Implications of Hydraulic Geometry Exponents of Zambian Rivers

Abstract

At-a-station and downstream hydraulic geometry exponents of rivers in Zambia, located in the central-southern African tropics, were analysed based on three power relations of width, depth and velocity plotted against discharge. Channel cross-sectional form and discharge data, for 55 gauging stations monitored by Department of Water Affairs between 1948 and 1984, the period with good data, were used. Of these stations, 12 were located in the Zambezi River sub-basin while 37 and 6 others were in the Kafue and Luangwa River basins, respectively. For downstream analysis, 14 stations located on the main channel of Kafue River were investigated. The objectives were to: (i) determine the hydraulic geometry relations of Zambian rivers for comparison with those of other regions of the world; (ii) assess the factors accounting for variations in the observed hydraulic geometry exponents; and (iii) draw some implications of Zambian rivers’ hydraulic geometry exponents. Results of analysis showed that the obtained average at-a-station hydraulic geometry exponents for width (b), depth (f) and velocity (m) of Zambian rivers were 0.15, 0.38 and 0.47, respectively, and accorded well with similar results in the literature. Similarly, ranges of downstream exponents on the Kafue River of b = 0.50, f = 0.30 and m = 0.20 were also within the ranges of reported values. The plotting of most of at-a-station hydraulic geometry exponents on the right side of the tri-axial (ternary) diagram (f>b), implies that Zambian rivers are generally adapted to the transportation of fine-grained sediment ostensibly caused by the entrenchment of the river channels. The observation of m>f in some cases was interpreted as evidence that such streams possessed greater capacity of transporting large calibre bed load sediment. It is concluded that the behaviour of Zambian rivers is comparable to others in different physiographic regions of the world. However, unlike in temperate areas, the existence and influence of dambos in Zambia provides a complicating factor in the understanding of the behaviour of rivers.