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

Ratings curves are conventional means to continuously provide estimates of discharges in rivers. Among 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 looped rating curves (also labeled hysteresis). Neglecting the unsteadiness of the flow when this is large can significantly affect the accuracy of the flow 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.

Therefore, the overarching goal of this study was to explore the uncertainty of the conventional approaches for constructing stage-discharge rating curves (hQRCs) and to evaluate methodologies for accurate and continuous discharge monitoring in unsteady open channel flows using analytical inference, index velocity rating curves (VQRCs), and continuous slope area method (CSA) with considerations on discharge measurement uncertainty. The study will demonstrate conceptual and experimental evidences to illustrate some of the unsteady flow impacts on rating curves and suggest the development of a uniform end-to-end methodology to enhance the accuracy of the current protocols for continuous stream flow estimation for both steady and unsteady river conditions. Moreover, hysteresis diagnostic method will be presented to provide the way to conveniently evaluate when and where the hysteresis becomes significant as a function of the site and storm event characteristics. The measurement techniques and analysis methodologies proposed herein will allow to dynamically tracking both the flood wave propagation and the associated uncertainty in the conventional RCs.