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154 Peter Allan Wood

154 Peter Allan Wood PARTICLE SIZE (after Baker & Ritter, !975! mm 30 50 100 200 300 h 1 1 1- ' 0.2 1 10 20 SHEAR STRESS kg/m 2 FIGURE 5. Competence as determined by equations (1), (2) and (3). and this model is applied to location S on the Hope River. Shear stress was determined using equations (1), (2) and (3), and the results are indicated in Fig. 5. Equations (2) and (3) give higher shear stress values than those obtained by the DuBoys equation (1), indicating that for the Hope River, both D and 2D/3 are poor approximations for R. The data suggest that during a discharge of 20 m 3 /s, material of approximately 200 mm may be in motion on the bed. Magnitude and frequency of flood events is important in a study of bed material transport as it is during the high magnitude, low frequency events that the larger particles of the bed material would be transported. One of the highest magnitude events within the Hope basin during recent times were the flows associated with the high intensity rains of Hurricane Flora, 1963 (Eyre, 1968). Figure 6 indicates the estimated peak discharge associated with Flora, and it can be seen that material of over 400 mm may have been in motion. Flows of the magnitude associated with the Flora rains have a recurrence interval of about 14 years. CONCLUSION The Hope River is characterized by periodic high magnitude flows. During such events suspended sediment concentrations of over 60 000 mg/1. occur. A general relationship has been established between discharge and suspended sediment concentration, although for greater accuracy two different rating curves are presented; one for rising stage, and one for falling stage. Rating curves for five individual hydrographs show that for one hydrograph with a recurrence interval of 4—5 years, over 8000 tonnes of suspended

1 000 100 a ce < 10 estimated position of peak discharge during Flora rains 0 / Sediment transport in the Hope River, Jamaica 10 100 SHEAR STRESS kg/m 2 -1000 FIGURE 6. Estimated competence during the passage of Hurricane Flora, 1963. sediment were transported in just over 30h. Shear stress and competence determinations for the Hope River vary widely, indicating that/) and 2D/3 are poor approximations of R. The data suggest that material of over 400 mm may be set in motionduring flows of about 200 m 3 /s which have a recurrence interval of 14 years. Acknowledgements. I thank the Water Resources Division, Ministry of Mines and Natural Resources, Jamaica, for allowing consultation of stage charts for the Hope River. A. Wood, A. Eva and M. Hendry assisted in the field. REFERENCES Arnborg, L., Walker, H. J. and Peippo, J. (1967) Suspended load in the Colville River, Alaska, 196 2. Geografiska A nnaler 49, 131 -141. Baker, V. R. and Ritter, D. F. (1975) Competence of rivers to transport coarse bedload material. Bull. Geol.Soc. Amer. 86,975-978. Beverage, J. P. and Culbertson, J. K. (1964) Hyperconcentrations of suspended sediment. /. Hydraul. Div., Proc. Amer. Soc. Civ. Engrs. HY6, 117-128. Bradley, W. C, Fahnestock, R. K. and Rowekamp, E. T. (1972) Coarse sediment transport by flood flows on Knik River, Alaska. Bull. Geol. Soc. Amer. 83, 1261-1284. Condie, R. (1973) Regional rainfall depth-duration-frequency relationships for Jamaica. Ministry of Mining and Natural Resources, Water Resources Division, Jamaica, Bull. no. 1, 1—32. Ehlmann, A. J. (1968) Clay mineralogy of weathered products and of river sediments, Puerto Rico. /. Sed. Pet. 38, 885-895. Evans, C. J. (1972) Estimates of maximum 24-hour rainfall amounts for return periods from 5 to 100 years. /. Scientific Research Council, Jamaica, 3, 25—45. Eyre, L. A. (1968) Precipitation from Hurricane Flora, 1963. /. Tropical Geog. 26, 29—36. Gessler, J. (1971) Beginning and ceasing of sediment motion. In River Mechanics (edited by H. W. Shen), pp.1-22: Fort Collins. Graf, W. H. (1971) Hydraulics of Sediment Transport: McGraw-Hill, New York. Gupta, A. (1975) Stream characteristics in Eastern Jamaica, an environment of seasonal flow and large floods. Amer. J. Sci. 275, 825-847. Guy, H. P. (1964) An analysis of some storm-period variables affecting stream sediment transport. US Geol. Surv. Professional Paper 462E, 1—46. Heidel, S. G. (1956) The progressive lag of sediment concentration with flood waves. Trans. Amer. Geophys. Un. 37, 56-66. -500 200 -100 50 E E 155

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