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Lecture No -20: Riverbank Stabilization

Good morning all of you for these presentations as discussed in the last class, we talk about Rivers stabilizations. Today I will discuss about Rivers stabilizations of Part 2 and you know there is a lot of experience on how to stabilize the riverbank throughout the world most especially in the Brahmaputra basins or the alluvial rivers in our country but elsewhere in the world like in river Mississippi, river yellow there is a lot of experience the field experience used to protect the river bank.

So mostly I am following it this part of the book as river bank stabilizations with a series of case studies from different part of the world. Looking that if you look at today contents will again talk about how to design the riprap revetment which you can see it is very easy to dumping the stones on the river bank, but that is not the correct we have to properly design the river bank revetment protections work which may be a stone riprap and nowadays we also talk about the river health so that is the reasons the new approach of our natural way to protect the river bank by planting the vegetations that the concept also is going on and he is also improved the river health.

We also very briefly we will talk about in presentation forms what it happens the history of the riverbank protections works in Brahmaputra rivers and small rivers like Brahmani rivers. Then we will talk about the windrows and trenches, Gabions and mattresses and the retaining walls, which a part of the geotechnical engineering. So here I will go contextually that what we need to do as a riverbank protections work.

But most probably we do not encourage to have a retaining walls for a bigger river like
Brahmaputra rivers and all, but very site specific we can have a retaining walls.
(Refer Slide Time: 03:07)Let us go for the next slides which is the repeating the slides of the last class, if you know the
design velocity so most of the times, we can find out what could be the design velocity that from
the design flood our experience running the HECRAS models we can know it what could be the
maximum velocity or that the velocity or the extreme velocity. We can consider the design
velocity, so if I look at this curve really design chart.
If you look it is a very clear cut is a velocity versus the diameters the equivalent spherical stone
diameters and the stone weight and you have a different degree of the slopes, different degree of
the bank slopes, so we can find out for example if I am considered to design velocity 4 and I
have where 2 is to 1 and that is the representative the size of the stones which is coming as closer
to 0.5 meter that is what the d50 or representative diameters but we can have the gradations.
So look with this design chart prepared by US crop of armies which gives us a very interesting
figures, that if I know the design velocity I can compute what will be the equivalent or d50 or the
representative value for to obtaining the riprap size also we follow the guidelines, the what
should be the gradations and what should be the packing of these materials.
(Refer Slide Time: 05:13)Looking that part let us go for the next part which is very easy part is that we talk about the
filters, so when you do stone riprap or the gabion mattress, you can have a stone riprap and
gabion mattress just below of that, you should have a the finer materials below of that you should
have a geotextiles. If you look at that the geotextile materials are there which acts like a filter
between base and revetment the riprap zones.
So there is a best materials and these filters act like prevent it should be fine enough to prevent
the base materials from escaping through the filters that is what is the idea, but it should allow
the water can seeps from these filters comes out, that is what it will take care of as we have the
increasing the level of the rivers and the ground water table fluctuations will be there and the
water fluctuations can seep through this but not the base material.
That is the reasons is to be fine enough to prevent the base material from escaping through the
filters and it must be more permeable than the base material, so water can seep through that the
gravel filter or synthetic geosynthetic cloth filters nowadays commonly we use. So like that the
basically the revetment its allow water to drain it and it should have a stability and permeability
the properties for better filter materials.
(Refer Slide Time: 07:04)Now if you look at the next slides which we talk about what should be the filters, that is what the
US army crop of engineers guideline design guidelines what they had said it should have a size
of gravel in the filter blanket should be from 3/16 inch (5mm) to a upper limit maximum size
approximately 90 mm, the filter thickness should not be less than 20 cm, the filters are one half
of the thickness of the riprap is quite satisfactory plus it should satisfy these 3 conditions.
The ratio between the d50(of filter)/d50(of the base) should have a lesser than the 40, the ratio between
d15 (of filter)/ d15 (of base) should range between 5 to 40 and d15 (filter)/d85 (base) should be the less
than 5. So if you look at that as we know what could be the best materials that is the best already
you can geotechnically test it what is the best material particle size distribution curve from that
we can find out d50, d15 and d85 and based on these criteria, we can find it what should be d50
for filter d15 for filters.
And these two criteria we have to hold get it which will be allowed to have a proper drainage
proper water escape from the best to the of through the filters to the out from the base that is the
criteria and from the design guideline from US army corps of engineers, they decide this design
criteria that is what we should use it to know it what should be the and many of the times we also
follow multiple layers instead of single layers filters.
