Evaluation of methodologies for continuous discharge monitoring in unsteady open-channel flows

Abstract

Kim, and all my tennis club players! I owe my success to my parents, who always respected my opinion through my life, and supported me emotionally and financially. There is the one last person who I cannot fully express my appreciation. I love you, Hunmin and my two sons, Hojin and Hodoo. v ABSTRACT Ratings curves are conventional means to continuously provide estimates of discharges in rivers. Irrespective of their approach, the most-often adopted assumptions in building these curves are the steady and uniform flow conditions for the open-channel flow that in turn provide a one-to-one relationships between the variables involved in discharge estimation. The steady flow assumption is not applicable during propagation of storm-generated waves hence the question on the validity of the steady rating curves during unsteady flow is of both scientific and practical interest. Scarce experimental evidence and analytical inferences substantiate that during unsteady flows the relationship between some of the variables is not unique leading to loop rating curves (also labeled hysteresis). Neglecting the unsteadiness of the flow when this is large can significantly affect the accuracy of the discharge estimation. Currently, the literature does not offer criteria for a comprehensive evaluation of the methods for estimation of the departure of the looped rating curves from the steady ones nor for identifying the most appropriate means to dynamically capturing hysteresis for different possible river flow conditions and events.The overarching goal of this study is to explore the uncertainty in the conventional stage-discharge rating curves (hQRCs) during unsteady flows and to evaluate methodologies for accurate and continuous discharge estimation. The study addresses these issues using experiments, modeling and analytical inference applied to hQRCs, index velocity rating curves (VQRCs), and continuous slope area (CSA) method.Included in the analysis is the implementation of a standardized framework for uncertainty assessment of the direct discharge measurements that are the building blocks of the rating curve construction. The study demonstrates using conceptual and analytical inferences that there are possibilities to develop a uniform end-to-end methodology to enhance the accuracy of the current protocols for continuous stream flow estimation for

Revisiting hysteresis of flow variables in monitoring unsteady streamflows

“…5d). Previous laboratory experiments (Nezu et al, 1997;Tu, 1992), field measurements (Holmes, 2016), and simulations (Lee, 2013) indicate that the loop thickness is proportional to the index-velocity change rate. It should be noted that the IVRC method can also develop loops from factors other than flow unsteadiness, i.e.…”

“…This method has been tested by a series of field experiments in a small stream located on a mild-sloped stream (Lee, 2013;Muste et al, 2019). The CSA implementation entails the following sequence (see also Fig.…”

“…Previous studies demonstrate both analytically (e.g. Lee, 2013;Moots, 1938) and experimentally (e.g. Graf & Qu, 2004;Graf & Song, 1995;Hunt, 1997;Nezu et al, 1997;Nezu & Nakagawa, 1995;Qu, 2002) that the peak phasing is commensurate with the strength of the hysteresis, that in turn is directly dependent on the site conditions and dynamic characteristics of the wave.…”

“…The results for the first and third cases were obtained from measurements conducted in a small natural stream, while the second case shows selected results from a field campaign in a medium-size stream reach located downstream from a hydropower station. The cases one and three illustrate that hysteresis is observable even in a small stream exposed to frequently occurring events (ISJP, 2016;Lee, 2013;Muste et al, 2019). The second case demonstrates the capabilities and limitations of the conventional HQRC and IVRC methods applied for unsteady flows developing at a site with a complex measurement environment, whereby the flow unsteadiness is significant and frequently occurring (Cheng et al, 2019).…”

“…These values indicate that all the storms propagating through this gauging station are of diffusive nature. Table 1 Hysteresis diagnostic approaches (Lee, 2013)…”