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!