To give you schematic picture of a river and then the tributaries that are coming in and
there are some locations where the waste water from
the nearby cities is treated to certain level and then it’s the treated waste water or
untreated waste water or partially treated waste water is mixed with the river flow,
there is one waste disposal point here then there is a waste disposal point here like
this, whereas on the downstream side we take water from certain locations, these are water
withdrawal points for our daily use.
When we dispose the waste here and as the waste is going through the river, the river
has a self-cleansing ability, self-cleansing nature of the river is there because the bacteria
which is there in the river they do biodegrade, the biodegradable material utilizing the oxygen
that is available in the dissolved oxygen that is available, and as the dissolved oxygen
gets depleted by the bacteria for biodegrading this biodegradable material, oxygen from the
atmosphere enters into the water through re-aeration process, so there is a balance, and probably
if I introduce the waste here and that kind of stabilizers I mean the utilization of oxygen
and then the re-aeration and so on, and when I take the water from here it’s not contaminated
by the waste which is discharged into the river on the upstream side, this re-aeration
rate or the self-cleansing ability of the river depends on the flow of conditions, if
the velocity is very high if the turbulence levels are very high then there is more mixing,
there is more re-aeration and the self-cleansing ability of the river will be high if the velocities
are high.
On this let’s say if I put a dam here, what this dam would do is it would increase the
water level on the upstream side and if it increases the water level on the upstream
side then the velocity gets reduced, if the velocity gets reduced then the self-cleansing
ability of the river gets reduced and for the same level of contaminant loading at this
location for the same level of contaminant loading at this location the water may not
be remaining uncontaminated at this location that is after constructing the dam all though
at this point was relatively free of contamination earlier, but because of construction of this
dam this becomes contaminated now.
So if that happens then I should not load the river with pollution at this location
if I want to use the water here for any useful purpose, if I don’t want to load the river
with contaminated domestic waste water or contaminated water or domestic waste water
then I have to treat it and that’s the reason why whenever we construct these dams we have
to see what it does to the water quality on the upstream side and accordingly take the
necessary actions, there is the effect is not only on the upstream side the effect is
going to be felt on the downstream side too because of the construction of the dam the
amount of water available on the downstream side is less, if the water availability on
the downstream side is less for the same level of contaminant
loading you will not have dilution effect and so the concentrations of the contaminants
will go up and that again will spoil the river health, so that’s why they had to build
more than 70 waste treatment plants, they had to spent 12 billion Yuan to stabilize
the transformed geology of the area, in fact Wan Xiaofeng who was a director
in charge of building this dam, he’s supposed to have commented we cannot win by achieving
economic prosperity at the cost of the environment, regarding the displacement 1.2 million people
it was estimated that 1.2 million people were forced to leave their homes during the construction
itself, and 300 thousand more people were found to be at risk due to landslides and
water pollution.
I will like to now discuss the story of the dams on Columbia River system in United States,
Columbia River originates I mean it has a catch bend both in United States as well as
Canada, it’s located in the North Western part of United States and you can see here
so many small, large, and medium dams have been constructed on the Columbia River, this
is the Columbia River here, this is hatchery and then this is the Columbia River and on
its tributaries like snake river, clear water river, and then you know Jordan river and
so on and so forth.
Now what is the effect of constructing all this dams on fish population, you have lot
of riverine fishes like steelhead and salmon in this river which are migratory fish, and
they are used to be abundance of the fish in Columbia River system before they constructed
this dams, and as I mentioned these are migratory fish that is after they born and they grow
up they swim all the way from the mountainous areas through these rivers into the pacific
ocean and when they want to rare or lay eggs they swim back all the way to their homes,
and this was possible before the construction of the dams, but once you construct these
dams and you’re obstructing their paths and the fish cannot jump over the dams then
it had an effect on their population, the fish population had come down significantly,
of course the once very abundant fish are not found any, I mean not found in same quantities
now, not only because of construction of this hydro-dams, because the changes in harvesting
methods, changes in ocean conditions, habitat alterations and so on, but construction of
dams is one of the major reasons for decline in the fish population in the Columbia River
system, so people want to increase their population.
How do you want to increase their population?
