Contents preface (VII) introduction 1—37

Fig. 12.9 Scour patterns for different spurs
12.5.3. Guide Banks
While selecting a site for bridge on an alluvial river, certain site requirements are always kept in mind. These requirements include straight reach of the river and small width of the river at the bridge site. The crossing reach between two successive bends of a meandering site is suitable from these considerations. However, the meandering pattern itself migrates and, hence, steps must be taken to ensure that the flow path does not change through the waterway at the bridge site, and also that the approach road embankment is not endangered due to the smaller waterway provided. For this purpose, earthen embankments are provided on one or both sides of the river at the bridge site. These embankments are known as guide banks (or guide bunds).
Guide banks are artificial embankments meant for guiding the river flow past a bridge (or other hydraulic structures such as weirs or barrages) without causing damage to the bridge and its approaches (3). Guide banks are built along the flow direction both upstream and downstream of the structure on one or both sides of the river as desired. Guide banks for a bridge restrict the waterway at the bridge site and prevent the outflanking of the bridge by the changing course of the river. The design criteria of guide banks are based on the works of Spring (18) and Gales (19).

424 IRRIGATION AND WATER RESOURCES ENGINEERING
The first step in the design of a bridge on an alluvial river is the estimation of the minimum and also a safe waterway. A reasonable estimation of clear waterway to be provided between guide banks can be obtained by equating it to Lacey’s regime perimeter given by Eq. (8.29) . The overall waterway between the guide banks is obtained by adding the thickness of piers to the clear waterway. Sharma and Asthana (20), based on their studies of design and performance of 20 bridges constructed during 1872 - 1966, recommended a waterway width (in

metres) varying from 3.3 Q to 7.1 Q in the alluvial stage and from 2.4 Q to 4.8 Q in the

boulder stage. Here, Q is the river discharge in m ³ /s. Obviously, a smaller waterway would cause a large afflux resulting in danger of outflanking.

Figure 12.10 shows the plan and sections of a typical guide bank. In plan, the guide bank can be either parallel, converging upstream or diverging upstream. In general, guide banks diverging upstream need to be longer than the straight or converging guide banks (5) . Flows with acute curvature result in shoal formation (21) near the shanks [Fig. 12.11 ( a )]. Hence, one may consider providing elliptical shanks, [Fig. 12.11 ( b )] instead of straight shanks. However, there is a possibility of shoaling, away from the elliptical guide bank, due to the divergence and this should be studied carefully in a model before finalising the design.

Flow

0.25 L
Downstream
curved head	Apron

	45° to 60°	Shank
Downstream	C of work (Bridge etc.)
nose
	H.F.L.	Pitching

1.25 L to 1.50 L

		Upstream curved
Pitched slope	A	head, or
impregnable head
	Crest		Pitched
		slope
		Front
Slope with		apron
vegetal cover	A	Rear
		apron
Plan		Pitched
		slope
Reserve of stones	120° to 145°
Crest

0.5 D

2.25 T
D

0.5 D

1.5 D	2:1
	T

1.5 T

1.25 T
Deepest scour level
Pitching
H.F.L.

1.5 D	2:1
	T

2:1

Section at A–A
Reserve of stones
Pitching

2:1 Launching

_T appron

B B
Apron

Upstream nose

2.25 T
	1.5 T	Launched
D	1.25 T
position
		Deepest	of apron
		scour level	Section at B–B

Fig. 12.10 Guide bank

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425

The length of guide banks upstream of the bridge should be 1.1 times the length of the bridge (18). Gales (19), however, has recommended that this length should be between 1.25 and 1.5 times the bridge length for flood discharges ranging from 7000 to 70,000 m³/s. The length of guide banks downstream of the bridge should be about 0.25 times the bridge length

(5).

Flow

120° to 150°

R
R = 0.4 To 0.5 P_W

P_W = Lacey¢s perimeter

Shoal
Axis
W
0.3 to 0.5 R
Fig. 12.11 (a) Flow near a straight guide bund with circular head

Flow

R = 0.25 W 30° to 45°

x	2	+ y	2
2	2 = 1
a	b

		a
Axis	b	Origin
W		0.25 to 0.4 W

45° to 60°

Fig. 12.11 (b) Flow near an elliptical guide bund

426 IRRIGATION AND WATER RESOURCES ENGINEERING
Radius of the upstream curved head R₁ should be equal to 2.2 Q metres. A smaller radius is permissible for smaller rivers. The sweep angle generally varies between 120° and 145° for the upstream curved head. The radius of the downstream curved head R₂ is generally kept equal to 1.1 Q metres. A sweep angle of 45° to 60° for the downstream curved head is considered satisfactory.

