My dissertation sought to improve our understanding of mountain stream stability. The research was conducted in partnership with consultants and government through a NSERC Strategic Grant secured by Dr. M. Church. The project began with a thorough review of the literature (Church and Zimmermann, 2007) which identified the main research questions. Then I designed, built and instrumented a unique artificial stream channel (flume). The methods developed during this project have resulted in two major publications (Zimmermann et al., 2008-a; Zimmermann et al., 2008-b).
The results from the experiments cover three specific topics:
- The pattern and controls on channel morphology in steep streams
- Modelling steep stream channel stability using detailed flow, channel form and sediment transport information (Zimmermann et al, in press)
- Field applicable flow resistance relationships for steep streams (Zimmermann, in review)
I finished my PhD in March of 2009 and am now working at Northwest Hydraulic Consultants. To download a copy of the entire thesis go to www.geog.ubc.ca/~azimmer/Zimmermann_2009_PhD.pdf. This version has 10 videos embedded in it and is 420 MB. A version without the videos is available at the UBC information repository website (97 MB). The videos can be downloaded individually from www.geog.ubc.ca/~azimmer/videos/
NOTICE: I have discovered that the data in Figure 51 (pg 118) are flipped! The plot on the left is actually H/Ls/S, and the plot on the right are actually H/L/S. As a result the discussion of the results on page 119 is wrong. The end result is that my data conform with the Abrahams et al (1995) 1<H/L/S<2 theory.
Abstract:
The stability of steep streams with step-pool and cascade morphologies cannot be assessed using knowledge from lowland streams due to the episodic delivery of sediment and the structured nature of headwater streams. Thus there is a need for experimental studies examining the stability of such channels. The structuring of these channels occurs as a result of boulders and cobbles jamming across the width of the channel and the more typical pattern of armouring and imbrication which results from degradation and a relatively low rate of sediment supply.
To conduct such a study, new experimental techniques were developed and an artificial stream channel (flume) was designed and built. Channel width, bed grain size and channel gradient were varied and step-pool bedforms were created and subsequently destroyed. The variables governing the dimensions, frequency and form of step-pools were observed to be channel slope, bed grain size and channel width. Video records show that on occasion groups of larger stones moved together as a coherent group forming a force chain across the width of the channel. The failure of a step most frequently occurred when the downstream scour pool undermined the step forming stones and was often associated with headward migrating instabilities. Stable beds with smaller jamming ratios (channel width/D84steps) persisted at larger Shields numbers (ratio of shear stress to grain size) confirming that such channels do gain stability by having grains jam across their width. The failure of the bed was shown to be a stochastic process with nearly half of the failures occurring within the first minute following an increase in discharge, while 26 % of the failures did not start to occur until tens of minutes or more after the flow was increased.
During the experiments detailed bed morphology, channel grain size and flow velocity measurements were made and these suggest that a dimensionless hydraulic geometry approach is likely the best method of predicting flow velocities in headwater channels. The bed stability criterion in combination with a dimensionless hydraulic geometry approach provides a means of assessing the stability of mountain stream channels.
Giveout Creek, Nelson BC |
Desolation Sound, Cortez Island |
Finishing a lamp shade on the lathe |
