APPLICATION OF THE FAST THEORY TO THE SEDIMENTOLOGY OF THE PAWMPAWM RIVER, GHANA
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Transcript of APPLICATION OF THE FAST THEORY TO THE SEDIMENTOLOGY OF THE PAWMPAWM RIVER, GHANA
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APPLICATION OF THE FAST THEORY TO THE SEDIMENTOLOGY OF THE
PAWMPAWM RIVER, GHANADivine Odame Appiah,
Dept. of Geography and Rural Dev’t,
KNUST, KumasiGGA/GGTA ANNUAL CONFERENCE
Held at KNUST, KumasiFrom 1st to 4th August, 2012
Theme: environmental sustainability and development; the Geographer’s
Perspective
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Introduction Stream channel morphology is a function of sediment
entrainment
Is it also a function of discharge and sediment yield. At different sub-links of the river (capacity, competence)
Interaction of precipitation, vegetation, runoff and erosion
Climate and geology are also essential controls on river channels
This was evident for the pawmpawm river system
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Flow And Sediment Transport Theory (FAST) Approach
FAST: Flow And Sediment Transport
Intensive interrelationship between riverbed adjustment and bed load transfer (competence & capacity)
Both river bed geometry and bed load transfer are controlled by the development of flow cells
The FAST theory considers links in continuum as the operative criteria of sediment generation and discharge
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Problem justification Small rivers have been glossed over in many fluvial
geomorphologic studies
Pawmpawm river demonstrates peculiar hydrologic characteristics that makes it amenable for studies
Coupled with the increasing changing human landuses in the basin.
Have tendencies to affect the sediment and water inputs (Kirkby and Rice, 1994)
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Materials and methods Current meter and wadding rods used for V, A, s =Q data,
relevant for the estimation of the sediment concentration
Two gauges were installed at Huhunhya and Oterkpolu and daily-monitored.
In all, five flow measurements were taken at each station
Sediments in Petri dishes in ovens at 70C. To determine gross and net weights in mg/l WRI Laboratory. Decantation and heating of samples for the suspended
Generation of discharge and sediment rating curves (EXCEL)
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Study area
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Morphometry of the Pawmpawm river Parameters Measurements
Total Length 452.69 km
Area of the 21 km2
Length of Channel 40.60 km
Slope of Channel 4. 87 m/km
Maximum Length of Basin 8.20 km
Areas of Circle with Same Perimeter 22.79 km2
Total No. of Streams (Ordered) 217
Amplitude (456-91) m 365m/ 0.365 km
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RESULTS
• Where, Qs = suspended sediment transport (tones/day); Qw
= water discharge (m3/s); Cs = suspended sediment concentration (mg/l) and 0.0864 = conventional constant
• (Ǿrstein, 1979)
daytonnesxk CQQ swS/
daytonnesxCQQ swS/0864.0
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Results cont...• The sediment rating curves with water discharge
for days only water levels were measured. • Generally, the power function derived from the
log–log slopes is commonly used to provide a fit between measured water and sediment discharges in the following relationship:
• (Gosseline and Craig, 2002)
QQm
sc Q
baL
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A Rating Curve for Osubo
y = 0.6554Ln(x) + 0.9915
R2 = 0.9859
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Discharge (m3/s)
Gau
ge H
eigh
t (m
)
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Results cont…
• The lowest and highest discharge values recorded for the Osubo were 0.1953 m3/s and 0.3555 m3/s,
• Corresponding to gauge heights of 0.30 m and 0.38 m respectively.
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Rating Curve for Huhunya
y = 0.5814Ln(x) + 1.3644
R2 = 0.9669
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Discharge (m3/s)
Gau
ge H
eigh
t (m
)
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• Discharges measured from Huhunya were 0.44 m3/s and 0.8366 m3/s, with gauge heights of 0.20 and 0.40 m respectively
• The highest and lowest recorded water levels were 0.84 m and 0.40 m respectively. These values yielded discharge values of 1.5841 m3/s and 0.7130 m3/s respectively.
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Rating curve for Oterpkolu
y = 2.301Ln(x) + 2.7145
R2 = 0.9734
0
0.5
1
1.5
2
2.5
3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Discharge (m3/s)
Gau
ge
Hei
gh
t (m
)
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SST for Huhunya
y = 2.8712x2.346
R2 = 0.654
0.1
1
10
0.1 1
Water Discharge (m3/s)
Sus
pend
ed S
edim
ent
Tran
spor
t (to
nnes
/day
)
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SST for Oterkpolu
y = 2.036x1.1716
R2 = 0.898
1
10
0.1 1 10
Water Discharge (m3/s)
Su
sup
end
ed S
edim
ent
Tra
nsp
ort
(to
nn
es/d
ay)
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Results cont…
• The minimum and maximum sediment discharges at the Oterkpolu gage station were 1.4066 and 8.0248 tonnes/day respectively
• Typically, sediment-rating curves plotted on
logarithmic scales have large slopes at low discharges and smaller slopes at high discharges.
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Conclusions/implications
The likelihood for the sediment load of the river to increase if... basin degradation
The implication on the Volta lake sedimentation at the confluence…
‘Suspended load of the river does not necessarily change with discharge downstream’ rejected by the t-test analysis.
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Vegetable farm in close proximity to the river
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Implications• Anthropogenic influences on the river basin were
considerably high e.g. Farming
• Some farms were seen located closer than the buffer zone of usually 30-40 metres from the river banks.
• De-vegetation activities for fuelwood were identified.
• Coupled with other land use activities within the basin, the River basin considerably impacted.
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Heavily silted bridge
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Abandoned Bridge
Tack so mycket!