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Chapter 9 Soil Erosion and Sedimentation
187 CHAPTER 9
SOIL EROSION AND SEDIMENTATION INTRODUCTION serious
A
problem
exists
in
many
developing
p a r t i c u l a r l y where a g r i c u l t u r e i s e x p a n d i n g .
parts
of
the
The problem under
world
reference
i s r u r a l resource d e g r a d a t i o n a n d more p a r t i c u l a r l y s o i l erosion. The problem o f soil erosion i n developing very serious and d i f f e r s o n l y is far
worse t h a n i s g e n e r a l l y
deieriorating. and
its
realised and
I t has enormous social,
impact on
adverse.
The
is
the
economic
the more developed
problem
countries
i s ubiquitous and
i n degree from p l a c e to place.
sectors
inextricably
situation
The problem
appears
and p o l i t i c a l can o n l y
linked
with
to be
implications
be s u b s t a n t i a l l y development
and
education. Orthodox engineering aspects a r e b u t a m i n o r facet. The M a i n Causes (Venn, 1988). The dominant b a s i c cause,
a n d the u b i q u i t o u s cause,
of s o i l erosion i s
excessive pressure on the resource base in a m i l i e u of
ignorance,
poverty
and l a n d use malpractices. The
dominant
vegetation
specific
denudation the eventual
the matter,
of
the
worst
soil
r e s u l t i n g from o v e r - g r a z i n g
communal g r a z i n g system. hindmost:
cause
is
erosion as
generally
aggravated
by
the
( E v e r y stockowner f o r himself a n d d e v i l t a k e the
rssult
being essentially
i s BUREAR t e r r a i n ) . related
to
plant
Note
cover,
that
is a
technicallv, biological
as
opposed to an engineering problem. Attitudes Erosion
is
to n a t u r a l resources and
not
regarded
by
most
a g r i c u l t u r e a g g r a v a t e the problem.
rural
people
as
National o r t r i b a l w i l l to conserve n a t u r a l resources so
Generally the leadership, to the conseration ethos. of
livestock
coupled
an
important
i s generally minimal.
i n f l u e n t i a l in o t h e r f i e l d s ,
pays
Complicating f a c t o r s a r e the important
with
the economic
sense of
issue.
investment
in
lip
service
social
role
livestock.
There i s also a general lack of u n d e r s t a n d i n g of l a n d management a n d the reasons f o r degraaa t ion. Facts
The problem i s worst
i n the medium r a i n f a l l
areas
(400-600 mm p.a.1
188 that
are
marginal
capacity
is
not
for
crop
matched
production
by
reduced
i n s t a b i l i t y of many a r i d zone soils. number of landless i s growing. years.
(a)
livestock
reduced
numbers
carrying (b)
the
A v a i l a b l e l a n d i s f i n i t e i n extent.
and
The
Populations w i l l a p p r o x i m a t e l y double i n 2C
This i s a useful time frame f o r p l a n n i n g .
The
dominant
supplies. and
because
Fuel
dung
need f e l t
of most
i s a matter of
is
used
rural
growing
instead,
communities
importance as
thereby
further
is
improved
trees
reducing
water
a r e cut
down
fertility
and
vege t a t ion. Rates of erosion from farmed
lands are
He i n d i c a t e s r a t e s as h i g h a s 500t/ha/a 50mm/a
from
100t/ha/a lt/ha/a
poor
grassed
arid
by
Zachar
w h i c h corresponds
areas.
f o r cropped l a n d w i t h slope
summarized The
less
rate
than
a
to
drops
to
(1982). depth of
less
10% a n d even
than
less t h a n
f o r well tended f i e l d s .
I f these r a t e s a r e compared w i t h the r e g e n e r a t i v e c a p a c i t y of
the s o i l ,
they a r e i n d i c a t i v e of l a r g e scale dissappearance o f w o r k a b l e l a n d i n less t h a n a c e n t u r y . Rates of s o i l formation c a n be 0.1
to lOt/ha/a
to
more
1
mm
soils,
depth/a.
but
steeper
these
slopes
weathering,
Generally
tend or
soil
reformation
to be more erosive
more
poorly
is
becsuse
managed.
they
o r 0.01 mm
rapid are
Regeneration
may
in
shallow
probably be
due
on to
o r deposits b y w i n d o r water o n f l a t t e r l a n d .
TRAl N I NG ASPECTS There a r e few colleges o r schools equipped o r even a s y l l a b u s designed to
teach
virtually
conservation. meaningless
Therefore
Conservation
and
in
the
programmes
programmes.
conservation
poor,
except
conservation
development rural
For
and
Rural
ill
educated
context must
development
progammes
of
be
people must
development
people
conservation
personal
direct
benefit.
with
suitable
integrated
be s u b s t a n t i a l l y
planning must
counter the fundamental causes of problems a n d
as
and far
is
involved
in
implementat ion. as
( b ) manifest
possible attention
(a) to
cornmun it y needs. The f o l l o w i n g methods do n o t appear to h a v e worked: Externally
imposed d i s c i p l i n e i s r e q u i r e d
ie.g.
paddocking,
rotational
g r a z i n g , stock r e d u c t i o n ) . Appeal to the emotions ("conserve f o r f u t u r e generations")
o r appeal
to
logic ( " i f you do not conserve your g r a s s a n d s o i l y o u r
livestock
will
d i e i n times of d r o u g h t " ) . T h i s indeed i s p a r t of
Widespread reform in l a n d tenure.
an
ultimate
solution b u t under present circumstances i s a p i p e dream. Any s u b s t a n t i a l conservation proposal t h a t does not d i r e c t l y b e n e f i t the local people. O l d s t y l e "development constructed
by
work"
outsiders
based on e n g i n e e r i n g works p l a n n e d a n d
that
today
litter
the
sub-continent
as
monuments of f a i l u r e . There a r e no simple solutions to the problems of
resource d e g r a d a t i o n
Solutions to these problems w i l l
i n developing areas.
a n d must be multi-faceted
a l w a y s be complex,
a n d h o l i s t i c a n d deal w i t h fundamentals.
The dimensions of these problems a r e such t h a t there can be no t o t a l solution
i n our
time.
preclude
this.
There
i n t e r n a t i o n a l scale, problem.
C o n s t r a i n t s of
time,
is
shortage
not
yet
a
money, of
manpower
or
land
food
an
The f a c t i s t h a t the problem w i l l surface w i t h a more a b r u p t j a r
necessary
programmes outwards
on
a n d w o r l d a t t e n t i o n h a s therefore not focussed on the
t h a n many environmental problems r e c e i v i n g the a t t e n t i o n of is
and o t h e r s
to
initiate
selected
in
thereafter.
integrated
small
areas
Education
conservation the
in
and
first
training
are
'Greens'.
and
instance key
It
development and
factors
expand in
the
reduce.
By
sol u t ion.
I m p o r t a n t Facets in t h e Solution By
promoting
urbanisation,
embarking on multi-facet a n d development
the
pressures
on
the
land
will
programmes of l a n d use p l a n n i n g f o r conservation
i n selected areas,
preferably
a t the head of
catchments,
i n the f i r s t instance, the erosion w i l l reduce. H a v i n g r e g a r d t o the f a c t s t h a t (b) typically,
: ( a ) water
i s a major cause of erosion,
improved water supplies a r e the dominant f e l t
need of r u r a l
communities in developing areas a n d ( c ) the f a v o u r a b l e cost b e n e f i t r a t i o of
i n n o v a t i v e water
based
s o i l and water conservation
appropriate in research,
technologies,
one
could
integrate
p l a n n i n g a n d implementation a n d
implement on the b a s i s of l a b o u r i n t e n s i v e systems.
d
W 0
TABLE 9 . 4 (Source:
R i v e r s of t h e World
Holeman,
R a n k e d b y Sediment Y i e l d
Water R e s o u r c e s Research,
1968. C o p y r i g h t b y Am.
Geophysical
Union)
~~
Average annual suspended l o a d Average Drainage basin. River Ye1 low Ganges Brahmaputra Yangtze Indus Ching
Amazon
Mississippi Irrawaddy Missouri
Lo
Kosi Mekong Colorado Red Nile
Metric tons
Metric
I03km2
x 106
tons/km
673 956 666 942 969 57 776 222 430 370 26 62 795 637 119 978
1 887 1 451 726 499 4 36 408 36 3 31 2 299 218 190 172 170 135 130 111
2 804 1 518 1 090 257 449 7 158 63 97 695 159 7 308 2 774 214 21 2 1 092 37
1 5 3 1
2
discharge a t 2
3 mouth, 10 c f s 53 415 4 30 770 i96 2 6 400 6 30 479 69
-
64 390
5.5
138 1DO
191
f e w u n i v e r s i t i e s produce the product we need.
The developing
r e g i o n s need
a l a r g e number of Conservation and Development g r a d u a t e s a n d t e c h n i c i a n s whose
training
eng ineer in g
,
should s t a n d on
four
soc io logy a n d econom ics
legs:
.
e.g.
elements of
Orie should enable local people to make b e n e f i c i a l use of erosion e.g.
"farm a donga" and/or
store water i n i t ;
m a t e r i a l s , fuel and fodder in s i l t deposits,
agriculture,
the p r o d u c t s of
produce c o n s t r u c t i o n
a n d so on.
It
i s necessary to
undertake more research i n t o social a n d economic costs a n d b e n e f i t s of s o i l erosion
and
conservation,
a
field
to
which
orthodox
evaluations
are
d i f f i c u l t to a p p l y .
RESERVOIR S E D I M E N T A T I O N Construction of a dam a n d r e s e r v o i r on a r i v e r equ i I ib r i urn
by
modi f y i n g
streamf low
t r a n s p o r t c a p a c i t y of the stream.
and,
i n t e r f e r e s w i t h stream
of sedimentation (sediment d e p o s i t i o n ) , a n d e v e n t u a l l y fill
w i t h sedirllent.
Some w i l l f i l l f a s t e r
t h a n others,
o n sediment y i e l d of the t r i b u t a r y d r a i n a g e b a s i n , the stream,
,
consequent I y
A l l r e s e r v o i r s a r e subject all
the
reservoirs w i l l
depending
resource p l a n n i n g
y i e l d of the d r a i n a g e b a s i n , the r e s e r v o i r ,
t r a n s p o r t c a p a b i l i t y of
the
problem
is
to
estimate
the
the
time
before deposition
enroaches
the useful storage space in the r e s e r v o i r to the p o i n t where design of r e s e r v o i r s ,
it
in on
interferes
I n p l a n n i n g and
i t i s essential t h a t p o t e n t i a l problems associated w i t h
considered.
Sufficient
w i l l not
sediment deposition
sediment
the r a t e a n d amount of sediment deposition
and the l e n g t h of
w i t h the system o p e r a t i n g a s i t was designed to operate. be
primarily
a n d size of the r e s e r v o i r .
I n water
sediment
sedimen t
to some degree
storage
impair
must
be
provided
reservoir operation d u r i n g
that
so
the
useful
l i f e of the p r o j e c t o r the p e r i o d of time used in economic a n a l y s i s . B a s i n Sediment Y i e l d Sediment i s d e r i v e d from erosion ( w e a r i n g a w a y ) of the l a n d surface b y n a t u r a l forces - water,
wind,
enters the d r a i n a g e system, the
basin.
