Description
Deflection walls or berms are also called "debris flow direction controlling works" in Japan. They are similar to lateral berms, with the only difference that they deviate the flow path to another area of the debris fan avoiding to go straight. These works are used to protect a structure at the base of the slope changing the direction of the flow; in California, deflection walls are used to decrease the angle of impact on a structure (Hollingsworth and Kovacs 1981).
Deflection berm plan view and lateral view (VanDine, 1996)
Usually these structures are built with concrete, local materials or can be a composite of more materials. There exist also some example of earth deflection berms constructed in British Columbia (VanDine, 1996).
Deflecting structures, Austria (from mearsandwilbur.com)
Earthen splitting wedge, Austria (from mearsandwilbur.com)
Design methods
In order to ensure the effectiveness of these structure, the location plays a substantial role; they have to be placed high on alluvial fan. In this way, the deflection structure allows to protect a bigger area and decrease the probability of avulsion and bypassing the structure. In addition, on top of alluvial fan the flow velocity is expected to be higher and thus the measure is able to drive the debris downslope avoiding deposition within the works.
With regards to the size of the deflection walls, some instructions are recommended by FEMA; for the height the following formula is proposed:
hB=h+ ∆h+0.9m
where hB is the height of debris-flow deflection berm, h is the depth of flow, ∆
h is the super elevation or run and 0.9m is the debris-flow freeboard recommended.
Moreover, concerning the cross section, the top of the berm should be at least 3 m wide considering also maintenance and cleanout access (Sherard et al., 1963). A narrower top width may be used if maintenance and cleanout access is provided from behind the berm rather than from on top of it, but narrower widths will make placement and compaction of the fill near the berm crest more difficult (Fell et al., 2005).
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 4 | Will be updated soon |
| Topple | 4 | |
| Slide | 3 | |
| Spread | 3 | |
| Flow | 9 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 9 | Will be updated soon |
| Debris | 9 | |
| Rock | 6 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Will be updated soon |
| Shallow (0.5 to 3 m) | 9 | |
| Medium (3 to 8 m) | 4 | |
| Deep (8 to 15 m) | 2 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 9 | Indicated for rapid movement. |
| Slow | 2 | |
| Very slow | 0 | |
| Extremely slow | 0 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 7 | Will be updated soon |
| High | 7 | |
| Low | 7 | |
| Absent | 7 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 9 | Will be updated soon |
| Snowmelt | 7 | |
| Localized | 7 | |
| Stream | 7 | |
| Torrent | 7 | |
| River | 3 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 6 | Will be updated soon |
| Feasibility and Manageability | 8 | It must be located high on the alluvial fan where the slope is still steep. On one hand, it means no accumulation of material close to the structure. On the other hand, reaching the site could be not easy. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Will be updated soon |
| Environmental suitability | 6 | In some cases, the structure can be covered with vetegation. |
| Economic suitability (cost) | 5 | Will be updated soon |
References
- Debris Basin and Deflection Berm Design for Fire-Related Debris-Flow Mitigation – A. B. Prochaska, P. M. Santi, J. D. Higgins Environmental & Engineering Geoscience, Vol. XIV, No. 4, November 2008, pp. 297–313
- Debris Flow Control Structures for Forest Engineering - D.F. VanDine 1- Res. Br., BC Min. For., Victoria, BC, Work. Pap, 8, 1996.
- A framework for landslide risk assessment and management - R. Fell, K.K.S. Ho, S. Lacasse, E. Leroi. 2005
- Earth and Earth-Rock Dams - Sherard, J. L., R. J. Woodward, S. F. Gizienski, and W. A. Clevenger. 1963
- Engineering Principles and Practices for Retrofitting Flood Prone Residential Buildings - FEMA 259, Federal Emergency Management Agency. 1995
- Soil Slumps and Debris Flows: Prediction and Protection – R. Hollingsworth, G. S. Kovacs - Environmental and Engineering Geoscience (1981) xviii (1): 17-28.
- Destructive mass movements in high mountains: hazard and management – G. H. Eisbacher, J.J. Clague. Geological Survey of Canada Paper 84-16, Ottawa. 1984
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