The challenge of small reservoir sedimentation to water resources development: The case of multi-purpose Chamakala II small earth dam in Malawi.

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22 févr. 2014 (il y a 3 années et 1 mois)

37 vue(s)

1



The challenge of small reservoir sedimentation to water resources development: The case of multi
-
purpose
Chamakala II small earth dam in Malawi.


Sidney L. Kamtukule
a*

and Eng. Evans Kaseke
b


a
Water Resources Board Secretariat, Private Bag 390, Lilongwe

3, Malawi.

b
Department of Civil Engineering, University of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe.


*
Corresponding author’s email:

slkamtukule@gmail.com




ABSTRACT


The contribution of smal
l multi
-
purpose earth dams towards sustaining water dependant rural livelihoods cannot be
over emphasized. The continued loss of these small water storage facilities due to accelerated sedimentation is a
critical challenge to the achievement of goals and o
bjectives pertaining to improving rural livelihoods through water
development. In Malawi, the issue of sediment transport and the respective sedimentation of small reservoirs are
given insignificant attention despite the Malawi Government having implemente
d a national small earth dam
construction program to develop 350 new facilities. Malawi’s Ministry of Irrigation and Water Development
confirmed concerns over the small reservoirs’ sedimentation problem and the resultant loss of storage capacity and
dimin
ishing available water to sustain a diversity of ever growing and competing rural livelihoods’ water demands.
Resultant anxieties over diminishing available water are further exacerbated by lack of an adequate catchment
management program.


A 2008 study of

the Chamakala II Small Earth Dam on assessing the impact of sedimentation on water availability
revealed a dependence of 8200 people, 1400 cattle, 1100 goats and 200 donkeys especially during the annual dry
season. After major dam rehabilitation and reser
voir de
-
silting works in 2002, the Chamakala II reservoir has since
been de
-
silted twice due to unabated sediment accumulations. A key finding through hydrographic surveys is that
the Chamakala II reservoir is loosing capacity to siltation rates of 2250 m
3
/year. The study established that the
reservoir’s capacity had been reduced by 39% while its life expectancy was reduced by 50%. Also determined is that
the Chamakala II reservoir will silt up by the year 2015 at the current siltation rates if no effective

land management
measures are implemented. Catchment aspects such as poor cultivation and land management practices,
deforestation and resultant poor land cover, livestock over stocking and fragile soils were observed to exacerbate the
rapid sedimentation
problem of the Chamakala II reservoir.


The study established that there is an inverse relationship between sedimentation rates and reservoir water yield
which indicates that sedimentation may lead to water scarcity if not controlled. The study recommends

comprehensive sediment yield assessments for sustainable water resources development through small dams
intended to sustain rural livelihoods.


Key words:

Small multipurpose earth dam, sedimentation, water availability, rural livelihoods.



1.0

Introducti
on


The ultimate destiny of all reservoirs is to be filled with sediment with the obvious consequences of sedimentation
being the decrease or outright loss of the reservoir’s live storage capacity, designed life span and the respective
functions (Linseley
et al., 1992). However studies from around the world show that large dams tend to stand the
threat of sedimentation especially where there is effective catchment management
.

The contrary are small dams
whose sedimentation problem is a significant IWRM chal
lenge in that the loss of live storage capacity adversely
impacts on the sustenance of water dependent rural livelihoods. In this study, a small dam is defined as a structure
whose vertical height is less than 8m to the wall’s non
-
overflow crest with a res
pective storage capacity of less than
3000 * 10
3

m
3

while a large dam is defined as a structure whose vertical height is greater than 15m and less than
30m to the wall’s non
-
overflow crest with the respective storage capacities of between 3000 * 10
3

m
3

and

20000 *
10
3

m
3

(Muyambo, 2000). Please note that the term “small earth dam” is used synonymously with “small reservoir”.
2


Thus this paper focuses on the sedimentation problem of small dams in Malawi with the small Chamakala II Dam
being of specific referen
ce.


