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, GHANA

• Divine Odame Appiah,
• Dept. of Geography and Rural Devt,
• KNUST, Kumasi
• GGA/GGTA ANNUAL CONFERENCE
• Held at KNUST, Kumasi
• From 1st to 4th August, 2012
• Theme environmental sustainability and
  development the Geographers 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
• (?rstein, 1979)

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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
  loglog slopes is commonly used to provide a fit
  between measured water and sediment discharges in
  the following relationship
• (Gosseline and Craig, 2002)

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A Rating Curve for Osubo
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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
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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
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SST for Huhunya
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SST for Oterkpolu
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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!