APPRAISAL OF METHODS FOR ESTIMATION OF DISCHARGE AT UNGAUGED RIVER SITES

Abstract

Reliable discharge estimations at a river site depend on local hydraulic conditions, which are usually defined by recording water level data. In fact, stage monitoring is relatively inexpensive, especially when compared to the cost necessary to carry out discharge measurements. However, there is a need of converting stage records into discharge values. This task is addressed through a reliable rating curve, which is unknown in the case of ungauged sites, where velocity measurements cannot be carried out. In addition, a rating curve differs from one-to-one relationship when unsteady flow effects occur, thus producing a loop, the size of which depends on the influence of inertial and pressure frccs on the flood propagation process. In order to relate stage and discharge hydrographs at a selected river section also during unsteady how conditions, one-dimensional hydraulic modelling can be adopted. Nevertheless, a reliable use of hydraulic modelling is strictly related to the accuracy of calibration methods. A new procedure called the Velocity Method for discharge measurements in river sites and natural channel is proposed in the study, assuming a direct proportionality between upstream velocity and downstream velocity of the reaches without the use of flood routing techniques. The proposed procedure, coupled with calibration strategy, allows covering practically all the possible scenarios occurring in real ungauged site. Different study areas, located in Italy along with several flood events have been used to test the procedure. The results at the downstream gauged sites for the shortest reach are considered to be good with the Nash-Sutcliffe efficiency greater than 90 % for all the simulated flood events, relative peak error of only 10 % and mean time to peak error in absolute value of about 0.64 hr, but for the other two reaches studied, it showed overestimation of peak due to the presence of lateral in flows.