THE HYDROLOGIC ALTERATION OF THE TANA RIVER AND THE ... Kitheka... · INTRODUCTION •The Tana...
Transcript of THE HYDROLOGIC ALTERATION OF THE TANA RIVER AND THE ... Kitheka... · INTRODUCTION •The Tana...
Dr. Johnson U. Kitheka(Senior Lecturer and Research Hydrologist)
School of Environment, Water and Natural Resources Department of Hydrology and Aquatic Science
South Eastern Kenya University
THE HYDROLOGIC ALTERATION OF THE TANA RIVER AND IMPACTS IN THE LOWER TANA
BASIN
INTRODUCTION
• The Tana river is the most important river in Kenya in view of its role in national development.
• Five HEP dams in the Upper Tana Basin -Kindaruma, Kiambere, Kamburu, Gitaru and Masinga.
• The Masinga Dam is the most significant in terms of regulating the hydrology of the Tanariver due to its size.
• Studies on the effects of soil erosion and impacts in the Upper Tana Basin (e.g. Dunne and Ongwenyi, 1976; Ongwenyi, 1978; Edwards, 1979; Ongwenyi, 1985; Brown et al., 1996; Schneider and Brown, 1998; Pacini et al., 1998; Maingi and Marsh, 2001; Maingi and Marsh, 2002).
• Studies on the effects of landuse change and water abstraction on streamflow in the Upper Tana basin (Njoguet al., 2018) and Mwendwa et al 2019).
• Studies on the environmental impacts of the hydrologic alterations of the flow of the Tana river on the riverineforests (Pacini et al., 1998; Maingi and Marsh, 2001; Maingiand Marsh, 2002).
• Few studies on the hydrology of the Tana river in the Lower Tana Basin (Kitheka et al., 2003 and 2005).
Objectivesi. Determine whether there has been a
significant change in the patterns of streamflow and sediment discharge in the River Tana following construction of dams in the basin.
ii. Establish the impacts of alteration of streamflow and sediment discharge in the lower Tana Basin/Tana Delta.
iii. Provide recommendations for sustainable development of the Tana Basin.
Drainage Areas of Kenya
Area = 132,000 km2.
River runoff = 32% of the total river runoff
Source of water and electricity for nearly 65% of the Kenya’s population.
HEP Reservoirs in the Upper Tana Basin
Existing and potential reservoirs in the Upper Tana
Basin
Reservoir Gross
volume
(106 m3)
Surface Area
(km2)
Catchment
area (km2)
Mean
river
discharge
(m3s-1)
Year of
completion
Masinga
Kamburu
Gitaru
Kindaruma
Kiambere
Karura
Mutonga
Grand Falls
Usueni
Adamson’s Falls
Kora Hills
1,560
147
20
16
315
74
1,580
3,600
330
1,730
3,800
113
15
3.1
2.4
13
8
46
119
26
102
190
7,335
9,520
9,520
9,807
11,975
97.2
149.5
120.9
155.9
121.8
1981
1975
1978
1968
1988
Planned
Planned
Planned
Planned
Planned
Planned
Planned
Tana river in the Lower Tana Basin-Hola
Lower Tana Basin and Tana Delta
•Tana Delta area -225,000 ha•Important for biodiversity and ecosystem services•Population of approx. 200,000 •Low human development index (0.509) and high poverty level (77%)•High dependence on natural resources -the main sources of livelihoods•Degradation of ecosystems due to changes in the hydrology of Tanariver (Nature Kenya SEA & LUP 2012)
Global Significance of the Tana Delta
• Habitat to globally significant wildlife: Red Colobus monkeys and Mangabeymonkeys, Dugong dugong(globally categorized as vulnerable, regionally as critically endangered)
• Key Biodiversity Area (KBA) and a Global Biodiversity Hotspot
• Part of the Coastal Forests of Eastern Africa Hotspot.
• Provides dry season grazing ground for both wildlife and domesticated animals.
• (Nature Kenya SEA & LUP 2012)
Tana River important for sustainability of mangrove forests
-High plant diversity - 320 plant taxa some of them Critically Endangered globally. -21% of the plants in the delta are of conservation concern-Home to 7 plants on the IUCN Red List of Threatened Species.
