In‐situassessment of the spatial arrangement of step–pool units on eroded rills

Abstract

Rills are generated on homogeneous hillslopes by the action of different discharges and evolve morphologically over short timescales due to a strong interaction between the flow and bed morphology. Such an interaction generates a reconfiguration of the bed geometry. Previous works suggest that bed geometry is often characterized by alternation between pools and flat reaches (steps). Each step–pool unit may contribute to hydraulic resistance and affects flow behaviour. The objectives of this work are (i) to assess different (innovative) techniques for the in‐situ assessment of rill bed geometry, (ii) to use these techniques to assess the geometry of eroded rills in situ in order to determine the spatial arrangement in the bed macro‐scale roughness and (iii) finally to analyse the role of slope and discharge as driving factors associated with the development of these macroforms. Roughly rectilinear, long rills were formed in the field as a result of combining different slope and discharges. Photogrammetry provided detailed digital elevation models (DEMs) before and after the experiments. The rills were morphologically characterized from the DEMs. In each rill, the presence of step–pools was identified from long profiles according mainly to morphological criteria published elsewhere, but with ad hoc critical threshold values more appropriate for small eroded channels. The minimum slope required for the development of step–pool units seems to be somewhere between 5 and 15%. Discharge seems to affect pool size or roughness amplitude. There does not seem to be a clear step–pool periodicity. However, external factors could have affected the normal growth and alternation of these structures. Identification of steps and pools from longitudinal elevation profiles can be objectively accomplished using a series of geometric rules originally proposed for rivers and large channels, and adapted to rills. © 2019 John Wiley & Sons, Ltd.

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

“…The authors, carrying out experiments on step–pool rills with 15% slope and shaped on a silty loamy soil with a slight‐to‐moderate stoniness, obtained an average ( H / L )/ S m value equal to 2.3. This value agrees roughly with the findings of Abrahams et al (1995) (Giménez et al, 2019), as it is slightly greater than the upper limit of the range 1–2 suggested for occurrence of maximum flow resistance. In any case, step–pool units in rills do not meet the fundamental hypothesis of spatial regularity present in the model of maximum flow resistance by Abrahams et al (1995).…”

“…These results are affected by the use of an expression of the steepness ratio for rills, ( H / L )/ S , which is different from that used for streams, ( H / L )/ S m , in terms of variable representative of slope. Instead, the result obtained in the field study by Giménez et al (2019) is properly expressed in accordance with this last ratio. The authors, carrying out experiments on step–pool rills with 15% slope and shaped on a silty loamy soil with a slight‐to‐moderate stoniness, obtained an average ( H / L )/ S m value equal to 2.3.…”

“…Step–pool units were also detected in rills (Deng et al, 2020; Di Stefano, Nicosia, Palmeri, et al, 2019; Giménez & Govers, 2001; Giménez et al, 2004, 2019), which are erosive channels characterized by a smaller spatial scale than that of streams. If the formation processes of step–pool units in rills were similar to those occurring in streams, then the applicability of Equation for rills would be expected.…”

“…According to Giménez et al (2019), pools in rills occur because of scouring of cohesive soil by the erosive flow. Giménez and Govers (2001) suggested that supercritical flow in rills causes local flow instabilities (hydraulic jumps), which lead to significant local bed scour.…”

“…Research examining step–pool structures in rills is still limited and mainly deals with the relationship between flow pattern and rill topography (Giménez & Govers, 2001; Giménez et al, 2004), the automatic identification of steps and pools (Giménez et al, 2019), the study of flow resistance (Di Stefano, Nicosia, Palmeri, et al, 2019) and the dynamics of the morphological parameters (Deng et al, 2020). These studies are based on both field and laboratory observations.…”