Contents preface (VII) introduction 1—37

Cross-section of the Guide Bunds :
The top width of guide bunds varies from 6.0 m to 9.0 m and the side slopes for the two faces of guide bunds are generally kept 1V:2H or flatter. A freeboard of 1.5 m to 1.8 m above the maximum flood level is provided. For the guide bunds, the top width is proposed as 8.0 m, side slopes 1V:2H and the freeboard as 2.5 m. The freeboard is kept on higher side to accommodate afflux. Cross-sections at two locations of the guide bunds are shown in Fig. 12.12.

428 IRRIGATION AND WATER RESOURCES ENGINEERING

					River Flow		L2	= 165 m
	Upstream curved			L1 = 725 m			Downstream
	head, or impregnable	A						curved head
	head (R1 = 250 m)		Pitched slope	Apron		(R2 = 125 m)
	Pitched				Crest
	slope
	Front
	apron				Slope with	Shank	R2
		Rear	A		vegetal cover		1 = 60°
		apron	^R1						R2
		Pitched					C of	Downstream
		slope					L	nose
						bridge
	1 = 145°
B	B	R1
					PLAN
		Apron
		Upstream nose
					Pitching	Reserve of stone
		HFL 88.50 m Free board 2.5 m		Crest 91.0 m
				2H:1V
		0.5 Ds	1.5 Ds			8.0 m
		2.25 T = 2.7 m			T = 1.2 m			River bed
	R
					1.5 T = 1.8 m
			Ds		1.25 T = 1.5 m
			Deepest	Section A–A
			scour level

Crest 91.0 m

H.F.L. 88.50 m

0.5 D_S

25. 
    T =
    
    7. 
       m
       

R
^Ds

Pitching
Free board 2.5 m

2H:1V
1.5 DS	T = 1.2 m
1.5 T = 1.8 m
1.25 T = 1.5 m
Deepest
scour level

Reserve of stone
Pitching

2H:1V T = 1.2 m

Launched position of apron

Launching apron

River bed

Section B–B
0.2 to 0.25 m gap filled with gravel

(Not to scale)

Concrete blocks interconnected with pre-grouted hooks
Details of laying of concrete blocks for launching apron

Fig. 12.12 Details of guide bund (plan, cross-sections of shank and curved head)

RIVERS AND RIVER TRAINING METHODS

429

Bank Revetment :
The thickness T required for the bank revetment is related to the high flood discharge by the following empirical equation proposed by Inglis (2, 5):
T = 0.04 to 0.06 Q^1/3 in SI units Adopting an average value of 0.05
T = 0.05Q^1/3 = 0.05 (13100)^1/3 ≅ 1.2 m

A geosynthetic filter or a conventional sand-gravel inverted filter of 0.30 m thickness may be placed on the sloping surface of the guide bunds facing the river flow and the revetment provided over this filter; see Fig. 12.13.

Guide bund

1 Geosynthetic or sand gravel filter 0.3 m
2 & 3 Hand-placed boulders 0.30 m
4 & 5 Dry random rubble masonry 0.30 m
5 4 3 2 1
Details of bank revetment

6.0 m	6.0 m	6.0 m	6.0 m
0.3 m
			Masonry wall

Front elevation of guide bund from the river side (Drawing not to scale)

Fig. 12.13 Bank revetment
For the revetment itself, four layers of 0.30 m size boulders are suggested. The first two layers should be of about 0.30 m size hand-placed boulders are suggested. The first two layers should be of about 0.30 m size hand-placed boulders followed by third and fourth layers of thickness 0.30 m each of dry random rubble masonry on top of the hand-placed boulders. The rubble may be placed in a closely-packed formation inside a grid formed by masonry walls of 0.30 m width along the bank slope at a spacing of about 6.0 m measured in the direction of the river flow. The revetment should extend to 0.5 m above the HFL of the river.
Design of Launching Apron :
Design of launching apron requires estimation of scour depth. Discharge for the design of foundation is usually taken as 1.1 to 1.2 times maximum flood discharge. Adopting the value of 1.2Q i.e., 15,720 m³/s, the normal scour depth below HFL, R may be computed from Lacey’s equation, Eq. (8.32).

430 IRRIGATION AND WATER RESOURCES ENGINEERING
F QI ^1/3

R = 0.48 ^{G J}

H f K

where, f = 1.76 d = 1.76 0.3 = 0.96

F ₁₅₇₂₀I ^1/3

Thus, R = 0.48G J = 12.19 m

H 0.96 K
Actual scour depth below HFL is increased to the following values for different parts of the guide bund following the recommendations of Inglis (2):

Upstream nose	:	2.25R = 2.25 × 12.19 = 27.43 m
Downstream nose	:	1.625R = 19.81 m
Straight portion	:	1.5R = 18.29 m

The average thickness of the launching apron in its launched position is generally taken as 1.25 T. Since T = 1.2 m, the required thickness of the launching apron in its launched position is 1.5 m.

The minimum size of the boulder d_min, to be placed in the apron (so that it is not washed away by the flow), is related to the maximum velocity U_max in the vicinity of the guide bund
(22).
d_min = 0.023 to 0.046 (U_max)² in SI units = say, 0.0345 (U_max)²
For the estimation of U_max, the value of Q is taken as 15720 m³/s and Lacey’s silt factor is taken as 0.96.
Using Lacey’s regime equation, the regime velocity, U, can be computed as

F Qf 2 I ^1/6	F	(15720)(0.96)(0.96)I	1/ 6
U = G	J	= G	140	J	= 2.17 m/s
H	140 K	H		K

Assuming the maximum velocity to be 50% higher than the regime velocity,
U_max = 1.5 × 2.17 = 3.26 m/s
d_min = 0.0345 (U_max)² = 0.0345 (3.26)² = 0.37 m, say 0.4 m
The stone requirement for the launching apron is computed on the assumption of uniform apron thickness of 1.25 T in its launched position. Thus, if D_s is the depth of scour below the river bed, the quantity of stone required for launching apron of 1 m length (along the guide
bund) would be 5 D_s × 1.25T, i.e., 2.8D_sT m³, i.e., 3.36D_sm³ for T = 1.2m. This volume of stone

is provided in the form of a wedge (to account for nonuniformity of stone layer thickness in launched condition) as shown in Fig. 12.12. Boulders of size ranging from 0.3 m to 0.4 m may be used to construct launching apron. Alternatively, one may provide cement concrete blocks of size 1.0 m thick, 1.5 m long and 1.5 m wide instead of the boulders. The concrete blocks are to be laid leaving a gap of about 0.2 to 0.25 m between them as shown in Fig. 12.12. This gap would be filled with gravelly material. Further, these blocks are to be interconnected with the help of pregrouted hooks to prevent dislodging of individual blocks and ensure that these blocks launch as a monolithic apron into the scour hole.

RIVERS AND RIVER TRAINING METHODS