Soil and Water Conservation Engineering

Prof. Rajendra Singh

Department of Agricultural and Food Engineering
Indian Institute of Technology, Kharagpur

Lecture - 05
Water Erosion Control Measures

Hello friends. Welcome back to NPTEL online certification course entitled Soil and
Water Conservation Engineering. I am Rajendra Singh professor in Agricultural and
Food Engineering Department, IIT Kharagpur and we are in last week that is lecture 5 of
week 1 and the topic is Water Erosion Control Measures. Just to give you a reminder of
the course content of this week, we started in lecture 1 with introducing the soil and
water conservation, erosion etcetera.

In lecture 2 we went through causes and types of soil erosion, in lecture 3 we saw factors
affecting soil erosion and effects of soil erosion. And lecture 4 we went through soil
erosion mechanics and in today lecture we will see some of the control measures that can
be adopted to control water erosion.

(Refer Slide Time: 01:15)

Now, erosion control is basically we want to control erosion or in other words we want to
conserve soil. So, the aim of soil conservation is to obtain maximum sustained level of
production from a given area of land. So, that is the that is the thing and for that you
remember when we discuss about the when we discussed about the sheet erosion, we saw
that it is the top fertile soil that is primary carried away that is at that is transported
during the erosion process. So, once with the fertilize soil is lost then obviously, the level
of production will be affected and in order to get the maximum sustained level of
production we must conserve our top fertile soil.

And of course, the second aim is to maintain the soil loss below a threshold level. And
also third-one is to permit the natural rate of soil formation to keep pace with the rate of
soil erosion. The ideal case is the case of geological erosion where the rate of soil
formation and the rate of soil erosion, they are naturally balanced. In the case of
accelerated erosion or manmade erosion, we know that the rate of soil erosion will
always surpass the rate of soil formation because, rate for soil formation is a very slow
process that we have already seen.

So, but it still we would like to keep the rate of soil erosion to be as close as possible to
the rate of soil erosion and that is why we want to maintain the soil loss below a
threshold level so, that so, that the soil formation page could be as close as possible to so,
soil erosion bright.

(Refer Slide Time: 03:29)

Now, coming to solve loss tolerance theoretically soil erosion should be maintained at a
rate that equals to or is below the natural rate at which new soil forms, that is what
basically happens. There is a balance between the soil formation and the soil erosion rate
in the geological erosion and that is why that that we say that is a very slow process, we
cannot really notice that with our naked eyes. On the other hand when the soil erosion
rate is much higher as compared to soil formation rate, then it is quite naturally
noticeable and that is where we call it accelerated erosion or manmade erosion.

Now, it is difficult to know whether this balance exist or not means very difficult. Basic
reason behind this is that though it is possible the rate of soil erosion the rate of though
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However, it is difficult to know whether this balance exist or not and the reason is very
simple that though, the rate of soil loss can be measured and wills we will spent 1 lecture
entire lecture on in the coming week on how to measure soil erosion soil soil erosion of
soil loss in the field conditions that will see. So, but let us assume that soil loss rate can
be measured at the moment with our knowledge, whatever knowledge we have that is
that is possible. But, the rate of soil formation is so slow the natural rate of formation is
so slow that it cannot be easily determined.

So, because one of them because we want to make a keep a balance between rate of soil
loss some rate of soil formation and because, it is not possible determine one of them so,
it is very difficult for us to know whether there is a balance between the soil formation
rate and soil loss rate or not. And the average rate of soil formation as you can see the
number it is 0.1 mm per year.

So, from this itself you can visualize that it is very very difficult really to be able to
measure or know the rate at which soil is getting formed. Now, soil loss tolerance that is
why we finally, defined a soil terms calls soil loss tolerance and then soil loss tolerance
basically defined as the maximum permissible rate of erosion at which soil fertility can
be maintain over 20 to 25 years. So, basically we want to keep the soil loss within a
permissible limit which we are calling as soil loss tolerance so, that the soil fertility is
maintained.

