Discharge estimation combining flow routing and occasional measurements of velocity

A new procedure is proposed for estimating river discharge hydrographs during flood events, using only water level data at a single gauged site, as well as 1-D shallow water modelling and occasional maximum surface flow velocity measurements. One-dimensional diffusive hydraulic modelis used for routing the recorded stage hydrograph in the channel reach considering zero-diffusion downstream boundary condition. Based on synthetic tests concerning a broad prismatic channel, the “suitable” reach length is chosen in order to minimize the effect of the approximated downstreamboundary condition on the estimation of the upstream discharge hydrograph. The Manning’s roughness coefficient is calibrated by using occasional instantaneous surface velocitymeasurements during the rising limb of flood that are used to estimate instantaneous discharges by adopting, in the flow area, a two-dimensional velocity distribution model. Several historical events recorded in three gauged sites along the upperTiber River, wherein reliable rating curves are available, have been used for the validation. The outcomes of the analysiscan be summarized as follows: (1) the criterion adopted for selecting the “suitable” channel length based on synthetictest studies has proved to be reliable for field applications to three gauged sites. Indeed, for each event a downstreamreach length not more than 500m is found to be sufficient, for a good performances of the hydraulic model, thereby enabling the drastic reduction of river cross-sections data; (2)the procedure for Manning’s roughness coefficient calibration allowed for high performance in discharge estimationjust considering the observed water levels and occasional measurements of maximum surface flow velocity during theCorrespondence to: G. Corato (g.corato@irpi.cnr.it) rising limb of flood. Indeed, errors in the peak dischargemagnitude, for the optimal calibration, were found not exceeding 5% for all events observed in the three investigatedgauged sections, while the Nash-Sutcliffe efficiency was, on average, greater than 0.95. Therefore, the proposed procedurewell lend itself to be applied for: (1) the extrapolation of rating curve over the field of velocity measurements (2) discharge estimations in different cross sections during the same flood event using occasional surface flow velocity measurescarried out, for instance, by hand-held radar sensors.