Sediment
ice,
and
gravity.
b u t what does
transported
by
streams
erosion of the streambed a n d b a n k s a s well surface and r i l l s of slope,
land
use,
the d r a i n a g e b a s i n . vegetative
cover,
Not a l l
eroded m a t e r i a l
i s termed the sediment is
Sediment
soil
derived
from
as from erosion of type,
yield varies amount
and
y i e l d of
scour
and
the
land
with
land
type
of
192 precipitation,
c l i m a t i c factors,
a n d n a t u r e o f the d r a i n a g e system.
erosion r a t e s a r e accelerated b y human a c t i v i t i e s , urbanization, farming, Worldwide,
i n c l u d i n g deforestation,
g r a z i n g a n d c h a n n e l i z a t i o n o f streams.
the a n n u a l sediment y i e l d from d r a i n a g e
i n southeast
Asia,
the southeastern U n i t e d States,
shown in F i g . 9 . 1 . h i g h sediment
Loess deposits,
yield.
as i n central
H i g h l a t i t u d e areas,
r u n o f f , h a v e low sediment y i e l d . a n d i n the t r o p i c s ,
Natural
vegetation
with
China, low
i s highest tropics,
as
also have a very
p r e c i p i t a t i o n and
I n the m i d - l a t i t u d e s reduces surface
basins
a n d in the
(55O
erosion
-
(Fig.
low
N a n d S)
30'
9.2).
Areas
w i t h a marked d r y season h a v e high sediment y i e l d s because desiccation of grassland (Petts,
produces
much
erosion
in
the
early
part
of
the
SEWHI WELD Il.km~2y?)
CLASS 1
season
3
I 5 6 7
F i g . 9.1 A
RUNOFF Imm )
-
(50 50 500 500
ARID 0-50 0 -50 50- 100 50- 100 100 t 100
2
general
50-5M)
5001 50 500 500
-
classification
of
world
rivers.
Sediment
i n d i c a t e d i n tonnes p e r s q u a r e k i l o m e t r e p e r y e a r
In t r o p i c a l a n d semi-tropical season,
wet
1984).
lasting
several
remainder of the year;
areas
months,
and
there
is usually
lesser
rainfall
yield
(Petts, a
is
1984).
distinct throughout
rainy the
i n such areas sediment y i e l d i s moderate to h i g h ,
193 as i n c e n t r a l A f r i c a .
Walling's
(1984) estimate of suspended sediment y i e l d
on the A f r i c a n continent i s shown on F i g u r e 9.2.
Runoff from thunderstorms
c a r r i e s l a r g e r concentrations of sediment t h a n r u n o f f from general r a i n s . Two processes a r e
involved
erosion and r i l l erosion. (1)
Geomorphological
steepness of t e r r a i n , and
agricultural
ir, soil
erosion
from
The i n t e n s i t y o f sheet erosion
characteristics
of
the
and
(4)
C I imatic (1)
basin
surface:
is a
Rill
and
( 3 ) L a n d use
precipitation
(3)
of:
erodibility,
Amount
factors.
( 2 ) Seepage
characteristics;
forces that may cause s l o u g h i n g of r i l l borders;
sheet
function
(soil
( 2 ) Soil types;
and l e n g t h of slopes);
practices;
Erosion from r i l l s i s a f u n c t i o n of: c l a y s i n the s o i l ;
land
and
( 4 ) Amount of o r g a n i c m a t e r i a l in the soil;
t y p e of
( 5 ) Size of
soil p a r t i c l e s ; a n d (6) C l i m a t i c and p r e c i p i t a t i o n factors.
DESERT REGIONS
Fig. 9.2 A
tentative
and
generalized
map
of
the
sediment y i e l d s w i t h i n the A f r i c a n continent.
pattern
of
(Walling,
suspended
1984).
194 Schumm (1977) d i v i d e s the f l u v i a l system i n t o three zones t h a t primary
areas
sediment
of
production,
transfer,
the predominate source of sediment
a n d water.
for
point
from
source
significant trubutary sediment
figure.
Zone 3
to
of
i n f l o w downstream of
-
stable,
area
inflow a t
i s an
A
point
will
Zone 2 i s a deposition.
Zone
equal
sediment
and
1
outflow
deposition,
transfer
If if
at
such
there
zone is
the channel point as
B
no is
in the
an
alluvial
t r a n s p o r t sediments a s both suspended a n d bed load.
Where a
p l a i n , a l l u v i a l fan,
a r e a of
serve a s
deposition,
Zone 1 i s the d r a i n a g e b a s i n t h a t i s
r e s p e c t i v e l y , as shown in F i g u r e 9.3. sediment
and
i n l a n d delta, o r estuarine delta.
-
ZONE 1 DRAINAGE BASIN SEDIMENT PROMlCNON AREA REKRVUR DELTA OEWSITS
REKRVOIR LIMITS
_--_--
A
ZONE Zta) -CHANNEL DECAAWTKIN IBED AND BANKS ARE SOURCE OF NEW SEDIMENT Lop9 1
--------
ZONE ZIb)-KDIMENT TRANSFER ZONE
F i g . 9.3 F l u v i a l system w i t h dam a n d r e s e r v o i r Effects of Impoundments o n Sediment T r a n s p o r t Streams
r i v e r flows i n t o a depth rather
and
l a r g e body o f
cross-sectional
rapidly,
thus
area
reducing
water,
such
as
increase
and
stream
the
sediment
stream and r e s u l t i n g
in deposition of
reservoir.
some of
With
time,
the
system.
reservoir,
in
deposits
the
velocities
transport
sediment
fine
r e s e r v o i r a n d deposit a g a i n s t the dam,
a
the
move
water
decrease of
the
headwaters of
capacity
the
down
the
through
a n d some a r e f l u s h e d t h r o u g h the
T y p i c a l deposition p a t t e r n s a r e shown i n F i g u r e 9.4.
Most f i n e suspended load w i l l
pass
short detention time o r a r u n - o f - r i v e r
through low-head
a
small
reservoir.
reservoir However,
with
a
larger
195 reservoirs
that
impound water
t r a p much of the suspended i n f l o w i n g sediment
for
long
time periods
load as well
deposited
in
a
can
be expected
as the bed load.
reservoir
(the
trap
to
The percent of
efficiency
of
the
r e s e r v o i r ) i s a f u n c t i o n of the r a t i o of r e s e r v o i r c a p a c i t y to t o t a l inflow.
,TURBID
INFLOW
DENSITY CURRENT-
FINE SEDIMENTS
F i g . 9.4 Sediment Accumulation i n a T y p i c a l Reservoir Releases
from
a
dam
usually
carry
r e s u l t of deposition in the r e s e r v o i r . w i l l p i c k up sediment from
relatively
little
sediment
as
a
Downstream from the dam the stream
the bed a n d b a n k s u n t i l
it
regains
a
normal
sediment load; d e g r a d a t i o n occurs a n d r i v e r slopes decreases p r o g r e s s i v e l y in
a
downstream
direction
t r a n p o r t a r e reached. wide armor
size g r a d a t i o n . layer
of
until
the
limiting
I f m a t e r i a l comprising natural
coarse
s o r t i n g may
material
that
conditions
for
sediment
the channel b o u n d a r y has a
result
limits
in the formation
the
extent
of
d e g r a d a t i o n process begins immediately below the dam a n d proceeds downstream d i r e c t i o n u n t i l
the e q u i l i b r i u m sediment
load f o r
of
an
scour.
The in
a
l o c a l slopes
a n d velocities i s reached (Vanoni, 1946). For the case of a dam and r e s e r v o i r ,
the zones
identified
f o r a f l u v i a l system can be modified a n d defined a s shown
b y Schumm
i n F i g u r e 9.3.
Zone 1 i s the d r a i n a g e b a s i n t r i b u t a r y to the r e s e r v o i r w i t h dam a t p o i n t
A.
Typically,
deposited
in
much of the
Zone 2 has been d i v i d e d water
the
reservoir
released from
sediment
yield
a n d the b a l a n c e
i n t o two reaches.
the dam p i c k s u p a
over some distance below the dam.
from
the
tributary
i s passed t h r o u g h
basin the
is
dam.
In zone 2 ( a ) r e l a t i v e l y c l e a r new
equilibrium
sediment
load
The extent o f d e g r a d a t i o n v a r i e s w i t h a
196 number of f a c t o r s among the most i m p o r t a n t of w h i c h a r e m a g n i t u d e of dam releases a n d size a n d g r a d a t i o n of bed a n d b a n k m a t e r i a l s .
Zone 2 ( b )
a n d zone 3 i s the a r e a of
comparable to Schurnm's t r a n s f e r zone,
is
sediment
deposition. Importance of Sediment Problems in Water Resource Planning Sediment
problems
associated
with
reservoirs
(2) distribution
the r e s e r v o i r ,
in
reservoir,
( 3 ) a g g r a d a t i o n upstream from the r e s e r v o i r ,
efficiency,
( 5 ) r e s e r v o i r sediment s u r v e y s ,
reservoir,
(7)
degradation
downstream
of
(1 )
include:
deposition
sediment
sediment
deposits
in
the
(4) r e s e r v o i r t r a p
( 6 ) removal of deposits from the
from
the
dam,
(8) sediment
and
a b r a s i o n of h y d r a u l i c machinery. A l l of
the adverse effects associated w i t h sediment r e s u l t
i n increased
p r o j e c t costs. Some f a c t o r s , such as p r o v i d i n g a d d i t i o n a l storage volume to a l l o w f o r sediment deposition o v e r the p r o j e c t
l i f e w i t h o u t encroaching
useful r e s e r v o i r c a p a c i t y , a r e r e f l e c t e d in p r o j e c t f i r s t costs. cannot
b e completely
upstream from the abrasion
damage
foreseen
reservoir,
or
occur
over
degradation
to h y d r a u l i c
such
these
Accordingly,
are
Other f a c t o r s
as
downstream from
machinery;
o p e r a t i o n a n d maintenance costs.
time,
on
aggradation
the dam,
reflected
in
and
annual
a l l d e t r i m e n t a l effects h a v e
a n effect on economic a n a l y s i s o f a p r o j e c t . P r o b a b l y the most c r i t i c a l problem associated w i t h sediment i s d e p l e t i o n of
reservoir
storage than
storage
capacity
projected
benefits.
due
that is
to
deposition
occurs more
a
For example,
in
rapidly
very
serious
if
conservation
the than
reservoir. projected
in
consideration storage
Depletion or
of
is
greater
estimating
project
is significantly
decreased
over the f i r s t 20 years of p r o j e c t o p e r a t i o n r a t h e r t h a n as p r o j e c t e d n e a r the
end
of
decreased
the
and
conservation
100-year
future storage
project
life,
benefits w i l l available.
be
average less t h a n
Similarly
b e n e f i t s i s based on flood c o n t r o l space, over time,
annual projected
projection
and
if
yield
that
f u t u r e benefits w i l l be less t h a n projected
Where sediment deposits in the v i c i n i t y of the dam,
will
with
of
the
flood
space
be full
control
i s decreased
i n p l a n n i n g studies.
such deposits may c l o g
low level o u t l e t s o r power p l a n t intakes. Delta deposits from
the
at
reservoir
the will
head o f result
e l e v a t i o n s f o r s p e c i f i c flows. damage
agricultural
land,
a
in
reservoir a
rise
and in
aggradation
upstream
T h i s may r a i s e the g r o u n d interfere
with
increase local flood damages, block water
gravity
water
storm
upstream
water
surface
table
drainage
i n t a k e s a n d sewer o u t f a l l
and and
lines,
197 a n d present problems w i t h b r i d g e clearance i f the r i v e r i s n a v i g a b l e . Scour drain
downstream from
valuable
b r i d g e piers,
wetland
roads,
dams
may
habitat,
a n d other
lower
cause
the
ground
streambank
structures
along
water
level
erosion,
the
river
and
undermine
bank,
cause
f a i l u r e of levees, a n d so on. Abrasion of h y d r a u l i c machinery i s o f special concern f o r h y d r o e l e c t r i c plants.
Guide vanes a n d r u n n e r s of r e a c t i o n t u r b i n e s a n d c o n t r o l
a n d seats of sediment,
impulse t u r b i n e s
sustain
significant
damage from
nozzles
suspended
a n d costly a n n u a l maintenance i s sometimes r e q u i r e d .
