Hydraulic geometry of Alpine streams channels

Abstract

This paper aims to analyze and discuss what controls the geometry of incised alpine streams. For this, we construct a dataset comprising 39 cross-sections and the associated grain size measured in 10 rivers. We then examine each geometry to discuss the definition of bankfull and the associated hydraulics. We conclude that in these incised cross section, width remains nearly constant for a wide range of discharge, including discharges as low as discharge with an exceedance probability of 0.01 (discharge exceeded 3.65 days/yr). In particular, the hydraulics associated with geometric markers located in the lower part of each cross-section suggest that Parker's theory of a threshold forming Shields stress ratio τ*/τ_c* ≈ 1.5 established at bankfull for non-incised gravel bed rivers still keep its significance within the incised section. We suggest that the width W_1.5 associated with the condition τ*/τ_c* ≈ 1.5 can be used as a reference width in alpine streams for river restoration or for risk management.