(Refer Slide Time: 09:27)And if you look at that part where you have and it can have a synthetic filter clothes or
geosynthetic materials, nowadays so often use it which will be the consist of plastic cloth and
woven plastic material or geosynthetic materials, nowadays so oftenly used to put it this once
and if you look at this figures, we should have a degradations. If you look at that if the bigger
material smaller than that d1 is increasing d2, d3 and d4.
So we can have put the coarser to finer and finer grade so that is what is there and this is what
your best materials or you can put a geotextile filters in between that to avoid this gradations
from coarser to finers we can have a geosynthetic filters. So there are the design guidelines are
there how to use geosynthetic materials what should be the property of geosynthetic materials
which provide as a protective blanket and for the sand and gravel on the of filters the sides and
the toe of the filter fabric must be sealed it.
The trenched so that the base material does not leach out around the filter cloth, the care also
should take in joining adjacent section the soft filter fabric together. So we it can have a
monolithic to stitching these geosynthetic geotextile materials, so the geosynthetic textile filters
we have been reading it but we need to do a proper geotechnical design to know it how we
develop such a way that this base material does not leach out around the filter cloth also the
joining adjacent sections the sub filter fabric together.So we should take care of that so you can see that how geosynthetic materials as a filter cloths
we use for as a filter materials for revetment and the base soil.
(Refer Slide Time: 11:53)
As we designed the revetment with a filter and best compactions and changing the slope of the
bank, we also should know it what type of failure can happens it? What type of riprap failures
will can happen like it can have a particle erosions, that means if as you put this stones so it can
erode the particles wise one by one as you see that that type of erosions can happen is or it can
sliding it you can see it this figures.
So as a block the all these riprap material can slide it or it can slump it or it can have a sides
slope failures, many of the times we have this type of failures if you look at the revetment you
will have a either riprap slump will be there or side slope failure is there if you do not design the
riprap accurately or properly or you cannot do the layout of the riprap effectively.
Then you can have a side slope failures we can have a riprap slump or the particle erosions and
the slides that is what nowadays we try to avoid it, because we follow a gabion mattress concept
that is what I will show to you. Nowadays we follow this gabion mattress concept, so these type
of particles erosions or the slides does not happen it but it can happen the riprap slumping or the
side slope failures and if you look at that if these ground water tables are fluctuating more you
can have a saturated zone and that the slope failures can happens it.So you can have a geotechnical study more details about the bank materials and the hydraulic
characteristics, if you know that very well then you can avoid the side slope failures of the
revetment structures, that is the point and if you look at that again just to have today that the
particle erosions is a most common erosion mechanisms but we try to avoid nowadays because
you have gabion mattress concept, that is what the individual stones are removed by the impact
the steep bank side.
And riprap gradation that is too uniform as that the ripraps should have the gradations it should
not be the uniform size that is what we discussed in the last class the same things, if you look at
that the riprap gradations is if is too uniform you can have a particle erosions same concept is
that.
(Refer Slide Time: 15:01)
If you look at this sliding failures which is a down slope mass movement of the stones, its
happens like that and this slides usually initiate by channel-bed degradations, what it happens is
that if the channel bed is like this and it is degraded. So its if erosions are happening the
deepening of the channels are happening it can have it the channel bed degradations happening it
which undermines the two of the riprap blanket then you can have a this type of slide.The presence of filter blanket may provides a potential failure plane for the translation side, so
thus you try to understand it if you have a of your filter blankets and which can also a failure
plane for the sliding or that because after that steep banks slideslope, so presence of excess
hydrostatic pore pressures, loss of the material at the two of riprap blankets. We just try to
understand it how the structure fails it if you have a degradations and if you have a excess
hydrostatic pore pressure develop in the bank soils.
And steep bank side slope is basically when you protect the bank we do the slope reductions we
cannot put it in a steep banks.
(Refer Slide Time: 16:35)
So if you look at the next one which is a riprap slump that we can understand it that side slope
failure is a rotational and the gravitational failure movement of materials that is what it happens
it is concave upward curve, because slump failure is related to the shear failure of underlying
best materials basically its giving about a geotechnical way the failure part excess pore pressures
in the base of materials and steep side slope.
Now if you look at that what to avoid these failures what we do it we always have a tie back, so
we extend it the mattress the gabion mattress are the riprap mattress inside the bank which act
like a tieback, also when you do a launching apron if you look at thickness is a t we put itadditional height is 2.5 times of the t, that is what initially we put it that once as the rivers covers
happens it this materials goes down it and falls in like this.
