River Training and Riverbank Protection Works (Part - 2) - Civil Engineering (CE) PDF Download

Afflux bund 

Afflux bunds extend from the abutments of guide bunds (usually) or approach bunds as the case may be. The upstream afflux bunds are connected to grounds with levels higher than the afflux highest flood level or existing flood embankments, if any. The downstream afflux bunds, if provided, are taken to such a length as would be necessary to protect the canal/approach bunds from the high floods. 

Afflux bunds are provided on upstream and downstream to afford flood protection to low lying areas as a result of floods due to afflux created by the construction of bridge/structure and to check outflanking the structure. 

Layout of afflux bund 

The alignment of the afflux bund on the upstream usually follows the alluvial belt edge of the river if the edges are not far off. In case the edges are far off, it can be aligned in alluvial belt, but it has to be ensured that the marginal embankment is aligned away from the zone of high velocity flow. Since the rivers change their course, it is not necessary that a particular alignment safe for a particular flow condition may be safe for a changed river condition. Hence the alignment satisfactory and safe for a particular flow condition (constructed initially) has to be constantly reviewed after every flood and modified, if necessary.

Top width of afflux bund 

Generally the top width of the afflux bund is kept as 6 to 9 m at formation level.

Free board for afflux bund 

The top level of the afflux bund is fixed by providing free board of 1 to 1.5 m over the affluxed highest flood level for a flood of 1 in 500 years frequency.

Slope pitching and launching apron Generally the afflux bunds are constructed away from the main channel of the river. Hence they are not usually subjected to strong river currents. In such cases, provision of slope pitching and launching apron are not considered necessary. However, it is desirable to provide a vegetal cover or turfing. In reaches where strong river currents are likely to attack the afflux bunds, the slopes may be pitched as for the guide bunds.  

A typical layout and section of afflux bund are shown in Figure 4. 

Figure 4. Typical layout and section of an afflux bund.

Approach embankment 

Where the width of the river is very wide in an alluvial plain, the diversion structure is constructed with a restricted waterway for economy as well as better flow conditions. The un-bridged width of the river is blocked by means of embankments called Approach embankments or tie bunds. 

Layout of approach embankment 

In case of alluvial plains, the river forms either a single loop or a double loop depending upon the distance between the guide bunds and the alluvial belt edges. Hence the approach embankments on both the flanks should be aligned in line with the axis of the diversion structure up to a point beyond the range of worst anticipated loop. Sometimes the approach embankments may be only on one flank depending on the river configuration. 

Top Width of approach embankments 

The top width of the approach embankment is usually kept as 6 to 9m at formation level. 

Free Board of approach embankment 

Free board for approach embankment may be provided similar to that for guide bunds. 

Side slopes of approach embankment 

The side slopes of the approach embankment 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 

Velocities for 40 percent of the design discharge would be estimated and the size of stone for pitching would be determined as for guide bunds discussed in Section 6.1.1.

Thickness of pitching 

The Guide lines for determining the thickness of pitching would be the same as for guide bunds in Section 6.1.1. The velocities would be estimated for 40 percent of the design discharge.

Provision of filter 

Generally filters are not provided below the pitching stones in the case of approach embankments. However, if the section of embankment is heavy, filter may be provided as mentioned for guide bunds discussed in Section 6.1.1.  

Launching apron 

The provisions of size of stone, thickness of apron and slope of launched apron would be similar to those of guide bunds mentioned in above paragraphs. But the depth of scour for the approach embankment may be taken as 0.5 to 1.0 D_max and beyond that the width may be increased to 1.0 D_max with suitable transition in the former reach. 

Groynes or Spurs

Groynes or spurs are constructed transverse to the river flow extending from the bank into the river. This form of river training works perform one or more functions such as training the river along the desired course to reduce the concentration of flow at the point of attack, creating a slack flow for silting up the area in the vicinity and protecting the bank by keeping the flow away from it.  

