, the safe exit gradient is kept lower than the critical gradient mainly due to non-uniformity of the soil conditions. Besides, the presence of faults and fissures in the subsoil, possibility of scouring up to the bottom of the vertical cutoff, sudden changes in head due to sudden dropping of gates, and similar factors suggest that the safe exit gradient be kept lower than the critical gradient. The recommended values of the safe exit gradient are 1/4 to 1/5 for shingles, 1/5 to 1/6 for coarse sand, and 1/6 to 1/7 for fine sand (2).
Thus, knowing the value of the safe exit gradient G_E, the head H, and the depth of the downstream cutoff d, one can determine the total floor length required, i.e., b, using Eq. (9.66) which is

G = ^H	1	(13.4)
E	d π	λ

where,	λ =	1	L	+	1 + (b / d)	2	O
2	M^1		P
			N					Q

The thickness of the impervious floor is decided from considerations of the uplift pressures which can be determined using the method proposed by Khosla et al. and as explained in Chapter 9. The thickness of floor in the jump trough should be examined for different stages of flow. For other parts of the downstream floor, however, the maximum uplift would occur when the water is at the pond level on the upstream without any tail-water (or flow) on the downstream side.