TO THE SEDIMENTOLOGY OF THE

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22 Φεβ 2014 (πριν από 3 χρόνια και 8 μήνες)

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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 1
st

to 4
th

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 km
2

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 km
2

Total No. of Streams (Ordered)

217

Amplitude (456
-
91) m

365m/ 0.365 km

RESULTS









Where, Q
s

= suspended sediment transport (tones/day); Q
w
= water discharge (m
3
/s); Cs = suspended sediment
concentration (mg/l) and 0.0864 = conventional constant


(
Ǿ
rstein, 1979)




day
tonnes
x
k
C
Q
Q
s
w
S
/



day
tonnes
x
C
Q
Q
s
w
S
/
0864
.
0

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)


Q
Q
m
s
c

Q
b
a
L

A Rating Curve for
Osubo

y = 0.6554Ln(x) + 0.9915
R
2
= 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 (m
3
/s)
Gauge Height (m)
Results cont…


The lowest and highest discharge values
recorded for the Osubo were 0.1953 m
3
/s and
0.3555 m
3
/s,



Corresponding to gauge heights of 0.30 m and
0.38 m respectively.


Rating Curve for
Huhunya



y = 0.5814Ln(x) + 1.3644
R
2
= 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 (m
3
/s)
Gauge Height (m)


Discharges measured from Huhunya were 0.44
m
3
/s and 0.8366 m
3
/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 m
3
/s
and 0.7130 m
3
/s respectively.

Rating curve for
Oterpkolu

y = 2.301Ln(x) + 2.7145
R
2
= 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 (m
3
/s)
Gauge Height (m)
SST for
Huhunya

y = 2.8712x
2.346
R
2
= 0.654
0.1
1
10
0.1
1
Water Discharge (m
3
/s)
Suspended Sediment
Transport (tonnes/day)
SST for
Oterkpolu

y = 2.036x
1.1716
R
2
= 0.898
1
10
0.1
1
10
Water Discharge (m
3
/s)
Susupended Sediment
Transport (tonnes/day)
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
!