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Lecture - 17: Jet Scour and River Navigation
Welcome all of you for this course on River Engineering and if you look at this course, already we discussed about bridge scour. Today, we will further discuss about jet scour as well as we will also discuss about the national waterways or how we can have an advanced technology for navigating the rivers or the national waterways. So that is the concept. We will discuss in more detail today on what I have emphasized.
We are following these books which is having a chapter on river engineering that part what we are following it here as well as you know it those are the books also we have a lot of reference on that. Now if you look at that today what we will cover it, jet scour. We will cover about waterways. We will talk about some of the very relevant case studies and we also will talk about the locks and dams and also you know the dredging. So those are the concepts. Today, we will discuss that how we can do the dredging to maintain a river channels for navigations point of view or other utility point of view. We can also look for option for dredging. Those things we will discuss today more detail.
Looking for the first part, jet scours. Let us look at these four figures which gives very interesting things that what type of scour can happens it, like if you have the dam structures and you have the water jet here. Because of the water jet, initially whatever the soil part that what will be scour it and you can see there is a vortex patterns, what will be created in and that vortex will be dissipated as go up.
So if you look at that it also depends upon ϴj, the angle of jet water jet what is impeding on the
waters. It also depends upon the tail water depth, how much of the water depth at the tail water
levels. It also depends upon the bed materials, what type of bed materials are there. So this is the
scour happens it, just the downstream of a dam or the waste where you have a water jet impeding
on a tail waters.
We need to find out what is the scour depth ∆z, the scour depth and you can see the process.
Mostly again, we go for the physical model and establish the relationship with this ∆z with the
flow with the water depth at the tail end, the bed material characteristics, all we do it with
function of h. We try to establish the relationship with the ∆z with the functions of all this part,
that is what we look at when you talk about this.
As we discuss for the scouring bridge pier, the same concept also followed it the plunging jet
scours. That part was also follow it. Same way if you look at mini gap case, you can have the
dam structures and you can have a sluice gate. As you have the sluice gate and that the sluice
gate will have a V1 velocity and Yj depth and you can understand it how the scour mechanism is
going to form it here and there will be a scour depositions.
And because of that, we try to know it, what will be the scour depth ∆z. What is the scour depth?
It is a functions of V1, V2, Yj and the bed material at the bed material. Again the similar way,
conducting a series of physical experiments scaled down models and try to establish the
empirical relationship between the flow characteristics with the scour depth. That is what the
basic idea is there, but many of the times you can have a great control structure scours.
That means you may have a higher level of, then you may have the low level of scouring
materials. It can happen in the rivers. You may have regions where you have a stones followed
by you may have the sand. Because of that if this stone and sand or if you have a weir and the
riverbed, we can have great control structure scour. So you will have the scour, because as theflow the vortex sense generates in it and this is what that scour it and here you will have a
deposit mode.
That means whatever the scour materials also deposit here and then you have the tail end depth,
this to velocity and V1. So just try to understand the conceptually how the processes are happens
it. How the scour depths are happening and most of the times we conduct the series of physical
experiments and then we establish the relationship between ∆z with the hydrodynamic
parameters like the velocity, ϴj, the ht, the tail end depth as well as the flow velocities.
Those empirical equations are really necessary for us to know it, how we can effectively design a
hydraulic structure like a weir or having a gate with a sluice gate or you have a graded control
structures or you can talk about the culvert. So many of the times, we do not bother about culvert
scours, but culverts also have the scour during extreme flood events. During extreme flood
events, you can have the scour depth.
That is the things I used to say that you have to understand it there will be the scour, when you
have the culvert, like if you look it, the culvert can make a water jet when you have a very high
flow the Q amount of this size is going into that. It can create the jet and that jet can impede it
and create the bed scours. So just downstream of the culvert, we can have the scouring and
whenever we design the culvert, we should take care of how we have to protect the scour or what
should be the strategy for scour counter measures; that we should do it.
