In this article we will discuss about the layout of river training work, with the help of suitable diagrams.
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 4.42, 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.
1. 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 4.43.
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 30°. 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 R times the radius of the curved head.
According to the river curvature, the angle of sweep of curved upstream head ranges from 120° to 145°. The angle for the curved tail usually varies from 45° to 60°.
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 multi-radii 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 45° to 60° and the smaller radius equivalent to 0.25 times the radius of curve head for straight guide bund with sweep angle of 30° to 40°.
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 0.25 m. 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:
(ii) For Graded Material of Sub-Rounded Particles:
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.
2. 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 4.45.
Design of Guide Bunds:
After fixing up the layout of the guide bunds, 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:
a. 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.
b. 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 (1 in 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.
c. 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).
d. 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 Fig. 4.44. 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.
3. Approach Embankments:
Where the width of the river is very wide in an alluvial plain, the diversion structure is constructed with a restricted waterway for economic 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.
a. Top Width of Approach Embankments:
The top width of the approach embankment is usually kept as 6 to 9 m at formation level.
b. Free Board of Approach Embankment:
Free board for approach embankment may be provided similar to that for guide bunds.
c. 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).
d. 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.
e. Thickness of Pitching:
The Guide lines for determining the thickness of pitching would be the same as for guide bunds. The velocities would be estimated for 40 percent of the design discharge.
f. 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 for guide bunds.
g. Launching Apron:
The provisions of size of stone, thickness of apron and slope of launched apron would be similar to those of guide bunds. But the depth of scour for the approach embankment may be taken as 0.5 to 1.0 Dmax and beyond that the width may be increased to 1.0 Dmax with suitable transition in the former reach.
4. 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:
(a) Permeable or impermeable
(b) Submerged or non-submerged
(c) Attracting, deflecting repelling and sedimenting
(d) T-shaped (Denehey), hockey (or Burma) type, kinked type, etc.
The different types of spurs are shown in Fig. 4.46.
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 10° to 30° 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.
a. Top Width of Spur:
The top width of the spur is kept as 3 to 6 m at formation level.
b. 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.
c. 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.
d. Size of Stone for Pitching:
The guide lines for determining the size of stone for pitching for guide bunds hold good for spurs also.
e. 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 throughout the entire length of the spur. It can be progressively reduced from the nose.
f. 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 45 m from the nose. The thickness of the same may be 20 to 30 cm. The thickness of filter for the next 30 to 45 m 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 Fig. 4.47.
5. Cut-Offs:
Cut-offs as river training works are to be carefully planned and executed in meandering rivers. The cut-off is artificially induced with a pilot channel to divert the river from a curved flow which may be endangering valuable land or property or to straighten its approach to a work or for any other purpose.
As the cut-off shortens the length of the river, it is likely to cause disturbance of regime upstream and downstream till readjustment is made. A pilot cut spreads out the period of readjustment and makes the process gradual. Model tests come in handy in finalising this form of river training works wherever needed.
A typical instance of a cut-off is shown in Figure 4.48 and Fig. 4.49.
6. River Bank Protection Works:
This aspect of river engineering considers methods and techniques for protecting the banks of rivers from collapsing. Hence, certain structural interventions are required to be implemented, which are termed as the riverbank protection works or alternately as bank stabilization structures. Generally these are simple to construct though, the specific hydraulic and geomorphic process associated with these structures are quite complex and challenging.
Hence, the type of bank protection work has to be in accordance with the conditions of the specific site – a method suitable for one location of a river may not be so far another location of the same river or at another river. For a proper appreciation of the techniques of bank stabilization, one has to have an awareness about fluvial geomorphology and channel processes.
Nevertheless, geomorphic analyses of initial morphological response to system disturbance provides a simple qualitative method for predicting the channel response to an altered condition. Another complicating factor in assessing the cause and effect of system instability is that very rarely is the instability a result of a single factor.
In a watershed where numerous alterations (dams, levees, channelization, land use changes, etc.) have occurred, the channel morphology will reflect the integration of all these factors. Unfortunately, it is extremely difficult and often impossible to sort out the precise contributions of each of these components to the system instability.
The interaction of these individual factors coupled with the potential for complex response makes assessing the channel stability and recommending channel improvement features, such as bank protection, extremely difficult. There are numerous qualitative and quantitative procedures that are available. Regardless of the procedure used, the designer should always recognize the limitations of the procedure, and the inherent uncertainties with respect to predicting the behavior of complex river systems.
Local Instability:
Local instability is a term that refers to bank erosion that is not symptomatic of a disequilibrium condition in the watershed (i.e., system instability) but results from site specific factors and processes. Perhaps the most common form of local instability is bank erosion along the concave bank in a meander bend which is occurring as part of the natural meander process. Local instability does not imply that bank erosion in a channel system is occurring at only one location or that the consequences of this erosion are minimal.
Erosion can occur along the banks of a river in dynamic equilibrium. In these instances the local erosion problems are amenable to local protection works such as bank stabilization measures. However, local instability can also exist in channels where severe system instability exists.
In these situations the local erosion problems will probably be accelerated due to the system instability, and a more comprehensive treatment plan will be necessary. Local instability of a riverbank may be due to either streambank erosion or erosion due to meander bends. These are explained below.
Streambank Erosion and Failure Processes:
The terms streambank erosion and streambank failure are often used to describe the removal of bank material. Erosion generally refers to the hydraulic process where individual soil particles at the bank’s surface are carried away by the tractive force of the flowing water. The tractive force increases as the water velocity and depth of flow increase.
Therefore, the erosive forces are generally greater at higher flows. Streambank failure differs from erosion in that a relatively large section of bank fails and slides into the channel. Streambank failure is often considered to be a geotechnical process.
Meander Bend Erosion:
Depending upon the academic training of the individual, streambank erosion may be considered as either a hydraulic or a geotechnical process. However, in most instances the bank retreat is the result of the combination of both hydraulic and geotechnical processes. The material may be removed grain by grain if the banks are non-cohesive (sands and gravels), or in aggregates (large clumps) if the banks are composed of more cohesive material (silts and clays).
This erosion of the bed and bank material increases the height and angle of the streambank which increases the susceptibility of the banks to mass failure under gravity. Once mass failure occurs, the bank material will come to rest along the bank toe. The failed bank material may be in the form of a completely disaggregated slough deposit or as an almost intact block, depending upon the type of bank material, the degree of root binding, and the type of failure.
If the failed material is not removed by subsequent flows, then it may increase the stability of the bank by forming a buttress at the bank toe. This may be thought of as a natural form of toe protection, particularly if vegetation becomes established. However, if this material is removed by the flow, then the stability of the banks will be again reduced and the failure process may be repeated.
As noted above, erosion in meander bends is probably the most common process responsible for local bank retreat and, consequently, is the most frequent reason for initiating a bank stabilization program. A key element in stabilization of an eroding meander bend is an understanding of the location and severity of erosion in the bend, both of which will vary with stage and plan form geometry.
As stream-flow moves through a bend, the velocity (and tractive force) along the outer bank increases. In some cases, the tractive force may be twice that in a straight reach just upstream or downstream of the bend. Consequently, erosion in bends is generally much greater than in straighter reaches. The tractive force is also greater in bends with short radius than those with larger ones. The severity of bank erosion also changes with stage.
At low flows, the main thread of current tends to follow the concave bank alignment. However, as flow increases, the streamlines tend to cut across the convex bar to be concentrated against the concave bank below the apex of the bend. Because of this process, meanders tend to move downstream, and the zone of maximum erosion is usually in the downstream portion of the bend level due to the flow impingement at the higher flows.