Sediment Deposition in Reservoirs E s t i m a t i n g Sediment Borland sediment
I n f l o w Volume
(1971 )
discussed
three
procedures
for
estimating
i n f l o w volume to a r e s e r v o i r f o r p l a n n i n g studies.
are applicable
to
areas o f
l i t t l e data;
the
third
annual
The f i r s t
procedure
two
i s based on
d e t a i l e d f i e l d measurements, as follows: 1.
Field
inspection of
the d r a i n a g e b a s i n
sediment
sources
(sheet
erosion);
comparison
of
erosion, physical
to
determine s o i l
gullying,
characteristics
those of other s i m i l a r areas f o r w h i c h sediment year)
i s known;
and
flood
applying that
yield
types,
erosion,
of
study
main
channel
area
with
y i e l d r a t e (volume p e r
r a t e to
the s t u d y
area
to
estimate average sediment i n f l o w volume p e r year.
2. Determine t h e a n n u a l sediment y i e l d r a t e f o r e x i s t i n g r e s e r v o i r s in the general a r e a u s i n g f i e l d d a t a from p e r i o d i c sediment
s u r v e y s of
those
r e s e r v o i r s ; a p p l y that y i e l d r a t e to the study a r e a to estimate average sediment i n f l o w volume p e r year.
3.
Compute the total a n n u a l sediment load a t the proposed dam s i t e u s i n g field
measurements to
discharge
and
determine
computing
sediment
sediment
transport
discharge
in
as
tons
a per
function day
as
of a
function of d a i l y streamflow a n d t o t a l volume of inflow p e r y e a r . Sediment Movement and Deposition The movement of
sediment
within
a n d c i r c u l a t i o n p a t t e r n s which,
a
in turn,
reservoir
i s governed
by
current
a r e determined b y t h e effects of
198 river
inflow
currents,
solar
c a r r y i n g a heavy sediment heavy
density
current
h e a t i n g of
load,
the
underflow.
the
water,
and
Solar
heating
s t r a t i f i c a t i o n of a r e s e r v o i r and e v e n t u a l l y complete of
the
reservoir
water.
Wind
In
wind.
rivers
i n f l o w may e n t e r the r e s e r v o i r as a
generates
surface
results
in
thermal
mixing
and
turnover
waves
that
rework
and
resuspend f i n e sediments deposited in shallow water. Solar
heating
temperate
is
climate.
more In
isothermal i n l a t e s p r i n g , reservoir
depth.
radiation,
and
temperature rests
above
a
waters
density
increases.
of
Finally
deeper
the
mid-latitudes reservoirs
warm
in
the
near-surface
an
of
zone
Between these two zones
in
mid-latitudes
summer
waters
upper epilimnion
heavier
cold
i s the thermocline
water
essentially
throughout due
to
decreases
of
light
in
of
areas
in
are
w i t h water temperature about 4°C
Surface the
important
the
the
the
solar as
warm
the
water
hypolimnion.
w i t h a h i g h density
gradient,
as shown in F i g u r e 9.5.
F i g . 9.5
the
T u r b i d i t y Currents a n d Reservoir Thermal S t r a t i f i c a t i o n
I n the f a l l
when
epilimnion
also
air
temperatures
drops,
and
drop,
the
the
temperature of
thermocline
disappears.
water As
in air
temperatures continue to lower,
the s u r f a c e waters e v e n t u a l l y become colder
t h a n water i n the hypolimnium,
and the c o l d s u r f a c e water s i n k s below the
hypolimnion,
resulting
in
thermal p a t t e r n continues a g a i n i n the s p r i n g ,
of
"turnover"
throughout
water
the w i n t e r
a n d the process repeats.
in
the
reservoir.
This
u n t i l s u r f a c e waters warm
Tropical reservoirs t y p i c a l l y
a r e permanen t I y s t r a t i f ied. Where r i v e r
inflows
in
the
summer
are
warmer
a n d of
lesser
density
199
t h a n i n the e p i l i m n i o n , the i n f l o w may flow surface,
as
shown
in
the foregoing
between that of the hypolimnion a
path
along
colder than high
the
thermocline
the e p i l i m n i o n o r
density,
it
may
flow
across
the r e s e r v o i r n e a r the
I f the
figure.
inflow
and the epilimnion, as a n
ivterflow.
carries
a
heavy
sediment
the
bottom
of
along
temperature
is
the i n f l o w may follow
If
the
the
incoming
flow
load a n d reservoir
is
has a as
ar!
underflow. The p a t t e r n of sediment deposition depends on the size a n d t e x t u r e the i n f l o w i n g sediment,
size and shape o f the r e s e r v o i r ,
relationship,
the
and
how
(sands a n d g r a v e l s )
reservoir
operated.
in the backwater
deposit
a n d in the headwaters of
is
the
reservoir,
The
area
building
coarsest
above delta
up
materials
the
reservoir
deposits.
f i n e r s i l t s a n d c l a y s a r e c a r r i e d downstream b y d e n s i t y c u r r e n t s , cases
as
far
as
the
dam,
and
deposit
on
the
of
the inflow-outflow
i n some
floor
reservoir
The
or
are
discharged t h r o u g h the dam. Backwater
deposits r a i s e the
Such deposits tend to be eroded,
streamhed
upstream
from
a n d some m a t e r i a l
pool when the r e s e r v o i r operates a t a
!ow pool
the
reservoir.
moves down
level.
into
the
Backwater deposits
r a i s e water surface p r o f i l e s upstream from the r e s e r v o i r . The U.S.
Bureau of Reclamation (1977) found that
topset slope o f r e s e r v o i r d e l t a original
charlnel
slope
in
deposits
the d e l t a
closely
area.
in most r e s e r v o i r s the
approximates one-half
The
pivot
point
the
elevation
at
which the slope changes a b r u p t l y approximates the water surface e l e v a t i o n a t which the r e s e r v o i r operates f o r a
l a r g e percent of time.
The
foreset slope observed in Bureau of Reclamation r e s e r v o i r s i s 6.5 topset
slope,
steeper
than
deposited
in a
although this. delta
In
some
reservoirs
computing
should
sizes and l a r g e r transported
have
foreset
deposition,
agree w i t h
the
slopes volume
the vo!ume of
b y the i n f l o w
average times the
considerably of
sediment
material
of
sand
i n the time p e r i o d considered,
assuming a d r y weight of about 1200 kg/m’. The s i l t s a n d c l a y s a r e c a r r i e d f a r t h e r downstream
i n t o the
reservoir
a n d deposited along the bottom of the r e s e r v o i r a n d in the v i c i n i t y dam.
of the
The location of these deposits depends p r i m a r i l y on the shape of the
reservoir, chemistry.
the
mineral
characteristics
Where f l o c c u l a t i o n occurs,
of
the
clays,
and
the c l a y s a r e deposited
the in or
water near
the upstream reach of the pool.
Estimating Reservoir Sediment Deposition The r a t e o f sediment
deposition
depends
primarily
on:
capacity/inflow
200 r a t i o of the r e s e r v o i r ; of the r e s e r v o i r .
sediment
content of
the
inflow;
and t r a p efficiency the i n f l o w i n g
The r a t e also depends on c h a r a c t e r i s t i c s of
sediment a n d r e s e r v o i r o p e r a t i n g procedures. Trap
efficiency
is
the
r a t i o of
sediment
deposited
in
a
reservoir
to
t o t a l sediment e n t e r i n g the r e s e r v o i r a n d depends on the r a t i o of r e s e r v o i r volume to i n f l o w (see F i g . 9 . 6 ) . Basic
steps
in
estimating
reservoir
sediment
deposition
for
planning
studies a r e as follows:
1.
Estimate sediment example,
i n f l o w to r e s e r v o i r
for
specific
time
increments,
for
yearly.
2.
Determine t r a p e f f i c i e n c y of the r e s e r v o i r f o r successive time p e r i o d s .
3.
Determine specific
weight of
deposited sediment,
noting
it
will
change
w i t h time due to compaction a n d w i t h r e s e r v o i r o p e r a t i n g procedures.
4.
Project d i s t r i b u t i o n o f sediment
within
the
reservoir
if
distribution
is
important i n p l a n n i n g studies.
5.
Estimate loss of r e s e r v o i r c a p a c i t y a s a f u n c t i o n of time project
throughout
I ife.
100
90 W
g 2
80 70
s
4 50
EU % 30 W I 20 0
NORMAL WNOED RESERVOIRS WITH SLUICING OR VENTING
:10
c
8 0
g g g s z
2:
0 6
F i g . 9.6 T r a p e f f i c i e n c y ,
z g g 5 z d d
0 0
3 2 32-
CAPACITY INFLOW RATIO a f t e r Brune,
1953.
the
201 B r u n e ' s (1953) c u r v e i n d i c a t e s t h a t v i r t u a l l y a l l incoming sediment
will
be deposited in l a r g e r e s e r v o i r s , b u t h i s method g i v e s less r e l i a b l e r e s u l t s f o r smaller r a t i o s of c a p a c i t y / i n f l o w to
topography,
importance.
hydrology,
and
where s i t e specific c o n d i t i o n s r e l a t i n g
sediment
characteristics
are
of
greater
The r a n g e i n flows e s p e c i a l l y flood r a t e s i s a major f a c t o r .
Empirical methods such as those of Brune
(1953)
(1948)
and Churchill
a r e adequate f o r estimates of sediment deposition over time a n d estimates of r e s e r v o i r l i f e f o r p l a n n i n g studies,
a n d the e m p i r i c a l method o f B o r l a n d
and M i l l e r
(1962)
(1958)
modified
by
Lara
is
adequate
estimates of the s p a t i a l d i s t r i b u t i o n of deposition. detailed
studies
are
needed,
and
periodic
for
preliminary
For p r o j e c t design, more
sedimentation
surveys
are
an
part
the
r e q u i r e d f o r project o p e r a t i n g decisions. Sediment Deposition Surveys in Reservoirs
A
sedimentation
overall
program
investigation
for
operation
program
of
a
is
dam
and
integral reservoir.
program i s based on p e r i o d i c r e s u r v e y s of the r e s e r v o i r reduction
in
storage
capacity
over
time,
the
of
The
sediment
to determine
distribution
of
the
deposits
throughout the r e s e r v o i r , a n d so on. Resurveys i n c l u d e f i e l d measurements, o f f i c e studies,
and
can be analyzed
laboratory
analysis
of
sediment
samples.
Field
data
to determine specific
weights of the deposited m a t e r i a l s ,
g r a i n size d i s t r i b u t i o n of the deposits,
sediment y i e l d r a t e of the d r a i n a g e
area,
reservoir
trap
efficiency,
density
currents,
and
so
forth
information t h a t i s v i t a l to operation o f the r e s e r v o i r a n d u s e f u l ,
-
all
as well,
f o r the design of f u t u r e r e s e r v o i r s . How f r e q u e n t l y r e s e r v o i r s a r e resurveyed depends on the estimated r a t e of
deposition
and
reservoir capacity.
on
how
critical
the
need
is
for
data
on
change
Resurveys a r e u s u a l l y scheduled f o r i n t e r v a l s of from 5
Sometimes p a r t i a l o r special r e s u r v e y s a r e made a f t e r
to 10 years.
in
major
floods. I f sediment
proportion records
of
depletions facilities. of
to
inflow volume a n d deposition a r e estimated to be l a r g e storage
actual far
capacities
storage
enough
in
Such information
remaining
reservoir
corresponding
changes
advance
facilities;
are to
space
pool
revision
to
project
needed
plan
i s needed f o r
l i m i t a t i o n s a n d other c r i t e r i a recreation
various
depletions
storage
in
for
and
decisions the
elevations;
for
reservoir
in
accurate
forecasting
construct
future
replacement
r e g a r d i n g reallocation
various
purposes
estabi ishment
to r e g u l a t e construction of
purposes,
regulation
of
of
boat plans
and
the
elevation docks
to
and
assure
optimum u t i l i z a t i o n of r e m a i n i n g r e s e r v o i r storage;
p o s s i b l e m o d i f i c a t i o n of
the
and
regulating
out!ets,
adversely
affected
backwater
effects
water
by
supply
sediment
upstream
from
intakes,
deposits. the
similar
Accurate
reservoir
are
facilities
data
needed
concerning there
if
are
problems l e a d i n g to l e g a l c l a i m s upstream from the r e s e r v o i r a r i s i n g from operation. The location of r e s e r v o i r sediment either
contour
or
method u s u a l l y Permanent
range
requires
ranqes
data
or
a
deposits can b e i d e n t i f i e d b y
using
combination
range
less time a n d
a r e mnnumented
is
in
time to time to o b t a i n p r o f i l e d a t a
of
the
less c o s t l y ;
the f i e l d
that
are
and
used
two.
it
The
i s widely
are
used.
resurveyed
from
to compute changes
in
volume of sediment deposits. An accurate r e s e r v o i r contour map for c o n d i t i o n s p r i o r to closure of dam serves as the b a s i s f o r e s t i m a t i n g sediment
deposition
with
surveys;
contour
determining network
maps
based on
initial
after
cross
the
future
section
beginning
topographic
p r o f i l e s of of
ranges
impoundment;
by as
added
and
as
the
comparison a
basis
to
the
a
basis
for
range for
d e t e r m i n i n g l e n g t h f a c t o r s to be used i n computing volume of deposition. A
sediment
elevations required, in
range
are
is
a
determined
fixed
line
initially
across
and
to measure the depth of sediment
elevation.
