Analysis of rill step-pool morphology and its comparison with stream case

摘要

In this paper, the morphology of step-pool features is analysed using rill measurements and literature data for streams. Close-range photogrammetry was used to carry out ground measurements on rills with step-pool units, shaped on a plot having slope equal to 14, 15, 22, 24 and 26%. Data were used to compare the relationships between H/L, in which H is the step height and L is the step length, and the mean gradient of the step-pool sequence, S-m, for streams or the slope of the step-pool unit, S, for rills. The relationship of H/L against S-m is widely used to test the occurrence of the maximum flow resistance condition in streams, which is associated with the range 1 <= (H/L)/S-m <= 2. Further analyses were carried out to compare both the formation process and the profile of the pool in rills with those related to streams. Moreover, for a single rill channel, an analysis of flow characteristics expressed in terms of Darcy-Weisbach friction factor and Froude number was developed. The results allowed us to state: (i) the relationships of H/L versus S-m and S are quite similar and the steepness ratio for streams, (H/L)/S-m, and for rills, (H/L)/S, generally ranges from 1 to 2; (ii) the formation process and the profile of the pool in rills are not consistent with those occurring in streams; (iii) in the rills, the longitudinal size of the pool is dominant with respect to the maximum scour depth; (iv) the presence of a sequence of step-pool units within a rill segment noticeably increases flow resistance compared to segments with a flat bed; (v) the Froude number of the flow over the sequence of step-pool units in rills is slightly below the range of 0.8-1 corresponding to the maximum flow resistance in step-pool units.