So if you look at this culvert scours which depends upon the diameters, the discharge and the
flow patterns, what is happen that is the ∆z is the scour depth and you have the flow. So in
summary, I can say that the jets scour happens in case of plunging case, that means you are
impacting plunging it inside a tail end or it can have a submerged jet scours. It can have a culvert
scours or great control structure. So now let us come to the empirical equations, which are
derived by conducting a series of lab experiment or the physical experiment, the scaled down
experiment setups.
(Refer Slide Time: 08:45)Now if you look it, first once that is what basically we release the water periodically in the down
streams and the plunging jets or submerged jets needs to be considered to look at the stability
analysis of hydraulic structures and if you look at that, the Fahlbusch 1994, he established the
relations with the scour depth with V1 velocity of z velocity, ϴj stands for the angle measured to
horizontal at the water surface, the jet angles which is measured from horizontally at the water
surface.
Then you have it the depth, the tail end water depth and you have a Kp stands for here the
coefficients for plunging jet. So that is the reason if you look it, it depends upon jet
characteristics in terms of the jet velocity theta z. It depends upon tail water depth that is what is
if you have higher the tail water depth, you will have a delta z is less. That is what is most of the
times is there that they create a stilling basins or can have another control structure, such a way
that we can increase the tail water depth and we can reduce the scouring depth.
That is what is being followed many of the downstream of a bridge or the downstream of weir,
when you measure that we try to have the tail water depth. So if you have a tail water depth is
sufficient, you can know the scour depth will be that and this Kp depends upon the bed material
characteristics and you can look it, these are what it all depends upon the sin ϴj, q is the flow
rate per unit width.So what I want to say it, if you look at the empirical equations always we try to understand it and
that understanding will give us a guiding principle how to manage that hydraulic structures
during the scour formation, extreme flow conditions. That is the biggest idea that if you have
very extreme flow conditions are coming it, we need to have either tail end water depth at the
major bridge dam locations or the weir locations, such a way that we can reduce the scour depth.
Next one is that we can look it, how we can have a Kp value variations.
(Refer Slide Time: 11:37)
If you look at that the Kp is a plunging coefficients is a multiplication factors, that is what you
try to understand it which is a 20 for the silt. So in the downstream you have the silt, this factor is
20. 5 - 20 the range for the sand and you have gravels is 3-5. So we just interpreted it that if you
have the silt, the scouring depth will be 20 times or the almost 10 times, close to the 10 times as
compared to the gravels. So what it indicates? From this experiment data, so we can know it
what type of downstream bed materials.
We can either armoring it or we can put it such a way that we can reduce the scour depth. That is
what is here. So you can see that you have a gravel, so this coefficients is just range from 3 – 5,
but in case of the silt, it is as high as the 20. So that is almost 8 – 9 times higher. That is what it
happens it in case of the silt and the gravels, because the more the scour depth it will be
dangerous to the hydraulic structures, because that the depth it will go.Again I have to repeat it, the extreme scours happen during the extreme floods. So we do not
know it what type of flood is going to come and that period how do we manage it or what could
be the our strategy to look at the safety of the dam and the weir structures, because we have to
protect the scouring mechanisms, protect the scour holes. To protect the scour holes, we should
have this knowledge of the scouring that how we can manage it with gravels or tail water depth.
So that is the reasons the submerge jets entirely under the free surface flow under the sluice gate.
So you have the sluice gate, you would also have the considerable scouring potentials. That is
what the Hoffmans and Verheij 1997 established the equilibrium scour depth. It followed the
Newton's second law. We have been discussing about applying the linear momentum equations
in a control volume, exactly same concept with using a coefficients.
It has to find out that ∆z is a functions of yj, y is a inflow, z thickness depth, Ksj is a scouring
coefficients. Again it depends upon the bed material characteristics and V1, V2 inflow and
outflow velocity respectively.
