Form and stability of step‐pool channels: Research progress

Abstract

[1] Research examining the hydraulics, morphology, and stability of step-pool mountain streams has blossomed in the last decade, resulting in more than a dozen dissertations. These, along with other research projects, have transformed our understanding of step-pool channels. Contributions have been made toward understanding depositional step formation and destruction, scour downstream of steps, step-pool hydraulics, and the effect of sediment transport on step stability. We propose that depositional steps exist in a jammed state whereby the boulders are structurally arranged within the channel and thereby stabilize it. Once a step has formed, a scour pool with a characteristic length and depth develops downstream, creating a zone where additional steps are unlikely to occur. Downstream of the scour hole, steps are more likely to occur as the high energy associated with the plunge pool has dissipated. Data suggest that the presence of cobbles or boulders limits pool scour as well as the degree to which well-defined, channelspanning step-pools form. We propose a state-space for step-pools in which conditions for a step to form include (1) the ratio between width and boulder diameter (the jamming ratio), (2) the ratio between applied shear stress and the stress needed to mobilize the bed (relative Shields number), and (3) the ratio between bed material supply and discharge (bed sediment concentration). Available data suggest this model is plausible. Emerging critical research questions are discussed.

On kinematics and flow velocity prediction in step‐pool channels

“…For section ''s'' (Figure 2) in TA and TB, the verification of the critical flow conditions (the unit Froude number) was conducted by measuring the water depth upstream of the step lip and at a distance precisely equal to 3.5 times the depth at the step lip. According to hydraulic considerations [Chow, 1973], the depth in this section is equal to the theoretical critical depth-d c 5 (q 2 /g) 1/3 , where q 5 Q/w-only when the flow regime approaching the step is subcritical. Because agreement between the measured flow depth and theoretical critical depth was good (the maximum absolute difference between the two equaled up to 10%), the local velocity at the TA-TB step crest was calculated using the critical-condition formula: u c 5 (g d c ) 1/2 and assuming the value of d c provided by Q and the measured flow width w.…”

“…The sequences dissipate flow energy due to the stepped longitudinal topography and alternation of the coarsest (steps) and finest (pool) particles [Keller and Swanson, 1979;Montgomery and Buffington, 1997;Chin and Wohl, 2005;Church and Zimmermann, 2007]. These morphological units are able to almost maximize the energy losses due to the high flow resistance they generate [Whittaker and Jaeggi, 1982;Abrahams et al, 1995].…”

“…Given water depth and energy gradient in a prismatic channel, the mean flow velocity depends on the hydraulic resistance of the channel, which can be expressed using Chezy's (C) [1769[ , in Chow, 1973, or Darcy-Weisbach's friction factor (f) [1854[ -1845[ , in Chow, 1973 or Manning's roughness coefficient (n) [1889[ , in Chow, 1973 and, accordingly, the relative normal depth equations. Techniques predicting flow resistance coefficients from field data exist for Chezy's C [e.g., ASCE Task Force, 1963], DarcyWeisbach's f [Bathurst, 1985;Kaufmann, 1987;Comiti et al, 2007;Kaufmann et al, 2008], and Manning's n [Jarrett, 1984;Rickenmann, 1994].…”

“…Instead, addressing the natural diversity of the step-pool channels is still partly lacking [Church and Zimmermann, 2007], because their full interpretation needs to extend the understanding of a single step/riffle-step pool to a structured continuous sequence of step pools. Furthermore, the uncertainties are more manifest if the overall kinematic behavior of the sequence at different flow stages is under investigation, e.g., to achieve a robust and not too complicated estimation for practical design/restoration purposes.…”

A Multi-Faceted Debris-Flood Hazard Assessment for Cougar Creek, Alberta, Canada

“…Hillslope processes are intimately linked to channel processes with some channels being supply-limited while other being supply-unlimited (Bovis and Jakob [15], Rickenmann [39]). As pointed out by Church and Zimmermann [40], steep mountain creeks can display a multitude of grain sizes, variable sediment sources, and rough and structured stream beds with step and pool morphology. Large boulders (keystones), woody debris and occasional bedrock sections further create a significant variation in channel geometry, flow velocity and roughness, all of which render theoretical or flume-derived sediment transport equations questionable (Gomi and Sidle [41]).…”