So we have to take care of because of the scouring mechanism because of river degradation
mechanisms, the river bank protections what we have done it thus should take care of the scour
formations, that is the reasons if you look it even launching apron, we provide it which will have
a thickness t is here the on the bank on the bed we put it the thickness which is 2.5 times of the t
and after the expected scour this materials can again can have a fall like this.
So similar way if you look at the planform how you do the revetment so if you have a river going
like this you can see this you should protect the river bank in this side because there will be point
bar formations and also this side. But if you look it there should be overlapped zone there should
be tie back this should be designed properly the tie backs would be there, the overlap should be
there and the protections of both the outer banks should be put in.
Such a way that it should not have a failure mechanisms will happen it because of if you protect
the partly it is not the successful story, so we have to protect the river as a single entity as the
river curvatures are there we should have the revetment in both the sites, we should have a
tiebacks and we should have at a overlap. Lot of design guidelines are there what should be the
overlap, how to make the tieback or you can have a more detailed geotechnical.
And hydraulic experiment the laboratory experiment to find out what should be the dimensions
of this planform of these revetment structures, so if you visit any of the river bank you can see
this revetment structures where you have a bank erosion is quite dominated.
(Refer Slide Time: 20:06)Next points what we are talking about when you look at this launching apron you should have
very much knowledge about the scouring mechanisms, how much of scour is going to happen it
like this is a riprap apron which we provided this is the water surface slope. So we can see that
there will be the failure things is changing it is this failure surface is changing it because of the
scouring regions, so the scour you have to anticipate it what could be scour happening because of
these river protections work.
And hydrodynamic force and the river curvatures that way you can find out what could the
failure surface will happen it and based on that we should have the protection mechanisms. The
launching apron is a flexible apron can be laid horizontally on the bed and foot of the revetment,
so this is the launching apron is what you will it is very flexible and the scour occurs and the
material will settle it over the side of the scour hole on a natural slope that is what we plan it.
So that is what we should anticipate it if there is a scour holes the launching aprons should be
very flexible, so that the launching apron the materials can cover up on the scour whole holes
and the scour zones can be protected naturally by the river dynamics you can settle down with
that is the reasons, it needs to have a lot of experience in the river levels we design something
some river revetment work, implement the river revetment work, monitor river revetment, work
to try to know it what should be the criteria follow for launching apron.So that the river bank can protect it during the steam scour formation states that is a quite
challenging one, it is not a just dumping the stone on the river bank, that is my point to tell it is
not a just dumping the stone on the river bank then eventually that part will be valid. We should
have a proper designing of the river revetment work.
(Refer Slide Time: 22:37)
If you look at that what the experience the river the stabilization work is not very easy task, when
you talk about river like Brahmaputra it is a large dimensions, we cannot think it the river bank
protections are a simple job, so if you look at these figures you can understand it what could be
the height of the bank the height of the bank is a 6 feet tall person is standing here and you can
understand with this scale which could be more than 30 feet that is what is above the free surface
there could be below any another maybe 30 feet.
So if you look at this river bank, so gigantic, so high river bank of 60 feet and if you look at the
river erodible bank materials which are close to the sand and the silt, they does not have are
some clay compositions and if you look at the river bank protections which is a porcupines
structures which are standing over that and which is falling down it and you can see that how
erosions processes are happening it very dynamically instantly say eroding the toe materials
collapsing these bank materials.And at the top the river bank protection standing it trees are standing it, so if you look at that the
bank erosion process what it happens in Brahmaputra rivers in the scale that does not happen in
many part of the world, so we should try to understand what type of bank erosion process are
happening it and we have a small structures like porcupines if you look at this concrete structures
the basic idea for this concrete structure is to reduce the velocity.
If I reduce the velocities that is what can start the sedimentations depositions the siltations will
start it and that siltations can facilitate river to divert from that locations, so this type of smaller
structures we have been putting in river bank of Brahmaputra rivers just to have a expected that
the velocity near the bank is going to decrease it and as it is going to decrease it can initiate the
sediment depositions and because of the sediment depositions as we expected it river can change
its course.
It can go from the locations where it is, but some success stories and failure stories are there but
this is what the riverbank protections for this alluvial river like this where the bank erosions are
really it is a quite challenging process what it offends it, some of the cases we can have a protect
the rivers with a soil cement a compositions of soil and cement bag you can see it and the top
you can have a protections work.