The
exact
locations
of
a
reservoir
redetermined
in
along
the
which
future,
as
accumulations o r o t h e r changes
ranges
should
be
identified
by
permanent monuments a n d v e r t i c a l a n d h o r i z o n t a l c o n t r o l surveys. Ranges a r e u s u a l l y are
spaced
using
the
so
that
located normal
volurne,
cross-sections
of
volume between the ranges. of
accuracy
across
the
tributaries. identify spacing
desired mouths
A
ranges
Sediment a
verify
typical in
large
adjacent
by
the
average
ranges,
end-area
reasonably
They
method
represents
the
The s p a c i n g of ranges v a r i e s w i t h the degree
the
volume
tributaries layout
and
reservoirs
estimate. and a
Ranges
should systematic
uses
pairs
should
extend
be
up
numbering
of
beacons
located
the
major
system
at
to
200-1000m
. ranges
s t a b l e channels, only
in of
to the stream a n d the v a l l e y .
computed
few
are
also
established
downstream
c h a r a c t e r i z e d b y erosion-resistant
ranges over
a
short
t h a t d e g r a d a t i o n i s not
d i s t a n c e below
a problem.
For
from
rock the
dam
alluvial
the
beds
dam.
and
are
In
banks,
needed
channels,
to
ranges
should be closely spaced n e a r the dam a n d s h o u l d e x t e n d downstream to a p o i n t where measurable d e g r a d a t i o n
15 to rivers
20 years of i s influenced
operation. by
the
i s not expected to occur
Location location
of
r a n g e s below
of outlet
channels,
in
dams on
the
first
alluvial
tributaries,
and
203
the location of erosion r e s i s t a n t controls. F i e l d measurements g e n e r a l l y include:
1.
Survey of established sediment ranges; p r e p a r a t i o n of topographic of special problem areas,
etc.,
to
maps
determine e l e v a t i o n s a n d depths
of
sediment deposits.
2.
Measurements
needed
to
compute
sediment
densities,
and
samplifig
r e q u i r e d to determine c h a r a c t e r i s t i c s o f the deposited m a t e r i a l .
3.
Observations,
probings,
established ranges, Laboratory deposited
analyses materials
and
other
measurements
not
related
such as photographs, d a t a on d e l t a areas, are
to
limited
largely
determine
size
to
of
analyses
gradation
and
to
etc.
samples
other
of
pertinent
characteristics. Sediment Management Measures The problem of sediment deposition in r e s e r v o i r s can be addressed i n a number of ways, 1.
Consider
some more e f f e c t i v e a n d some more economical t h a n others:
the
sediment
yield
of
deposition problems i n selecting
the a
drainage
reservoir
sediment i n f l o w w i l l be r e l a t i v e l y s m a l l . topography
permits,
select
basin
site;
select
and a
potential s i t e where
I f the r e s e r v o i r i s small
an off-channel
site
if
the r i v e r
and
carries a
heavy sediment load.
2.
Provide
excess
storage
capacity
in
the
reservoir
for
the
sediment
accumulation estimated over the p r o j e c t l i f e .
3.
Implement watershed management measures to reduce sediment p r o d u c t i o n on the t r i b u t a r y b a s i n .
4.
Bypass h e a v i l y
5.
Construct reservoirs.
laden flood flows a r o u n d the r e s e r v o i r .
debris
dams
to
trap
sediment
upstream
from
major
storage
204 6.
P r o v i d e f a c i l i t i e s such a s low-level
s l u i c e s to d i s c h a r g e some sediment
t h r o u g h the r e s e r v o i r .
7.
Use
mechanical
means
such
as
dredging
and
siphoning
to
remove
deposits from the r e s e r v o i r .
Reducing Sediment Inflow The p r i m a r y means t o reduce sediment improved
watershed
management
inflow
flood flows a r o u n d the r e s e r v o i r ;
and
to
(2)
measures;
a
reservoir
bypassing
(1)
are:
heavily-laden
( 3 ) c o n s t r u c t i o n of upstream
debris
dams to t r a p sediment before i t reaches the m a i n storage dam. 1.
Improved erosion
Watershed include
Management
appropriate
Measures
to
agricultural
reduce
methods,
sheet
and
rill
strip
planting,
I f the d r a i n a g e b a s i n
terracing,
crop r o t a t i o n ,
and reafforestation.
small (1-5
k m 2 ) , such measures c a n reduce sediment
is
y i e l d b y 90 to 95
percent. However, f o r l a r g e d r a i n a g e b a s i n s , w i t h numerous landowners, it
is usually
yield
not
physically
significantly
Ye1 low River
by
such
i n China,
were expected
to
o r economiczlly methods.
for
example,
significantly
At
feasible
Sanmenxia
watershed
reduce
sediment
to
reduce b a s i n
Reservoir
on
the
management
techniques
yield,
they
but
have
been r e l a t i v e l y ineffective. 2. B y p a s s i n g H e a v i l y Laden Flood Flows a r o u n d a r e s e r v o i r r e d u c i n g r e s e r v o i r sediment areas.
inflow,
particularly
in a r i d
i s effective and
in
semi-arid
T h i s was done a t the Hushan Reservoir ( i r r i g a t i o n water s u p p l y )
i n China where i n seven years abcut 54 percent of the storage was due to sediment deposition.
A
the head of
and flood f l o w s
reservoir, percent
the reservoir,
reducing
of
what
the it
small
annual
was
diversion
rate
of
originally
dam
were
was
constructed
diverted
sedimentation (UNESCO,
!ost
around about
to
19851.
The
at the
eight UNESCO
p u b l i c a t i o n also r e p o r t s t h a t bypasses h a v e been used i n the USSR a n d i n Switzerland. the
However,
topography,
feasible
for
not
small
such p l a n s a r e expensive
always
feasible.
impoundments
for
It
is
most
hydropower
and,
depending
often where
on
economically bypassing
sediment also reduces maintenance costs associated w i t h a b r a s i o n of
the
h y d r a u l i c machinery. 3.
Debris Dams a n d Sedimentation Basins
have
been
constructed
to
trap
205
a n d permanently reservoirs.
In
store sediment the
U.S.,
Conservation
Service
areas.
are
They
mountain basins
foothills
are
both
have
essentially and
are
maintained
by
that the
otherwise Corps
constructed small
of
Engineers
debris
reservoirs
designed
trap
to
periodically
w o u l d enter basins
and
removing
the
Soil
in
mountainous
in
canyons
located coarse
downstream
sediments. the
in
Some
sediment
by
models
for
mechanical means. They a r e a n e f f e c t i v e c o n t r o l measure.
Future Trends There
is
increasing
simulating sediment
runoff
confidence
(ASCA,
1982).
in
and
use
Modelling
of
the
mathematical erosion
i s not as easy a n d e m p i r i c a l models such a s
loss equation (ASCE,
1986). The l a c k of d a t a , ( P a l i n g et a l . , Pitman,
and
particularly interpolation
rainfall
are
based models
makes continous
generally
necessary
1989). Water resources models based on m o n t h l y r a i n
1973) h a v e not p r o v e d o f use f o r sediment modelling.
physically
based
erosiodi
models
( Y a l in,
1963)
of soil
(Stephenson a n d Meadows,
intensities,
procedures
deposition
the U n i v e r s a l
1975) a r e b e i n g r e p l a c e d b y p h y s i c a l l y
where water r u n o f f v e l o c i t i e s a r e accounted f o r model I i n g d i f f i c u l t
and
coupled
(e.g.
In a n y case
with
hydrualic
models appear most promising. REFERENCES American Society of A g r i c u l t u r a l Engineers, 1982. H y d r o l o g i c M o d e l i n g of Smal I watersheds. 1975. Sedimentation E n g i n e e r i n g . American Society of C i v i I Engineers, Manual on E n g i n e e r i n g Practice, 54. B o r l a n d , W.M., 1971. Reservoir Sedimentation, in R i v e r Mechanics, H.W. Shen, ed. Water Resources Pub1 ications. Colorado. B o r l a n d , W.M. a n d M i l l e r , S.P. 1950. D i s t r i b u t i o n o f sediment i n l a r g e r e s e r v o i r s , ASCE Proceedings. Vol. 84, HY2. Brune, G.M., 1953. T r a p e f f i c i e n c y o f r e s e r v o i r s . T r a n s Am. Geophys. Union. 34 ( 3 ) . C h u r c h i l l , M.A., 1948. A n a l y s i s a n d use o f r e s e r v o i r sedimentation d a t a . Proc. Fed. I n t . Sed. Conf. USBR, Denver. L a r a , J.N.M., 1962. Revision o f procedures to compute sediment d i s t r i b u t i o n in l a r g e r e s e r v o i r s . U.S.B.R. (1989). M o d u l a r r a i n f a l l P a l i n g , W.A.J., Stephenson, D. a n d James, C.S. r u n o f f a n d erosion model l i n g . Water Systems Research Group, U n i v e r s i t y of the Witwatersrand, Johannesburg. Petts, G.E., 1984. Impounded Rivers, John Wiley a n d Sons,. 1973. A mathematical model f o r g e n e r a t i n g m o n t l y r i v e r Pitman, W.V., flows from meteorological d a t a in S.A. Hydrol. Research U n i t , U n i v e r s i t y of the Witwatersrand. Schumm, S.A., 1977. The F l u v i a l System, John Wiley a n d Sons. M.E., 1986. Kinematic H y d r o l o g y and D. a n d Meadows, Stephenson, Modelling, E l s e v i e r , Amsterdam.
206
U.S. Bureau of Reclamation, 1977. Design of Small Dams. UNESCO, 1985. Methods of computing sedimentation i n Lakes a n d Reservoirs, Stevan B r u k , Rapporteur. Vanoni, V., 1946. Transport o f Suspended sediment b y water. Trans. Am. SOC. C i v . Engrs. I I I . Venn, A., 1988. Notes on soil conservation. Continuing Engineering Education Course on Water Resources i n Developing Countries, U n i v e r s i t y of the Witwatersrand. 1984. The sediment y i e l d s of A f r i c a n r i v e r s , i n Challenges W a l l i n g , D.E., i n A f r i c a n Hydrology a n d Water Resources, IAHS P u b l i c a t i o n No. 144. 1963. An expression o f bed load t r a n s p o r t a t i o n . Proc. ASCE., Y a l i n , T.S., J. H y d r a u l i c s Div., 89 (H 7 3 ) . 221 - 250. Zachar, D . , 1982. Soil Erosion, E l s e v i e r , Amsterdam, 547p.
