Academia Brasileira De Ciencias

Enhancing Water Management Capacity in a Changing World.

Sao Paulo June 25-28

Lake Chivero: Case study of urban water supply.

Issues and possibilities

C.H.D. Magadza University of Zimbabwe

Magadza.christopherhd@gmail.com

In a nutshell

• The Harare Municipality supplies water to close to 7 million people, i.e close to 50% of current Zimbabwe population.

• The Waterworks can only supply 60% of demand.

• 20% of produced water is lost through leakages

• 40% of consumption is paid for; i.e. 32% of production

• Lake Chivero, supplemented from Lake

Manyame, is principal source of water.

Google earth view of L. Chivero watershed with overlays

Cleveland Dam

Seke Dam

Manyame R

Mukuvisi R

Marimba R

L. Chivero

Chitungwiza City

Seke Communal

lands

Epworth informal settlement

H A R A R E C I T Y

Harahwa Dam

Dziwaresekwe T/ship

Table 1.Hydrological and morphometric features of Lake

Chivero

Full supply volume 250 x 106m3

Full supply surface area 26.30 km2

Catchment area 2227 km2

Shoreline length 74 km

Maximum depth 27.43 m

Mean depth 9.4 m

Maximum breadth 8.0 km

Mean breadth 1.68 km

Length 15.7 km

Catchment : lake area ratio 8.9

Shoreline development 4.1

Year construction 1952

Paintings by the Xan people

Hydrology

Runoff, Manyame River Half Normal Probability Plot of RUNOFF (MANYAME.STA 10v*64c)

y=0.362+2.144e-6*x+eps

Observed Value

Exp

ect

ed N

orm

al V

alu

e

-0.2

0.4

1.0

1.6

2.2

2.8

-1e5 1e5 3e5 5e5 7e5 9e5 1.1e6

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

1970

1975

1980

1985

1990

1995

2000

2005

2010

Sta

nd

ard

devia

tio

ns

Year

Fig 4. Standardised annual runoff; Manyame River, with six yr moving average

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

1950 1960 1970 1980 1990 2000 2010

Flu

sh

ing

tim

es

Season

Fig 5. Flush rate per anumFlush

Mean Pre 1980 runoff

Mean Post 1980 runoff

Comparative run-off flow and wastewater flow into L. Chivero

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Pro

po

rtio

n

Year

Fig 6. Proportionate flows of sewage bearing streams and Manyame runoff

Manyame Sum of tributaries 6 per. Mov. Avg. (Manyame) 6 per. Mov. Avg. (Sum of tributaries)

y = 2E-33e0.0391x

R² = 0.5967

0

5

10

15

20

25

1950 1960 1970 1980 1990 2000 2010% s

ew

ag

e o

f L

ak

e v

olu

me

Year

Fig.7. % contribution of sewage to lake volume from Ferle & Croborough sewage works

Reconstructed Harare population: doubling period ~12 years

y = 5E-60e0.0721x

R² = 0.932

0.0

1000.0

2000.0

3000.0

4000.0

5000.0

6000.0

7000.0

1920 1940 1960 1980 2000 2020

Popula

tion x

1000

Year

Diffuse source

Breached sewer effluent flowing along suburban road in a middle to high income area

Dog on uncollected refuse heap

The facilities

Ratio of wastewater carrying stream flows to Manyame River flow.

y = 8E-37e0.0417x

R2 = 0.2396

0.00

1.00

2.00

3.00

4.00

5.00

6.00

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Rati

o (

Urb

an

str

eam

s/M

an

yam

e)

Untreated wastewater outflow from a sewage treatment plant, Harare

Table. 2. Waste water treatment capacity (MLd

) at some of the Manyame watershed sewage works

(excluding Chitungwiza, Ruwa and Norton)

Plant Trickle

filter

Year Activated

sludge

(BNR)

Year Ponds Year Total

capacity

Present flow

Ferle 36 1960 18 1982 144 250

18 1974

72 1998

Crowborough 36 1957 18 1982 54 120

Donnybrook 2.3 1953

-

1972

2.3 10

Marlborough 2 Post

1980

2 7

Chitunwiza 54

Total 202.3 387

Capacity deficit 441

Pressures on the Water supply Reservoirs

Table 4 Historical trend in P loading to Lake Chivero. (Magadza 2003,

Nyumbu 2012).