But that is what also can be possible where you have not these compositions are you want to
protect where you do not have much secondary current is there you can think it to protect the
river bank with the soil cement and with a some sort of gabion or this surface protection systems
like putting the some stones or even the bricks to protect these ones, so if you look at that
implementations what is going on
(Refer Slide Time: 26:53)The same way if you look it this is what I explaining it the huge porcupine structures that is what
is called porcupine fields and this is what because of these porcupine structures it may be
facilitating depositions. If you can see that porcupine structures and this the depositions and
because of that river might have changed its course from these protections to other part so these
are the structures what is implement is it is a simple tetrahedral structures and it is quite stable.
And those structures we use it just protect which are the vulnerable region success and failure
stories are there and I am not going to discuss much detail on that but this is what the basic idea
when you do it.
(Refer Slide Time: 27:50)But if you look at this river bank protections in Brahmani rivers where it is a smaller rivers as
compared to the Brahmaputra and you can see this rivers bank, you can see the embankment, you
can see this floodplain cultivations so as we have been discussing the same concept is there is a
floodplain, there is a embankment, so we have enough flood plain we have given it to rivers and
it has the agriculture activities within the floodplain and that is what is there.
And the cases where you have a river bend, the channel bend is there in Brahmani and you can
see these spur structures from even from Google data you can see this river spur structures which
is facilitating the river to go like this not to attacking this bank. So you can see this a series of
spur structures are there which is divert these flow stream lines away from the bank that is you
can see it and there are earlier there used to have a weir structures to protect the maximum flow
or protect the maintain the navigable or flow depth.
So if you look at that way our success stories for the smaller rivers is quite impressive but bigger
river like Brahmaputra and partly Ganges are not that successful because the dimension of the
problems are you as we have been discussing it is still it is a complex task to protect the river
bank in Brahmaputra where you have a so dynamic processes are helping it.
(Refer Slide Time: 29:50)
Like this is the concept many of the peoples in the world wide they are talking about river health.
The basic idea is that to bring the river as close to natural conditions that means any rivers youcan have protect with a vegetations, protect with natural vegetations and because of this
protection of natural vegetations what is going to happens is that the velocity reduces as we
increase the roughness Manning’s roughness coefficients putting this vegetations.
What is happens is velocity reduction happens which is facilitated for the sedimentations which
we need it during the high flood periods if there is a sedimentations and that what will facilitate
also the growth of the vegetations. So we try to look it how we can reintroduce the vegetations
on the river banks and the it is potential plot plane area so that we can improve this increase this
Manning’s roughness coefficient.
Which we can know it because of increase of Manning’s roughness coefficient velocity
reductions will be there and which can facilitate the sedimentations. And not only that near
boundary near bed and the bank the flow structures will change it. Flow and sediment structures
will change it which is quite complex that way so if you try to understand it that is what is there
and if you look at that the examples which is there.
That if you have a river bank like this and if you put the vegetations and you can see that how it
is a very natural vegetations can have an effectiveness in terms of protecting the bank but be
remember it is may happen for a smaller rivers where that significant order of flow is not there or
the velocity is not there so smaller rivers we can protect naturally with help of the vegetations
changing the near bed and bank flow and sediment structures changing the flow resistance and
changing the velocity that is what we facilitated it and variations grow it.
So this is what basically environment conditions which is as I said that it will improve the river
ecosystems, root system may increase the overall bank stabilities, reduce the flow velocity,
induce the depositions that is what I explain it. And there are limitations there is a failure of
vegetation subject to undermining, may not have withstand alternative periods of wetting drying
periods, uprooted by freezing thawing of ice suffer wildlife livestocks.
It is just expected if you grow the vegetations it may not effective for all the cases it can effective
for certain range of the flow the velocities and all but if you have a extreme flood event it maynot sustain that way or whether these vegetations remain grow throughout the years taking care
of wetting and drying of different durations.
(Refer Slide Time: 33:57)
Now if you look at next part which is very interesting that what type of vegetations we can put it
for your river bank if you look in like, this is the flow but there is a zone you will have the flow
where you have the flow which is called the high stress zone which is inundated throughout the
most of the years that is what this is a low flow this normal flow so this part of the vegetations
which is a splash zone.
Where we can have a throughout the year you can have the waters in that case you can have a
semi aquatic plants you can have reeds, rushes and sedges. So this is the area so often the water
will spreading it there and there you can put the some vegetations will be semi-aquatic plants.