SOIL EROSION AND SEDIMENTATION INTRODUCTION serious
A
problem
exists
in
many
developing
p a r t i c u l a r l y where a g r i c u l t u r e i s e x p a n d i n g .
parts
of
the
The problem under
world
reference
i s r u r a l resource d e g r a d a t i o n a n d more p a r t i c u l a r l y s o i l erosion. The problem o f soil erosion i n developing very serious and d i f f e r s o n l y is far
worse t h a n i s g e n e r a l l y
deieriorating. and
its
realised and
I t has enormous social,
impact on
adverse.
The
is
the
economic
the more developed
problem
countries
i s ubiquitous and
i n degree from p l a c e to place.
sectors
inextricably
situation
The problem
appears
and p o l i t i c a l can o n l y
linked
with
to be
implications
be s u b s t a n t i a l l y development
and
education. Orthodox engineering aspects a r e b u t a m i n o r facet. The M a i n Causes (Venn, 1988). The dominant b a s i c cause,
a n d the u b i q u i t o u s cause,
of s o i l erosion i s
excessive pressure on the resource base in a m i l i e u of
ignorance,
poverty
and l a n d use malpractices. The
dominant
vegetation
specific
denudation the eventual
the matter,
of
the
worst
soil
r e s u l t i n g from o v e r - g r a z i n g
communal g r a z i n g system. hindmost:
cause
is
erosion as
generally
aggravated
by
the
( E v e r y stockowner f o r himself a n d d e v i l t a k e the
rssult
being essentially
i s BUREAR t e r r a i n ) . related
to
plant
Note
cover,
that
is a
technicallv, biological
as
opposed to an engineering problem. Attitudes Erosion
is
to n a t u r a l resources and
not
regarded
by
most
a g r i c u l t u r e a g g r a v a t e the problem.
rural
people
as
National o r t r i b a l w i l l to conserve n a t u r a l resources so
Generally the leadership, to the conseration ethos. of
livestock
coupled
an
important
i s generally minimal.
i n f l u e n t i a l in o t h e r f i e l d s ,
pays
Complicating f a c t o r s a r e the important
with
the economic
sense of
issue.
investment
in
lip
service
social
role
livestock.
There i s also a general lack of u n d e r s t a n d i n g of l a n d management a n d the reasons f o r degraaa t ion. Facts
The problem i s worst
i n the medium r a i n f a l l
areas
(400-600 mm p.a.1
188 that
are
marginal
capacity
is
not
for
crop
matched
production
by
reduced
i n s t a b i l i t y of many a r i d zone soils. number of landless i s growing. years.
(a)
livestock
reduced
numbers
carrying (b)
the
A v a i l a b l e l a n d i s f i n i t e i n extent.
and
The
Populations w i l l a p p r o x i m a t e l y double i n 2C
This i s a useful time frame f o r p l a n n i n g .
The
dominant
supplies. and
because
Fuel
dung
need f e l t
of most
i s a matter of
is
used
rural
growing
instead,
communities
importance as
thereby
further
is
improved
trees
reducing
water
a r e cut
down
fertility
and
vege t a t ion. Rates of erosion from farmed
lands are
He i n d i c a t e s r a t e s as h i g h a s 500t/ha/a 50mm/a
from
100t/ha/a lt/ha/a
poor
grassed
arid
by
Zachar
w h i c h corresponds
areas.
f o r cropped l a n d w i t h slope
summarized The
less
rate
than
a
to
drops
to
(1982). depth of
less
10% a n d even
than
less t h a n
f o r well tended f i e l d s .
I f these r a t e s a r e compared w i t h the r e g e n e r a t i v e c a p a c i t y of
the s o i l ,
they a r e i n d i c a t i v e of l a r g e scale dissappearance o f w o r k a b l e l a n d i n less t h a n a c e n t u r y . Rates of s o i l formation c a n be 0.1
to lOt/ha/a
to
more
1
mm
soils,
depth/a.
but
steeper
these
slopes
weathering,
Generally
tend or
soil
reformation
to be more erosive
more
poorly
is
becsuse
managed.
they
o r 0.01 mm
rapid are
Regeneration
may
in
shallow
probably be
due
on to
o r deposits b y w i n d o r water o n f l a t t e r l a n d .
TRAl N I NG ASPECTS There a r e few colleges o r schools equipped o r even a s y l l a b u s designed to
teach
virtually
conservation. meaningless
Therefore
Conservation
and
in
the
programmes
programmes.
conservation
poor,
except
conservation
development rural
For
and
Rural
ill
educated
context must
development
progammes
of
be
people must
development
people
conservation
personal
direct
benefit.
with
suitable
integrated
be s u b s t a n t i a l l y
planning must
counter the fundamental causes of problems a n d
as
and far
is
involved
in
implementat ion. as
( b ) manifest
possible attention
(a) to
cornmun it y needs. The f o l l o w i n g methods do n o t appear to h a v e worked: Externally
imposed d i s c i p l i n e i s r e q u i r e d
ie.g.
paddocking,
rotational
g r a z i n g , stock r e d u c t i o n ) . Appeal to the emotions ("conserve f o r f u t u r e generations")
o r appeal
to
logic ( " i f you do not conserve your g r a s s a n d s o i l y o u r
livestock
will
d i e i n times of d r o u g h t " ) . T h i s indeed i s p a r t of
Widespread reform in l a n d tenure.
an
ultimate
solution b u t under present circumstances i s a p i p e dream. Any s u b s t a n t i a l conservation proposal t h a t does not d i r e c t l y b e n e f i t the local people. O l d s t y l e "development constructed
by
work"
outsiders
based on e n g i n e e r i n g works p l a n n e d a n d
that
today
litter
the
sub-continent
as
monuments of f a i l u r e . There a r e no simple solutions to the problems of
resource d e g r a d a t i o n
Solutions to these problems w i l l
i n developing areas.
a n d must be multi-faceted
a l w a y s be complex,
a n d h o l i s t i c a n d deal w i t h fundamentals.
The dimensions of these problems a r e such t h a t there can be no t o t a l solution
i n our
time.
preclude
this.
There
i n t e r n a t i o n a l scale, problem.
C o n s t r a i n t s of
time,
is
shortage
not
yet
a
money, of
manpower
or
land
food
an
The f a c t i s t h a t the problem w i l l surface w i t h a more a b r u p t j a r
necessary
programmes outwards
on
a n d w o r l d a t t e n t i o n h a s therefore not focussed on the
t h a n many environmental problems r e c e i v i n g the a t t e n t i o n of is
and o t h e r s
to
initiate
selected
in
thereafter.
integrated
small
areas
Education
conservation the
in
and
first
training
are
'Greens'.
and
instance key
It
development and
factors
expand in
the
reduce.
By
sol u t ion.
I m p o r t a n t Facets in t h e Solution By
promoting
urbanisation,
embarking on multi-facet a n d development
the
pressures
on
the
land
will
programmes of l a n d use p l a n n i n g f o r conservation
i n selected areas,
preferably
a t the head of
catchments,
i n the f i r s t instance, the erosion w i l l reduce. H a v i n g r e g a r d t o the f a c t s t h a t (b) typically,
: ( a ) water
i s a major cause of erosion,
improved water supplies a r e the dominant f e l t
need of r u r a l
communities in developing areas a n d ( c ) the f a v o u r a b l e cost b e n e f i t r a t i o of
i n n o v a t i v e water
based
s o i l and water conservation
appropriate in research,
technologies,
one
could
integrate
p l a n n i n g a n d implementation a n d
implement on the b a s i s of l a b o u r i n t e n s i v e systems.
d
W 0
TABLE 9 . 4 (Source:
R i v e r s of t h e World
Holeman,
R a n k e d b y Sediment Y i e l d
Water R e s o u r c e s Research,
1968. C o p y r i g h t b y Am.
Geophysical
Union)
~~
Average annual suspended l o a d Average Drainage basin. River Ye1 low Ganges Brahmaputra Yangtze Indus Ching
Amazon
Mississippi Irrawaddy Missouri
Lo
Kosi Mekong Colorado Red Nile
Metric tons
Metric
I03km2
x 106
tons/km
673 956 666 942 969 57 776 222 430 370 26 62 795 637 119 978
1 887 1 451 726 499 4 36 408 36 3 31 2 299 218 190 172 170 135 130 111
2 804 1 518 1 090 257 449 7 158 63 97 695 159 7 308 2 774 214 21 2 1 092 37
1 5 3 1
2
discharge a t 2
3 mouth, 10 c f s 53 415 4 30 770 i96 2 6 400 6 30 479 69
-
64 390
5.5
138 1DO
191
f e w u n i v e r s i t i e s produce the product we need.
The developing
r e g i o n s need
a l a r g e number of Conservation and Development g r a d u a t e s a n d t e c h n i c i a n s whose
training
eng ineer in g
,
should s t a n d on
four
soc io logy a n d econom ics
legs:
.
e.g.
elements of
Orie should enable local people to make b e n e f i c i a l use of erosion e.g.
"farm a donga" and/or
store water i n i t ;
m a t e r i a l s , fuel and fodder in s i l t deposits,
agriculture,
the p r o d u c t s of
produce c o n s t r u c t i o n
a n d so on.
It
i s necessary to
undertake more research i n t o social a n d economic costs a n d b e n e f i t s of s o i l erosion
and
conservation,
a
field
to
which
orthodox
evaluations
are
d i f f i c u l t to a p p l y .
RESERVOIR S E D I M E N T A T I O N Construction of a dam a n d r e s e r v o i r on a r i v e r equ i I ib r i urn
by
modi f y i n g
streamf low
t r a n s p o r t c a p a c i t y of the stream.
and,
i n t e r f e r e s w i t h stream
of sedimentation (sediment d e p o s i t i o n ) , a n d e v e n t u a l l y fill
w i t h sedirllent.
Some w i l l f i l l f a s t e r
t h a n others,
o n sediment y i e l d of the t r i b u t a r y d r a i n a g e b a s i n , the stream,
,
consequent I y
A l l r e s e r v o i r s a r e subject all
the
reservoirs w i l l
depending
resource p l a n n i n g
y i e l d of the d r a i n a g e b a s i n , the r e s e r v o i r ,
t r a n s p o r t c a p a b i l i t y of
the
problem
is
to
estimate
the
the
time
before deposition
enroaches
the useful storage space in the r e s e r v o i r to the p o i n t where design of r e s e r v o i r s ,
it
in on
interferes
I n p l a n n i n g and
i t i s essential t h a t p o t e n t i a l problems associated w i t h
considered.
Sufficient
w i l l not
sediment deposition
sediment
the r a t e a n d amount of sediment deposition
and the l e n g t h of
w i t h the system o p e r a t i n g a s i t was designed to operate. be
primarily
a n d size of the r e s e r v o i r .
I n water
sediment
sedimen t
to some degree
storage
impair
must
be
provided
reservoir operation d u r i n g
that
so
the
useful
l i f e of the p r o j e c t o r the p e r i o d of time used in economic a n a l y s i s . B a s i n Sediment Y i e l d Sediment i s d e r i v e d from erosion ( w e a r i n g a w a y ) of the l a n d surface b y n a t u r a l forces - water,
wind,
enters the d r a i n a g e system, the
basin.
