Marian Muste, Kyeongdong Kim, Dongsu Kim, Gabor Fleit

This paper demonstrates that the multivariate monitoring methods are capable to underpin the systematic investigation of the hysteretic behavior occurring during gradually-varied flows. For this purpose, we present simultaneous measurements of stage, index velocity, and free-surface slope acquired continuously with high-frequency sampling instruments deployed at several river gauging sites exposed to a range of storm magnitudes. The experimental evidence reveals intrinsic features of unsteady open-channel flow mechanics that are hinted by pertinent governing equations but rarely substantiated with in-situ measurements. The illustrations are intentionally made for fluvial waves propagating at sites located in lowland areas where the relationships among flow variables are most likely displaying hysteretic loops and phasing in the hydraulic variable progression. The set of presented measurements highlights that: a) the hysteretic behavior is apparent in both time-independent and time-dependent graphical representations of any two of the hydraulic variables; b) the severity of the hysteresis is commensurate with the geomorphic, hydraulic, and hydrological characteristics of the measurement site; and c) there is a pressing need for changing the flow paradigms currently used in tracking flow variables during gradually-varied flows. Also discussed are research needs associated with flow hysteresis for advancing the understanding of the mechanisms underlying the movement and storage of water in the lowland river environments as well as for increasing the accuracy of streamflow monitoring, modeling, and forecasting.