(Refer Slide Time: 14:45)
So again if you look at that Ksj it varies from the 50 to again this range of values for the gravels
and for the sand and for the silts. So you can see that you have a bed materials in different, like a
silt, sand or the gravels the scouring you can know it what is happening. Now let us come in tothe scour below the great control structures. There is a drop structures. That is what we are
talking about that. They have a great structures below that I have the materials.
As z happens it how we can estimate the scour depth, how we can estimate the ∆z, the scour
depth. So if you look at that again the same experimental works, but the equations looks bit
lengthy, but if you look at that ∆z is a function of sin ϕ ϕj, G is specific gravity of the bed
materials. You have a q V1 and the Dp. So it is a complex equations, but it is not that complex
todays world. If you look at that the Dp is a drop height of the grade-control structures.
If you look at the figures here, so Dp stands from this part ϴj is the impinging angles, you have a
V1 the velocity, V2 velocity you have a tail water depth, then you have a original bed. So
looking this you can understand it that we can have a relationship between the scour depth with a
functions of Dp. The ds that is what particle size distributions d50 most of the times you consider
it, then you have a theta z V1 and q. V1 is a unity discharge per unit width.
V1 is approach velocity, ds is a particle size and G is a specific gravity of bed materials and ϕ is
angle of repose of the bed materials. ϴj is angle measured from the horizontals. So if you look it,
these are all the experimental studies plus the dimensional analysis to establish these empirical
equations, but these relationships tell us that what type of the dependencies of the scour hole
depth in terms of the flow characteristics, in terms of the drop height, in terms of the bed material
characteristics. That is what how are these things.
(Refer Slide Time: 18:16)Now if you look it that scour below the circular culverts outlets. Many of the times, we generally
avoid it. We do not bother about the scouring mechanisms for a culvert, but I do believe it many
of the time, whenever you have a 10 years or 20 years floods most of the time the culvert fails it
and that is what is a lifeline of the road networks and during the flood when it fails, all sort of the
problems we face it, because of the floods, not because of the waters, because of failure of the
culverts.
So whenever you design the culvert designing, at that time we should consider what amount of
the scouring happens it during the extreme floods. That is the basic idea. So if you look at this Q
is the discharge, D is the diameters for the circular outlets. This is the scouring depth. So as D is
the culvert diameters, Q is the discharge. So basically we always have this empirical equations to
estimate the scour hole depth.
But whenever you go for major structures, we conduct separately the physical modeling studies
followed by the dimensional analysis from case to case, because that is what gives us the more
the field data and the laboratory data we combine it and we try to modify these equations or try
to locate whether these equations are enough for designing scour holes, how to protect the scour
holes in major structures like a dam, like culvert, like you have a sluice gate or the grade
controls. That is the things. So let us go for a very simple examples, what is there.
(Refer Slide Time: 20:14)Let us say you have a broad-crested weir which is built across a 30 meter wide river. It has a
drop height is 2.25 and the phase angles of the structure is 60 degrees. That is what is given it
here the 60 degrees and the scour hole approximately 1V:2H non-cohesive materials. It is a non
cohesive materials having D50 2 mm D90 2.5 mm, estimate the scour depth when the river
discharge is 150 m3/s. This is very simple problem.
Only we have to give it the basic things. So as you have this weir, it will have a critical flow
depth is here. So that you know from the critical flow concept. So you will have the critical flow
here, so that we need to compute what is the critical flow at this and there will be some sort of
hydraulic jump formations, then followed by this. So we need to compute the Q, discharge per
unit width. We have the drop height ds is given to us.
ϴj we compute it as the impedance that we have a ϕ value 40 degrees and we have a G equal to
the 2.65 and we try to compute first the critical depth. That is what comes as the critical depth.
