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Chapter 6 Preventative Measures
118
Chapter 6
PREVENTATIVE MEASURES The enormous investments usually required by water resources projects demand consideration of ways to limit accumulation of sediment in reservoirs.
Such
measures can be subdivided into the following categories:
-
-
Control over the volume of sediment which can flow into a reservoir. Creating flow conditions within reservoirs which will prevent deposition of sediment.
-
Techniques which can be used to remove sediment that has already been deposited in reservoirs.
6.1
CONTROLLING SEDIMENT DISCHARGE
The most logical way to control discharge of sediment into a storage reservoir is to place the reservoir in a catchment with low sed’iment yield. Although this seems to be a very easy way of eliminating the problem, it may not always be feasible. The availability of suitable reservoir
sites in low
sediment yield areas close to the consumer may be limited, and in the final analysis a decision on economic grounds will have to be made in order to establish whether it is feasible to attempt to prevent deposition of sediment in the reservoir under consideration and transport water over long distances, or build the reservoir closer to the consumer and allow it to silt up over a period of time. Another way of controlling the discharge of sediment into reservoirs is to design the reservoir in such a way that floods transporting large quantities of sediment can be guided to flow past and not through the reservoir. designs often require unique topographies, e.g. the Nagle reservoir.
Such
the topography surrounding
The plan view of this reservoir (Figure 6.1) indicates
that i t was placed in a horse-shoe bend of the river.
By constructing a
flood diversion weir, flood control gates and a diversion canal at the inflow to the reservoir, it is possible to guide floods containing large quantities of sediment past the reservoir.
Once such a flood has passed, the flood
control gates are closed and water is allowed to flow into the reservoir again. Construction of so-called “silt dams” upstream of major reservoirs can also be considered to control sediment discharge in high sediment yield areas.
119
1
0
7
2 ' , 3
4
5krn
SCALE
Fig. 6.1
Plan view of the Nagle reservoir.
The aim of providing such dams is to force sediment to be deposited in them and in so doing decrease the sediment load of the river, allowing clear water to flow into the major downstream reservoir.
The final decision regarding
the implementation of this alternative will however also depend on the economic feasibility thereof.
6.2
CONTROLLING SEDIMENT DEPOSITION
An approach which can be followed to limit sediment deposition in a reservoir is to attempt to control sediment deposition once it has been discharged into the reservoir.
This can only be achieved if the sediment carrying capacity
of the stream flowing through the reservoir is kept as close to the original carrying capacity of the river as possible. A s soon as a river flows into a reservoir the retarding effect the latter has on flow normally results in the sediment carrying capacity of the stream being the limiting factor of sediment transport.
This is in contrast with river flow where the availability
120
of s e d i m e n t i s u s u a l l y t h e c o n t r o l l i n g f a c t o r d e t e r m i n i n g t h e sediment l o a d of
t h e s t r e a m (Rooseboom,
I f t h e sediment c a r r y i n g c a p a c i t y of t h e
1975).
c u r r e n t moving t h r o u g h t h e r e s e r v o i r i s t h e r e f o r e c o n t r o l l e d i n such a way as t o b e a b l e t o m a i n t a i n d i s c h a r g e of t h e sediment l o a d e n t e r i n g t h e r e s e r v o i r , deposition thereof can be prevented.
However,
i t is n o r m a l l y v e r y d i f f i c u l t
t o m a i n t a i n a d e q u a t e sediment c a r r y i n g c a p a c i t y as sediment d i s c h a r g e i s v e r y s e n s i t i v e t o changes i n f l o w c o n d i t i o n s , a s u b j e c t t h a t w i l l b e covered more d e t a i l
i n Chapter 8 dealing with
sediment p r o f i l e s .
techniques
Such d e s i g n s u s u a l l y
require
in
t o c a l c u l a t e shapes of a
capability
to
remove
v i r t u a l l y t h e complete dam w a l l d u r i n g t i m e s of f l o o d , a g o a l which c a n o n l y b e a c h i e v e d by p r o v i d i n g l a r g e g a t e s i n r e l a t i v e l y s m a l l dams such as b a r The a c i d t e s t of t h e s u c c e s s o f such a p r o j e c t depends on how suc-
rages.
cessful the reservoir
i s o p e r a t e d , which i n
c a p a b i l i t i e s and judgement.
is
turn
a f u n c t i o n of
human
But t h e complex n a t u r e of t h e problem i n ques-
t i o n p l a c e s s e r i o u s d o u b t s o n t h e s u c c e s s of t h i s approach.
