Perhaps there are two ways to think of the problem:
1) If you trust the design of the berm to be adequate, you could just make sure that the new foundation could support at least the strength lost by the excavated soil plus the pressure of the soil on the berm side of the foundation. I would think that given a normal concrete foundation and its supported weight that it wouldn't need to be much larger than a normal retaining wall type of application.
2) Otherwise you need to find an equation that gives you the energy of impact of a debris flow. You will have to assume a flow cross-section (height and length), speed, mass and impact angle. These things are very speculative, and that is why if you can go back to the design of the berm that should give some clues. Was any analysis used in the berm design?

I would not expect an "impact force" from a debris flow, moving way too slow and not rigid in order to transmit that type of force, especially if you do not expect large boulders or trees. however, it could be a very significant force to deal with. you will need to estimate the depth of the debris which could be higher than your berm or your wall. and it would impart a load due to the saturated weight for the full depth (I would assume your berm is fully saturated and becomes part of the debris flow). if it is moving rapidly, you might also want to consider shear if it is moving past your structure.

See photo of a road fill that became part of the debris flow. This was a catastrophic event, not failure through incremental erosion which then led to failure. Debris flow came from higher up the gulley and blew out this fill.

• http://files.engineering.com/getfile.aspx?folder=a9abeddf-dbdc-46fc-91f7-54

another photo of the results of debris flow. The pipe used to be buried in a trench in a road. The scour caused by the debris flow was at least 10 feet deep and only stopped because it reached a rock layer

• http://files.engineering.com/getfile.aspx?folder=7217e2f3-0ae6-4b2d-90b3-ac

Please pardon the length of this post.
The questions are difficult in that several methods have been used (older & newer) yet all require validation, usually backfiguring from events and the associated failures. The geology, topography, debris source (surface debris, slope failure or rockfall), climate & storm patterns have enormous influences on the catastrophes. What I am going to provide is some guidance from regions of Western Colorado, USA. Much of the following has undergone little change from the mid 1970's. The real changes have been much better mapping and definition of potential events. You MUST Understand your situation.

Debris Flow Hazard means low, moderate and severe debris flow activity, to include mudflows.

• Low debris flow hazard poses possible minor damage to structures and slight risk to life. Mitigation is usually limited to site grading and slight elevation of the building foundation.

• Moderate debris flow hazard poses possible moderate damage to structures and risk to life. The building site location, relative to the Debris Flow Feature determines the mitigation criteria, as described in Hazard Mitigation. Mitigation is recommended and major cleanup is probable.

• Severe debris flow hazard poses possible severe damage to structures and risk to life.. The building site location, relative to the Debris Flow Feature, determines the necessary mitigation criteria, as described in Hazard Mitigation, (3)a. The building site or parcel may include areas which should be avoided for some types of construction or in extreme circumstances, placed in a Hazard Avoidance District.

In the case of moderate to severe debris flow hazard the following criteria is applied:

a. Within six hundred (600) feet of gully mouths, as shown on the geologic hazards mapping, the following performance specifications shall be met unless a report by a qualified geologist and a qualified engineer which provides computations supporting other performance specifications for the specific area in question. Protection walls or constructions may be implemented which are separate from the building walls and meet the appropriate design criteria.

1. Within three hundred (300) feet (90 m) of gully mouths, all buildings shall have the uphill wall designed and constructed to resist a horizontal force of 900/lbs/ft2 (43kPa) to a height of six (6) feet above undisturbed or finished ground level, whichever is higher.

2. Between three hundred (300) and six hundred (600) feet (90-180 m) from gully mouths, all buildings shall have the uphill wall designed and constructed to resist a horizontal force of 400/lbs/ft2 (19kPa), to double the anticipated debris height ( six [6] feet maximum) above the undisturbed or finished ground level, whichever is higher.

3. The uphill wall shall be considered that wall most likely vulnerable, in terms of direction, to debris flow.

Building design which places sleeping quarters on the downhill or less vulnerable side is recommended within hazard mitigation districts with a high Hazard Zone designation.

Hey emmgjld, those values you show there are building code "defaults"? So one might consider them conservative for your situation in CO? That is interesting that you have actual stated forces to use in design in risk situations..... We don't have those in OR.

ORUSA Not Building Code defaults but incorporated into a municipal Development Manual. Yes they are 'conservative' for some topographies & geologies. These values have been effective on the East & North valley sides of this municipality. On the West & South sides of the valleys, these valleys these values are known to greatly exceed previous experience. The higher values are for events which travel at feet/sec, the lower values are for events which travel at feet/minute. The potential of Rock Fall/Rolling is a real & separate issue.

As I noted above ' You MUST Understand your situation'
There are areas in Western Colorado which I would not commit to 'report values', far too little historic information to properly define the events.