River discharge gauging using slope-area method and comparison of the results with those obtained from direct discharge gauging methods: A case study

The slope-area method is an indirect method for river discharge gauging. Although it is less accurate than direct river discharge gauging methods, it is a simple and elementary method to estimate the discharge. In this paper, the field measurements, data analysis, and interpretation of calculated results obtained using this method in the River Derwent, southwest of Newcastle, will be discussed. The measurement reach or field site in the River Derwent should satisfy the assumption which the method is based on (i.e. the flow is uniform). Therefore, an almost uniform part of the river, in terms of geometry and shape, was selected, and then, three cross sections, as upstream, mid-point and downstream cross sections, were considered for taking measurements. Basically two kinds of measuring in the field site were carried out. One was surveying and the other was to measure the sizes of bed samples along their median axis. Surveying was carried out in all three cross sections to obtain the water surface level at the river banks and also bed level at every metre distance of cross-sections. Since all the three sections could not be surveyed from the same position, and the level had to be moved, a datum point at inland next to the river was chosen, and at each cross section, in addition to the surveying data, the datum point level was measured to calculate the fall in the water surface elevations between the sections. Taking sample particles from the bed surface was also carried out in the field site to obtain particle size distribution. For this, every two paces, a sediment particle was taken on a grid basis from the bed surface in the site, and its median or intermediate diameter was measured. In this way, totally 100 sample particles were taken and their size distribution was analysed by number rather than weight. Using the Darcy- Weisbach approach to flow resistance, we calculated the resistance coefficient, and then, conveyance for each section. Finally the water discharge was obtained using a relevant equation. A comparison between the results from this method and two main direct river discharge gauging methods, i.e. the current meter and salt dilution methods, indicated that the slope-area method has less accuracy or more error than the other two methods in estimating the discharge.

Keywords:

River discharge gauging , slope-area method , flow velocity , roughness , Manning equations , conveyance , salt dilution method