And for that we are what we are saying is that the soil fertility should be maintained over
a period of 20 to 25 years. And based on the studies a mean involves soil loss of 11 tones
per hectare is generally accepted as per as soil loss tolerance is concerned. That means,
that simply means that if you are able to maintain annual soil loss below 11 tons per
hectare for a given area, then it will be problem possible to maintain the soil fertility for
few coming years may be 20-25 years or coming year; that is what we are interested in.
(Refer Slide Time: 08:25)

Now, this table basically describes a criteria that could be used to set the level of soil loss
tolerance and this has been given taken from Morgan 2005. According, to this table the
soil loss is divided into 7 different classes and 7 course are given 1 to 7; 1 is when the
class referred to as very slight where the erosion rate is less than 2 2 tons per hectare to
7, when it is referred is catastrophic where the erosion rate is greater than 500 tons per
hectare. And as we can see that for most purposes the level should be maintained within
code 1, 2, 3; that means, this is the acceptable course. That means, the class should be
between very slight to moderate and that simply means the erosion rate should be less
than 10 tones per hectare.

And this probably will give an idea why the soil tolerance rate in the previous slides we
saw that was taken as 11 tons per hectare because, then that simply means that we will be
able to keep our soil loss within moderate levels. And that means, that will help us in
maintaining keeping the soil productivity maintained for a long period of time.
(Refer Slide Time: 10:02)

Now, coming to erosion control the strategies with could be adopted for conserving soil
should be based on certain factors. And these are 4 major factors, the first one is covering
the soil to protect it from raindrop impact. Second is increasing the infiltration and
capacity of soil to reduce runoff. Third is improving the aggregate stability of soil and
fourth is increasing surface roughness to reduce velocity of runoff.

In our previous class when we discussed about the water erosion mechanics, we saw that
there are 3 processes involved detachment, transportation and deposition. Now,
detachment about detachment when we discussed we saw that it is the raindrop impact
effect or it is the flowing water that is responsible for detachment process. On the other
when hand with come should transportation it is the runoff or the overland flow. That
simply means if you want to check erosion, we have to keep a check on detachment and
erosion processes transportation processes.

And that simply means that two agents which are responsible for these two processes
that is raindrop impact and flowing water that has to be checked. And that is that is
basically the concept behind the strategies which are adopted for erosion controller. If
you look here covering the soil to protect it from raindrop impact. So, if we cover the soil
obviously, we know we have seen the rain drop impact erosion that when the raindrop
hits with its kinetic energy to the base soil surface that breaks down the soil aggregates
and that causes detachment of soil particles.
So, if the soil is covered by some kind of plantation or some kind of grass then;
obviously, that cover will absorb the kinetic energy of raindrop impact. That means,
aggregate disaggregation of the soil mass will not be there that mean detachment will be
limited. Similarly, if we call if the soil cover is there then; obviously, water will be whole
for a longer period time on the soil surface and that will allow more of infiltration to take
place. And that simply means as a result the total runoff that could be generated that will
be reduced..

Then the third is improving the aggregate stability of soil and for that we have to add
some kind of organic matter into our soil. And for that what are the steps, the what is
done will see little later when we talk about the conservation measures. And, lastly if we
increase the surface roughness that is by covering the soil then; obviously, the velocity of
runoff will be conserved. That means, the kinetic energy of the flowing water will be less
that means, the erosive capacity will be less and that means, erosion could be controlled.
So, these are the sum of the strategies which are adopted for controlling erosion.

(Refer Slide Time: 13:33)

Now, coming to erosion control measures water erosion control measures; they are
broadly classified into two groups: agronomical and biological measures and engineering
measures.
(Refer Slide Time: 13:35)

And we will discuss first about the agronomical and biological control measures and
obviously, is the name itself suggest erosion is controlled through crop or vegetation. So,
agronomical and biological the name itself suggests that is the crops or vegetation that
are basically use for controlling erosion. And basically in that means that we need to do
our cultivation in such a way which shall minimize erosion. And obviously, it aims at
providing a suitable crop cover for as long period as possible during rainy season. That
whenever, rain rainy is taking place if our soil surfaces is covered so; obviously, is our
several lectures we have seen now, that that simply means that will observe the kinetic
energy of the falling rain drops or it will offer resistance to the flowing water.