6.3
REMOVAL OF DEPOSITED SEDIMENT
An approach which c a n a l s o be f o l l o w e d t o r e s t o r e a s t o r a g e r e s e r v o i r t o i t s original basin.
state
is
t o remove
sediment which
has
already deposited
in its
T h i s c a n b e a c h i e v e d by d r e d g i n g a n d / o r by f l u s h i n g sediment from t h e
reservoir.
Dredging of
d e p o s i t e d sediment seems t o be a n obvious s o l u t i o n
b u t h a s two main problems a s s o c i a t e d w i t h i t .
Feasibility studies usually
i n d i c a t e t h a t d r e d g i n g i s a v e r y e x p e n s i v e o p e r a t i o n and t h a t even i f owner of t h e r e s e r v o i r d e c i d e s t o d r e d g e ,
s t o r a g e of
the
t h e dredged m a t e r i a l
u s u a l l y p o s e s a problem a s l a r g e r e s e r v o i r s c o u l d y i e l d m i l l i o n s of t o n s of sediment.
These two problems o f t e n l e a d t o t h e c o n c l u s i o n t h a t
i t i s more
economical t o b u i l d a new r e s e r v o i r t h a n t o a t t e m p t d r e d g i n g . The a l t e r n a t i v e s o l u t i o n ,
viz.
the
flushing
of
sediment
from r e s e r v o i r s ,
seems s i m p l e i n c o n c e p t b u t h a s p r a c t i c a l l i m i t a t i o n s . A s t u d y performed by
applying
this
White
approach
can
and
Bettess
be
achieved
(1984) in
indicates
very
small
R e s u l t s of s i m u l a t i o n s t u d i e s o n t h e Kamativi r e s e r v o i r has
a
length
of
approximately
2,5
km
and
an
initial
that
success
in
reservoirs
only.
i n Zimbabwe,
which
c a p a c i t y of
only
7 x 106m3, i n d i c a t e t h a t s t o r a g e loss due t o sediment d e p o s i t i o n a f t e r t e n years
can be
reduced
from a p p r o x i m a t e l y 29
approximately 7 per cent with flushing. c a n t i t must b e e v a l u a t e d
per
cent without
flushing to
Although t h i s seems t o b e s i g n i f i -
i n c o n j u n c t i o n w i t h t h e e f f e c t i t has on r e l i a -
b i l i t y , a n e f f e c t which, a l t h o u g h mentioned i n p r i n c i p l e by White and B e t t e s
121
( 1 9 8 4 ) , is not quantified in terms of probability of failure.
In semi-arid
countries the latter can be the determining factor, especially in water supply projects where reliability is of the utmost importance.
As a rough
practical guide White has stated in personal communication that he is of the opinion that the reservoir capacity : mean annual runoff ratio must be much less dhan 1:50 for flushing measures to be feasible. 6.4
CONCLUSION
In considering the three approaches which can be followed to limit the volume of sediment deposited in reservoirs, control of
sediment deposition and
removal of already deposited sediment are the least feasible.
Sensitivity of
sediment discharge to small changes in carrying capacity of streams makes it virtually
impossible to prevent sediment from depositing in reservoirs.
Removal of deposited sediment on the other hand is generally not economically feasible and also creates environmental problems.
The only alternative
solution that remains therefore, is to attempt to build reservoirs in areas with low sediment yield and in so doing eliminate the problem. A strategy in the planning of water resources projects as far as sedimenta-
tion is concerned, is to estimate the sediment discharge at the site under consideration (with methods presented in Chapter 5) and after calculating the reduction in capacity over the economic life of the reservoir and evaluating other effects resulting from reservoir sedimentation, to decide on economic grounds whether it is feasible to build a particular reservoir. Although preferable, it is not always possible to build a reservoir in a low sediment yield area and under such conditions the water resources engineer must be able to quantitatively evaluate effects of sediment on the environment, economics and design of a reservoir.
Quantitative techniques for
calculating sediment profiles in and scour downstream of reservoirs are therefore presented in the following chapters.