You have a bank zone which is above the normal to the high water levels this is the bank zone
which above the normal high water levels.
It is inundated at least a 60 days durations once in every 2 to 3 years. Inundation does not happen
so often here it is inundation at least a 60 days durations that you can have a herbaceous or
woody plants we can put it that. If you terracing zone beyond that you have a terracing zone you
have a splash zone and you have bank zone which is usually not subject to the erosive action less
upon flooded native grasses, herbs, shrubs and trees can be planted it.Most oftenly I can take it in natural for river systems mostly follows like this whether we can
facilitate to have a same natural conditions by artificially planting these trees, native grasses
woody plants or semi-aquatic plants so basic idea as a river engineer is that we try to improve the
river health as well as we can protect the bank protections because we change the near bed or
near bank hydraulic conditions that is we try to understand it.
But how effective it is only at the big questions marks when you talk about extreme plot
conditions that is where do we look at more technically how effective it is but there are the cases
we have been advocating to have a ecosystem based river protections work which can be
implemented for the smaller rivers where we are not we have modified the river ecosystems so
those reasons we can plan it to have a different type of vegetations like a splash zone, bank zone
terrace zone.
We can put the different vegetation and we can make it river ecosystem as closer to the natural
conditions and by the way we can improve the river health and also we can protect the river
bank.
(Refer Slide Time: 37:33)
Let us look at river bank protections what we look at very traditional way to protect the river
bank like wind row or the trench if you look at these figures it is very simple way you just dumpthose stones on the river bank or you create a trench and fill up the stones. What is our idea is
that as the river eroded as it is widening the channels this stone materials will fall down it after
that it will be naturally have a protections it create a natural formations of river blankets.
Bank blanket you will create it its natural. So that is what is the windrows just you dump the
stones of different sizes and you just wait to river to erode that part as its river erode this part and
its fall it during the natural conditions and it will create a blanket of river protections work its
naturally by river dynamic forces but this is what nowadays we are not practicing it because it is
quite expensive.
And we do not know whether river can come to that locations or not and it is same way we can
have at the trench facility to have a protections of the bank and or you can have a launching
aprons so you can have a launching this materials from the top and you can go like this so we can
have windrow revetments is piling a sufficient supply of erosions resistance material on the
existing land surface along the riverbank that is what is.
Earlier we used to do it similar to windrows but material is buried here the same concept we also
depend upon the size of the stone, well graded stones and we also talk about the relative
thickness of the final revetment the stone diameters greater the velocity steeper the side slopes.
Basically this is the practice used to do from the Brahmaputra Ganges rivers in Bangladesh and
they have been very old practice to do this type of riverbank protections using windrow or the
trench concept to protect these things.
But nowadays we cannot do those type of bank material we cannot put it huge and we cannot
wait for the river to erode it and have other protections but we go for the launching way and
proper river bank protections work.
(Refer Slide Time: 40:29)Now if you look it sacks and blocks which most oftenly use it one is that you can have a one by
one like this we when you have a the side slope ratios like this or you have a one over one by one
this is a stacking will be there or you can have like or you can have a stacking like this then you
have a two protections and the scour depth and after the scour the filling materials. So if you
look at this once which in generally if you go to any of river bank protections you can see that.
Depending upon the slope you can stack it the sack and blocks that is what is that and most
probably you have to look at what you have been doing it and protecting the river bank.
(Refer Slide Time: 41:23)Most of the nowadays we talk about the gabion mattress because as you know it the individual
stone if you put it there could be a particle erosions there could be a chance to have a slidings to
avoid that we last few decades is introduced the concept of the gabion boxes if you only see that
there could be a box with a length and height and you can fill up the stone here and box with a
wire mass mesh so if you have a box with wire mesh.
So you can put these stones and those stones can have a particle level stability as well as it can
act like a mattress. So if you look at this act like a mattress so that is more stables will come it
when you combine the riprap stones with a gabion boxes and if you tie off all these given boxes
it can act like a mattress it is a big mattress, we can put a stone gabion mattress on the river bank
which is more much more strength as compared to the individual stones.
So that is the reasons nowadays we have been using this concept of gabion mattress on riverbank
protections where we need to protect it because the cost is quite effectiveness in terms of
reduction of the thickness of the stone revetment and as well as it creates a monolithic structures
single structures so that any sort of the failure mechanisms can be prevented because of gabion
mattress.