Tana River important for sustainability of riverine forests
Tana Delta has important riverine forests (3,700 ha) which are important for carbon sequestration and biodiversity conservation.
METHODOLOGY
Rainfall Data
• Old rainfall data obtained from Kenya Meteorological Service.
• Rainfall data obtained for Sagana and Nyeristations.
• Establish the response of the Tana river to rainfall in its headwaters in Central Kenya.
Determination of the river discharge
• The first approach: acquisition of old data archived at the Ministry responsible for Water, WARMA and TARDA.
• The second approach: field measurements of river discharges at the river gauging stations -Kipini (RGS 4G03), Garsen (RGS 4G02), and Garissa (RGS 4G01).
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Sampling on the Tana at Hola
River Gauging at Kipini
Determination of sediment load
• The first approach: data archived at the Ministry responsible for Water resources , WARMA and TARDA.
• Data obtained for Kiambere, Garissa, Hola and Garsen.
• The second approach: involved field measurements at Garissa, Bura, Hola, Garsen and Kipini.
• Sediment concentrations were determined in the KMFRI laboratory by filtering water-sediment samples using Whatman G/F filters according to APHA methods.
Analysis of rainfall –runoff data
• River discharge data for the period 1948-1968/1972 before the damming of the river were compared to periods following construction of Masinga dam.
• The continuous rainfall and river discharge data are not readily available for most of the river gauging stations.
• Thus, only years of relatively good continuous data were used in the analysis of flow variability.
RESULTS • Shifts in the frequency of occurrence of low flows (baseflows) and
maximum (peak) flows
• Shift in the frequency of occurrence of river discharges of both base flows and peak flows.
• Before damming: The probability of occurrence of low flows <50 m3s-1 was 80% of while that for the high flows >150 m3s-1 was 20%.
• After the construction of Masinga dam: The probability of occurrence for low flows of < 50 m3s-1 was 70% while that of high flows > 150 m3s-1 is 30%.
• After damming, the probability of occurrence of high flows has increased by 10% while that of low flows has reduced by 15%.
Tana River Discharge and TSSC variability at Garsen(2002-2003)
0
0.5
1
1.5
2
2.5
0
100
200
300
400
500
600
700
800
TS
SC
(g
/l)
Dis
ch
arg
e (
m3/s
)
Discharge TSSC
maximum river discharge during long rains= 750 m3s-1. The maximum river discharge during the short rainy season =300 m3s-1.
The relationship between rainfall at Sagana and Tana River discharge at Garsen (1960-1972)
0
100
200
300
400
500
600
700
0
500
1000
1500
2000
2500
3000
3500
4000
Jan
-60
MA
Y
SEP
Jan
-61
MA
Y
SEP
Jan
-62
MA
Y
SEP
Jan
-63
MA
Y
SEP
Jan
-64
MA
Y
SEP
Jan
-65
MA
Y
SEP
Jan
-66
MA
Y
SEP
Jan
-67
MA
Y
SEP
Jan
-68
MA
Y
SEP
Jan
-69
MA
Y
SEP
Jan
-70
MA
Y
SEP
Jan
-71
MA
Y
SEP
Jan
-72
Rai
nfa
ll (m
m)
Dis
char
ge (
m3
/s)
Year-Months
Q RAIN
Significant inter-annual variabilities . River discharge largely follows rainfall pattern in Central Kenya . Phase lag of about one month to reach Tana Delta.
The relationship between rainfall (Sagana) and Tana River discharge at Garsen (1960-1972)
y = 2.039x + 170.9R² = 0.359
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700
Dis
char
ge (
m3
/s)
Rainfall (mm)
Longitudinal changes discharge of the Tana
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50
100
150
200
250
300
350
400
450
500
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Dis
ch
arg
e (
m3/s
)
Tana at Garsen Tana at Garissa
Tana at Grand Falls
River discharge significantly reduces as the river flows into the Lower Tana Basin
River Discharge and TSSC at Garsen (2002-2003)
0
0.5
1
1.5
2
2.5
0
100
200
300
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TS
SC
(g
/l)
Dis
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arg
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m3/s
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Discharge TSSC
River Discharge and Sediment load at Garsen(2002-2003)
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5000
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15000
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25000
30000
35000
0
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Sed
imen
t lo
ad
(t/
d)
Dis
ch
arg
e (
m3/s
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Discharge TSSload
The influence of rainfall and river discharge on TSSC
y = 0.003x + 0.068R² = 0.546
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 50 100 150
TS
SC
(g
/l)
Discharge (m3/s)
Changes in the sediment load of the Tana
• 1950-1957 Data (Before damming): Sediment load of 8.74 x 106 tons.yr-1 (Ongwenyi, 1979)
• 2002-2003 Data: Sediment load of 6.8 x 106 tons.year-1
at Garsen (Kitheka et al., 2003b and 2005).
