Description
Debris racks are used to separate coarse debris from fine debris and water of the debris flow to encourage the deposition of the coarser material (VanDine, 1996).
"The function of a debris rack is to essentially create a barrier across the stream channel to trap light and medium floating debris" (Bradley et al., 2005). This control measures may need a storage area adequate to retain quantity of debris expected to be accumulated during any intensive rainfall event. If a large debris storage area is provided upstream of the rack location, the frequency of maintenance can significantly be reduced and added safety is provided against overtopping of the installation during a single storm.
Figure 5 Planar and oblique view of a racks located before a bridge/culvert (VanDine, 1996)
There exist different types of debris racks. Most commons are:
With reticular structure
With vertical and parallel piles
Design methods
Debris racks can be constructed out of various types of material; they are usually built of steel sections. They may be vertical or inclined. Since vertical racks receive the full impact of floating debris and boulders, their structural design should incorporate retaining elements in concrete (Bradley et al., 2005).
If a culvert is present, spacing between vertical members should be between 1/2 and 2/3 the minimum culvert dimension. This spacing permits the lighter debris to pass through the rack and the culvert. Unfortunately, the close spacing of the bars creates a debris trap and increases the maintenance required (Bradley et al., 2005).
Figure 7 Debris rack for light to medium floating debris installed 3,5 meters upstream of culvert
(Bradley et al., 2005)
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 3 | Will be updated soon |
| Topple | 3 | |
| Slide | 2 | |
| Spread | 2 | |
| Flow | 9 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 5 | Will be updated soon |
| Debris | 9 | |
| Rock | 5 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 7 | Will be updated soon |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 8 | |
| Deep (8 to 15 m) | 2 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 7 | Will be updated soon |
| Slow | 4 | |
| Very slow | 2 | |
| Extremely slow | 0 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 7 | Will be updated soon |
| High | 9 | |
| Low | 5 | |
| Absent | 3 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Will be updated soon |
| Snowmelt | 6 | |
| Localized | 5 | |
| Stream | 8 | |
| Torrent | 8 | |
| River | 7 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 5 | Will be updated soon |
| Feasibility and Manageability | 4 | Will be updated soon |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Will be updated soon |
| Environmental suitability | 6 | will be updated |
| Economic suitability (cost) | 6 | Will be updated soon |
References
- Debris Flow Control Structures for Forest Engineering - D.F. VanDine 1- Res. Br., BC Min. For., Victoria, BC, Work. Pap, 8, 1996.
- Debris Control Structures Evaluation and Countermeasures Third Edition - Publication No. FHWA-IF-04-016 October 2005 - Hydraulic Engineering Circular No. 9 - J.B. Bradley, D.L. Richards, C.D. Bahner
- Sistemazioni idraulico forestali - Benini G., ed.UTET
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