1.1

Background


In Malawi, numerous multi
-
purpose small earth dams have been constructed since the 1940s for the primary purpose
of storing productive water which is aimed at improving and sustaining rural livelihoods’ demand uses such as
nutritional
vegetable gardening, livestock watering, brick making and fishing. A survey carried out on the state of
small earth dams by the Ministry of Water Development (MWD, 2004) revealed that
more than 60%

of Malawi’s
small earth dams had lost their storage capaci
ties mainly to sedimentation or siltation. Factors such as breaching of
poorly constructed embankments by flood flows, piping and neglect due to lack of maintenance were observed to be
some of the causes which exacerbated the loss of storage capacity by sm
all dams. Case examples are those of the
Masambanjati and Nyakamba small reservoirs which respectively failed due to human activity flood flows within a
year of their construction (MWD, 2004).


However, of concern with respect to the issue of sedimentation

are the short periods of less than
10 years

over which
most small dams silted up. The worst recorded cases of rapid small dams degradation are those that were constructed
between 1950 and 1970 with environmental degradation due to poor catchment managemen
t being cited as the main
causes (MWD, 2004). The overall consequence of the severe depletion and out right loss of small dams’ storage
capacity to sedimentation is the currently experienced significant shortage of available productive water to sustain
the

ever growing, diverse and competing rural livelihood demand uses.


The continued loss of these small water storage facilities due to accelerated sedimentation is a critical challenge to
the achievement of goals and objectives pertaining to improving rural

livelihoods through water development. In
Malawi, the issue of sediment transport and the respective sedimentation of small reservoirs are given insignificant
attention despite the Malawi Government having implemented a national small earth dam constructi
on program to
develop 350 new facilities. Malawi’s Ministry of Irrigation and Water Development confirmed concerns over the
small reservoirs’ sedimentation problem and the resultant loss of storage capacity and diminishing available water to
sustain a div
ersity of ever growing and competing rural livelihoods’ water demands. Resultant anxieties over
diminishing available water are further exacerbated by lack of an adequate catchment management program.


2.0

Description of the study area


The multi
-
purpose C
hamakala II Small Earth Dam is located in the Dwangwa Catchment on Chamakala River
which is a tributary of the Mpasadzi River that drains into Lake Malawi. The dam lies between latitude 12º45’0
South and longitude 33º28’0 East and at an altitude of 1038 me
ters above sea level. It was constructed in 1949 and
has a catchment area of 5.3 km
2
. Refer to
Figure 1

for location details. The catchment receives an annual average
rainfall of 831 mm with temperatures ranging from 15ºC to 30ºC. The dam was rehabilitated

to a full storage
capacity of 35 * 10
3

m
3

and a life of 30 years in 2002 (Mchazime, 2002).


By 2008, the dam had a rural dependence of 8200 people, 1400 cattle, 1100 goats and 200 donkeys especially during
the annual dry season which is from April to Sept
ember. Known livelihood water demand uses include domestic
needs, livestock watering, nutritional vegetable gardening, small irrigated tobacco production and brick making.


Worthy of note is that the Chamakala II small dam has a history of silting up and s
ubsequent rehabilitation
(more
than 8 times)

through de
-
silting by way of sediment excavation. Each time it silts up, the consequence is loss of
livelihoods through non
-
availability of productive water for the local rural communities who have to walk long
distances of between 10 and 15 km to fetch water for domestic purposes as well as livestock watering. For example,
after rehabilitation in 2002 to the stated storage capacity of 35 * 10
3

m
3
, Chamakala II small dam had lost most of its
storage capacity to s
edimentation by 2006. Loss of storage capacity which was compounded unabated diverse water
demands have the causes of the dam drying up quite early in the annual dry seasons of 2005, 2006 and 2008 (MWD,
2008).


3




Fig
ure
1
:

Locatio
n of
Chamakala II small dam

on Chamakala River


Despite acknowledgement by decision
-
makers (Ministry of Irrigation and Water Development, 2008) of the
incessant threat posed to achieving sustainable rural livelihoods due to sedimentation of small dams and
the resultant
diminished to non
-
availability of productive water, the challenge seems to remain marginal in water development
and catchment management planning initiatives.


