the effects of sedimentation in malawi case study.pdfintroduction of mudi dam mudi dam is located in...
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THE EFFECTS OF SEDIMENTATION IN MALAWI:
A CASE STUDY OF MUDI DAM IN BLANTYRE
PETER STEWART KADEWERE
HYDROLOGIST
MINISTRY OF IRRIGATION AND WATER DELOPMENT
MALAWI (16/10/07)
INTRODUCTION OF MUDI DAMMudi dam is located in the commercial city of Blantyre (≈ 750,000 People)
The dam is 15m high and has a storage capacity of 1.5 million m3 with a treatment plant of design capacity of 45,000m3 per day.
Blantyre Water Board (Parastatal body) uses water from the dam and Walkers ferry on Shire River to supply water to the city and surrounding areas with a 75% coverage.
Currently, the annual average daily demand from the two water sources is 70,700m3 per day.
However, the operating costs from the Walkers ferry are very high due to:
Pumping cost (from a 780m head and for a distance of 48km)
The river carries a high silt load in rainy season causing sedimentation at the intake, high treatment costs and frequent pump wear.
Mudi dam is therefore an important source of supply for its low operating cost.
0 10 20kms
Discharge MeasurementWith Recorder
Discharge MeasurementWithout Recorder
Level MeasurementWith Recorder
Level MeasurementWithout Recorder
Elephant Marsh
NkombedziwaFodya
Mwanza
Mwampanzi
Lika
bula
Nswadzi Thuc
hila
Ruo
Samb
oni
Lake Chilwa
SH
IRE
Mwanza
Nku
rum
adzi
L:is
ungw
e
Rivi Rivi
Rivi Rivi SHIR
E
SHIRE
Lunzu
NamadziPhalo
mbe
Phalombe
Thondwe
Naisi
Damasi
Likangala
Likwenu
Lake
Malom
be
Nkasi
Hydrological Network – Southern Malawi
Dual Level & DischargeWith Recorder
Dual DischargeWith Recorder
IR183E7
IR19
IR21IR22 NTCHEU
IR3BALAKA
LIWONDEIB1IB7
IS7
2C3
2C82C10
2B21
2B222B6
2B33
1C1
1C9
1O1
1K31M5
1M41L18
1E19
1E414A3
14A214B2
14B314D3
1F20
1E11L12
1K2
14C514C2
14C1 & C9
14C8
14C714C414C6
2B37
1F1
1M1
1P6
1P22B11
14B8
14A114B4
14B9
1E17
ZOMBA
BLANTYRE/LIMBE
CHIKWAWA
MWANZA
TOWNHYCOS Stn
MWANZA
MULANJE
Mott MacDonaldSource: Ministry of Water Development
MUDI RESERVOIR
PART OF RESERVOIR INTAKE STRUCTURE
CAUSES OF SEDIMENTATION PROBLEMS ON MUDI RESERVOIR
Deforestation in the catchment (Ndilande Forest reserve
Cultivation crops in the catchment area
Vandalism and lack of maintenance of sediment preventing structures (silt traps and weirs).
Reservoir operation – Mainly impounding the water at all times hence allowing accumulation of more sediments.
Consequently, the dam became vulnerable to increased siltation as a result of soil erosion in the catchment and its direct release into the reservoir.
LAND USE IN THE CATCHMENT
LAND USE IN THE MUDI CATCHMENT
EFFECT OF SEDIMENTATION ON MUDI1999 hydrological survey revealed silt accumulation of 2m from the original reservoir bed (1955); marking at estimated total loss of about 30% of the reservoir capacity.
Use of agricultural inputs (i.e. fertilizer and pesticides):
blossoming of water weeds in the reservoirs (very significant on Hynde dam)
affecting the water quality. Mudi reservoir only supplies about 8000m3/day (20% of potential yield and 10% of the water demand), .
The Board now largely depends pumping water from Walkers Ferry and spending huge sums of money on power consumption.
More water problems in the rainy season (siltation at walkers ferry)
RESULTS OF HYDROGRAPHIC SURVEY ON MUDI RESERVIOR
RESULTS CONT……
SEDIMENTATION PROBLEMSON MUDI DAM
ACCUMULATED SILT ON BANKS SILT ACCUMULATING TOWARDS INTAKE
INFESTATION OF WEEDS ON HYNDE DAM
WATER WEEDS ON THE HYNDE RESERVOIR
PROPOSED REHABILITATION WORKS
The Government through the Blantyre Water Board and Ministry of Irrigation and Water Development, has lined a number of projects to address the current problems.
Catchment rehabilitation and protection works
Under this initiative, the Mudi‐Ndirande catchment areas have been targeted for rehabilitation. The works would involve the following:
Reforestation of the Mudi‐Ndirande catchment and construction of contour ridges to improve soil and water conservation
Rehabilitation of check dams and other silt trap structures long the Mudi River to prevent excessive deposition of silt into the Mudi reservoir
Conduct awareness campaigns on catchment protection and establish local institutions for monitoring the catchment rehabilitation works.
PROPOSED REHABILITATION WORKSRehabilitation of the actual reservoir
The Government through the Ministry of Irrigation and Water plans to conduct a comprehensive study on the sedimentation problem on the Mudi and Hynde dams using hydrographic surveys. The study is expected to establish the volume of the silt in the reservoirs and propose methods of removing the silt.
Enforcement of rules regarding catchment protection
The Board is looking at ways of enforcing the rules already in place that aims at protecting the Mudi‐Ndilande forest reserve which is now under encroachment despite being declared a reserve and compensating the communities in the catchment area.
EXPECTED BENEFITS
• The proposed works are expected to yield the following benefits:
improve the contribution of the Mudi reservoir and become a reliable water supply source for the Board and hence enable the Board to significantly increase savings on power required for pumping water from Walkers Ferry at Shire River.
Rehabilitation of the catchment is also expected to reduce algae count and hence reduce treatment cost of water in terms of chemicals required for purification to attain the recommended standards for human consumption.
THANK YOU
ZIKOMO!