Abstract
River flow discharge is commonly estimated from measured water surface velocity with floats, or noncontact current meters, or water surface slope; eg. Slope-area method is introduced in International Organization for Standardization (ISO, 1992) with assuming rigid-bed and constant Manning's roughness coefficient. On the other hand, fluvial hydraulics community indicates that the bed roughness composed of grain roughness and form roughness, and that form roughness depends on meso-scale bed forms. These kind of studies were well verified by experimental studies, and contributed to theoretical studies, though it is not easy to verify in the river reach. Observations during flood had been considered as almost impossible, because of high velocity and water surface fluctuation. However, due to development of the measurement technique, especially a tethered boat mounted ADCP, flood observational results have accumulated data of mentioning phenomena about fluvial hydraulics about bed evolution and the roughness change. Summarizing above mentioned aspects, it is inferred that the fluvial characteristics of river channels are major components of uncertainties in estimating flow discharge. The authors in this paper describe estimation of the flow discharge under erodible condition.
Based on the ADCP measurements, the authors investigate how the fluvial characteristics under erodible bed condition affect to estimate flow discharge using the following time-series data: flow area, bed evolution, the Manning's coefficient, and flow discharge obtained by ADCP and Slope-area method. As a results, flow discharges obtained by ADCP and Slope-area method were identical excepting peak flow discharge period, though river bed and bed roughness fluctuated in great extent. The authors also discuss about not-identical parts at peak using relation between hydraulic resistance and micro-bed form. As a results, when the bed roughness changes due to deformation of bed form from duen to transition, Slope-area method always overestimate discharge value.