2.1

The study problem


In Malawi, the challenge posed to the sustenance of rural l
ivelihoods by the depletion to outright non
-
availability of
productive water due to sedimentation of multi
-
purpose small earth dams remains a grey, unquantified and primarily
uncharacterized entity.


The characterization of this challenge is essential in l
ight of Malawi’s water development plans that aim to construct
350 more small dams to sustain rural livelihoods through improved availability of water. Information on the
behaviour of the small reservoir sedimentation challenge is anticipated to be invalua
ble input to Malawi’s current
initiatives on producing and implementing IWRM based catchment outline plans which are anticipated to benefit the
sustenance of water dependent rural livelihoods through improved the availability of productive water.


2.2

Stud
y objectives


The main objective of the study is to characterize the sedimentation of small earth dams as a challenge and threat to
the sustenance of water dependent rural livelihoods through the improved availability of productive water.


This is achieved

through:




Carrying out literature review and field surveys in order to gain insight to the situation that is antecedent to
the Chamakala II small dam’s sedimentation challenge and a threat to sustainable rural livelihoods.



Carrying out a hydrographic surv
ey of the Chamakala II small earth dam in order to determine
sedimentation/siltation rates and impact on availability of productive water over time.



Predicting the impacts of sedimentation on the Chamakala II small earth dam’s efficiency and reliability to

sustain rural livelihoods’ water demands over time.

4




Recommendations on the management of the sedimentation challenge for sustainable productive water
availability to support rural livelihoods.



3.0

Methods and materials


In order to achieve the study obj
ectives, several data gathering methods and analytical techniques were used as
outlined in the proceeding sub
-
sections.


3.1


Desk study and field surveys


For the purposes of establishing an understanding on the situation that is antecedent to the reservo
ir sedimentation
challenge and the threat to productive water for sustaining rural livelihoods in the Chamakala area, a desk study
which entailed reviewing published information and records was done. Reconnaissance surveys of the study area
were also done
on such parameters as land use, ground cover, rainfall patterns and farming practices. Consultations
based on structured and unstructured interviews were done with key stakeholders such as public policy makers, local
traditional leadership and communities,

agricultural and water experts.


3.2

Hydrographic surveys of Chamakala II dam



A hydrographic survey was done on the Chamakala II dam in order to determine the reservoir storage capacity.
Refer to Figure 2 for a schematic layout of survey lines.





Figure 2:

Schematic diagram with survey lines for the Chamakala II small dam


A temporary benchmark (TBM) valued at 50m was established as the reference for all readings pertaining to the
Chamakala II reservoir survey. A total of f
ifteen lines were surveyed Line number 1 being on top of the main
embankment (dam wall) and the final Line number 15 running from top of the embankment running across the
reservoir longitudinally to the point where Chamakala River enters the reservoir. Sur
vey stations were marked at 10
-
metre intervals on top of the embankment while those for the middle Line number 15 were at 20
-
metre intervals.
The remaining 13 lines (Lines 2 to 14) were surveyed across the reservoir with each of them extended to about 2 m
above the spillway level

of the reservoir on each side.












S

urve
y

point in reservoir











D

ry

point survey



Spillway

le

vel





Survey lines



Dam

Em

b

ankment



5


The materials used during the survey were a small boat, 2 tape measures, nylon ropes 250m long, wooden pegs,
floaters, a Geographical Position Set (GPS), a theodolite, 2 graduated staffs, 2 stadia ro
ds, stationery, protective
clothing, a 1:50,000 map sheet, a shovel and a pail.


Water depths at each of the stations were determined through vertical dipping of a graduated staff into the water
with the top reading being recorded with respect to the crest

of the spillway while the bottom on which the staff
rested was coincident with the sediment level for the respective station. Refer to Figure 3 for an illustration on how
depth measurements were taken from the boat using the graduated staff. In addition t
o the hydrographic
(bathymetric) survey for each line, a topographic (dry land) survey was carried out around the reservoir using a
theodolite so that each of the lines was extended to a level above that of the spillway on both banks. Level readings
for th
e topographic survey were reduced from the temporal benchmark (TBM) of value 50m described above.

