That is what we discussed earlier, how to estimate the critical flow in terms of flow Froude
number is equal to 1 and we can substitute this is V=q/h. So you can compute it these equations
at the critical flow, which will come it with a flow Froude number is equal to 1. So you can
compute it easily just to remember these things. We can compute it.
(Refer Slide Time: 22:30)Now if you look it, if you go for, so V1 is we are getting the V1 for the critical depth, then we
are just substituting these values. As it is given it to estimate it what will be the ∆z. So here it
comes out to be 5.14 meters and this is for the drop height. This is what the drop height is there
and we are computing it, what will be the scour depth.
(Refer Slide Time: 23:02)
Let us come in to very interesting topics what we are talking about that is national waterways.
You all know about national highways. So similar way we have national waterways. It starts
from national waterways one, the Ganga rivers from Haldia to Allahabad, we have the river
systems where we can use it. We have been using it as a national waterways one to transport thegood from Haldia to the Allahabad and that is what covers the state like UP, Bihar, Jharkhand
and the West Bengal.
Same way, we have a national waterways 2, that is what is the Brahmaputra rivers from Dhubri
to Sadiya, which is about 891 kilometer length. So that is the national waterways starting from
Sadiya to Dhubri. We also have a national waterway 6 which is connecting for the Barak rivers
121 kilometers. We have a national waterway 5, which are Brahmani projects, that is what we all
we demonstrate you as mathematical modeling of Brahmani river is a part of this national
waterways 5 projects, where we have a Brahmani and the deltatic regions.
That is what is the national waterways 5, then we have a national waterways 4, which is in the
Andhra Pradesh, Godavari, Krishna and the canal systems. That is a combined systems of
connecting from Pondicherry, Kakinada all these are connectivity with the Godavari, Krishna
and this. That is what is 1000 kilometers and it covers the state like Andhra Pradesh, Tamil Nadu
and also Puducherry. So if you look at that this is what the national waterways 4.
Then similar way, we have a national waterways 3, which is the West Coast canals, which is
around 205 kilometers. See if you look at this national waterways systems what we needed that
to have navigable, we need to know always have the waters at least a minimum water depth; that
is a minimum water depth should be there the river should be navigable. That is not always
possible to have that and what way we can maintain this national waterways.
Such as the way that all throughout the seasons throughout the years, we can have the ship, the
vessels can go through these channels, goes through this national waterways. These are very
challenging tasks for us, because as you know this river morphology changes it and the river is
quite dynamics and maintaining the water depth and all the bigger issues.
So looking that let me talk about the contest that to navigate the river to have enough water to
maintain as a national waterways, you need to require for the commercial navigations for the
goods from one point to other points. For example, from Haldia to Allahabad, we need to have a
channel depth and the channel width. The sufficient depth should be there; sufficient widthshould be there such a way that the ship, the vessels can start from Haldia and can go to
Allahabad or can start from the Dhubri can go to the Sadiya.
So we need to have a minimum the standard channel depth and the width, such a way that river
will be the navigable or the commercial navigations can be done. So that what we look at that it
also depends upon the factors like the type of volume of the tonnage, what the goods is carrying
is there from one point to others. It is just a transport processes. We should know it how much
goods we need to carry from a point to b point and b point to a point through national waterways.
We should know this what type of vessels we are using and what is the size of vessels and tows
in generally used to connecting these waterways. So the basically let us try to understand it how
this national waterways we have and how we need to make all the national waterways are the
commercially navigable and that is the reasons we should know it what is required channel width
should be required and channel depth is required throughout the years and that all depends upon
what type of goods we are having and what is the volume of that, the type and size of vessels and
tools connecting the waterways.
(Refer Slide Time: 28:33)
Now if you look at the next point. There are the barge are there which is carrying the goods from
one point to other point in rivers or the canal transports for the transporting the bulk goods. One
is open hopper barges. So if you can see that this type of barges can be good for the carryingopenly, that mean upon deck it can have a mineral, it can have the coal. So the length is 175 feet,
breadth will be 26 feet, the draft the depth of the water requirement to carry to have the shipping
through this river, it is needed 9 feet.