Sediment
ice,
and
gravity.
b u t what does
transported
by
streams
erosion of the streambed a n d b a n k s a s well surface and r i l l s of slope,
land
use,
the d r a i n a g e b a s i n . vegetative
cover,
Not a l l
eroded m a t e r i a l
i s termed the sediment is
Sediment
soil
derived
from
as from erosion of type,
yield varies amount
and
y i e l d of
scour
and
the
land
with
land
type
of
192 precipitation,
c l i m a t i c factors,
a n d n a t u r e o f the d r a i n a g e system.
erosion r a t e s a r e accelerated b y human a c t i v i t i e s , urbanization, farming, Worldwide,
i n c l u d i n g deforestation,
g r a z i n g a n d c h a n n e l i z a t i o n o f streams.
the a n n u a l sediment y i e l d from d r a i n a g e
i n southeast
Asia,
the southeastern U n i t e d States,
shown in F i g . 9 . 1 . h i g h sediment
Loess deposits,
yield.
as i n central
H i g h l a t i t u d e areas,
r u n o f f , h a v e low sediment y i e l d . a n d i n the t r o p i c s ,
Natural
vegetation
with
China, low
i s highest tropics,
as
also have a very
p r e c i p i t a t i o n and
I n the m i d - l a t i t u d e s reduces surface
basins
a n d in the
(55O
erosion
-
(Fig.
low
N a n d S)
30'
9.2).
Areas
w i t h a marked d r y season h a v e high sediment y i e l d s because desiccation of grassland (Petts,
produces
much
erosion
in
the
early
part
of
the
SEWHI WELD Il.km~2y?)
CLASS 1
season
3
I 5 6 7
F i g . 9.1 A
RUNOFF Imm )
-
(50 50 500 500
ARID 0-50 0 -50 50- 100 50- 100 100 t 100
2
general
50-5M)
5001 50 500 500
-
classification
of
world
rivers.
Sediment
i n d i c a t e d i n tonnes p e r s q u a r e k i l o m e t r e p e r y e a r
In t r o p i c a l a n d semi-tropical season,
wet
1984).
lasting
several
remainder of the year;
areas
months,
and
there
is usually
lesser
rainfall
yield
(Petts, a
is
1984).
distinct throughout
rainy the
i n such areas sediment y i e l d i s moderate to h i g h ,
193 as i n c e n t r a l A f r i c a .
Walling's
(1984) estimate of suspended sediment y i e l d
on the A f r i c a n continent i s shown on F i g u r e 9.2.
Runoff from thunderstorms
c a r r i e s l a r g e r concentrations of sediment t h a n r u n o f f from general r a i n s . Two processes a r e
involved
erosion and r i l l erosion. (1)
Geomorphological
steepness of t e r r a i n , and
agricultural
ir, soil
erosion
from
The i n t e n s i t y o f sheet erosion
characteristics
of
the
and
(4)
C I imatic (1)
basin
surface:
is a
Rill
and
( 3 ) L a n d use
precipitation
(3)
of:
erodibility,
Amount
factors.
( 2 ) Seepage
characteristics;
forces that may cause s l o u g h i n g of r i l l borders;
sheet
function
(soil
( 2 ) Soil types;
and l e n g t h of slopes);
practices;
Erosion from r i l l s i s a f u n c t i o n of: c l a y s i n the s o i l ;
land
and
( 4 ) Amount of o r g a n i c m a t e r i a l in the soil;
t y p e of
( 5 ) Size of
soil p a r t i c l e s ; a n d (6) C l i m a t i c and p r e c i p i t a t i o n factors.
DESERT REGIONS
Fig. 9.2 A
tentative
and
generalized
map
of
the
sediment y i e l d s w i t h i n the A f r i c a n continent.
pattern
of
(Walling,
suspended
1984).
194 Schumm (1977) d i v i d e s the f l u v i a l system i n t o three zones t h a t primary
areas
sediment
of
production,
transfer,
the predominate source of sediment
a n d water.
for
point
from
source
significant trubutary sediment
figure.
Zone 3
to
of
i n f l o w downstream of
-
stable,
area
inflow a t
i s an
A
point
will
Zone 2 i s a deposition.
Zone
equal
sediment
and
1
outflow
deposition,
transfer
If if
at
such
there
zone is
the channel point as
B
no is
in the
an
alluvial
t r a n s p o r t sediments a s both suspended a n d bed load.
Where a
p l a i n , a l l u v i a l fan,
a r e a of
serve a s
deposition,
Zone 1 i s the d r a i n a g e b a s i n t h a t i s
r e s p e c t i v e l y , as shown in F i g u r e 9.3. sediment
and
i n l a n d delta, o r estuarine delta.
-
ZONE 1 DRAINAGE BASIN SEDIMENT PROMlCNON AREA REKRVUR DELTA OEWSITS
REKRVOIR LIMITS
_--_--
A
ZONE Zta) -CHANNEL DECAAWTKIN IBED AND BANKS ARE SOURCE OF NEW SEDIMENT Lop9 1
--------
ZONE ZIb)-KDIMENT TRANSFER ZONE
F i g . 9.3 F l u v i a l system w i t h dam a n d r e s e r v o i r Effects of Impoundments o n Sediment T r a n s p o r t Streams
r i v e r flows i n t o a depth rather
and
l a r g e body o f
cross-sectional
rapidly,
thus
area
reducing
water,
such
as
increase
and
stream
the
sediment
stream and r e s u l t i n g
in deposition of
reservoir.
some of
With
time,
the
system.
reservoir,
in
deposits
the
velocities
transport
sediment
fine
r e s e r v o i r a n d deposit a g a i n s t the dam,
a
the
move
water
decrease of
the
headwaters of
capacity
the
down
the
through
a n d some a r e f l u s h e d t h r o u g h the
T y p i c a l deposition p a t t e r n s a r e shown i n F i g u r e 9.4.
Most f i n e suspended load w i l l
pass
short detention time o r a r u n - o f - r i v e r
through low-head
a
small
reservoir.
reservoir However,
with
a
larger
195 reservoirs
that
impound water
t r a p much of the suspended i n f l o w i n g sediment
for
long
time periods
load as well
deposited
in
a
can
be expected
as the bed load.
reservoir
(the
trap
to
The percent of
efficiency
of
the
r e s e r v o i r ) i s a f u n c t i o n of the r a t i o of r e s e r v o i r c a p a c i t y to t o t a l inflow.
,TURBID
INFLOW
DENSITY CURRENT-
FINE SEDIMENTS
F i g . 9.4 Sediment Accumulation i n a T y p i c a l Reservoir Releases
from
a
dam
usually
carry
r e s u l t of deposition in the r e s e r v o i r . w i l l p i c k up sediment from
relatively
little
sediment
as
a
Downstream from the dam the stream
the bed a n d b a n k s u n t i l
it
regains
a
normal
sediment load; d e g r a d a t i o n occurs a n d r i v e r slopes decreases p r o g r e s s i v e l y in
a
downstream
direction
t r a n p o r t a r e reached. wide armor
size g r a d a t i o n . layer
of
until
the
limiting
I f m a t e r i a l comprising natural
coarse
s o r t i n g may
material
that
conditions
for
sediment
the channel b o u n d a r y has a
result
limits
in the formation
the
extent
of
d e g r a d a t i o n process begins immediately below the dam a n d proceeds downstream d i r e c t i o n u n t i l
the e q u i l i b r i u m sediment
load f o r
of
an
scour.
The in
a
l o c a l slopes
a n d velocities i s reached (Vanoni, 1946). For the case of a dam and r e s e r v o i r ,
the zones
identified
f o r a f l u v i a l system can be modified a n d defined a s shown
b y Schumm
i n F i g u r e 9.3.
Zone 1 i s the d r a i n a g e b a s i n t r i b u t a r y to the r e s e r v o i r w i t h dam a t p o i n t
A.
Typically,
deposited
in
much of the
Zone 2 has been d i v i d e d water
the
reservoir
released from
sediment
yield
a n d the b a l a n c e
i n t o two reaches.
the dam p i c k s u p a
over some distance below the dam.
from
the
tributary
i s passed t h r o u g h
basin the
is
dam.
In zone 2 ( a ) r e l a t i v e l y c l e a r new
equilibrium
sediment
load
The extent o f d e g r a d a t i o n v a r i e s w i t h a
196 number of f a c t o r s among the most i m p o r t a n t of w h i c h a r e m a g n i t u d e of dam releases a n d size a n d g r a d a t i o n of bed a n d b a n k m a t e r i a l s .
Zone 2 ( b )
a n d zone 3 i s the a r e a of
comparable to Schurnm's t r a n s f e r zone,
is
sediment
deposition. Importance of Sediment Problems in Water Resource Planning Sediment
problems
associated
with
reservoirs
(2) distribution
the r e s e r v o i r ,
in
reservoir,
( 3 ) a g g r a d a t i o n upstream from the r e s e r v o i r ,
efficiency,
( 5 ) r e s e r v o i r sediment s u r v e y s ,
reservoir,
(7)
degradation
downstream
of
(1 )
include:
deposition
sediment
sediment
deposits
in
the
(4) r e s e r v o i r t r a p
( 6 ) removal of deposits from the
from
the
dam,
(8) sediment
and
a b r a s i o n of h y d r a u l i c machinery. A l l of
the adverse effects associated w i t h sediment r e s u l t
i n increased
p r o j e c t costs. Some f a c t o r s , such as p r o v i d i n g a d d i t i o n a l storage volume to a l l o w f o r sediment deposition o v e r the p r o j e c t
l i f e w i t h o u t encroaching
useful r e s e r v o i r c a p a c i t y , a r e r e f l e c t e d in p r o j e c t f i r s t costs. cannot
b e completely
upstream from the abrasion
damage
foreseen
reservoir,
or
occur
over
degradation
to h y d r a u l i c
such
these
Accordingly,
are
Other f a c t o r s
as
downstream from
machinery;
o p e r a t i o n a n d maintenance costs.
time,
on
aggradation
the dam,
reflected
in
and
annual
a l l d e t r i m e n t a l effects h a v e
a n effect on economic a n a l y s i s o f a p r o j e c t . P r o b a b l y the most c r i t i c a l problem associated w i t h sediment i s d e p l e t i o n of
reservoir
storage than
storage
capacity
projected
benefits.
due
that is
to
deposition
occurs more
a
For example,
in
rapidly
very
serious
if
conservation
the than
reservoir. projected
in
consideration storage
Depletion or
of
is
greater
estimating
project
is significantly
decreased
over the f i r s t 20 years of p r o j e c t o p e r a t i o n r a t h e r t h a n as p r o j e c t e d n e a r the
end
of
decreased
the
and
conservation
100-year
future storage
project
life,
benefits w i l l available.
be
average less t h a n
Similarly
b e n e f i t s i s based on flood c o n t r o l space, over time,
annual projected
projection
and
if
yield
that
f u t u r e benefits w i l l be less t h a n projected
Where sediment deposits in the v i c i n i t y of the dam,
will
with
of
the
flood
space
be full
control
i s decreased
i n p l a n n i n g studies.
such deposits may c l o g
low level o u t l e t s o r power p l a n t intakes. Delta deposits from
the
at
reservoir
the will
head o f result
e l e v a t i o n s f o r s p e c i f i c flows. damage
agricultural
land,
a
in
reservoir a
rise
and in
aggradation
upstream
T h i s may r a i s e the g r o u n d interfere
with
increase local flood damages, block water
gravity
water
storm
upstream
water
surface
table
drainage
i n t a k e s a n d sewer o u t f a l l
and and
lines,
197 a n d present problems w i t h b r i d g e clearance i f the r i v e r i s n a v i g a b l e . Scour drain
downstream from
valuable
b r i d g e piers,
wetland
roads,
dams
may
habitat,
a n d other
lower
cause
the
ground
streambank
structures
along
water
level
erosion,
the
river
and
undermine
bank,
cause
f a i l u r e of levees, a n d so on. Abrasion of h y d r a u l i c machinery i s o f special concern f o r h y d r o e l e c t r i c plants.
Guide vanes a n d r u n n e r s of r e a c t i o n t u r b i n e s a n d c o n t r o l
a n d seats of sediment,
impulse t u r b i n e s
sustain
significant
damage from
nozzles
suspended
a n d costly a n n u a l maintenance i s sometimes r e q u i r e d .