Chapter 6
PREVENTATIVE MEASURES The enormous investments usually required by water resources projects demand consideration of ways to limit accumulation of sediment in reservoirs.
Such
measures can be subdivided into the following categories:
-
-
Control over the volume of sediment which can flow into a reservoir. Creating flow conditions within reservoirs which will prevent deposition of sediment.
-
Techniques which can be used to remove sediment that has already been deposited in reservoirs.
6.1
CONTROLLING SEDIMENT DISCHARGE
The most logical way to control discharge of sediment into a storage reservoir is to place the reservoir in a catchment with low sed’iment yield. Although this seems to be a very easy way of eliminating the problem, it may not always be feasible. The availability of suitable reservoir
sites in low
sediment yield areas close to the consumer may be limited, and in the final analysis a decision on economic grounds will have to be made in order to establish whether it is feasible to attempt to prevent deposition of sediment in the reservoir under consideration and transport water over long distances, or build the reservoir closer to the consumer and allow it to silt up over a period of time. Another way of controlling the discharge of sediment into reservoirs is to design the reservoir in such a way that floods transporting large quantities of sediment can be guided to flow past and not through the reservoir. designs often require unique topographies, e.g. the Nagle reservoir.
Such
the topography surrounding
The plan view of this reservoir (Figure 6.1) indicates
that i t was placed in a horse-shoe bend of the river.
By constructing a
flood diversion weir, flood control gates and a diversion canal at the inflow to the reservoir, it is possible to guide floods containing large quantities of sediment past the reservoir.
Once such a flood has passed, the flood
control gates are closed and water is allowed to flow into the reservoir again. Construction of so-called “silt dams” upstream of major reservoirs can also be considered to control sediment discharge in high sediment yield areas.
119
1
0
7
2 ' , 3
4
5krn
SCALE
Fig. 6.1
Plan view of the Nagle reservoir.
The aim of providing such dams is to force sediment to be deposited in them and in so doing decrease the sediment load of the river, allowing clear water to flow into the major downstream reservoir.
The final decision regarding
the implementation of this alternative will however also depend on the economic feasibility thereof.
6.2
CONTROLLING SEDIMENT DEPOSITION
An approach which can be followed to limit sediment deposition in a reservoir is to attempt to control sediment deposition once it has been discharged into the reservoir.
This can only be achieved if the sediment carrying capacity
of the stream flowing through the reservoir is kept as close to the original carrying capacity of the river as possible. A s soon as a river flows into a reservoir the retarding effect the latter has on flow normally results in the sediment carrying capacity of the stream being the limiting factor of sediment transport.
This is in contrast with river flow where the availability
120
of s e d i m e n t i s u s u a l l y t h e c o n t r o l l i n g f a c t o r d e t e r m i n i n g t h e sediment l o a d of
t h e s t r e a m (Rooseboom,
I f t h e sediment c a r r y i n g c a p a c i t y of t h e
1975).
c u r r e n t moving t h r o u g h t h e r e s e r v o i r i s t h e r e f o r e c o n t r o l l e d i n such a way as t o b e a b l e t o m a i n t a i n d i s c h a r g e of t h e sediment l o a d e n t e r i n g t h e r e s e r v o i r , deposition thereof can be prevented.
However,
i t is n o r m a l l y v e r y d i f f i c u l t
t o m a i n t a i n a d e q u a t e sediment c a r r y i n g c a p a c i t y as sediment d i s c h a r g e i s v e r y s e n s i t i v e t o changes i n f l o w c o n d i t i o n s , a s u b j e c t t h a t w i l l b e covered more d e t a i l
i n Chapter 8 dealing with
sediment p r o f i l e s .
techniques
Such d e s i g n s u s u a l l y
require
in
t o c a l c u l a t e shapes of a
capability
to
remove
v i r t u a l l y t h e complete dam w a l l d u r i n g t i m e s of f l o o d , a g o a l which c a n o n l y b e a c h i e v e d by p r o v i d i n g l a r g e g a t e s i n r e l a t i v e l y s m a l l dams such as b a r The a c i d t e s t of t h e s u c c e s s o f such a p r o j e c t depends on how suc-
rages.
cessful the reservoir
i s o p e r a t e d , which i n
c a p a b i l i t i e s and judgement.
is
turn
a f u n c t i o n of
human
But t h e complex n a t u r e of t h e problem i n ques-
t i o n p l a c e s s e r i o u s d o u b t s o n t h e s u c c e s s of t h i s approach.
