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

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

• 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

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

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

)

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

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

)

• 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

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

)

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

)

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)

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!