River Training and Riverbank Protection Works (Part - 1) - Civil Engineering (CE) PDF Download

Introduction 

For constructing a hydraulic structure across a river, a water resources engineer must also consider the effect of the structure on the hydraulics of the river and the best ways to train the river such that the structure performs satisfactorily and also there is no significant damage to the riverine environment. For example, if a barrage is located within a river, then its length may span from end to end of the river width or could be smaller, if the waterway is so calculated. In the latter case, that is, when the length of a barrage is smaller than the width of a river, then certain auxiliary structures in the form of embankments have to be constructed as shown in Figure 1, known as River Training Works. At times, people residing very close to the flood zones of a river may have to be protected from the river’s fury. This is done by providing embankments along the river sides to prevent the river water from spilling over to the inhabited areas.

In order to limit the movement of the bank of a meandering river, certain structures are constructed on the riverbank, which are called riverbank protection works. Sometimes, an embankment like structure, called a Groyne or a Spur, is constructed at right angles to the riverbank and projected into the river for attracting or deflecting the flow of the river towards or away from the riverbank.

This chapter discusses the layout and design of these River Training and Riverbank Protection Works, which can together be termed as aspects of River Engineering. Of course, river engineering includes much more, like dredging to keep the pathway of ships in a river navigable, or techniques of setting up jetties for ships to berth, but they are not discussed in this lesson.

Guide bunds or banks

Alluvial rivers in flood plains spread over a very large area during floods and it would be very costly to provide bridges or any other structure across the entire natural spread. It is necessary to narrow down and restrict its course to flow axially through the diversion structure. Guide bunds are provided for this purpose of guiding the river flow past the diversion structure without causing damage to it and its approaches. They are constructed on either or both on the upstream and downstream of the structure and on one or both the flanks as required.

Classification of Guide Bunds

Guide bunds can be classified according to their form in plan as (i) divergent, (ii) convergent, and (iii) parallel and according to their geometrical shape as straight and elliptical with circular or multi-radii curved head. These are shown in Figure 2.

Figure 2. Types of guide banks and typical dimensions.

In the case of divergent guide bunds, the approach embankment gets relatively less protection under worst possible embayment and hence divergent guide bunds require a longer length for the same degree of protection as would be provided by parallel guide bunds. They also induce oblique flow on to the diversion structure and give rise to tendency of shoal formation in the centre due to larger waterway between curved heads. However, in the case of oblique approaching flow, it becomes obligatory to provide divergent guide bunds to keep the flow active in the spans adjacent to them. 

The convergent guide bunds have the disadvantage of excessive attack and heavy scour at the head and shoaling all along the shank rendering the end bays inactive. 

Parallel guide bunds with suitable curved head have been found to give uniform flow from the head of guide bunds to the axis of the diversion structure. 

In the case of elliptical guide bunds, due to gradual change in the curvature, the flow is found to hug the bunds all along their lengths whereas in the case of straight guide bunds, separation of flow is found to occur after the curved head, leading to obliquity of flow. Elliptical guide bunds have also been found to provide better control on development and extension of meander loop towards the approach embankment.  

Length of Guide Bunds

The length of the guide bund on the upstream is generally kept as 1.0 to 1.5L where L is the width between the abutments of the diversion structure. In order to avoid heavy river action on the guide bunds, it is desirable to limit the obliquity of flow to the river axis not more than 300 as indicated in Figure 1. The length of the downstream guide bund is kept as 0.25L to 0.4L.

For wide alluvial belt, the length of guide bunds is decided from two important considerations, viz. the maximum obliquity of the current and the permissible limit to which the main channel of the river can be allowed to flow near the approach embankment in the event of the river developing excessive embayment behind the training works. The radius of the worst possible loop has to be ascertained from the data of the acute loops formed by the river during the past. Where river survey is not available, the radius of the worst loop can be determined by dividing the radius of the average loop worked out from the available surveys of the river by 2.5 for rivers having a maximum discharge up to 5000 cumecs and by 2.0 for a maximum discharge above 5000 cumecs.

Curved head and tail of Guide Bunds

The upstream curved head guides the flow smoothly and axially to the diversion structure keeping the end spans active. The radius of the curved head should be kept as small as possible consistent with the proper functioning of the guide bund. The downstream curved tail provides a smooth exit of flow from the structure.

From the hydraulic model tests conducted for a number of projects over the past years, it has been found that a radius of the curved head equal to 0.4 to 0.5 times the width of the diversion structure between the abutments usually provides satisfactory performance. The minimum and maximum values could be 150 m and 600 m respectively. However, the exact values are to be ascertained from model tests. The radius of the curved tail generally ranges from 0.3 to 0.5 times the radius of the curved head.

According to the river curvature, the angle of sweep of curved upstream head ranges from 1200 to 1450. The angle for the curved tail usually varies from 45⁰ to 60⁰.

In the case of elliptical guide bunds, the elliptical curve is provided upto the quadrant of the ellipse and is followed by multi-radii or single radius circular curve. In case of multiradii curved head, the larger radius adjacent to the apex of the ellipse is generally kept as 0.3 to 0.5 times the radius of the curved head for straight guide bund with the angle of sweep varying from 45⁰ to 60⁰ and the smaller radius equivalent to 0.25 times the radius of curve head for straight guide bund with sweep angle of 30⁰ to 40⁰. 

