Technical Luncheons
Avalanches: Turbidity Current Analogues that Kill
Speaker(s):
Dr. John Harper,
Harper Consulting International Inc.
Date/Time: Thursday, September 24, 2009 - 11:30am
Location: Telus
Convention Centre - Calgary, Alberta
The cut-off date for ticket sales is 1:00 pm, Monday, September 21, 2009.*
CSPG Member Ticket Price: $38.00 + GST.
NON-MEMBER Ticket PRICE: $45.00 + GST.
*Please note: Due to the popularity of talks, we strongly suggest purchasing tickets early, as we cannot guarantee seats will be available on the cut-off date.
ABSTRACT
Avalanches! The word conjures up images quite familiar to Canadians in winter, especially those in mountainous regions, although avalanches occur wherever snow can build up an accumulation. In reality, in geologic terms, avalanches are turbidity currents, and their deposits are snow turbidites. Avalanches are all about the instability of snow sediment overload. Instability occurs as a result of snow overloading on steepened slopes on which there occur defined slide surfaces. Sediment overload is the result of heavy snowfall, or wind loading (cornices are generally in evidence). Slope angles corresponding to instability range from about 30° to 50°. At higher angles it is difficult for the snow to accumulate and hold. At lower angles the snow stabilises very rapidly and holds in place. Interestingly a slope angle of 18° is considered steep by most skiers. Double-black runs tend to be in the high 20°s and steeper. Defined slide detachment surfaces result from extended periods of freeze and thaw, or rain and freeze, or sun melt and freeze.
Comparisons with modern deep-sea slides and slumps are presented which identify identical avalanche morphologies of fracture lines, slide scars, flow valleys, and runout fans. The main difference is scale. Whereas avalanches tend to run less than a kilometre, but occasionally can go much further, deep-sea slumps set off huge gravity mass flows which in the case of the Bengal Fan can run as far as 2000 km. The end products are similar in many cases; being debris flows, slab avalanches or slides, sluffs, channelized deposits, leveed margins, distal fans, meandering channels, and spillover lobes. Water content variations in avalanche deposits can result in significantly different surface character to the deposits which can be likewise identified in the deep marine. Inner bend and the thicker major outer bend deposits occur in like fashion between modern turbidity currents and avalanches.
To watch an avalanche is to see a flume study in motion. The huge plume reacts in air as it does in flume water studies. The internal motion of the plume is identical to that seen in the water versions. The slide initiates at an uppermost fracture line of varying height depending on how much of the sediment load is involved, whether it is only a surface slide or it fractures to the underlying ground. In the latter case significant volumes of rock can become incorporated; so much so that geologic studies after melt have tried to characterize the fabric, texture, and bed stacking relationships of the sediments. Almost instantaneously, after the fracture line initiates, a huge area downslope of the fracture detaches and the deposit begins to move as a mass of snow blocks of all sizes, loose snow, maybe rocks, and trees, and too often humans and their skis or snowmobiles, down the chutes and bowls, over the rock cliffs, and channelized around bends. The front of the flow begins to mix with the air, thereby creating the plume. Flow lobes race and spurt out in front of the main flow, picking up and incorporating downslope snow in the avalanche's body. Velocities can reach as high as 200 km/hour. Outer bends, base of slope fans, and accumulation lows are where the bodies commonly are deposited. As the flow begins to slow, it fans into its proximal and distal portions which are commonly defined by channels and levees just as are the marine equivalents.
Once caught in the mass flow, unless trapped at the margin, there is absolutely no chance of escaping other than trying to keep as near to the surface as possible by swimming with the current. Mouth and nose become filled with snow in an instant -- almost before the individual is aware of what has happened. Every attempt has to be made to retain one's senses, to hold one's breath, keep one's mouth closed, protect the face and to preserve an air pocket once the avalanche "quick phases", whereby it essentially converts to a rigid immoveable mass as the finer grained snow metamorphoses and freezes. Once having been entombed the trapped individual suffocates from either the packed snow or the subsequent ice mask which forms as exhaled air melts the surface of the pocket and then freezes.
Recognition of conditions and potential dangers (risk analysis is something to which we are accustomed) can be addressed at many levels from Google imagery, to use of Canadian Avalanche Association area reports, to Alberta Environment snowfall (snow pillow) records, to use of the "Avaluator", to awareness of local conditions, and to analysis of snow pits. Training courses and the purchase of appropriate rescue equipment is imperative. Of course once it is necessary for you to have to use the equipment it almost too late anyway. So the best advice is to exercise restraint in exuberant situations and not become a victim. In a risk sense the equipment is necessary "once you get on the curve'. The trick is to avoid getting on the curve!
We are fast approaching a new winter season. Snow has already begun to accumulate. Please let this be a reminder to renew your training, practice your skills, evaluate the degree of risk you are willing to accept, and if necessary make sure your will is up to date.
I would like to extend my thanks to the producers of the avalanche film "The Fine Line" which won awards at the Banff film festival, and for which they gave permission for use of the leader I used to introduce this talk. The film which is a superb training tool can be purchased at www.rockymountainsherpas.com, or at MEC here in Calgary for nominal cost, as can the "Avaluator".
BIOGRAPHY
John D. Harper, PhD, P. Geol., FGSA, FGAC: previously Senior Geological Advisor, ConocoPhillips Canada Ltd.; Retired Full Professor, Petroleum Geology, and the first Director of the Centre for Earth Resources Research at Memorial University of Newfoundland to Jan 1, 1998, and Adjunct Professor to 2002; formerly with Shell Development, Shell Oil, Shell Canada, and Trend Exploration. He has operational, management, and research credentials over the past 38 years in reservoir characterization and basin analysis for Canadian, US, and International onshore and offshore basins. His most recent activities have been in the Canadian Arctic, Mackenzie Delta - Beaufort, the Scotian Shelf and Deep Water, Labrador and Grand Banks, and the West Coast of Newfoundland.
For 40 years Harper was a member of the Canadian Ski Patrol System and related organizations (National Ski Patrol System, Ventura County Sheriff's Search and Rescue, Rhode Island Civil Air Patrol rescue training) holding a variety of positions from Patroller to Instructor to National VP - Training. He has held courses in Winter Survival for the Newfoundland and Labrador Junior Forest Ranger Instructors. In the last 10 years he has been involved with presentation of avalanche awareness programmes in Calgary schools and in ConocoPhilllips Canada in the hope that a little bit of information can have a large impact.