And the result the erosive capacity of either the raindrop or the flowing water will be less
and that means, the detachment in transportation processes will be under control. And the
measures typically which are adopted under agronomical and biological control
measures are crop rotation, crop covering, contour farming, strip cropping and mulch
tillage or stubble mulching. That these are the 5 different types of measures which are
normally adopted under agronomical and biological control measures.
(Refer Slide Time: 15:21)

Now, let us start with crop rotation. So, crop rotation is defined as a more or less regular
succession of different crops being grown on the same piece of land. And this is done
because; rotation of crop reduces erosion and increases the fertility of soil. So, that
simply means that as you can see here example there is a plot having 4 different fields.
So, here one the crops chosen are canola, wheat, field peas and oats and when we go in
to year 2, the same crops are being grown, but there are other fields are change. So, oats
has come from this plot to this plot, field peas has come from here to here, canola has
gone from here to here and wheat has so; that means, there is a circular rotation.

Similarly, if we go into year 3 then again the it is circularly rotated. So, oats comes here
canola, wheat, field peas and year 4 again wheat comes here, wheat, field peas, oats, and
canola. So, this is how the crops are rotated. So, that is crop rotation that is regular
succession of different crops being grown on the same piece of land. And basically, this
helps in two possible ways, one is that the different crops have different root root growth
pattern and because the root growth pattern is different so, each plant or each crop has a
tendency root take nutrients from a particular zone of the soil.

So, if you keep on changing the crop that simply mean the entire the root root zone of the
soil it can be utilized and nutrients are extracted from different places. If we grows same
crop year after year that simply means that the same portion of the root zone is being
utilized by crops. And that means, the remainder of the portion whatever nutrients are
there they remain unutilized, where in the where it is being used that is you being used
excessively and that means, there no nutrient left. So, that one advantage of of crop
rotation.

And of course, it varies with land because all crops cannot be in everywhere. Economic
condition of farmers because, if you have to change crop then obviously, you should
have some cash in hand and cropping system prevalent in area this is very important
point because, if our talk about say Bengal where IIT Kharagpur is located then farmers
here have a tendency to grow paddy during the Kharif season, followed by paddy giving
the Rabi seasons.

So, paddy and paddy is followed whereas, you can see that it is against the principles of
crop rotation. If you go to North-India mostly you will find that paddy or rice is grown
during the Kharif season, followed by wheat in the winter season or Rabi season. And
then some kind of legume crops are taken in the (Refer Time: 18:23) or summer season.

So that means, there is a natural rotation being followed by farmers in Northern India
and suitable crops are used in rotation are legumes and grasses. Basically, because these
are nitrogen fixing crops; legumes especially are nitrogen fixing crops that means, they
add nutrients or organic matter to the soil. So, this them they strengthen the soil from
nutrient point of view and that means, the productivity or production will be it can be
expected to be much higher from such soils which are healthy in nature.

(Refer Slide Time: 19:04)

Then next measure is cover cropping at in the cover cropping it is grown as a
conservation measure, either cover crops are cover crops are grown either during off-
season or in-between rows and for ground protection under trees. So, in your previous
class when we were talking about the ill effects of erosion or erosion then we saw in
between rows if faulty agricultural pads in between rows.

If we allow bears soil then obviously, when raindrops hits there will be detachment and
because, there will be flow velocity here without any kind of resistance so, the erosion
will be much much higher. So, when we say cover cropping then in between rows certain
crops are grown and that simply means that the soil area or the land between the rows
crop rows is protected.

Then common crops are which are used for its cover crop are rye, oats, beans, peas,
alfalfa and grass because of two reasons, one is there provide good cover against erosion.
So, because they spread on the surface those say they provide the cover the soil surface
the that is the purpose that is cover cropping and they furnish hay or fodder and serve as
a soil building crops. So, if you grow alfalfa and grass so obviously, not only covering,
but your also getting some hay or fodderanimals in the area. And if you go for beans,
peas or leguminous crops then obviously, as already I mentioned that they are nitrogen
fixing crop so, they add organic matter to the soil.

