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152 Peter Allan Wood

152 Peter Allan Wood 100000 ! 10 000 1000 10 DISCHARGE FIGURE 3. Individual rating curves. on the rising limb. Rating curves have been constructed for five hydrographs of the Hope River (Fig. 3) and three of these exhibit hysteresis. Using these curves it is possible to estimate suspended sediment discharge for each hydrograph: hydrograph A — 347 tonnes in 38 h, hydrograph B — 20 tonnes in 30 h, hydrograph C -8125 tonnes in 32 h, hydrograph D — 33 tonnes in 8 h, hydrograph E — 7 tonnes in 12 h. Note that a large magnitude event (hydrograph C) is responsible for the transport of over 8000 tonnes in just over 30 h, while the other four events total less than 500 tonnes in over 80 h. Flows of the magnitude of hydrograph C have a recurrence interval of 4—5 years. The cause of hysteresis in the suspended sediment concentration and discharge relationship has been much discussed (eg. Arnborg et al., 1967; Guy, 1964; Heidel, 1956; Walling and Teed 1971; Wood, 1977). The data presented here indicate that short lived, lower magnitude events are characterized by reduced or no hysteresis (hydrographs B, D and E) while events of longer duration and higher magnitude are characterized by more pronounced hysteresis (hydrographs A and C). This is in agreement with the model outlined for a basin in the UK (Wood, 1977) where sediment availability is important. During events of longer duration and higher magnitude the sediment supply becomes exhausted during the recession flow and hysteresis results. m J /s

200 £ 150- 6 100- iu 50- Sediment transport in the Hope River, Jamaica 153 GRAIN SIZE RANGE OF SUSPENDED SEDIMENT phi • 9 »7 '5 .3 .1 0 -I -3 -5 I i ' I I I I I I I I I i I {broken line indicates a few grains only) 1 5 RIVER DISCHARGE m 3 /s •— FIGURE 4. Velocity, discharge and grain size relationship. — i , T , r— 10 15 20 River velocity at section S was determined for several conditions of stage, and this has been plotted against discharge (Fig. 4). Discharges of over 20 m 3 /s were obtained where velocities of over 150 m/min would be expected. Figure 4 also indicates the size range of suspended sediment samples collected at different velocities. Whereas there appears to be a rapid increase in maximum grain size as velocity increases to 15 m/min, any further increase in velocity is accompanied by a less rapid increase in the maximum grain size. BED MATERIAL TRANSPORT The competence of rivers to transport bed-load material may be determined in several ways. Of the possible methods, competence has often been considered in terms of shear stress which can be expressed by the DuBoys equation T = JRS (0 where r is the critical mean shear stress to initiate particle motion, 7 is the specific weight of the transporting fluid (here taken as 1000 kg/m 3 ), R is the hydraulic radius, and S is the energy slope, usually taken as the slope of the water surface, but here taken as the channel gradient. Limitations of the DuBoys equation have been outlined byGraf(1971)andGessler(1971). Bradley et al. (1972) use a similar equation where T = yDS (2) and R in equation ( 1) is replaced by D which is the maximum depth of flow. Baker and Ritter (1975) also suggest that/? is a close approximation toi? when considering streams transporting bed-load material. However, Ogrosky and Mockus(1964) indicate thati? can be approximated by 2D/3 for shallow parabolic channels so that T = y(2Dl3)S (3) In this instance D is described as the depth of flow in the centre of the channel. Baker and Ritter (1975), using the DuBoys equation (1), constructed a model of particle size versus mean shear stress for coarse bed-load material transported by rivers, 6

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IAHS Publication no. 269