Design of guide bunds 

After fixing up the layout of the guide bunds in accordance with the guidelines mentioned in the foregoing paragraphs, the details of the guide bund sections have to be worked out. The various dimensions worked out are top width, free board, side slopes, size of stone for pitching, thickness of pitching, filters and launching apron. The guide lines for the same are given below. 

Top width of guide bund 

At the formation level, the width of the shank of guide bunds is generally kept 6 to 9 m to permit carriage of material and vehicles for inspection. At the nose of the guide bunds, the width is increased suitably in a bulb shape to enable the vehicles to take turn and also for stacking reserve of stone to be dumped in places whenever the bunds are threatened by the flow. 

Free board for Guide Bund 

A free board of 1 to 1.5 m above the following mentioned two water levels has to be provided and the higher value adopted as the top level of the upstream guide bund:

(i) Highest flood level for 1 in 500 years flood

(ii) Affluxed water level in the rear portion of the guide bank calculated after adding velocity head to HFL corresponding to the design flood (1in 100 year frequency) at the upstream nose of the guide bank.

On the downstream side also, a free board of 1 to 1.5 m above the highest flood level for 1 in 500 years flood is to be adopted. 

Side slopes of guide bund 

The side slopes of guide bund have to be fixed from stability considerations of the bund which depend on the material of which the bund is made and also its height. Generally the side slopes of the guide bund vary from 2:1 to 3:1 (H:V). 

Size of stone for pitching 

The sloping surface of the guide bund on the water side has to withstand erosive action of flow. This is achieved by pitching the slope manually with stones. The size and weight of the stones can be approximately determined from the curves given in Figure 3. It is desirable to place the stones over filters so that fines do not escape through the interstices of the pitching. For average velocities up to 2 m/sec, burnt clay brick on edge can be used as pitching material. For an average velocity upto 3.5 m/sec, pitching of stone weighing from 40 to 70 kg (0.3 to 0.4 m in diameter) and for higher velocities, cement concrete blocks of depth equal to the thickness of pitching can be used. On the rear side, turfing of the slope is normally found to be adequate.

Figure 3. Graph for determining pitching and stone aprons.

Thickness of Pitching 

The thickness of pitching is to be kept equal to the size of the stone for pitching determined. However, it should not be less than 0.25m. wherever the velocities are high for which the size of stone is greater than 0.4 m, cement concrete blocks of thickness 0.4 to 0.5 or 0.6 m may be used. 

Provision of filter 

It is always desirable to provide an inverted (graded) filter below the pitching stones to avoid the finer bund materials getting out through the interstices. The thickness of the filter may be 20 to 30 cm. Filter has to satisfy the criteria with respect to the next lower size and with respect to the base material: 

(i) For uniform grain size filter, 

R₅₀ = D₅₀ of filter material / D₅₀ of base material = 5 to 10

(ii) For graded material of sub-rounded particles, 

R₅₀ = D₅₀ of filter material / D₅₀ of base material = 12 to 58

R₅₀ = D₁₅ of filter material / D₁₅ of base material = 12 to 40

Launching apron 

Just as launching apron is provided for the main structure both on the upstream and downstream it has to be provided for guide bunds also in the bed in continuation of the pitching. The different aspects to be looked into are the size of the stones, depth of scour, thickness, slope of launched apron, shape and size of launching apron. 

The required size of stone for the apron can be obtained from the curves. In case of non-availability of required size of stones, cement concrete blocks or stone sausages, prepared with 4 mm GI wire in double knots and closely knit and securely tied, may be used. 

The scour depths to be adopted in the calculations for the launching apron would be different along the length of the guide bund from upstream to downstream, as given in the following table. The value of R, that is the normal depth of scour below High Flood Level may be determined according to Lacey’s scour relations.

While calculating the scour values, the discharge corresponding to 50 to 100 years frequency may be adopted. However, after construction and operation of the diversion structure, the portions of the guide bund coming under attack of the river flow should be carefully inspected and strengthened as and when necessary. 

The thickness of apron of the guide bund should be about 25 to 50 percent more than that required for the pitching. While the slope of the launched apron for calculation of the quantity can be taken as 2:1 for loose boulders or stones, it may be taken as 1:5:1 for c.c blocks or stone sausages. 

From the behaviour of the guide bunds of previously constructed diversion structures, it has been observed that shallow and wide aprons launch evenly if the scour takes place rapidly. If the scour is gradual, the effect of the width on the launching of apron is marginal. Generally a width of 1.5 R has been found to be satisfactory. For the shank or straight portions of the guide bunds, the thickness of the apron may be kept uniform at 1.5 T where T is the thickness of the stone pitching. To cover a wider area, for the curved head, the thickness is increased from 1.5 to 2.25 T with suitable transition over a length of L1 equal to one fourth of the radius of the curved head and provided in the shank portion only. On the rear side of the curved head and nose of the guide bund, the apron should be turned and ended in a length equal to about one fourth of the respective radius.