A field investigation of the influence of sediment supply on step-pool morphology and stability
Etude de l'influence des forçages sédimentaires sur la stabilité des torrents en step-pool
Résumé
The effect of the sediment supply on step-pool morphology and stability was studied through the investigation of 42 step-pool reaches (eight in the Vosges and 34 in the Alps, France). Step-pool geometry (step wavelength L and height H) was measured for each reach and the data set split into two distinct groups comprising 25 connected (to a sediment source) step-pool reaches and 17 disconnected step-pool reaches. The geometry analysis showed differences between the two groups, with the shape factors k = L/Hr and the H/Hr ratios (Hr is the residual pool depth) being lower for disconnected step-pools. This was interpreted to result from progressively changing geometry over time, with disconnected step-pool geometry defining the probable asymptotic characteristic associated with this long-term process. The measured step-pool dimensions were in good agreement with equations based on maximum flow resistance (Abrahams et al., 1995), energy of a falling jet (Comiti et al., 2005), and channel spanning deposition (Chartrand et al., 2011). However, the average shape factor for the Comiti et al. equation was k = 17 instead of 8, as the authors had proposed. The data set was extended to 255 values with data from the literature, which confirmed this average value for k. Field evidence (moss cover and stone imbrication) indicate that connected step-pools are mobile whereas disconnected step-pools are stable. This was investigated by a hydraulic analysis of threshold motion conditions for the boulder steps. Comparison with discharges measured at nearby gauging stations indicates that remobilization return periods are ~ 20-50 years for connected step-pools (and even less when considering scouring effects) and greater than 50 years for disconnected step-pools. This difference in step mobility was ascribed to the fact that, with sediment supply, the channel can freely adjust its local slope, allowing a relatively efficient transfer downstream of large boulders by scouring effects.