Stage—discharge rating method considering backwater effect in river channel

To quantify stage variations in the middle reaches of the Yangtze River that are affected by backwater effect and channel adjustments, the hydraulic characteristics of selected typical reaches were analyzed and the applicability of previous stage-discharge rating methods was studied. Furthermore, a hydrological data-based stage-discharge rating method for non-uniform flow was proposed. Results indicated that the influences of channel adjustments and backwater effect cannot be separated using stage-discharge rating methods for uniform flow. Moreover, previous rating methods for non-uniform flow were difficult to use when only stage, but not discharge, should be estimated. Although channel morphology is irregular in the middle reaches of the Yangtze River and the flow roughness coefficient varies at different discharges, evident laws for channel hydraulic geometry factors and roughness-discharge relation were observed. The momentum equation for steady non-uniform flow can be simplified on the basis of the hydraulic features of the channel to obtain the rating function between the upstream and downstream stages and discharge. By using the proposed rating method, the upstream stage can be estimated via the rating function based on long term data. Meanwhile, the residuals between the estimated and measured stages represent the effects of channel adjustments. Lastly, the proposed method was applied to three typical reaches to examine its accuracy and applicability. The results showed that the coefficient of determination (R²) between the measured and estimated stages exceeded 0.99 when the channel was not altered. Therefore, the proposed method is capable of identifying stage changes caused by channel adjustments.