APPLICATION OF THE FAST THEORY
TO THE SEDIMENTOLOGY OF THE
PAWMPAWM RIVER, GHANA
Divine Odame Appiah,
Dept. of Geography and Rural Dev’t,
KNUST, Kumasi
GGA/GGTA ANNUAL CONFERENCE
Held at KNUST, Kumasi
From 1st to 4th August, 2012
Theme: environmental sustainability and
development; the Geographer’s Perspective
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
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
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)
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)
Study area
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
RESULTS
Q
S
Q
S


 k Q x C s tonnes / day

w

 0.0864 Q x C s tonnes / day
w
• 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)
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:
m
Q cQ
s
b
L  aQ
• (Gosseline and Craig, 2002)
A Rating Curve for Osubo
1
0.9
0.8
y = 0.6554Ln(x) + 0.9915
R2 = 0.9859
Gauge Height (m)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
Discharge (m3/s)
0.6
0.7
0.8
0.9
1
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.
Rating Curve for Huhunya
0.9
0.8
y = 0.5814Ln(x) + 1.3644
R2 = 0.9669
0.7
Gauge Height (m)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.05
0.1
0.15
0.2
0.25
Discharge (m3/s)
0.3
0.35
0.4
0.45
• 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.
Rating curve for Oterpkolu
3
2.5
Gauge Height (m)
2
y = 2.301Ln(x) + 2.7145
R2 = 0.9734
1.5
1
0.5
0
0
0.1
0.2
0.3
0.4
0.5
Discharge (m3/s)
0.6
0.7
0.8
0.9
SST for Huhunya
Suspended Sediment
Transport (tonnes/day)
10
y = 2.8712x2.346
R2 = 0.654
1
0.1
0.1
1
Water Discharge (m3/s)
SST for Oterkpolu
10
Susupended Sediment
Transport (tonnes/day)
y = 2.036x1.1716
R2 = 0.898
1
0.1
1
10
3
Water Discharge (m /s)
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.
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.
Vegetable farm in close proximity to the river
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.
Heavily silted bridge
Abandoned Bridge
Tack so mycket!