89-Velocity Profiles Assessment in Natural Channels During High Floods

Abstract

The accuracy of three different approaches for velocity profiles assessment during high floods has been investigated. For high floods is here intended the flow condition when the velocity points sampling is carried out only in the upper portion of the flow area. The first two methods assume the classical logarithmic law with additional terms, to take account of the dip-phenomenon in the curvature of the velocity profile. In this case the two methods need the estimation of three and four parameters, respectively. The third one is based on the entropy theory and uses a velocity profile distribution depending on the maximum flow velocity occurring in the flow area. This approach requires the assessment of one parameter. A sample of velocity measurements carried out during a period of 10 years at Pontelagoscuro gauged section, located along the Po River, northern Italy, has been considered for the analysis. Four flood events have been selected and for each of them the high flood condition has been surmised in terms of sampling. The accuracy of the investigated methods has been evaluated in terms of mean errors in estimating both the mean velocity along each sampled vertical and the mean flow velocity. Results showed that for high floods, the first two methods were not able to accurately reproduce the velocity profiles, showing a mean error on mean flow velocity greater than 20%; whereas the entropic approach was found more accurate showing a mean error which did not exceed 5%. Based on the obtained results, a procedure for addressing the velocity measurements during high floods is also proposed. The procedure can be conveniently adopted for practical applications allowing both to short remarkably the time of the velocity measurements sampling and to estimate quickly the discharge.