So you can understand it needs a 9 feet depth and its carrying capacity is 1000 tons. Same way, it
can have a super jumbo which is 250 to 290 length is more, the width or the breadth will be 40-
90 and it can carry as many as 3000 tons. So you can compare with a particular truck. It carries
about 6 ton to 12 ton. So as equivalent you can compute it, how much of goods we can carry in a
barges, open hopper barge, which can carry from 1000 – 3000 ton.
And as you know it, it is more fuel efficient, no environmental pollution systems, but there are
the issues. So if you look at that way, you can see that as equivalent how much ton of materials
we can carry it having a standard or the super jumbo which will carry as high as 3000 tons, but
nowadays you have this integrated chemical and petroleum parts. So here the basic idea to carry
the chemicals and the petroleum things which can have a length 150 – 300, width will be 50 feet,
draft need is 9 feet and the capacity can vary from 1900 tons to 3000 tons.
Same way, if you look it covered hopper barge, the standard one is 175 length, 26 width, draft
feet is 9. It can carry the 1000 and there are the two barges, which will have either you would
have a length, breadth, depth and horsepower you needed. So the basically what I am to do that
there are different type of inland vessels are there and depending upon our requirements whether
you need an open or covered hoppers or integrated chemicals and petroleum, it can have it.
And you know it is much well efficient, environment friendly inland transport mechanisms, but
that is the way that we can carry as one as the 3000 tons, so that and it goes to 3000 tons of
goods we can carry from point a to point b through the rivers, which happens in many of the
developed country like United States. In Europe countries, they extensively use the inland
navigations to carry the goods from a to b using the river navigations or the inland navigations.
(Refer Slide Time: 32:31)Now if you look at that that is what you have discussed is that open hopper barges transporting
the coal, sand, the gravel and the sulfur. You can covered hopper base because it could be a grain
and mixed cargo in it and the tank barges for the petroleum and the chemicals. It needs channel
alignment whether it is a straight channel river or it is a mandarin channels, we cannot have the
straight channels. We need to know how you have to do it.
So for the straight channels what is the alignment is there, what type of is a straight channel; that
you know it for a river. If it is a canal, it can have a straight channels; for rivers we will have
some degree of the meandering and that is the reasons we should have the strategy for that. Size
of the tow whether one way traffic or the two-way traffic you can plan it. As you plan for
national highways, we can think it that whether you have a one-way traffics or the two-way
traffics.
That what you can plan it. So one way traffic basically we can have the signaling systems. You
can also look at the good visibilities and all the things. So one way traffic you have to know it
how much visibility we are happening it. The two way traffic, it heavy traffic to move fast, but
tows are meeting are the passing points. So basically you try to know it, as when you are going
through this thing how these two spacing and its giving it in case of the river meanders, river
bends.The wider navigation channels, we required it for bends which takes the oblique positions in
river. In a bend, the navigations vessels will have oblique positions for that we need a wider
channels as compared to the straight channel, navigation channel is width is a direct proportional
with drift angles. We will show it and we will discuss more. Look at the next figures.
(Refer Slide Time: 34:49)
What is the width is necessary? It is very simple figures. If you look it one way traffic, it is a tow
then you need to have a 40 feet, 40 feet both left and right side, but if you have a two way
traffics, you need to have a 20-20 and 50 feet, then you will have the both the tows. So this is for
the straight channels. As I said it the straight channel is possible for only the man made canals,
but the rivers we may not have the straight reaches. So we always have a bend.
(Refer Slide Time: 35:26)So if you look at this figure, you can understand it that when you have a radius of curvature with
3000 feet, degree of curvature is 90, the velocity is 3 ft/s and if you have that, so it is from the P.