Sediment Deposition in Reservoirs E s t i m a t i n g Sediment Borland sediment
I n f l o w Volume
(1971 )
discussed
three
procedures
for
estimating
i n f l o w volume to a r e s e r v o i r f o r p l a n n i n g studies.
are applicable
to
areas o f
l i t t l e data;
the
third
annual
The f i r s t
procedure
two
i s based on
d e t a i l e d f i e l d measurements, as follows: 1.
Field
inspection of
the d r a i n a g e b a s i n
sediment
sources
(sheet
erosion);
comparison
of
erosion, physical
to
determine s o i l
gullying,
characteristics
those of other s i m i l a r areas f o r w h i c h sediment year)
i s known;
and
flood
applying that
yield
types,
erosion,
of
study
main
channel
area
with
y i e l d r a t e (volume p e r
r a t e to
the s t u d y
area
to
estimate average sediment i n f l o w volume p e r year.
2. Determine t h e a n n u a l sediment y i e l d r a t e f o r e x i s t i n g r e s e r v o i r s in the general a r e a u s i n g f i e l d d a t a from p e r i o d i c sediment
s u r v e y s of
those
r e s e r v o i r s ; a p p l y that y i e l d r a t e to the study a r e a to estimate average sediment i n f l o w volume p e r year.
3.
Compute the total a n n u a l sediment load a t the proposed dam s i t e u s i n g field
measurements to
discharge
and
determine
computing
sediment
sediment
transport
discharge
in
as
tons
a per
function day
as
of a
function of d a i l y streamflow a n d t o t a l volume of inflow p e r y e a r . Sediment Movement and Deposition The movement of
sediment
within
a n d c i r c u l a t i o n p a t t e r n s which,
a
in turn,
reservoir
i s governed
by
current
a r e determined b y t h e effects of
198 river
inflow
currents,
solar
c a r r y i n g a heavy sediment heavy
density
current
h e a t i n g of
load,
the
underflow.
the
water,
and
Solar
heating
s t r a t i f i c a t i o n of a r e s e r v o i r and e v e n t u a l l y complete of
the
reservoir
water.
Wind
In
wind.
rivers
i n f l o w may e n t e r the r e s e r v o i r as a
generates
surface
results
in
thermal
mixing
and
turnover
waves
that
rework
and
resuspend f i n e sediments deposited in shallow water. Solar
heating
temperate
is
climate.
more In
isothermal i n l a t e s p r i n g , reservoir
depth.
radiation,
and
temperature rests
above
a
waters
density
increases.
of
Finally
deeper
the
mid-latitudes reservoirs
warm
in
the
near-surface
an
of
zone
Between these two zones
in
mid-latitudes
summer
waters
upper epilimnion
heavier
cold
i s the thermocline
water
essentially
throughout due
to
decreases
of
light
in
of
areas
in
are
w i t h water temperature about 4°C
Surface the
important
the
the
the
solar as
warm
the
water
hypolimnion.
w i t h a h i g h density
gradient,
as shown in F i g u r e 9.5.
F i g . 9.5
the
T u r b i d i t y Currents a n d Reservoir Thermal S t r a t i f i c a t i o n
I n the f a l l
when
epilimnion
also
air
temperatures
drops,
and
drop,
the
the
temperature of
thermocline
disappears.
water As
in air
temperatures continue to lower,
the s u r f a c e waters e v e n t u a l l y become colder
t h a n water i n the hypolimnium,
and the c o l d s u r f a c e water s i n k s below the
hypolimnion,
resulting
in
thermal p a t t e r n continues a g a i n i n the s p r i n g ,
of
"turnover"
throughout
water
the w i n t e r
a n d the process repeats.
in
the
reservoir.
This
u n t i l s u r f a c e waters warm
Tropical reservoirs t y p i c a l l y
a r e permanen t I y s t r a t i f ied. Where r i v e r
inflows
in
the
summer
are
warmer
a n d of
lesser
density
199
t h a n i n the e p i l i m n i o n , the i n f l o w may flow surface,
as
shown
in
the foregoing
between that of the hypolimnion a
path
along
colder than high
the
thermocline
the e p i l i m n i o n o r
density,
it
may
flow
across
the r e s e r v o i r n e a r the
I f the
figure.
inflow
and the epilimnion, as a n
ivterflow.
carries
a
heavy
sediment
the
bottom
of
along
temperature
is
the i n f l o w may follow
If
the
the
incoming
flow
load a n d reservoir
is
has a as
ar!
underflow. The p a t t e r n of sediment deposition depends on the size a n d t e x t u r e the i n f l o w i n g sediment,
size and shape o f the r e s e r v o i r ,
relationship,
the
and
how
(sands a n d g r a v e l s )
reservoir
operated.
in the backwater
deposit
a n d in the headwaters of
is
the
reservoir,
The
area
building
coarsest
above delta
up
materials
the
reservoir
deposits.
f i n e r s i l t s a n d c l a y s a r e c a r r i e d downstream b y d e n s i t y c u r r e n t s , cases
as
far
as
the
dam,
and
deposit
on
the
of
the inflow-outflow
i n some
floor
reservoir
The
or
are
discharged t h r o u g h the dam. Backwater
deposits r a i s e the
Such deposits tend to be eroded,
streamhed
upstream
from
a n d some m a t e r i a l
pool when the r e s e r v o i r operates a t a
!ow pool
the
reservoir.
moves down
level.
into
the
Backwater deposits
r a i s e water surface p r o f i l e s upstream from the r e s e r v o i r . The U.S.
Bureau of Reclamation (1977) found that
topset slope o f r e s e r v o i r d e l t a original
charlnel
slope
in
deposits
the d e l t a
closely
area.
in most r e s e r v o i r s the
approximates one-half
The
pivot
point
the
elevation
at
which the slope changes a b r u p t l y approximates the water surface e l e v a t i o n a t which the r e s e r v o i r operates f o r a
l a r g e percent of time.
The
foreset slope observed in Bureau of Reclamation r e s e r v o i r s i s 6.5 topset
slope,
steeper
than
deposited
in a
although this. delta
In
some
reservoirs
computing
should
sizes and l a r g e r transported
have
foreset
deposition,
agree w i t h
the
slopes volume
the vo!ume of
b y the i n f l o w
average times the
considerably of
sediment
material
of
sand
i n the time p e r i o d considered,
assuming a d r y weight of about 1200 kg/m’. The s i l t s a n d c l a y s a r e c a r r i e d f a r t h e r downstream
i n t o the
reservoir
a n d deposited along the bottom of the r e s e r v o i r a n d in the v i c i n i t y dam.
of the
The location of these deposits depends p r i m a r i l y on the shape of the
reservoir, chemistry.
the
mineral
characteristics
Where f l o c c u l a t i o n occurs,
of
the
clays,
and
the c l a y s a r e deposited
the in or
water near
the upstream reach of the pool.
Estimating Reservoir Sediment Deposition The r a t e o f sediment
deposition
depends
primarily
on:
capacity/inflow
200 r a t i o of the r e s e r v o i r ; of the r e s e r v o i r .
sediment
content of
the
inflow;
and t r a p efficiency the i n f l o w i n g
The r a t e also depends on c h a r a c t e r i s t i c s of
sediment a n d r e s e r v o i r o p e r a t i n g procedures. Trap
efficiency
is
the
r a t i o of
sediment
deposited
in
a
reservoir
to
t o t a l sediment e n t e r i n g the r e s e r v o i r a n d depends on the r a t i o of r e s e r v o i r volume to i n f l o w (see F i g . 9 . 6 ) . Basic
steps
in
estimating
reservoir
sediment
deposition
for
planning
studies a r e as follows:
1.
Estimate sediment example,
i n f l o w to r e s e r v o i r
for
specific
time
increments,
for
yearly.
2.
Determine t r a p e f f i c i e n c y of the r e s e r v o i r f o r successive time p e r i o d s .
3.
Determine specific
weight of
deposited sediment,
noting
it
will
change
w i t h time due to compaction a n d w i t h r e s e r v o i r o p e r a t i n g procedures.
4.
Project d i s t r i b u t i o n o f sediment
within
the
reservoir
if
distribution
is
important i n p l a n n i n g studies.
5.
Estimate loss of r e s e r v o i r c a p a c i t y a s a f u n c t i o n of time project
throughout
I ife.
100
90 W
g 2
80 70
s
4 50
EU % 30 W I 20 0
NORMAL WNOED RESERVOIRS WITH SLUICING OR VENTING
:10
c
8 0
g g g s z
2:
0 6
F i g . 9.6 T r a p e f f i c i e n c y ,
z g g 5 z d d
0 0
3 2 32-
CAPACITY INFLOW RATIO a f t e r Brune,
1953.
the
201 B r u n e ' s (1953) c u r v e i n d i c a t e s t h a t v i r t u a l l y a l l incoming sediment
will
be deposited in l a r g e r e s e r v o i r s , b u t h i s method g i v e s less r e l i a b l e r e s u l t s f o r smaller r a t i o s of c a p a c i t y / i n f l o w to
topography,
importance.
hydrology,
and
where s i t e specific c o n d i t i o n s r e l a t i n g
sediment
characteristics
are
of
greater
The r a n g e i n flows e s p e c i a l l y flood r a t e s i s a major f a c t o r .
Empirical methods such as those of Brune
(1953)
(1948)
and Churchill
a r e adequate f o r estimates of sediment deposition over time a n d estimates of r e s e r v o i r l i f e f o r p l a n n i n g studies,
a n d the e m p i r i c a l method o f B o r l a n d
and M i l l e r
(1962)
(1958)
modified
by
Lara
is
adequate
estimates of the s p a t i a l d i s t r i b u t i o n of deposition. detailed
studies
are
needed,
and
periodic
for
preliminary
For p r o j e c t design, more
sedimentation
surveys
are
an
part
the
r e q u i r e d f o r project o p e r a t i n g decisions. Sediment Deposition Surveys in Reservoirs
A
sedimentation
overall
program
investigation
for
operation
program
of
a
is
dam
and
integral reservoir.
program i s based on p e r i o d i c r e s u r v e y s of the r e s e r v o i r reduction
in
storage
capacity
over
time,
the
of
The
sediment
to determine
distribution
of
the
deposits
throughout the r e s e r v o i r , a n d so on. Resurveys i n c l u d e f i e l d measurements, o f f i c e studies,
and
can be analyzed
laboratory
analysis
of
sediment
samples.
Field
data
to determine specific
weights of the deposited m a t e r i a l s ,
g r a i n size d i s t r i b u t i o n of the deposits,
sediment y i e l d r a t e of the d r a i n a g e
area,
reservoir
trap
efficiency,
density
currents,
and
so
forth
information t h a t i s v i t a l to operation o f the r e s e r v o i r a n d u s e f u l ,
-
all
as well,
f o r the design of f u t u r e r e s e r v o i r s . How f r e q u e n t l y r e s e r v o i r s a r e resurveyed depends on the estimated r a t e of
deposition
and
reservoir capacity.
on
how
critical
the
need
is
for
data
on
change
Resurveys a r e u s u a l l y scheduled f o r i n t e r v a l s of from 5
Sometimes p a r t i a l o r special r e s u r v e y s a r e made a f t e r
to 10 years.
in
major
floods. I f sediment
proportion records
of
depletions facilities. of
to
inflow volume a n d deposition a r e estimated to be l a r g e storage
actual far
capacities
storage
enough
in
Such information
remaining
reservoir
corresponding
changes
advance
facilities;
are to
space
pool
revision
to
project
needed
plan
i s needed f o r
l i m i t a t i o n s a n d other c r i t e r i a recreation
various
depletions
storage
in
for
and
decisions the
elevations;
for
reservoir
in
accurate
forecasting
construct
future
replacement
r e g a r d i n g reallocation
various
purposes
estabi ishment
to r e g u l a t e construction of
purposes,
regulation
of
of
boat plans
and
the
elevation docks
to
and
assure
optimum u t i l i z a t i o n of r e m a i n i n g r e s e r v o i r storage;
p o s s i b l e m o d i f i c a t i o n of
the
and
regulating
out!ets,
adversely
affected
backwater
effects
water
by
supply
sediment
upstream
from
intakes,
deposits. the
similar
Accurate
reservoir
are
facilities
data
needed
concerning there
if
are
problems l e a d i n g to l e g a l c l a i m s upstream from the r e s e r v o i r a r i s i n g from operation. The location of r e s e r v o i r sediment either
contour
or
method u s u a l l y Permanent
range
requires
ranqes
data
or
a
deposits can b e i d e n t i f i e d b y
using
combination
range
less time a n d
a r e mnnumented
is
in
time to time to o b t a i n p r o f i l e d a t a
of
the
less c o s t l y ;
the f i e l d
that
are
and
used
two.
it
The
i s widely
are
used.
resurveyed
from
to compute changes
in
volume of sediment deposits. An accurate r e s e r v o i r contour map for c o n d i t i o n s p r i o r to closure of dam serves as the b a s i s f o r e s t i m a t i n g sediment
deposition
with
surveys;
contour
determining network
maps
based on
initial
after
cross
the
future
section
beginning
topographic
p r o f i l e s of of
ranges
impoundment;
by as
added
and
as
the
comparison a
basis
to
the
a
basis
for
range for
d e t e r m i n i n g l e n g t h f a c t o r s to be used i n computing volume of deposition. A
sediment
elevations required, in
range
are
is
a
determined
fixed
line
initially
across
and
to measure the depth of sediment
elevation.
