River Bank and River Training works
River training works are constructed for a
variety of reasons, but the fundamental objective is to restrict horizontal
movement of river channel by guiding and training the flow.
Objective of river training works
1.
To train the river to flow in straight reach both
U/S and D/S well as at the bridge site.
2.
To deflect the flow of the river from the banks to
prevent the erosion of the bank.
3.
To provide the guide bund so as to confine the
stream in the desired limits.
4.
To provide the required depth at the bridge in case
of navigational channels.
5.
To confine the span to minimum.
River Bank may be failed by
1.
Erosion of soil by river currents or waves
2.
Excessive hydrostatics pressure in the material in
the bank or sloughing of slop due to saturation of bank materials
3.
Under mining the toe of bank due to high current,
eddies etc.
4.
Liquefaction and subsequent movement of soil mass by
piping
5.
Formation of sink hole due to water in the
dispersive soil.
Type of river training works
1.
Embankments
2.
Guide banks or bund
3.
Groynes or spurs
4.
Pitching of banks and sub siding
5.
Revetment work
6.
Pitched islands
7.
Cut-offs
8.
Closing dykes
Techniques for Bank protection/Stabilization
Two broad ways such as the direct methods of
protecting the slope and the indirect way by providing structure that extend
into the stream channels and redirect the flow so that hydraulic forces at the
channel’s boundary are reduced to a non-erosive level. Among the direct method some of are
1.
Self-adjusting armour made for stone or other
materials.
2.
Trench fill revetment/windrow revetment/longitudinal
stone toe
3.
Slope pitching
4.
Lining with cement concrete
5.
Spur/stud
6.
Riprap
7.
Launching apron
8.
Flexible mattress etc.
Embankment
Embankment on either approach banks are
provided to protect the adjoining area getting submergence.
Guide Banks
The main purpose of these banks is to divert
the stream from comparatively larger area to the confined section under the
bridge. These bunds are constructed in pairs on either banks and provide a
streamline flow at the bridge site. It is made of earth and sometimes inner
face is covered with stone or concrete block. The U/s length of bund is in
between 1L to 1.1L times the length of the bridge while down stream length is
usually kept 0.2 times the length of the bridge.
The radius of U/S side
decide depth of scour as well as velocity of flow while D/S side is half of the
U/S side. A free board of 1.2 to 1.5 m is provided above the HFL. An apron
equal to 1.5 D should be provided at the toe of guide banks.
Pitching of banks and subsiding apron
The banks are very often protected by stone
pitching, bushes, wood, plants, concrete blocks or grass. The slope of such
pitching depends up on the material used for pitching. Proper care should be
taken that toe is properly protected otherwise the pitching would be
damaged/destroyed during flood.
The launching or subsiding apron of loose stone is therefore provided at
the top of the river bank up to the required length in the river bed. For
launching apron, if the average velocity up to 3.5 m/s, stone having weight 40
to 70 kg can be used and if the velocity is more than 3.5 m/s concrete block is
useful.
Revetments
Failure of a revetment is often result of
inadequate protection against scour at the toe of the revetment. There are
various types of material used for revetment such as gabion filled with stones,
sand bags, stone riprap, timber piles, bamboo piles, old tires, concrete slabs,
concrete blocks and also combination of two or more of these materials.
Pitched islands
Pitched island is artificial island
constructed in the river bed when it is required to prevent the flow of river
getting confined to one bank only. The stone pitching used to form the island
obstructs the flow of water which becomes turbulent. This results in the
scouring of the bed. As the bed gets lower, the water from the other bank gets diverted
to this bank and the flow along the far bank is reduced. Pitched island is
protected by stone pitching.
Cut-offs (to control the meandering of the
river)
Sometimes the river
develops horse shoe hair pin ben during floods the area between the horse shoe
gets submerged and the river starts flooding along the straight line. Due to
the reduction in the length of the channels the slope becomes steeper and the
portion U/S of the cut off undergoes erosion. As a result of this the D/S gets
silted up and disturbance are caused in the river. If, however the straight
portion is dug up, the river water will start along this straight portion and
curved portion would be get silted up.
The chances of the river changing the courses
are practically eliminated.
Closing Dykes
Sometimes the river flows in the main as well
as in a subsidiary channel and the river has a tendency to change its course
towards the subsidiary channel. This tendency of the river is checked by
providing a closing dyke. It may be solid, permeable natural soil like brush
wood, branches of trees et
The solid material stops the flow into
subsiding channel while permeable materials reduce the velocity of water. As a
result of this, the subsidiary channel gets silted up in course of time and the
flow of river gets confined to the main channel.
Groynes/Spur/Stud
Groynes deflect or guide flow away from the
vulnerable river bank. They can therefore be used both for bank protection and
help maintain a navigational channel. It is made of timber, masonry, concrete
or earth etc. Based on the orientation, it may be repelling, deflection or
attracting spurs. Porcupine (tetrapod) is permeable spur which helps for
siltation along the banks and made of bamboo, timber, gabion etc.
Point to be considered for Spur
1.
Height of spur must be more or equal to maximum
flood level
2.
Launching must be provided sufficiently
3.
Retard must be permeable.
4.
The length of retard line should correspond to the
amount of an eroded river bank that, it is to be recovered by deposition.
5.
For velocity reduction along an eroding bank, retard
should be placed in lines perpendiculars to the bank with line spacing about.
Design Consideration.
Spur length, L: generally, 1/3th of river
width but not more than 200m
Spur Spacing: 3 to 4 L or LcotA, A is an
angle deviation of spur orientation
Top width 3 to 6 m at formation level
Free board 1 to 1.5m above HFL or 1 in 500 yr
return period flood
Side slope 2:1 to 1.5:1
Thickness of pitching for spurs (t) 0.06 Q1/3
, Q in Cum/sec.
River Training
spur length
top width
free board
side slope
pitching
groynes
armour
permeable soil
bund
thickness of pitching
riprap
erosion
liquefaction
sink hole
excessive hydrostatic pressure
dispersive soil
piping
filter material
mattress
trench fill
window revetment
stone toe
guide banks
embankment
meandering
revetments
pitched island
subsiding apron
sour depth
bank protection
launching
retard
velocity of water
stud
closing dykes
water current
boulder
spur
cutoff