Figure
3:
Schema
tic diagram showing method of data collection

The following steps were used to determine storage capacity of the reservoi
r as outlined by Nelson, (1985).


a.

A contour was drawn using values obtained from the reduced levels for the entire reservoir. These were reduced
from the spillway level at 48.505 as a stable datum.


b.

Determining surface areas enclosed by the contours at 0.5
m intervals by using a planimeter and graphical
methods.


c.

Storage volume between two respective contours was determined by using the Prismoidal Method also known
as the Simpson’s Rule



C=










n
i
i
i
i
i
A
A
A
A
dh
1
)
1
(
)
1
(
3
/






Equation 1


Where

C =
reservoir capacity,

A
i
= surface area at contour
i,

A
(i+1)
=
Surface area at the next contour level above contour level
i.





dh

= contour interval


d.

The tabulated results were plotted on a graph of reservoir height against cumulative storage


e.

From
the graph, the water storage capacity was determined and its difference from the required capacity at
design level was attributed to volume of sediment yield.





Staff

Peg

Peg

Water Surface

Boat




Survey line

Survey line

6


f.

Total capacity of the water available in the reservoir was determined by summing up the calcul
ated volumes
between every two consecutive contours at 0.5m intervals.



3.3

Determining sediment yield



In a related study by Aynekulu et., al (2006), determination of sediment yield parameters was done using the
equations listed below. The method was ad
opted in this study after analyzing its reliability and proved that the
method could be applicable in this study as well.




SV = area*depth









Equation 2






SR =
y
SV









Equation
3



LE =
SR
RSC










Equation 4





S
Y = SV*dBD









Equation 5




SSY =
A
SY









Equation

6


Where,
SV

is sediment volume (m
3
);
area

is the area of contour of sediment thickness (m
2
),
Thickness

is the
thickness of the sediment measured from the pits (m);
SR

is rate

of sedimentation (m
3
y
-
1
);
y
is age of reservoir
(year);
LE

is the life expectancy of the reservoir (years);
RSC

is the reservoir storage capacity (dead) (M
3
);
SY

is
sediment yield (ty
-
1
);
dBD

is dry bulk density (tm
-
3
);
SSY

is Specific Sediment Yield (t
km
-
2
y
-
1
); and
A

is catchment
area (km
2
).


3.4

Determination of impacts of sedimentation on reservoir yields


The impact of sedimentation on reservoir yields was determined for specified reservoir volumes in relation to
analyses under sub
-
section 3.3 above
. In determining useful yields for the specified reservoir storage volumes, the
designed dead storage volume of 5000 m
3

was subtracted from the respective net yield for the respectively specified
storage volume.


The mathematical power r
elationship given
in Equation
7

was determined from the plot of storage volume versus net
yield. It was used to estimate net yields for the projected storage volumes for the years 2002 to 2017 which were
obtained after earlier projections of sediment yields for the respecti
ve time periods.




688
.
0
31
.
39
V
N
y



(R
2
= 0.98)



Equation 7


Where:


N
y

=

Net Yield (m
3
)



V

=

Storage volume (m
3
).


3.5

Estimation of dry season yields


Estimation of dry season yield starts by determining the
evaporation index for the reservoir under consideration.
Determination of the evaporation index (EI) for Chamakala II Small Earth Reservoir was determined by using the
projected values from a curve of surface area and storage volume. Therefore, the corresp
onding surface area and
volume at design stage were applied to equations 8 to 10 taking into account the value of evaporation values for the
study area during the six
-
months dry season which is from April to October of each year.


The following steps were
taken in estimating the dry season yields:

7



Determination of an Evaporation Index (EI)











max
max
001
.
0
RC
xRA
E
EI
D









Equation 8


Where;


E
D

=

evaporation over the dry season (mm)



RA
max


=

surface area of the reserv
oir at full supply level (m
2
)




RC
max
=

full supply capacity of the reservoir (m
3
)


Determine a K
-
factor as follows:




EI
e
K
9
.
0












Equation 9


Where: K is a parameter of variation between reservoir surface area (RA) and c
apacity (RC).


