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Transport of Bed Load and Suspended Load by Rivers from Low Rainfall Areas in Africa

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Transport of Bed Load and Suspended Load by Rivers from Low Rainfall Areas in Africa Transport of bed load and suspended load by rivers from low rainfall areas in Africa Peter R. B.Ward Abstract. Sediment yields from two basins in Rhodesia are compared. Large differences are found in the sediment yields from these basins because of differences in the hydrology and in the geology, although only a small difference exists in the mean annual rainfall. Charriages et transports en suspension dans des rivières recevant de faibles précipitations en Afrique Résumé. De grandes différences ont été observées dans la production sédimentaire de deux bassins versants de Rhodésie en raison de variations hydrologiques et géologiques car les précipita­ tions annuelles étaient identiques. INTRODUCTION A comparison is made between two basins in Rhodesia (Zimbabwe), separated by a distance of only 250 km. The mean annual rainfall for the Gwai basin (see Fig. 1) differs from that for the Umsweswe basin by a small percentage ( 18 per cent), but the geology of the two basins differs significantly. This, together with the difference in rainfall, causes the basins to be hydrologically different from one another. Table 1 summarizes the results. The figures for mean annual runoff are based on 15 years of data from a gauging station on the Umsweswe and on estimates based on gauging stations upstream and downstream of the measuring point (18 years of data) for the Gwai. The mean annual flood for the Umsweswe is based on measured data, and agrees with a recently developed flood formula for most rivers in Rhodesia (Mitchell, 1974). For the Gwai basin the mean annual flood is calculated from the cross-sectional area of the channel using the procedure described in the next section. MEAN ANNUAL FLOOD In order to compare sediment transport from the two basins the wash loads and bed-material loads were compared for discharges equal to various percentages of the mean annual flood. The determination of the mean annual flood at the Gwai sampling site was made difficult by the facts that (a) only two years of flow gauging records were available, and (b) other gauging points on the river were a long distance from the site. An estimate of the mean annual flood on the Gwai was made by accurately surveying the channel (using 12 sections spaced at 0.5 km intervals), and using the hydraulic calculation for sand-bed rivers given by Einstein (1950). The assumption was made that at a stage of 3.1 m the Gwai was carrying a flow equal to the mean annual flood. This value of stage was based on a comparison with the Umsweswe, for which an Einstein calculation was also carried out. This river was found to convey its mean annual flood (200 m3/s) at a stage of 3.1 m. The comparison was considered reasonable as the mean slopes of the two rivers in the reaches 142 Transport of bed load and suspended load 143 &/A soum Af/ucA SCAi.£ Aâ OOP OOP 0 50 tOO fSO 200XM FIGURE 1. Location of the sediment sampling stations for the two basins, 1975-1976 season. TABLE 1. Comparison of basins, geology and hydrology Sampling Area of Dominant Dominant Mean Mean Mean site basin geological land use annual annual annual [km2] formation rainfall runoff flood flow [mm] [mm] [m3/s] Umsweswe at Upper 1990 Granite Non-intensive 700 45 200 Claw flume ranch land Gwai at Tribal land Gwai- 14400 Kalahari and non- 580 15 320 Bembezi sand intensive flume ranch land 144 Peter R. B.Ward : * ••'• '** * * r 4 -• / «-/^ o o,S i,o ,tS KM ft, • — » • « ' —* FIGURE 2. Aerial photographs of reaches considered: Gwai (top) and Umsweswe (bottom). considered agreed with one another within 2 per cent and the cross-sectional areas were also in agreement. Figure 2 shows aerial photographs of the two rivers, both on the same scale. The similarity of channel widths of the two rivers is striking. This similarity exists despite the fact that the basin areas differ by a factor of 7.2. On the basis of this comparison, the mean annual flood on the Gwai at the sampling site was estimated to be 320m3/s. Transport of bed load and suspended load 145 UMSWESWE. 