Classification of Groynes or spurs 

Groynes or spurs are classified according to (i) the method and materials of construction (ii) the height of spur with respect to water level (iii) function to be performed and (iv) special types which include the following: 

These are 

• Permeable or impermeable      
• Submerged or non-submerged        
• Attracting, deflecting repelling and sedimenting and        
• T-shaped (Denehey), hockey (or Burma) type, kinked type, etc.  

The different types of spurs are shown in Figure 5.

Figure 5. Different types of spur

Impermeable spurs do not permit appreciable flow through them whereas permeable ones permit restricted flow through them. Impermeable spurs may be constructed of a core of sand or sand and gravel or soil as available in the river bed and protected on the sides and top by a strong armour of stone pitching or concrete blocks. They are also constructed of balli crates packed with stone inside a wire screen or rubble masonry. 

While the section has to be designed according to the materials used and the velocity of flow the head of the spur has to have special protection.

Permeable spurs usually consist of timber stakes or piles driven for depths slightly below the anticipated deepest scour and joined together to form a framework by other timber pieces and the space in between filled up with brush wood or branches of trees. The toe of the spur would be protected by a mattress of stones or other material. As the permeable spurs slow down the current, silt deposition is induced. These spurs, being temporary in nature, are susceptible to damage by floating debris. In bouldery or gravelly beds, the spurs would have to be put up by weighing down timber beams at the base by stones or concrete blocks and the other parts of the frame would then be tied to the beams at the base.

Layout of groynes or spurs

Groynes are much more effective when constructed in series as they create a pool of nearly still water between them which resists the current and gradually accumulates silt forming a permanent bank line in course of time. The repelling spurs are constructed with an inclination upstream which varies from 100 to 300 to the line normal to the bank. In the T-shaped groynes, a greater length of the cross groyne projects upstream and a smaller portion downstream of the main groyne.

Length of Groynes

The length of groynes depends upon the position of the original bank line and the designed normal line of the trained river channel. In easily erodible rivers, too long groynes are liable to damage and failure. Hence, it would be better to construct shorter ones in the beginning and extend them gradually as silting between them proceeds. Shorter and temporary spurs constructed between long ones are helpful in inducing silt deposition.

Spacing of Groynes

Each groyne can protect only a certain length and so the primary factor governing the spacing between adjacent groynes is their lengths. Generally, a spacing of 2 to 2.5 times the length of groynes at convex banks and equal to the length at concave banks is adopted. Attempts to economise in cost by adopting wider spacings with a view to insert intermediate groynes at a later date may not give the desired results as the training of river would not be satisfactory and maintenance may pose problems and extra expenditure. T-shaped groynes are generally placed 800 m apart with the T-heads on a regular curved or straight line.

Design of groynes or spurs

The design of groynes or spurs include the fixation of top width, free board, side slopes, size of stone for pitching, thickness of pitching, filter and launching apron.

Top width of spur

The top width of the spur is kept as 3 to 6 m at formation level. 

Free board

The top level of the spur is to be worked out by giving a free board of 1 to 1.5 m above the highest flood level for 1 in 500 year flood or the anticipated highest flood level upstream of the spur, whichever is more.

Side slopes 

The slopes of the upstream shank and nose is generally kept not steeper than 2:1 the downstream slope varies from 1.5 : 1 to 2:1.

Size of stone for pitching The guide lines for determining the size of stone for pitching for guide bunds hold good for spurs also.

Thickness of pitching 

The thickness of pitching for spurs may be determined from the formula T = 0.06 Q1/3 where Q is the design discharge in cumecs. The thickness of stone need not be provided the same through-out the entire length of the spur. It can be progressively reduced from the nose.

Provision of filters 

Provision of filter satisfying the filter criteria has to be made below the pitching at nose and on the upstream face for a length of 30 to 4 m for the next 30 to 45m from the nose. The thickness of the same may be 20 to 30cm. The thickness of filter for the next 30 to 45m on the upstream face may be reduced to about 15 cm and beyond that, it can be omitted.

A typical layout of a spur is shown is Figure 6.

Figure 6. Typical layout and section of spur.