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Scour holes and ripples occur below the hydraulic smooth to rough transition of movable beds

Kleinhans, Maarten G.; Leuven, Jasper R. F. W.; Braat, Lisanne; Baar, Anne


Maarten G. Kleinhans

Jasper R. F. W. Leuven

Lisanne Braat

Dr Anne Baar
Post-doctoral Research Associate


Jaco Baas


Scour holes often form in shallow flows over sand on the beach and in morphodynamic scale experiments of river reaches, deltas and estuarine landscapes. The scour holes are on average 2 cm deep and 5 cm long, regardless of the flow depth and appear to occur under similar conditions as current ripples: at low boundary Reynolds numbers, in fine sand and under relatively low sediment mobility. In landscape experiments, where the flow is only about 1 cm deep, such scours may be unrealistically large and have unnatural effects on channel formation, bar pattern and stratigraphy. This study tests the hypotheses that both scours and ripples occur in the same conditions and that the roughness added by sediment saltation explains the difference between the ripple–dune transition and the clear‐water hydraulic smooth to rough transition. About 500 experiments are presented with a range of sediment types, sediment mobility and obstructions to provoke scour holes, or removal thereof to assess scour hole persistence. Most experiments confirm that ripples and scour holes both form in the ripple stability field in two different bedform stability diagrams. The experiments also show that scours can be provoked by perturbations even below generalized sediment motion. Moreover, the hydraulic smooth to rough transition modified with saltation roughness depending on sediment mobility was similar in magnitude and in slope to ripple–dune transitions. Given uncertainties in saltation relations, the smooth to rough transitions modified for movable beds are empirically equivalent to the ripple–dune transitions. These results are in agreement with the hypothesis that scours form by turbulence caused by localized flow separation under low boundary Reynolds numbers, and do not form under generalized flow separation over coarser particles and intense sediment saltation. Furthermore, this suggests that ripples are a superposition of two independent forms: periodic bedforms occurring in smooth and rough conditions plus aperiodic scours occurring only in hydraulic smooth conditions.

Journal Article Type Article
Publication Date 2017-08
Journal Sedimentology
Print ISSN 0037-0746
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 64
Issue 5
Pages 1381-1401
APA6 Citation Kleinhans, M. G., Leuven, J. R. F. W., Braat, L., & Baar, A. (2017). Scour holes and ripples occur below the hydraulic smooth to rough transition of movable beds. Sedimentology, 64(5), 1381-1401.
Keywords Bedform stability; Landscape experiments; Ripples; Scaling; Scour holes
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