Identifying a dominant discharge for natural rivers in southern Italy

Natural rivers are subjected to continuous adjustments in response to any change in the environment. These environmental changes may occur naturally, as in the case of climatic variation or changes in vegetative cover, or may be related to human activities including channelization, damming, bank protection, and bridge construction. Identifying the value of that discharge ('dominant' or 'effective' discharge, QD) considered responsible for the main changes operated by a river has been a subject of great challenge to scientists and engineers during the last decades. In fact, this threshold value is largely adopted for stream-management decisions, for predicting the stable slope upstream of grade-control structures, and for designing moderate- to large-sized hydraulic structures. In this paper, a simple concept of dominant discharge, corresponding to that value of QD that accounts for at least 50% of the total suspended sediment load transported by the river, is introduced and discussed. The work is based on a long-term data set that includes measurements of monthly discharge and suspended sediment load in 27 stream gauge stations located in three different regions of southern Italy. The analysis showed that QD corresponds to values of the return time, T, ranging from 1 to 5 years, confirming previous findings by other authors in similar analyses. Also, as the values of QD were well correlated with the 1.5-year peak discharges of the same investigated rivers, an empirical approach is suggested in order to estimate QD in rivers with no sediment load measurements.