The elevation of the guide banks is obtained by adding a freeboard of 1.5 to 2.5 m to the high flood level of 100-year-flood. Alternatively, a freeboard of 1.0 m is added to the high flood level of a 500-year flood to determine the elevation of the top of guide bank. The top width of guide banks is generally fixed between 6 to 9 m (5). Both faces of the guide banks have side slopes of 1V:2H or flatter. Locally available sand, silt, and gravel are used for the construction of the core of the guide banks. To protect the face towards the river and the back of the curved heads of the guide banks from severe erosion, large stone pitching is provided. The embankment - side faces of the guide banks, however, do not need such protection. The stones used for slope protection must be large enough to withstand the force of current and stay in place. The minimum size of stones of relative density 2.65 required for this purpose can be calculated by the empirical relation (2, 22),

d = 0.023 to 0.046U^2

(12.6)

where, d is in metres and U is in m/s. Normally, angular and graded stones having the ability to interlock and weighing between 450 and 1800 N are used for slope protection. The thickness of stone pitching T in metres is related to river discharge Q (in m³/s) by an empirical equation as follows (2, 5, 22):

T = 0.04 to 0.06Q^1/3

(12.7)

For large streams, the constant 0.06 should be reduced to 0.04 (5). Thickness for pitching should be increased by 25 per cent for the curved head region. The pitching must be provided on both sides on the guide bank embankments in the curved head regions.
12.5.3.1. Launching Apron
Heavy scour of the river bed at the curved heads and shanks of guide banks can cause undermining of the stone pitching thereby resulting in failure of the guide banks. Such failure of guide banks can be prevented by providing launching aprons (consisting of stones) beyond the toe of the guide banks as shown in Fig. 12.10. As the scour continues, the launching apron is undermined and it eventually covers the face of the scour hole adjacent to the guide bank. The slope of the apron in the launched position varies between 1(V) : 1.25(H) and 1(V) : 2.5(H). For the purpose of design, a slope of 1(V) : 2(H) for loose boulders and 1(V) : 1.5(H) for concrete blocks can be assumed (23). The scour depths (below HFL) in the vicinity of the guide banks can be taken as K times Lacey’s normal scour depth given by Eq. (8.32) or Eq. (8.33). The value of K is taken as 2.25 to 2.75 at the upstream nose and 1.5 to 2.0 at the downstream nose of guide banks, 1.25 for transition from the straight portion to the nose of guide bank and 1.5 to 2.0 for straight reach of guide banks (2, 5, 22). The width of the launching apron is generally kept equal to 1.5 times the scour depth (below the bed) at that place. 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 is the depth of scour below the bed, the quantity of stone required for launching apron of 1 m length (along the guide bank) would be 5 D × 1.25T i.e., 2.8 DT m³. This volume is provided in the form of a wedge (to account for the non-uniformity of stone layer thickness in launched condition) as shown in Fig. 12.10. The launching apron

RIVERS AND RIVER TRAINING METHODS

427

should be provided on both sides of the guide bank embankements in the curved head regions. No spur should project from a guide bank. For maintenance purposes, a reserve of stones is usually kept ready on the top of the guide bank for dumping, if the bank is threatened.

Generally, a filter is always provided below the stone pitching provided for the bank protection. However, as per the earlier practices, the filter was generally, not provided between the launching apron and the river bed. It was believed that filter might hinder the launching of the apron. In the absence of a filter between the launching apron and the river bed, finer particles from the bed near the toe of the bank would escape through the voids in the apron which may result in vertical sinking of the apron and the toe of the bank gets exposed for possible failure of the bank protection. Hydraulic model studies (24) have indicated that the provision of filter below launching apron does not hinder its launching. During the launching of the apron, however, some filter material may get carried away downstream by the flow. Therefore, one should provide a relatively thicker filter layer. Or, alternatively, one may provide synthetic fibre filter.
Example 12.1 Design guide bunds (or banks) and launching apron required to be provided for a bridge across a river whose total waterway is 658.88 m. The design flood discharge is 13100 m³/s which may be increased by 20% for the design of launching apron. Mean size of the river bed material is 0.3 mm.
Solution: The geometric parameters of the guide bunds are calculated from the equations given by Spring (18).
Length of the guide bund upstream of the bridge, L₁ is given as L₁ = 1.1 L = 1.1 x 658.88 ≅ 725.0 m
Length of the guide bund downstream of the bridge, L₂ is given as L₂ = 0.25 L= 0.25 x 658.88 ≅ 165.0 m
The radius of the upstream curved head, R₁ is given as
R₁ = 2.2 Q = 2.2 13100 = 251.8 m ≅ 250 m The radius of the downstream curved head, R₂ is given as




R₂ = 1.1 Q = 1.1 13100 ≅ 125 m

The upstream curved head is extended to subtend an angle (θ₁) of 120° to 145° at its center, while the downstream curved head subtends an angle (θ₂) of 45° to 60°. For the present design, θ₁ and θ₂ are assigned values of 145° and 60°, respectively. Plan-form of the design guide bund is shown in Fig. 12.12.