Variable 1967 1978 1996 2010

P concentration

(mg/l)

P load g (m2)

2.8

27.4

0.13

1.5

1.8 (Manyame)

14

2.77

27.8

P load (tonnes/pa) 685.0 39.6 350.0 697

Table 2. Phosphorus and Nitrogen export through surface runoff

from Harare suburbs. Dry season; after Mufaro 2002 Phosphorus Nitrogen

Ratio

P: N

Total export

Catchment/

Suburb Type

Tonne

km-2

Kg

Capita-1

Tonne

km-2

Kg

Capita-1

P-

tonne N- Tonne

Gwebi Low 0.02 0.04 0.16 0.42 5.81 0.99 9.31

Muwisindale Low 0.07 2.31 1.22 41.39 5.99 5.24 93.96

Kuwadzana Medium 0.08 0.17 0.47 0.96 3.89 2.88 16.75

Mukuvisi

High/

Industria

l 10.28 1.00 39.98 3.89 3.61 98.99 385.04

Marimba

High/

Industria

l 0.13 0.77 0.86 4.98 3.68 9.28 60.31

Budiriro High 2.30 0.23 13.77 1.35 9.42 22.08 132.17

Epworth High 3.38 1.11 12.20 4.00 17.95 103.12 371.88

Glenview High 0.30 0.39 1.09 1.44 6.50 30.23 111.39

Total or mean 1.62 0.56 7.53 6.38 5.69 272.81 1180.81

P loading gm-2 7.6 gm-2

Conditions in Lake Chivero

y = 0.0728e0.1865x

R² = 0.7862

0

1

2

3

4

5

61

98

8

19

89

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

20

09

20

10

mg

/l

Year

Fig 7. Annual mean phosphate: 2010 data after Nyumbu (2012), 1988 - 1994 Harare Minucipality, other data by author.

Mean annual

Expon. (Mean annual)

Lake Chivero from Dam wall

Macrophytes in eutrophic waters: warmer climate promotes faster growth, therefore higher

management costs

Algal scum, Chivero (Photo C.H.D. Magadza)

Methane bubbles from the sediments: L. Chivero

Health issues

Enteric diseases deaths

Fig 7. Enteritis deaths per thosant infectd

y = 9.2206x - 1.3779

R2 = 0.6121

0.00

20.00

40.00

60.00

80.00

100.00

120.00

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Fig. 8. Liver cancer: incidence/1000. This disease is associated with

microcystin from cynobacteria

0

0.2

0.4

0.6

0.8

1

1.2

1995 1996 1997 1998 1999 2000

Unexplored health hazards

• Health hazards associated with chlorinating water with high dissolved organic content

• Heavy metals

• Other industrial pollutants.

• Parasites

Remedies??

Technology solution

• The 1970 innovative use of BNR technology proved that it was a feasible method of eutrophication control

• The infrastructure and technical know-how is still available

• It however needs extensive investment to rehabilitate increase its capacity

• Plans for expansion of these sewage works have been completed

Secchi disc chneges following restoration measures (data from Thornton 1982, Ndebele and Magadza 2006, Magadza

unpublished. The 2003 data refer to wet period.)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

1957-58 1970-1971 1974 -1975 1976 - 1977 1977-1978 1979-1980 2003 2006

Period

Se

cc

hi

de

pth

( c

m).

BNR Commisioning

Recovery period Deterioration

Google earth view: Ferle BNR sewage works. Green colour of reactors and

clarifiers indicate dysfunctional state.

Ecological approach

Diffuse source pollution control.

Cleveland dam: 1913

Acidic waste

Increasing diffuse source

Past sewage works

Wetlands distribution in Lake Chivero watershed: total estimated area = 40 000 ha (After Nyumbu 2012)

Time projections of lake response to three waste-water management scenarios: Top:

Business as usual. Middle: BNR technology; Bottom BNR +wetlands.

Adapted from (Nyumbu 2012

Historical, current and projected phosphorus loading on Lake Chivero

Parameter 1967 1978 1996 2010 2020

(Projections)

P

concentration

(mg/l)

P load gm-2

2.8

27.4

0.13

1.5

1.8

14

2.77

22.56

0.22

1.36

P load

(tonnes/pa)

685.0 39.6 350.0 564 34.1

CONCLUSION

Harare water supply issues 1. Declining runoff

2. Growing urban population: communal lands issues

3. Wastewater returns to supply reservoirs

4. Large diffuse source pollution component

5. Insufficient wastewater treatment capacity

6. An economic meltdown leading to dysfunctional wastewater treatment facilities

Results: 1. Hypereutrophication

2. High costs of potable water production

3. Inability to meet water demand

Consequences

1. Disease outbreaks

2. Water shortage in cities

Restoration strategy 1. The ILEC Integrated Lake Basin Management strategy.

The ILBM platform

Thank you for listening to us on Wetlands day