J. Julien books. If you look at that how the deflections angles are changing it and the channel
width requirement. So you have a two arrangement. You can see that as the vessels goes and
how these deflections angles are changing it and how the channel width requirements are
changing it.
That is what is showing to you. So that is the compositions it is showing from difference width
requirement for different angles and we also have a difference to width and the length that is
what is showing it that the drift angle is larger in down bound tows than the off bound tows that
varies it. The radius of curvatures, it depends upon the speed, power and design of the craft, wind
force, the tow is empty or the loaded, the traffic is going off or down.
All we do it and the flow patterns. Many of the times, we do a physical models try to know it if
the ship vessels are coming with it to that how things are happens it, whether it can smoothly
pass it without having any impact on the river banks, that is the canal bank so that is how we do a
physical models, try to know it the scale model, try to know it whether it will be navigables with
a particular river bank and what could be the width requirement and the deflections angles.
(Refer Slide Time: 37:26)Now if you look at the next part, the many of the times we have to make a river its alignment,
either as mild river bend or we can have the straight channels. The river as you know does not
follow exactly straight path. We have to confine the rivers. We have to make it fairly straight
channels or the bend with a larger radius of curvatures. That is our idea for a channel to make
navigable. So wider channel waterway alignment can be controlled through revetment.
You can see this revetment. You can have many places you can see the revetment, the structure
protecting the riverbank and as we discussed earlier. So you can have the revetment, you can
have a spur dykes to divert the flow. You can have the spur dykes. You can see this figure. You
can have the spur dykes to divert the flow from the bank and confining the flow or you can have
a stone riprap.
You can see these figures and if you visit any rivers, which is a navigable river, you can see that
stone riprap the revetment nowadays. You can have a longitudinal dykes. The construction
revetments for dykes, as you know it we discussed earlier you can have a different structures. So
that we can make the river or the canals is navigable maintaining its alignment.
(Refer Slide Time: 39:06)Now you talk about the basic way, the cut off. So you know from natural cases that when you
have a river bend, so river has bend like this, it increases the curvature, sharpens the bends. After
certain times, it cannot maintain more sharpening of bend, because overflow happens. If the flow
overflow happens like this, the bifurcation bend covered. So it form a headcut, chute bar. So it
start depositing here, then start making a chute bar and the headcut here.
And after that it blocks the path, then it can take up this channel, chute cutoff followed by the
triggered by the flood, then you can have this. This is what naturally cut offs happens. Most of
times when you design national waterways, we should understand this process what it happens
naturally. That means when you have the river bend, it bends grows it. After certain the bend
cannot grow it, at that time the overflow bifurcations happens it.
Followed by chute cutoffs are happening it with the trigger by the flood that is what will carry
the waters. So then this part remains as a cutoff, because this knowledge we need it as you know
it, sometime we have to man-made triggering this cut off process. That is what we try to do it
whether you can trigger the cutoff process, because natural cutoffs it takes lot of times. It
depends upon to have a major flood events to have a natural cutoffs.
We cannot wait till that, so we try to make the as resemble to as the natural cognitions. We try to
make artificial cutoffs following the behavior of natural cutoffs. So the basic ideas we learn fromthe river systems, we try to understand the morphology of the river systems of that particular
river and how the natural cut-off process happen. The same cut-off process mechanisms, we
artificially or man-made, we try to introduce to the rivers, then river will be safe.
That is the idea always we should have a morphology studies to try to understand it, how this cut
off process happening in a particular river or particular river reaches from the first historical
satellite imagery or any data set. That understanding is necessary to design artificial cutoff. That
is what my point to say that and we have to understand the river behaviors. If you understand the
river behavior, we can trigger that mechanisms same behaviors with some artificial cut off
process.
So that is the reasons, we do the cutoffs improve the alignment of the river, reduce the sinusity,
shorten the river length as it shortens the river length, it increases the slopes and reduce the flood
stages. It has other effects but let me have this things to that.
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