You create structures for fish to swim upstream across the dams, so when you construct a dam
you also construct what we call the fish ladders, fish ladders connect upstream of the dam to
the downstream of dam with probably some kind of a gentle slope and try to mimic the flows
that were existing before the construction of the dam, so all the dams when they were
constructing, they were accompanied by the construction of this fish ladder now to make
it easy for the fish to migrate from the downstream side to upstream side, and upstream side to
the downstream side, and there many different designs of fish ladders evolved over a period
of time through research and no, through the research like we have rock,
ramp and nature like fishways, these are all, this is a fishway made by humans to assess
fish to move up and down in the river across the dam but the design here is mimicking the
nature more closely than before, all this is nice you can construct this fish ladders
and fishways, but how do fish know that there is a fish ladder there or there is a fishway
there, and that they have to go through that then we need to understand fisheries biology
before we design these things, and how the altered flow conditions are restored, altered
flow conditions by the dam and somewhat restore flow conditions by these fish ladders and
so on and so forth whether they’re having any effect on the migration of the fish, one
has to study, one has to understand that, they may not be effective all the time, and
they may not be effective to the level that we desire, people in that region started demanding
the removal of the dams, the dam removal is a very serious issue,
many of this dams across the world are closed to their end of licensing agreements, so they
say anyway the licensing agreement is getting over, so why don’t we remove this dam so
that the fish can move more easily, approximately 500 dams have been removed in USA in the last
decade, Pacific Northwest as I mentioned Columbia River is one of the major river systems in
the Pacific Northwest is the most prominent in dam removal discussions, so I’m showing
a picture of putting some data nets and then trying to remove the dam here, okay, we would
like to remove the dam, but removing the dam will it achieve what we want.
So before we remove the dam we need to have some objectives.
The objectives are we have to identify, we have to do some projects, we have to study,
we have to plan, we identify impacts on the dam, impacts of the dam on the ecosystem,
we have to weigh the costs and benefits of this dam removal, what are the costs in removing
the dam, what is the benefit that we are going to get, then we have to determine without
removing the dam, if you put the fish ladders what is the effectiveness of fish ladders
and many other methods of increasing the fish population, do we really need to remove the
dam or can we do this, I mean increase the fish population
through some other means, and then based on the studies we have to recommend how several
methods can be used in conjunction to restore the fish population and convince the stakeholders
that that’s the best way to go about.
But before we go and then remove the dam, we ask one question, yes I remove the dam
then what do I do with all the sediment that has been deposited on the upstream side of
the dam in the reservoir over the past few decades, remember I was discussing about aggradation
on the upstream side of the dams are deposition of the sediments in the reservoirs, there
is huge volume of sediment that is existing on the upstream side of this dams.
Now if I remove this dam, what will happen to all that sediment, where does that go,
if I just remove the dam and don’t do anything else the answer is obvious all that sediment
which is there in the reservoir will eventually start moving to the downstream side along
with the water and as it starts moving downstream side and as the river starts widening on the
downstream side in the plain areas this sediment will start depositing because on the downstream
side velocities will be less as the river widens, the velocities gets reduced, this
sediment starts depositing on the channel bed so there will be a aggradation of the
channel bed now due to removal of this dam.
And if the channel bed is a grading and with the same amount of rains that are coming down
you will have a higher frequency of floods, so naturally the people who are leaving on
the downstream side they would demand that we do something about the sediment in the
reservoir before we bust the dam.
Now that’s not an easy question to answer because there is a huge amount of sediment
and if need to dredge all this sediment from the reservoir before we bust the dam that’s
going to cost lot of money, it’s not only the money that is the issue but what if we
do with this sediment that we have dredged from these reservoirs, if we take the sediment
out of the reservoir and simply dump on the banks of the river, on the sides of the, on
sides I mean on the sides then eventually that sediment will find its way into the river
again when there is rain and there is overland flow, so we cannot simply go and then address
the sediment and then dump it on the river banks, we have to take it somewhere else,
if you have to take it somewhere else then we have to think about the transportation
cost of this sediment, we have to find a proper location where we can utilize the sediment
or dumped the sediment and then we need to worry about how we take it to that location
and so on and so forth, so the management of dredged sediment becomes an important issue
and it could be a limiting factor 2.
There is one more question, we are assuming that by removing this dam we would go to the
same pristine conditions that existed before the construction of the dam, how true is that
assumption, is that assumption right?
The ecology has evolved to a particular state over the years because of the flow of water
and other hydrological conditions and climatic conditions and we have change that by constructing
this dam, we have changed those input conditions, so because there is a change in this input
conditions and these dams have been existing for let’s say 30, 40 years the ecology has
evolved to a new state, let’s say state B and it try to come to a balance to this
altered input.
Now we remove this dam and then again change the flow conditions and other input conditions,
how do we know that the ecology goes back from state B to original state A, it may go
to state C, so how the ecology of that area evolves over a period of time in response
to the removal of the dam, in response to the altered conditions now is a big question
that one has to answer.
So the thing is how to sustain a dam?
As we have seen earlier we need these dams for achieving food security for generating
power and then for protecting our cities from flooding and for recreation and so on and
so forth, but then the construction of the dams will have a significant impact on the
ecology and social life on the people who get displaced, there are some people who get
displaced or get disadvantage, then there are some people who get benefitted by this
dam on the downstream side because of increased availability of water, so how do we take this
decisions?
This is a very good example of the sustainability concept where if you build large dams the
economy of scales comes in and then the benefit cost ratio will be increasing if you increase
the height of the dam, but then if you increase the height of the dam more people on the upstream
side will get affected, that’s a social issue.
And in the last 50 minutes we also have seen what is the effect of the dam on the environment,
so when we design, plan, design and implement this dams we have to give due consideration
to sustainability aspects.