And of course, this cover crops are also grown under tall trees like rubber to protect the
soil from the raindrop impacts. So obviously, if the if the surface below tall tree is if you
can imagine in that from a tall tree if whatever water is intercepted it that falls the kinetic
energy will be much much higher. So, if the soil surface is protected with some kind of
covered then soil erosion will be minimized in that case. So, that is the second type of
agronomical major that is cover cropping.
(Refer Slide Time: 21:24)

The third agronomical measure which is adopted is contour farming and in the contour
farming it is defined as tillage practice of applying all treatments on contour. And I think
you know contour what contours are, that is the points having same allegation they are
referred to as contours. So, we identify that and we perform all the field operations like
ploughing, planting, cultivation, harvesting across the slope rather than up and down the
slope. So, if the slope is in this direction say for example of course, is very difficult to
say from this person say for example, in this direction.

So, our all our agricultural operations we do across this slope, not along the slope. So
obviously, in that case what is the what happens is that the flow path is broken and also
the velocity of flow will be checked to a larger extent. And it is generally reported that
contour farming reduces soil loss up to 50 percent, while enhancing the crop yield by 10
percent.

Because, we are allowed we do not allow water to flow away here so obviously, more
soil more water is conserved in the soil; that means, more water is available crop
production and that is why crop yield also is enhanced. Then contour cultivation saves
power and its effectiveness depends on slope steepness and slope length. It has, if saves
power because if you do cultivation practices along the slope then obviously, you can
imagine that while traversing up the slope the energy consumed by farm implements we
much much higher.
(Refer Slide Time: 23:16)

So, the allowable contour length because there are two things, one is length as well as
slope. The allowable contour length are 180 meters at 1 degree, 30 meters at 5.5 degree
and 20 meters at 8.5 degree and so on. So, larger the slope smaller will be this slope
length which is quite obvious. And then there are certain specific cases like for example,
on steep slopes or under conditions of high rainfall intensity and soil erodibility contour
forming alone may increase gulling because, row breaks may release store water.

So obviously, there is always a danger because there is a slope and one of the, if any one
of the each row they break that is simply means whatever, it store water is there that will
have tendency; that will flow with a much higher velocity and much greater impact. And
that means, it might result in row breakage and may cause gulling that is gully generation
it might start. So, usually under such circumstances suggest specific circumstances they
are supplemented by strip cropping which we will be seeing next.
(Refer Slide Time: 24:36)

So, strip cropping is basically a practice of growing alternate strips of row crops across
the slope of the land. So, that is why strip because, alternate strips of row crops are
grown across the land. So, as you can see here for example, they are strips this was one
strip, this is other strip, this is get another strip.

So corn, oats, and hay so, they are grown alternatively. And generally close growing
crops such as hay, wheat or other forages are alternated with strips of row crops such as
corn, soybean, cotton, sugar beets; which simply means that if you grow corn first then
next close growing crops say for example, if season permits then hay or wheat could be
grown depending upon the seasons.

And then again you could have another row crop that in means say for example, soybean.
So that means, you have a row crop followed by a close growing crop and then again a
row crop and what happens is it reduces runoff flowing through the crops rows. So, if
this is a row crop so, whatever water is generated this close good in crops will erase that
water that means, the flow velocity will be reduced.

And then it also increases the infiltration rate of the soil undercover conditions because,
soil water is get arrested here. So obviously, this will had tendency to check the check or
rather because water is store there so; obviously, the more water could in filtered into the
soil because of these regions.
(Refer Slide Time: 26:40)

Then strip cropping could be of 3 different types: buffer strip cropping, contour strip
cropping and field strip cropping. There are 3 types of strip cropping are possible. Let us
see one of one after the other, buffer strip cropping means the strips are grass or legume
crops laid out between contour strips of crops in regular rotation. So, whenever we have
the regular crops grown in between that them the grass or legume crops are laid out, that
is basically they are grown contour. The next is contour strip cropping where crops are
arranged the strips or bands on contours at right angles to natural slope of land.

