Description
Debris retention basins can be also called "catch areas" or "catchment areas" and they are engineered ditches that are designed to stop and capture falling material avoiding the impact on elements at risk. These catchment basins are commonly built at the base of slopes where debris flows are frequent. The use of catchment areas to mitigate rockfall hazards is one of the best and most effective rockfall protective measures (Pierson et al, 2001).
The retention basin is usually combined with other type of passive control works such as dissipation structure or barrier system placed forming a cascade, during which the energy of the flow is dissipated and at the end the debris material starts to deposit; periodically, this material must be removed. If large boulders are present, dump trucks or power shovels are used to carry the debris away. Especially where the space availability is limited, these additional works are often used.
Figure 1 Aerial photograph of a debris-flow basin in California, USA. (Photograph by Doug Morton, U.S. Geological Survey.)
Advantages
- Able to contain and stop very large quantity of debris
- Easy to construct from point of view of the access to the building site (downhill on the alluvial fan)
Disadvantages
- Periodical removal of debris accumulated behind the structure
- Need a very large area for the construction
Design methods
The main design features for catch areas are:
- Location: usually located at the base of a slope, however may be located in upslope or midslope areas depending on the nature of the source area, the slope geometry and where the at-risk facility is located on the slope. The location should consider access for debris removal.
- Width: should allow for the initial impact and subsequent rollout of the rock. The height and geometry of the slope above the ditch affect the width.
- Shape: commonly has a gentle backslope and a depth to minimise potential for rocks to roll out of a ditch.
- Substrate: a soft, loose material such as uncompacted sand or pea gravel may be used to help attenuate the energy of falling rock, noting that choice of substrate may be influenced by access for maintenance.
(from NZGS, 2016)
Figure 3- A: principle of debris flow breaker with retention basin; B: principle of overfall barriers with retention basin
(Hübl et al, 2008)
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 5 | will be updated |
| Topple | 5 | |
| Slide | 4 | |
| Spread | 3 | |
| Flow | 10 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | will be updated |
| Debris | 10 | |
| Rock | 5 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 8 | will be updated |
| Shallow (0.5 to 3 m) | 9 | |
| Medium (3 to 8 m) | 5 | |
| Deep (8 to 15 m) | 3 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 8 | will be updated |
| Slow | 6 | |
| Very slow | 4 | |
| Extremely slow | 3 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 9 | will be updated |
| High | 9 | |
| Low | 8 | |
| Absent | 8 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | will be updated |
| Snowmelt | 8 | |
| Localized | 8 | |
| Stream | 4 | |
| Torrent | 5 | |
| River | 3 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 10 | will be updated |
| Feasibility and Manageability | 4 | will be updated |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 2 | will be updated |
| Environmental suitability | 2 | will be updated |
| Economic suitability (cost) | 2 | will be updated |
References
- NZGS (2016), "Rockfall: Design considerations for passive protection structure" (https://www.building.govt.nz/assets/Uploads/building-code-compliance/b-stability/b1-structure/rockfall-design-consideration/rockfall-design-passive-protection-structures.pdf)
- Rockfall catchment area Design guide, final report - Pierson L., C. Gullixson, R. Charrie. 2001.
- Debris flow mitigation measures in Austria - J. Hübl, J. Suda. 2008
- The Landslide Handbook— A Guide to Understanding Landslides - Appendix C, Introduction to Landslide Stabilization and Mitigation, Lynn M. Highland, Peter Bobrowsky
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