ESTIMATION OF DISCHARGE AND ROUGHNESS BY MEASUREMENT OF WATER LEVELS ALONE. A Review of the Slope-Area Method

The Slope-Area method, braodly described in the technical literature, allows estimation of the discharge flowing through two river cross-sections located at a suitable distance and with no lateral inflow/outflow, when simultaneous water level measurements in the two sections and the roughness coefficient of the reach are available. After a summary of the theoretical basics and mode of application of the method, we highlight its sensitivity to roughness, the latter expressed by means of the Manning coefficient. Then we describe a procedure for estimation of this coefficient based only on knowledge of the levels measured in two adjacent sections. The procedure is based on the concept of kinematic celerity and assumes that the bathymetry of the two sections and the length of the reach are known. Finally, we present an application of the procedure to a reach of the Arno River which shows that the roughness is a function of water depth. Comparison with the known rating curve allows us to validate the procedure proposed for estimation of the roughness coefficient and confirms the possibility of using the Slope-Area method to estimate the discharge on a continuous time basis, using measurement of the water levels alone.