The
exact
locations
of
a
reservoir
redetermined
in
along
the
which
future,
as
accumulations o r o t h e r changes
ranges
should
be
identified
by
permanent monuments a n d v e r t i c a l a n d h o r i z o n t a l c o n t r o l surveys. Ranges a r e u s u a l l y are
spaced
using
the
so
that
located normal
volurne,
cross-sections
of
volume between the ranges. of
accuracy
across
the
tributaries. identify spacing
desired mouths
A
ranges
Sediment a
verify
typical in
large
adjacent
by
the
average
ranges,
end-area
reasonably
They
method
represents
the
The s p a c i n g of ranges v a r i e s w i t h the degree
the
volume
tributaries layout
and
reservoirs
estimate. and a
Ranges
should systematic
uses
pairs
should
extend
be
up
numbering
of
beacons
located
the
major
system
at
to
200-1000m
. ranges
s t a b l e channels, only
in of
to the stream a n d the v a l l e y .
computed
few
are
also
established
downstream
c h a r a c t e r i z e d b y erosion-resistant
ranges over
a
short
t h a t d e g r a d a t i o n i s not
d i s t a n c e below
a problem.
For
from
rock the
dam
alluvial
the
beds
dam.
and
are
In
banks,
needed
channels,
to
ranges
should be closely spaced n e a r the dam a n d s h o u l d e x t e n d downstream to a p o i n t where measurable d e g r a d a t i o n
15 to rivers
20 years of i s influenced
operation. by
the
i s not expected to occur
Location location
of
r a n g e s below
of outlet
channels,
in
dams on
the
first
alluvial
tributaries,
and
203
the location of erosion r e s i s t a n t controls. F i e l d measurements g e n e r a l l y include:
1.
Survey of established sediment ranges; p r e p a r a t i o n of topographic of special problem areas,
etc.,
to
maps
determine e l e v a t i o n s a n d depths
of
sediment deposits.
2.
Measurements
needed
to
compute
sediment
densities,
and
samplifig
r e q u i r e d to determine c h a r a c t e r i s t i c s o f the deposited m a t e r i a l .
3.
Observations,
probings,
established ranges, Laboratory deposited
analyses materials
and
other
measurements
not
related
such as photographs, d a t a on d e l t a areas, are
to
limited
largely
determine
size
to
of
analyses
gradation
and
to
etc.
samples
other
of
pertinent
characteristics. Sediment Management Measures The problem of sediment deposition in r e s e r v o i r s can be addressed i n a number of ways, 1.
Consider
some more e f f e c t i v e a n d some more economical t h a n others:
the
sediment
yield
of
deposition problems i n selecting
the a
drainage
reservoir
sediment i n f l o w w i l l be r e l a t i v e l y s m a l l . topography
permits,
select
basin
site;
select
and a
potential s i t e where
I f the r e s e r v o i r i s small
an off-channel
site
if
the r i v e r
and
carries a
heavy sediment load.
2.
Provide
excess
storage
capacity
in
the
reservoir
for
the
sediment
accumulation estimated over the p r o j e c t l i f e .
3.
Implement watershed management measures to reduce sediment p r o d u c t i o n on the t r i b u t a r y b a s i n .
4.
Bypass h e a v i l y
5.
Construct reservoirs.
laden flood flows a r o u n d the r e s e r v o i r .
debris
dams
to
trap
sediment
upstream
from
major
storage
204 6.
P r o v i d e f a c i l i t i e s such a s low-level
s l u i c e s to d i s c h a r g e some sediment
t h r o u g h the r e s e r v o i r .
7.
Use
mechanical
means
such
as
dredging
and
siphoning
to
remove
deposits from the r e s e r v o i r .
Reducing Sediment Inflow The p r i m a r y means t o reduce sediment improved
watershed
management
inflow
flood flows a r o u n d the r e s e r v o i r ;
and
to
(2)
measures;
a
reservoir
bypassing
(1)
are:
heavily-laden
( 3 ) c o n s t r u c t i o n of upstream
debris
dams to t r a p sediment before i t reaches the m a i n storage dam. 1.
Improved erosion
Watershed include
Management
appropriate
Measures
to
agricultural
reduce
methods,
sheet
and
rill
strip
planting,
I f the d r a i n a g e b a s i n
terracing,
crop r o t a t i o n ,
and reafforestation.
small (1-5
k m 2 ) , such measures c a n reduce sediment
is
y i e l d b y 90 to 95
percent. However, f o r l a r g e d r a i n a g e b a s i n s , w i t h numerous landowners, it
is usually
yield
not
physically
significantly
Ye1 low River
by
such
i n China,
were expected
to
o r economiczlly methods.
for
example,
significantly
At
feasible
Sanmenxia
watershed
reduce
sediment
to
reduce b a s i n
Reservoir
on
the
management
techniques
yield,
they
but
have
been r e l a t i v e l y ineffective. 2. B y p a s s i n g H e a v i l y Laden Flood Flows a r o u n d a r e s e r v o i r r e d u c i n g r e s e r v o i r sediment areas.
inflow,
particularly
in a r i d
i s effective and
in
semi-arid
T h i s was done a t the Hushan Reservoir ( i r r i g a t i o n water s u p p l y )
i n China where i n seven years abcut 54 percent of the storage was due to sediment deposition.
A
the head of
and flood f l o w s
reservoir, percent
the reservoir,
reducing
of
what
the it
small
annual
was
diversion
rate
of
originally
dam
were
was
constructed
diverted
sedimentation (UNESCO,
!ost
around about
to
19851.
The
at the
eight UNESCO
p u b l i c a t i o n also r e p o r t s t h a t bypasses h a v e been used i n the USSR a n d i n Switzerland. the
However,
topography,
feasible
for
not
small
such p l a n s a r e expensive
always
feasible.
impoundments
for
It
is
most
hydropower
and,
depending
often where
on
economically bypassing
sediment also reduces maintenance costs associated w i t h a b r a s i o n of
the
h y d r a u l i c machinery. 3.
Debris Dams a n d Sedimentation Basins
have
been
constructed
to
trap
205
a n d permanently reservoirs.
In
store sediment the
U.S.,
Conservation
Service
areas.
are
They
mountain basins
foothills
are
both
have
essentially and
are
maintained
by
that the
otherwise Corps
constructed small
of
Engineers
debris
reservoirs
designed
trap
to
periodically
w o u l d enter basins
and
removing
the
Soil
in
mountainous
in
canyons
located coarse
downstream
sediments. the
in
Some
sediment
by
models
for
mechanical means. They a r e a n e f f e c t i v e c o n t r o l measure.
Future Trends There
is
increasing
simulating sediment
runoff
confidence
(ASCA,
1982).
in
and
use
Modelling
of
the
mathematical erosion
i s not as easy a n d e m p i r i c a l models such a s
loss equation (ASCE,
1986). The l a c k of d a t a , ( P a l i n g et a l . , Pitman,
and
particularly interpolation
rainfall
are
based models
makes continous
generally
necessary
1989). Water resources models based on m o n t h l y r a i n
1973) h a v e not p r o v e d o f use f o r sediment modelling.
physically
based
erosiodi
models
( Y a l in,
1963)
of soil
(Stephenson a n d Meadows,
intensities,
procedures
deposition
the U n i v e r s a l
1975) a r e b e i n g r e p l a c e d b y p h y s i c a l l y
where water r u n o f f v e l o c i t i e s a r e accounted f o r model I i n g d i f f i c u l t
and
coupled
(e.g.
In a n y case
with
hydrualic
models appear most promising. REFERENCES American Society of A g r i c u l t u r a l Engineers, 1982. H y d r o l o g i c M o d e l i n g of Smal I watersheds. 1975. Sedimentation E n g i n e e r i n g . American Society of C i v i I Engineers, Manual on E n g i n e e r i n g Practice, 54. B o r l a n d , W.M., 1971. Reservoir Sedimentation, in R i v e r Mechanics, H.W. Shen, ed. Water Resources Pub1 ications. Colorado. B o r l a n d , W.M. a n d M i l l e r , S.P. 1950. D i s t r i b u t i o n o f sediment i n l a r g e r e s e r v o i r s , ASCE Proceedings. Vol. 84, HY2. Brune, G.M., 1953. T r a p e f f i c i e n c y o f r e s e r v o i r s . T r a n s Am. Geophys. Union. 34 ( 3 ) . C h u r c h i l l , M.A., 1948. A n a l y s i s a n d use o f r e s e r v o i r sedimentation d a t a . Proc. Fed. I n t . Sed. Conf. USBR, Denver. L a r a , J.N.M., 1962. Revision o f procedures to compute sediment d i s t r i b u t i o n in l a r g e r e s e r v o i r s . U.S.B.R. (1989). M o d u l a r r a i n f a l l P a l i n g , W.A.J., Stephenson, D. a n d James, C.S. r u n o f f a n d erosion model l i n g . Water Systems Research Group, U n i v e r s i t y of the Witwatersrand, Johannesburg. Petts, G.E., 1984. Impounded Rivers, John Wiley a n d Sons,. 1973. A mathematical model f o r g e n e r a t i n g m o n t l y r i v e r Pitman, W.V., flows from meteorological d a t a in S.A. Hydrol. Research U n i t , U n i v e r s i t y of the Witwatersrand. Schumm, S.A., 1977. The F l u v i a l System, John Wiley a n d Sons. M.E., 1986. Kinematic H y d r o l o g y and D. a n d Meadows, Stephenson, Modelling, E l s e v i e r , Amsterdam.
206
U.S. Bureau of Reclamation, 1977. Design of Small Dams. UNESCO, 1985. Methods of computing sedimentation i n Lakes a n d Reservoirs, Stevan B r u k , Rapporteur. Vanoni, V., 1946. Transport o f Suspended sediment b y water. Trans. Am. SOC. C i v . Engrs. I I I . Venn, A., 1988. Notes on soil conservation. Continuing Engineering Education Course on Water Resources i n Developing Countries, U n i v e r s i t y of the Witwatersrand. 1984. The sediment y i e l d s of A f r i c a n r i v e r s , i n Challenges W a l l i n g , D.E., i n A f r i c a n Hydrology a n d Water Resources, IAHS P u b l i c a t i o n No. 144. 1963. An expression o f bed load t r a n s p o r t a t i o n . Proc. ASCE., Y a l i n , T.S., J. H y d r a u l i c s Div., 89 (H 7 3 ) . 221 - 250. Zachar, D . , 1982. Soil Erosion, E l s e v i e r , Amsterdam, 547p.