6.3
REMOVAL OF DEPOSITED SEDIMENT
An approach which c a n a l s o be f o l l o w e d t o r e s t o r e a s t o r a g e r e s e r v o i r t o i t s original basin.
state
is
t o remove
sediment which
has
already deposited
in its
T h i s c a n b e a c h i e v e d by d r e d g i n g a n d / o r by f l u s h i n g sediment from t h e
reservoir.
Dredging of
d e p o s i t e d sediment seems t o be a n obvious s o l u t i o n
b u t h a s two main problems a s s o c i a t e d w i t h i t .
Feasibility studies usually
i n d i c a t e t h a t d r e d g i n g i s a v e r y e x p e n s i v e o p e r a t i o n and t h a t even i f owner of t h e r e s e r v o i r d e c i d e s t o d r e d g e ,
s t o r a g e of
the
t h e dredged m a t e r i a l
u s u a l l y p o s e s a problem a s l a r g e r e s e r v o i r s c o u l d y i e l d m i l l i o n s of t o n s of sediment.
These two problems o f t e n l e a d t o t h e c o n c l u s i o n t h a t
i t i s more
economical t o b u i l d a new r e s e r v o i r t h a n t o a t t e m p t d r e d g i n g . The a l t e r n a t i v e s o l u t i o n ,
viz.
the
flushing
of
sediment
from r e s e r v o i r s ,
seems s i m p l e i n c o n c e p t b u t h a s p r a c t i c a l l i m i t a t i o n s . A s t u d y performed by
applying
this
White
approach
can
and
Bettess
be
achieved
(1984) in
indicates
very
small
R e s u l t s of s i m u l a t i o n s t u d i e s o n t h e Kamativi r e s e r v o i r has
a
length
of
approximately
2,5
km
and
an
initial
that
success
in
reservoirs
only.
i n Zimbabwe,
which
c a p a c i t y of
only
7 x 106m3, i n d i c a t e t h a t s t o r a g e loss due t o sediment d e p o s i t i o n a f t e r t e n years
can be
reduced
from a p p r o x i m a t e l y 29
approximately 7 per cent with flushing. c a n t i t must b e e v a l u a t e d
per
cent without
flushing to
Although t h i s seems t o b e s i g n i f i -
i n c o n j u n c t i o n w i t h t h e e f f e c t i t has on r e l i a -
b i l i t y , a n e f f e c t which, a l t h o u g h mentioned i n p r i n c i p l e by White and B e t t e s
121
( 1 9 8 4 ) , is not quantified in terms of probability of failure.
In semi-arid
countries the latter can be the determining factor, especially in water supply projects where reliability is of the utmost importance.
As a rough
practical guide White has stated in personal communication that he is of the opinion that the reservoir capacity : mean annual runoff ratio must be much less dhan 1:50 for flushing measures to be feasible. 6.4
CONCLUSION
In considering the three approaches which can be followed to limit the volume of sediment deposited in reservoirs, control of
sediment deposition and
removal of already deposited sediment are the least feasible.
Sensitivity of
sediment discharge to small changes in carrying capacity of streams makes it virtually
impossible to prevent sediment from depositing in reservoirs.
Removal of deposited sediment on the other hand is generally not economically feasible and also creates environmental problems.
The only alternative
solution that remains therefore, is to attempt to build reservoirs in areas with low sediment yield and in so doing eliminate the problem. A strategy in the planning of water resources projects as far as sedimenta-
tion is concerned, is to estimate the sediment discharge at the site under consideration (with methods presented in Chapter 5) and after calculating the reduction in capacity over the economic life of the reservoir and evaluating other effects resulting from reservoir sedimentation, to decide on economic grounds whether it is feasible to build a particular reservoir. Although preferable, it is not always possible to build a reservoir in a low sediment yield area and under such conditions the water resources engineer must be able to quantitatively evaluate effects of sediment on the environment, economics and design of a reservoir.
Quantitative techniques for
calculating sediment profiles in and scour downstream of reservoirs are therefore presented in the following chapters.