Regional Estimates of Bed Load Sediment Yield in the French Alps

The movement and storage of bed load sediment is of first-order importance in determining the form and function of alluvial rivers. However, because of the costs involved, direct sampling of bed load is rarely done anymore, consequently, in most rivers, we have little idea of present-day transport rates and sediment yields. In this paper, we couple relations for bed load transport with relations for flow frequency to model the annual bed load sediment yield of rivers draining the Ecrins Massif in southeastern France. Sediment production in this part of the Alps appears to be very high, but contemporary sediment yields are largely unknown. Measurements of channel and bed material properties were taken in 15 alluvial reaches with upstream drainage areas ranging from 20 to 311 km2. The majority of peak flows in this region are generated by snowmelt, but rainfall-generated floods are not uncommon. Reach average slopes equal or exceed 1% at all sites. Our observations and results indicate that Shields numbers representative of active bed load transport are much higher than typical threshold values. Corrections for the effects of steep slopes and high relative roughness narrow the difference between high-flow Shields numbers and threshold conditions, but in many of our study reaches, high-flow Shields numbers are typically more than two times the threshold for motion. Consequently, unit-width bed load transport rates commonly exceed 1.0 kg m-1 s-1, which is comparable to measured loads in other river systems with high sediment supply. Annual sediment yields follow a log-normal distribution with a mean of ~80 t/km2/yr; expressed as an erosion rate this equates to 0.03 mm/yr. The morphologic expression of these high bed load transport rates is for many rivers in the French Alps to become braided as they exit more-confined valleys near their headwaters.