10 >o 100 WATER DISCHARGE M/SEC FIGURE 3. Wash load concentration versus water discharge. TRANSPORT OF SEDIMENT Estimates of bed-material load (transport of those sediment sizes represented in the river bed) were obtained by calculation, using Einstein's (1950) method for sand-bed rivers. The wash load was determined by field measurements from samples taken several times per day, so far for one season (1975—1976) only. At the maximum flow rates measured (151 m3/s for the Gwai, 117m3/s for the Umsweswe), a very small amount of the bed-material load was moving in suspension. Thus the suspended load samples all consisted of wash load. Wash load The 1975—1976 season was hydrologie ally close to the average for the Umsweswe basin, and below average for the Gwai basin. Eighty per cent of the wash load for the season passed the measuring stations in 13 d and 5 d for the Umsweswe and Gwai Rivers respectively. Figure 3 shows the wash load concentration versus 146 Peter R. B. Ward TABLE 2. Summary of data for bed-material load calculation River and site Mean cross- Mean slope of Mean bed-material sectional area at bed over reach size » 3.1 m 2 fm l ^35 D6S [mm] [mm] Umsweswe at 142 0.000595 0.71 1.25 Upper Claw flume Gwai at Gwai-Bembezi 156 0.000 587 0.36 0.52 confluence foatf, fount FIGURE 4. Bed-material load versus water discharge. discharge relationship for the two rivers. The concentration depends on whether the measurements are made on the rising or falling limb of the hydrograph; hence the scatter of readings for each value of discharge is large. The error bars of the points of Fig. 3 show the plus and minus one standard deviation values for all readings for a given discharge. Bed-material load Surveys were made of the beds of both rivers, and sand sampling was carried out to enable the bed-material load to be calculated. Twelve sections were surveyed, spaced at 0.5 km intervals, over reaches 5.5 km long. Table 2 gives a condensation of the data obtained. Values of discharge and bed-material load were calculated for various values of stage for both rivers using Einstein's method. Figure 4 presents the results of this calculation and it shows that at the same values of discharge the Gwai transports 4^6 times more bed-material load than the Umsweswe, the main reason for this being differences in the size of bed-material. Comparison of the two rivers Values of wash load and bed-material load were compared at equivalent water flows in the two rivers. These flows were in the range (0.1 — 1.0) times the mean Transport of bed load and suspended load 147 TABLE 3. Comparison of sediment transport of Umsweswe and Gwai Rivers River Basin Discharge: Wash load Bed-material Total load Bed- and site area mean annual [tonnes/d] load per unit area material [km2] flood [tonnes/d] of load to times catchment wash load factor [tonnes d"1 ratio km"2] Umsweswe at Upper 1.0 - 2 290 Claw 1990 0.3 3 060 158 1.62 0.05 flume 0.1 620 11 0.32 0.02 Gwai below Gwai- 1.0 - 20 900 Bembezi 14 400 0.3 10 800 2 510 0.92 0.23 confluence 0.1 1240 191 0.10 0.15 annual flood. An analysis for the Umsweswe River (not presented here) shows that more than 50 per cent of the sediment transported is carried by flows in this range. Flows larger than the mean annual flood are too rare in occurrence to contribute a large amount of sediment. Flows smaller than 0.1 times the mean annual flood carry only small amounts of sediment. Table 3 compares the sediment transport of the Gwai and Umsweswe Rivers. No values are given for wash load at discharges equal to the mean annual flood as the flows were consistently lower than this in the year selected for measurements. DISCUSSION Table 3 shows that the contribution of wash load outweighs the contribution of bed-material load by a large factor for both basins. On the basis of the yield of total sediment per unit area of basin for the flows analysed, the Umsweswe basin yields significantly more than the Gwai. This is in accord with findings such as those of Langbein and Schumm (1958) that as yearly rainfall amounts decrease, yields of sediment are reduced, because the erosive capability of the runoff is reduced. Although measurements over the single season selected did not provide the opportunity to measure values of wash load at discharges equal to the mean annual flood, the differences between basins listed in Table 3 are so striking that measurements at larger flow values are not expected to alter the present conclusions. Using an extrapolation of the results in Fig. 3, and using flow frequencies from 15 years of data at the Umsweswe site (not presented here), a long-term average sediment transport was computed. Although the value determined by this extrapolation is not expected to be accurate, it is probably correct to within a factor of 2. The time average transport so calculated, 24 tonnes year -1 km-2, was of the same order as the figure of 27 tonnes year-1 km"2 (Holeman, 1968), for the average annual yield of African rivers to the ocean.
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