Determine the maximum dry season yield (Y
max
) as follows
:







max
max
15
.
0
1
9
.
0
EIxRC
K
K
Y













Equation 10


Where: Y
max

is the maximum dry season yield.


A plot of the determined maximum dry season yiel
ds and values of sedimentation accumulation with time are used
to show the impact of the later on the former.


3.6

Determination of impact of sedimentation on reservoir trap efficiency


Reservoir trap efficiency is defined as the percentage of incoming sed
iment which is trapped by the reservoir or the
ability of a reservoir to trap and retain sediment, expressed as a percentage of sediment yields or inflowing sediment
(Bupe and Timble, 1986). Linsley et al, (1992) defines trap efficiency as the percent of i
nflowing sediment that
remains in the reservoir. Equation 11 is for determining reservoir trap efficiency (United States Army Corps of
Engineers


USACE, 1989):








)
(
/
in
Ys
out
Ys
in
Ys
E









Equation 11


Where:

E

=

Trap efficiency expressed as decimal



Ys

=

Sediment yield in weight units



in

=

inflow



out

=

outflow



Trap efficiency is of particular importance when determining the annual sedimentation rate or capacity
loss as
expressed by Equation
12.




C
EYs
CI
/








Equation 12


Wher
e:


CI

=

annual sedimentation rate



E

=

trap efficiency, in percent



Ys

=

annual net sediment yield from the drainage area



C

=

original reservoir storage capacity in same units as Ys


According to USACE, (1989) the reservoir storage capacity is decrea
sed as sediment is trapped and in turn the trap
efficiency decreases.
For practical purposes, the initial trap efficiency can be used as a constant up to 50 percent
8


storage depletion; however, if storage depletion is rapid, the trap efficiency should be up
dated at specified times
with the respective adjustment of C to reflect the sediment retained.


The trap efficiency of the dam at the time of this study was found by matching the ratio of gross storage and mean
annual inflow with the Brune’s Curve assuming

both the envelope and median curves. By using the earlier projected
reservoir volumes as gross storage volumes, the corresponding trap efficiencies are also determined. From a
logarithmic relationship of the capacity and trap efficiency, the expected stor
age volumes and trap efficiency of the
Chamakala II reservoir are estimated.


3.7

Impact of sedimentation on water availability for domestic uses


Domestic water demands for the communities were determined by using the daily per capita water requirements
as
obtained from interviews and literature reviews. Given that Chamakala has insignificant and unreliable groundwater
resources as an alternative source of fresh water for domestic uses, this study assumed that the reservoir is the
primary source of water
supply. The projected human population increases of 8.2 percent per annum for Chamakala
were taken into consideration in the analyses (National Statistics Office of Malawi, 2008) on projected water
demands and the respective reservoir yields for specific t
ime periods.



4.0

RESULTS AND DISCUSSIONS


4.1

Causes of Sedimentation at Chamakala Dam


4.1.1


Population pressure on natural resources



Rapid human population growth of 8.2% per annum which results in increased demand for land to sustain the ever
expanding human settlements and agricultural production has been observed to be the main cause of rapid loss of
natural vegetation hence land cover and accelerated land erosion and in rapid sedimentation of the Chamakala II
reservoir. The situation is comp
ounded by bad farming practices whereby over 50% of the subsistence farmers have
no land conservation or management measures such as contours in place. Stream bank cultivation is a rampant
activity which results in unstable river banks collapsing and incre
ased deposition of silt into the dam.


There is a problem of encroachment especially during the annual dry season whereby 75% of the land around the
reservoir is under gardening and other crop production within 10 m of the reservoir shoreline. Overall, 80%

of the
dam’s catchment is under agricultural production, 15% is barren land with the remaining 5% being covered by poor
scrub vegetation. Of concern is that the farmers (85% of those interviewed) felt that the responsibility for protecting
the reservoir b
elongs to the government and the politicians. Thus there is need for solutions to the sedimentation
problem which are anchored in community participation.