So, basically here as you can see the contour lines are being followed for cultivation. So,
there are row crops and in between these row crops some alternate crop will be grown.
So, grass or legume could be grown and third-one is the field strip cropping where the
crops are laid out in strips across the slope, but only approximately on the contour. So,
here the idea is same I mean idea is on 3 cases same that you first you have row crops
and in between that you grow the strips of grass or legume crops in between.

And the only thing is that in this case it is the contour lines which are followed in (Refer
Time: 28:12). Here contour lines are followed as close as possible where in this case
contour lines are not so, strictly followed. So, buffer strip cropping, contour strip
cropping or field strip cropping. So, here the buffer the grass or legume use as buffer
crop wherein, the these cases the crops itself are being used for alternating.
(Refer Slide Time: 28:45)

Then the last measure is mulch tillage or stubble mulching and obviously, it is a crop in
soil management practices that utilizes the residual mulches of preceding crops. So, as
you can see that if you when you harvest a crop then the stubble is left in the in the field
itself and that is stubble is used as mulch. That means, in between the rows that that
stubble is used to cover the soil and two major benefits these mulch as break the fall of
the raindrops and thus, dissipating kinetic energy and reduce the surface flow.

And very similar to what cover cropping, but what we are doing instead of growing
another crop we are using the stubble of the previous crop. And that this means that they
will be of no tillage or very minimum tillage that will be required for this cover cropping
kind of things. So, in case of mulch tillage so, that is that is the advantage, but very
similar to the cover cropping these mulch; mulch a stubble use is used as mulch very
similar to the concept of cover cropping.
(Refer Slide Time: 29:58)

Then we come to engineering measures and there are 4 types of engineering measures
which are normally are adopted bunds, terraces, vegetative waterways and gully control
structures. And in on each one of them will be spending at least a minimum of 1 week.
So, that is why I am not going to give you much detail today, but just to just a brief
introduction about these topics.

(Refer Slide Time: 30:28)

So, bunds are earthen embankments built across the slope of the land so, across this slops
that is very important. And of course, bunds could be of different types and typically
there also along all field boundaries you will find bonds where, they are referred to as
peripheral bunds. And two types of bonds which are commonly used for soil erosion
purposes or water erosion purposes they are referred to as contour bund and graded bund.
And week number 3 we will be spending entire week on the bunds, on discussing water
bunds, how to design them and things like that.

(Refer Slide Time: 31:05)

Then next engineering measure is terracing or terrace which is the practice of cutting flat
area the out a hilly or mountainous landscape to grow crops. So, wherever you go to any
of the hilly terraces you will find because there is a hill there is a steep slopes so,
cultivation cannot take place. So, in order to facilitate cultivation the flat areas are cut
and so, that some kind of steps kind of structures are there and so, the cultivation could
take place.

And the types two types of terraces which are commonly adopted are broad base terraces
and bench terraces. So, again week number 4 will be spent on terracing and designed of
terraces.
(Refer Slide Time: 31:54)

Next measure is vegetative waterways which are natural or constructed waterways

shaped to required dimensions and vegetated for safe disposal of runoff from a field,
diversion, terrace or other structure. So, they are nothing, but natural or constructed
waterways or channels where some kind of vegetation is allowed to grow so, that the
water could be taken away at a safe velocity or without causing any kind of erosion. And
week number 5, will be spent on vegetative waterways and then we will see the details of
how water there and how to design them.

(Refer Slide Time: 32:37)

And they last engineering measure is gully control structures and gully control structures
are adopted to control gully erosion. We have seen gully erosion already over this is most
severe type of erosion. And the measures which could be adopted are vegetative
measures, temporary gully control structures and permanent gully control structures. And
again we will spend several weeks on seeing what are the gully control structures and
how to design them in greater detail, if you remember that is what we have seen.

So, this was all about the water erosion control matter in brief. We spend more time on
agricultural and biological measures, a little less on engineering measures because in
coming weeks we will be spending a lot of time on engineering measures.

Thank you very much.