Steep mountain streams are the most numerous and ubiquitous river type in the world and represent an essential component of mountain environment and landscape. Coarse grained, steep channels typically show reaches with a staircase bed morphology, known as step-pool sequences, that may alternate with simpler streambed morphology reaches such as runs or plane beds. Though in the last decade the attention of scientists about step-pools has increased remarkably, their morphological characteristics and the processes originating them are still matter of debate. In order to contribute to the understanding of step-pools and their geomorphic significance, a large database has been set up using field and laboratory data reported on both published scientific journals and unpublished studies. The review of the existing data has been paired by some field experiment on large particle entrainment and step-pool dynamics in a mountain stream of the Northern Apennines. The literature data review has shown that very few concepts about step-pool morphology and flow hydraulics can be considered as well established. For instance, many authors report about the inverse relation between step wavelength and streambed gradient, but plot diagrams of field and laboratory data show no evidence of such a relationship. This observation implies that the hypotheses based on such an assumption need at least to be revised. The lack of correlation between step wavelength and slope seems also to be affected by the non homogeneous methods to measure the basic step-pool geometry parameters as the long tail of high values of the step wavelength to channel width ratio indicate compared to a significant clustering of data around the value of one. Many authors assume that step-pool sequences are originated only by long return interval (30-50 years) floods, in mountain streams characterised by sediment supply limited conditions. Field experiment in the northern Apennines have shown that large particles making up the steps can be entrained and steps can be completely removed or new ones formed by flood flows close to or just a bit higher than bankfull stage (2-5 years). Moreover, field observations suggest that the condition of limited sediment supply is not a prerequisite for the formation of step-pool since they were found on the bottom of ephemeral gullies or in small channels incised in very steep (35° - 40°) scree deposits of arid and semi-arid environments. All these findings have fundamental implications on the development of a general theory to explain the origin of step-pool sequences. Substantially, three main hypotheses are reported in the literature: antidunes, maximisation of flow resistance and large keystones control. These theories are analysed in the light of the field experiments carried out in our study mountain streams and compared with a further elaboration of the literature data sets. Improving our understanding of the morphology and dynamics of step-pools will result in a sounder management of steep mountain streams and in eco-compatible interventions for channel restoration in extreme floods or anthropogenic impact aftermaths.

Step-pool sequences of steep mountain streams. A review of their characteristics and origin

BILLI, Paolo;
2008

Abstract

Steep mountain streams are the most numerous and ubiquitous river type in the world and represent an essential component of mountain environment and landscape. Coarse grained, steep channels typically show reaches with a staircase bed morphology, known as step-pool sequences, that may alternate with simpler streambed morphology reaches such as runs or plane beds. Though in the last decade the attention of scientists about step-pools has increased remarkably, their morphological characteristics and the processes originating them are still matter of debate. In order to contribute to the understanding of step-pools and their geomorphic significance, a large database has been set up using field and laboratory data reported on both published scientific journals and unpublished studies. The review of the existing data has been paired by some field experiment on large particle entrainment and step-pool dynamics in a mountain stream of the Northern Apennines. The literature data review has shown that very few concepts about step-pool morphology and flow hydraulics can be considered as well established. For instance, many authors report about the inverse relation between step wavelength and streambed gradient, but plot diagrams of field and laboratory data show no evidence of such a relationship. This observation implies that the hypotheses based on such an assumption need at least to be revised. The lack of correlation between step wavelength and slope seems also to be affected by the non homogeneous methods to measure the basic step-pool geometry parameters as the long tail of high values of the step wavelength to channel width ratio indicate compared to a significant clustering of data around the value of one. Many authors assume that step-pool sequences are originated only by long return interval (30-50 years) floods, in mountain streams characterised by sediment supply limited conditions. Field experiment in the northern Apennines have shown that large particles making up the steps can be entrained and steps can be completely removed or new ones formed by flood flows close to or just a bit higher than bankfull stage (2-5 years). Moreover, field observations suggest that the condition of limited sediment supply is not a prerequisite for the formation of step-pool since they were found on the bottom of ephemeral gullies or in small channels incised in very steep (35° - 40°) scree deposits of arid and semi-arid environments. All these findings have fundamental implications on the development of a general theory to explain the origin of step-pool sequences. Substantially, three main hypotheses are reported in the literature: antidunes, maximisation of flow resistance and large keystones control. These theories are analysed in the light of the field experiments carried out in our study mountain streams and compared with a further elaboration of the literature data sets. Improving our understanding of the morphology and dynamics of step-pools will result in a sounder management of steep mountain streams and in eco-compatible interventions for channel restoration in extreme floods or anthropogenic impact aftermaths.
2008
Step-pool; Mountain streams; Flow resistance; Particle entrainment; River restoration
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/530392
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