4.1.2

Compromised Engineering Standards


The development of small reservoirs in Malawi is generally
not subject to rigorous engineering standards that
address problem issues like reservoir site selection, sedimentation, Environmental Impact Assessments (EIA) and
many others. Thus flaws were found in the designing of the Chamakala II dam with examples bei
ng that it is 5.2%
of mean annual runoff such that the dam is too small for the catchment. Normally a minimum of 10% of MAR is
recommended in order to significantly cater for the sedimentation problem.


4.2


Hydrographic survey results of Chamakala I
I dam in 2008



With reference to the Chamakala II reservoir’s designed capacity of 34760 m
3

and an average depth of 4m, highlight
results from the hydrographic survey show that the reservoir capacity in 2008 was found to be 21294±1000 m
3

with
an average w
ater depth of
1.3±0.5 m and a surface area of 15942±1000 m
2
.
Refer to Table 1 for details on the
surface area and capacity parameters as at 2008.





9


Table 1:


Chamakala II reservoir surface area and capacity in 2008


Contour

Height (m)

Surface Area by
ea
ch contour
(m
2
)

Computed
Volume (m
3
)

Cumulative
Volume (m
3
)

45.7

0.0

0

0

0

46.0

0.3

1286

563

563

46.5

0.8

2541

1707

2271

47.0

1.3

4371

3325

5595

47.5

1.8

9226

5668

11263

48.0

2.3

13585

7374

18637

48.5

2.8

15942

2657

21294


This translates into a cu
rrent capacity of 61% of the designed storage volume and a respective average water depth
of 32.5% of the original depth. Thus the loss of 39% of the designed capacity to silt sedimentation over a 6
-
year
period after reservoir
de
-
silting and rehabilitation

in 2002 is quite significant.
Also refer to Table 2 for a summary of
sedimentation information pertaining to the loss of reservoir capacity from 2002 to 2008.


4.3

Sedimentation yield and projected loss of reservoir storage capacity since 2002


From the r
esults of the hydrographic survey, it was determined that sediment volume in the reservoir amounted to
13466 m
3

or 39% of the designed storage capacity. With reference to information on sediment yield summarized in
Table 2, the rate of reservoir sedimentat
ion was estimated at 2244±500 m
3
/year resulting in the dam’s life span
being reduced to 16±2 years instead of the designed 30 years.


Without any interventions on the sedimentation process say through effective catchment conservation measures, the
Chamakal
a II reservoir is projected to fill up by 2017. Refer to Table 3 and Figure 4 for more details on the
projected complete sedimentation by the year 2017. The resultant loss of productive water will adversely impact on
the sustainability of water dependent l
ivelihoods.



Table 2:


Summary of sediment yield for Chamakala II dam from 2002 to 2008


Parameter

Measure

Error margin

Design reservoir capacity (m
3
)

34760


Designed reservoir life (yr)

30


Current reservoir capacity (m
3
)

21294

±1000

Sediment Volume
(m
3
)

13466

±1000

Sediment Volume as (%)

39

±1

Period of dam in operation (yr)

6


Rate of sedimentation (m
3
/yr)

2244

±500

Life Expectancy (yr)

16

±2

Percentage loss of reservoir life (%)

48


Dry Bulk Density (T/m
3
)

1.22

±0.1

Sediment Yield (Tons)

16429

±1000

Annual Sediment Yield (Tons/yr)

2738

±500

Specific Sediment Yield (Tons/yr/km
2
)

517

±100








10


Table 3:


Relationships between sediment accumulation and storage volumes


Year

2002

2003

2004

2006

2008

2010

2012

2014

2016

2017

Sed. Acc. In
m
3

----

4485

6730

8975

13466

17956

22447

26938

31428

33674

Stor. Vol. In m
3

34760

30275

28030

25785

21294

16804

12313

7822

3332

1087





Figure 4:

Projected reservoir sedimentation and loss of storage capacity since 2002


4.3

Impact of sedimentation on
reservoir water yields from 2002 to 2017


An analysis which involved plotting a graph of reservoir storage and net yields showed that reservoir sedimentation
has a very significant adverse impact on reservoir yields. A mathematical power relationship was d
etermined from
the plot as given in equation 2.7. Statistical analysis of the relationship between the two parameters indicated a
correlation coefficient of 0.98 with a significance level of 0.00004 and a confidence level of 95%. Equation 2.7 was
then appl
ied in the estimation of net yields for the projected storage volumes for the years 2002 to 2017 as in Table
3. The results are as shown in Table 4 and Figure 5.