• 2010-2012 Data: Sediment load in the Upper TanaBasin of 7.1 × 106 tons.year-1 (Njogu and Kitheka, 2018)
• A reduction of about 20% in sediment load following the construction of the Seven Folks dams.
Flow of the Tana at Grand Falls before (1963-1981) and after (1982-1996) the Masinga Dam
0
500
1000
1500
2000
2500
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Dis
ch
arg
e (
m3/s
)
Months
Mean flow before damming Mean flows after damming
Peak flow before damming Peak flow after damming
• The maximum flows and mean flows generally decreased due to damming. Significant augmentation of minimum flows (Maingi and Marsh, 2002).
• 75 - 98% of the terrigenous sediment load trapped in the Seven folk reservoirs (Maingi, 1991, Schneider, 2000).
• Flood flow alteration and changes in baseflow induces major changes in the river morphology and ecosystem (cf. Basson, 2013).
• Lack of significant dampening of the seasonal and interannual streamflow variability of the river (cf. Poffet al., 2007).
Degradation of mangrove forests
Degradation of riverine Forests
Reduced the influx of river water into the far edges of the riverine forests causes degradation of the forests (Maingi and Marsh, 2002)
Seawater intrusion into the Delta due to sea level rise
y = 0.127x + 2277.
2000
2050
2100
2150
2200
2250
2300
2350
2400
2450
2500
19
95
Jan
19
95
Ju
n
19
95
No
v
19
96
Ap
r
19
96
Sep
19
97
Feb
19
97
Ju
l
19
97
Dec
19
98
May
19
98
Oct
19
99
Mar
19
99
Au
g
20
00
Jan
20
00
Ju
n
20
00
No
v
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01
Ap
r
20
01
Sep
20
02
Feb
20
02
Ju
l
20
02
Dec
20
03
May
20
03
Oct
20
04
Mar
20
04
Au
g
20
05
Jan
20
05
Ju
n
20
05
No
v
20
06
Ap
r
20
06
Sep
20
07
Feb
20
07
Ju
l
20
07
Dec
20
08
May
20
08
Oct
20
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Mar
20
09
Au
g
20
10
Jan
20
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Ju
n
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10
No
v
20
11
Ap
r
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11
Sep
Sea
Leve
l (m
m)
Deepening of the river and tidal channel (10-30 m deep) (Kitheka et al., 2003a-b).
Seawater intrusion in the Kipini Branch
Sea water intrusion (>10km inland)
Coastal erosion
Intensification of the erosion of the delta
Modification of coastal sediment budget leading to accelerated erosion of the shoreline of the delta
Loss of dry season grazing fields that supports livelihoods in the Delta
Conflicts between farmers and pastoralists in the Delta
Collapse of flood-recession agriculture Loss of dry season pastures (77% poverty level)
• The ultimate consequence of the alteration of the flow of the Tana River is high level of absolute poverty in the Lower Tana Basin.
• 77% of the population living below poverty line.
• Poor management of irrigation schemes in the Lower Tana Basin affects livelihoods of people.
• A large segment of the population in the Lower Tana Basin is sustained by relief food (Red Cross and GoK).
Recommendations
• The proposed further damming of the Tana river should be scaled-down.
• The operating procedures of the Seven Folk dams should be reviewed so that the flow release from the dams mimics the original flow patterns.
• Invest on the more efficient irrigation systems in the lower Tana Basin to cushion local communities from changes in the flooding patterns of the river.
• Implement landscape restoration programmes in the Delta.
Conclusions
• The most important impact of damming of TanaRiver has been alteration of the sediment load and river discharge.
• The impact of damming on the Tana River flow has been particularly significant on the mean and maximum river flows.
• The impacts of alteration of the flow of the Tanariver would be even more serious with the planned further damming of the river.
Thank you