APPLICABILITY OF HYDRAULIC PERFORMANCE GRAPH FOR FLOW ESTIMATION IN OPEN CHANNELS

摘要

The hydraulic performance graph (HPG) is an efficient means for summarizing the backwater profiles of all possible gradually varied flow conditions in an open-channel reach, expressed in the form of water surface elevations (stages) or depths at the ends of the channel reach for different constant discharges. HPGs have been used for determining the flow carrying capacity of channel systems (Yen and Gonzalez, 1994, 2000). Further, Gonzalez and Yen (2000) have also carefully assessed the applicability of HPG for unsteady flow routing. Theoretically, the HPG for a channel reach is a graphical representation of the relation between discharge and the downstream and upstream stages in the reach according to gradually varied flow. In this paper we explore the applicability of HPGs for estimating the flow in an openchannel reach from observed water stages at its two end stations. We describe the methodology for developing the HPG for a site in South Florida based on actual channel geometric and roughness properties and compare the discharges estimated with the reach’s HPG for actual observations of upstream and downstream stages with field-flow measurements concurrently collected with an acoustic Doppler current profiler. This comparison shows that the HPG of channel reach is a plausible and reliable alternative for estimating flows in open channels. It is also shown that the accuracy of the HPG for flow rating is a function of Sf L / H, where Sf = friction slope, L = length of the channel reach and H is the error in estimating the water stages. HPG ratings are found relatively accurate for Sf L / H >> 1, but their accuracy decreases as Sf_ L / H→ 1.