Table 4:


Impact of sedimentation on reservoir yields from 2002 to 2017


Years

Sediment
Yield

(m
3
)

Storage
Volume
(m
3
)

Net Yield
(m
3
)

Useable
Yield (m
3
)

2002

0

34760

52332

47332

2003

4485

30275

47587

42587

2004

6730

28030

45130

40130

2006

8975

25785

42611

37611

2008

13466

21294

37355

32355

2010

17956

16804

31738

26738

2012

22447

12313

25626

20626

2014

26938

7822

18755

13755

2016

31428

3332

10426

5426

2017

33674

1087

4823

0


11


Sedimentation was found to have adverse impacts on reservoir storage, net yields, reservoir storage and usable
yields. Refer to Figure 5 for details. The same trend i
s applicable to dry season yields. The relationships show that
net yield depends on reservoir volume which in this case is affected by the problem of increasing sedimentation and
consequently useable yield is also affected by continuously decreasing storag
e with time.





Figure 5:

Impact of sedimentation on reservoir yields from 2002 to 2017


It is therefore clear that the Chamakala II small earth dam will be filled up with sediments within 8 years of the
study in 2008.


4.4

Impact of sedimentation on av
ailabilty of water for domestic uses



Dependence on the Chamakala II dam for domestic water supplies is most apparent during the annual dry season
from April to October. This is unlike during the annual wet season when water is abundant unless it is a dr
ought
season. Thus an assessment of the impact of reservoir sedimentation on domestic water demands for the dry season
was done using a quantity of 20.5 litres per capita per day. A population growth of 8.2% per annum was applied.
The results are as presen
ted in Table 5.


Table 5:


Projected domestic water demands from 2008 to 2018


Years

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

Population

4000

4320

4666

5039

5442

5877

6347

6855

7404

7996

8636

Demand
in m
3

15088

16295

17599

19007

20527

22169

23943

25858

27927

30161

32574


It can be concluded from Table 5 as population increases so does the demand for domestic. The adverse impact of
sedimentation on water availability hence that for domestic water requirements from the Chamakala II small eart
h
dam is shown in Figure 6.








12


Figure 6:

Projected impacts of sedimentation on water availability from 2008 to 2017


4
4.7
5.4
6.4
7.4
8
0
5
10
15
20
25
30
35
40
2008
2010
2012
2014
2016
2017
Volumes (m
3
)Thousands
Years
Projected Impacts of Sedimentation on Availability of Water Resources
Population
Demand
Capacity
Ymax
Useable Yield
Sed acc


With reference to Figure 6, population growth has an exponential growth over time with a correlation coefficient of
0.992 while wate
r demand will be increasing as a polynomial function with a correlation coefficient of 0.995. On the
other hand, both the useable yield and dry maximum yield are decreasing with time as polynomial functions with
correlation coefficients of 0.997 and 0.996,

respectively.


4.5

Relationship between storage capacity and trap efficiency of Chamakala Small Earth Dam


Based on the logarithmic functions derived from the two curves above, the estimated trap efficiency values for the
Chamakala II Small Earth Dam would be

a trap efficiency of 100 % with a storage capacity of at least 65000 m
3

assuming the envelope curve of Brune’s Curve. Refer to Table 6 and Figure 7 for details.


This situation supports the observation that a small dam with a capacity of about 65000 m
3

w
ould be more
sustainable in comparison the current small dam with a storage capacity of 34760 m
3
. Table 6 gives determined
results of the gross storage ratios (SR
g
) and gross dam capacities (DC) with change over time. The value of MAI
was found to be 673 a
s a product of catchment area and mean annual runoff.


