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Mud and Debris Flow analysis

Mud and Debris Flow analysis

Mud and Debris Flow analysis

As all of us in Southern California are well aware, this winter will, and already has produced catastrophic mud and debris flows on the hillsides denuded by the wild fires.

In determining the flow velocity, mud concentration and subsequent mud flow rate, I have been unable to find any uniformly accepted references for determining:

a) the thickness of potenially flowable soil on a slope,
b) a method of determining the anticipated first event percentage, and
c) the soil bulking percentage.

I have developed a spresdsheet that allows for changes in these factors to allow for determination of the channel capacity and detention wall height, however I would like input from others before I release the spreadsheet.

Any information and references on mudflow analysis and deflection wall and channel capacity design would be much appreciated.

RE: Mud and Debris Flow analysis

Might want to check out references from Canadian Geotechnical Society - there were a while back - several day seminars put on by the Vancouver Group which included such things as debris flows, etc. as this is big along the Whistler Highway corridor.  Also, check the journals - say in mid to late 80s and you will probably find some good information to start off with.  
   I would think that the volume of debris (or thickness of soil that could be stripped off) depends a lot on the nature of the soil deposit, its sensitivity, propensity of rooting systems and the like - in other words quite a few variables.

RE: Mud and Debris Flow analysis

BigH is right on target; you are discussing a very complicated issue.  The simplified spreadsheet seems to ignore an awful lot of factors.

It sounds like the model was developed without the benefit of guidance from an experienced geotechnical engineer...or are we missing something?

I do empathize with your problem; I lived in southern California for a few years (Corona/Riverside, Menifee/Sun City, San Juan Capistrano - in that order) and I've seen what can happen when the rains come after an area has been ravaged by fire.

One part of the answer may be that the cost of accurately predicting the slide areas isn't economically feasible, particularly in a tight economy.

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

RE: Mud and Debris Flow analysis

Many thanks to BigH & Focht3 for your comments.

Yes, I guess I certainly oversimplified the discussion in my question. I am aware of the extremely complex nature, causes and analysis of mud and debris flows.

The mud flow criteria I am examining is not related to very large native sites with huge watershed areas, but to small natural slopes that occur between graded hillside developments or relatively small natural hillside areas that have limited potential failure areas and may only have a few feet (3-5 maybe typical)of soil overlying a sloping bedrock profile. Programs like SINMAP or SHALSTAB with ArcView GIS tend to be too unwieldy for relatively small areas of potential instability.

One question is  - is the type of failure I'm looking at, a typical slope failure occurring under conditions of rapid saturation (to be analysed by conventional methods) or a small mud flow to be alalysed some other way. Maybe this is what I should determine first?

What I am trying find, are references for assessment of near surface permeability on sloping surfaces, how the very gradual or rapid saturation of a known soil profile reacts and influences the surficial stability (not simplistically like the Orange County surficial stability analysis)and how rapidly the reduction of the factor of safety during the saturation process will cause the the flow to commence (ie. a method of determining the anticipated first event percentage of failure of the soil profile). It seems more likely that the analysis of a mud flow is a multitude of small failures occuring one after the other rather than one static condition with specific paramters.

So, this is what I was trying to present in my question.

Another issue I mentioned was a method of calculating the soil bulking percentage during this complex process of inital failure. Does the soil bulking become less of an issue the higher the moisture content becomes ?

I hope this provides a better expalnation of my needs (other than a course in Mud Flow 101 !)

Thanks ahead of time

RE: Mud and Debris Flow analysis

Good.  Unfortunately, we get some flaky questions (and posters) - please excuse our initial questions.

In response to your questions (in no particular order):
  1. The importance of the bulking factor clearly decreases as the water content increases (i.e. as the soil approaches or exceeds the Liquid limit.)
  2. Surface permeability of slopes is heavily influenced by surface discontinuities i.e. cracks, animal burrows, root holes, etc.  Good luck finding a reference on this topic...
  3. Your characterization of the mud flow as a series of "small" events coincides with my expectations and experience.
  4. You need to determine which method of analysis to use as your first task.  My suggestion is to use the conventional slope stability analysis to identify likely "candidates" then use the mudflow analysis to evaluate the actual "event."  Pay close attention to those details that are infrequently addressed by conventional slope stability analyses - root pattern and depth, etc.
I hope that this has helped -

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

RE: Mud and Debris Flow analysis

with such limited soil thicknesses overlying sloping bedrock, infinite slope analyses are likely a very good step in getting a picture of the potential for slipping.  As Focht3 points out, though, in these kind of soils, the root systems, animal burrows, etc. are quite problematic to quantify.

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