Table 6:


Relationship of Sediment accumulation and the Gross Storage Ratio


Year

Sediment

DC

SRg

Trap Efficiency

Median

Envelope

2002


34760

0.052

77

86

2003

4485

30275

0.045

75

84

2004

6730

2
8030

0.042

74

83

2006

8975

25785

0.038

72

82

2008

13466

21294

0.032

70

80

2010

17956

16804

0.025

64

76

2012

22447

12313

0.018

58

69

2014

26938

7822

0.012

49

61

2016

31428

3332

0.005

28

42

2017

33674

1087

0.002

3

20




13


The values given in Table 6 we
re plotted in order to determine the impact of sedimentation on the trap efficiency of
the dam. The plot is given in Fig. 7.


Figure 7:

Relationship of storage capacity and trap efficiency





5.0

Conclusions and Recommendations


5.1

Conclusions



A comb
ination of rapid land use changes, lack of community empowerment and poor farming practices are major
causes of the accelerated sedimentation of Chamakala II Small Earth Dam. With 39% of the current designed
capacity already occupied by sediments after de
-
silting and rehabilitation in 2002, the reservoir is projected to fill up
by 2017. At the current rate of sedimentation which is compounded by rapid population growth and subsequent
increases in water demand, the study also found that there is a looming pr
oblem of water scarcity at Chamakala II to
sustain water dependent rural livelihoods such as domestic uses and livestock watering.


Currently the dam Chamakala II dam has a trap efficiency of 70% and 80% assuming the median and envelope
curves respectively

while at design stage the trap efficiency was 77% and 86% respectively. Thus a small dam with
a larger storage capacity of about 65,000 m
3

was assessed to be a more viable option for dealing with the reservoir
sedimentation problem if the objective of pro
viding sustainable productive water availability for Chamakala’s rural
livelihoods is to be realized.



5.2

Recommendations


The study recommends that since the construction of small dams constitutes an actual investment into facilities
whose goal is to im
prove rural lives through the provision of improved access to productive water, the issue of
reservoir sedimentation and resultant loss of storage capacity needs to be incorporated into the planning and
designing stages.


Another recommendation is that IWR
M based catchment outline planning is developed and implemented with
particular attention to such aspects as effective land use management and community participation if the threat from
sedimentation is to be turned into an opportunity for sustainable rura
l livelihoods through improved availability of
productive water.


Acknowledgements


We are thankful to the Government of Malawi’s Ministry of Water and Irrigation who were very supportive of this
study through provision of materials and manpower during fie
ld work. Our appreciation goes to Waternet who
provided the financial support to ensure successful completion of this study.


14


6.0

References


1.

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-
Scale Reservoir Sedimentation Rate Analysis for a
Reliable Es
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Northern Ethiopia), Mekelle University, Mekelle, Ethiopia.

2.

Bube, K.P. and Trimble, S.W. (1986). Revision of the Churchill Reservoir Trap Efficiency Curves Using
Smoot
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3.

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Accessed on 14

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2011. June Report 2006.

6.

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7.

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8.

Malawi Government, (1999). Water Resources Policy and Strategies, Ministry of Water Development,
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9.

Ministry of Water Development, (2007). Rep
ort on the Status of Nyakamba Dam, (Unpublished) Ministry
of Irrigation and Water Development, Blantyre. Malawi.

10.

Ministry of Water Development, (2002) Design Calculations of Construction Works for Chamakala Dam II
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11.

Mi
nistry of Water Development, (2004, unpublished). Report on the Malawi National Consultative
Meeting on the World Commission on Dams (WCD) Report. Dams and Development: A New Framework
for Decision Making.

12.

National Statistical Office of Malawi, (2007).
Web

page:
http://www.citypopulation.de/Malawi.html

Accessed on 27 November at 3:12pm.

13.

Nelson K.D., (1985). Design and Construction of Small Earth Dams, Globe Press, Brunswick,Victoria.

14.

USACE, (1995). S
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Engineers.Engineering and Design Manual.
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2
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