Description
This measure is used in gullies to control water flow and to prevent the intiation of debris torrenting. Usually live cutting are placed high in the channel to control the initiation of torrents rather than attempting to control the torrent once it gets moving. The aim of these breaks is to reduce the flow velocities and provide control of water movement, as well as to entrap soil sediments (Polster, 2003).
Additionally, they can be helpful in the revegetation and stabilization of debris scars that are already actively eroding or that are altered from previous erosion phenomena. In this case loose materials may be entrapped along the breaks facilitating the vegetation establishment (Osman, 2018). The cuttings forming the live break will continue to grow as sediment accumulates, optimally creating strong attachments to the substrate. As consequence, their effectiveness is strengthen with age, once the developed root system will help to hold soils in place and prevent recurrent events (Stokes et al., 2014). Live gully blocks act by creating numerous small structures in the ditches or high on the slopes rather than creating massive engineered structures to trap debris down below (Pacas, 1999).
Figure 1. Live gully breaks act to slow the velocity of water movement down a gully and thus to control erosion and trap sediments (http://www.dfo-mpo.gc.ca/Library/277708.pdf).
Advantages:
- Preventing recurrent debris flows/torrents events and slowing potential water flow;
- Helpful in the revegetation and stabilization of gullies (scars) that already experienced debris torrents by providing sites where vegetation can become established;
- The roots from the cuttings used in the live gullies breaks will provide substantial reinforcement of the soil in the longterm period;
Disadvantages:
- If not well designed the water can flow out from the lateral parts (the height of the center should be lower than the wings);
Design methods
Installation methods
Live gully breaks are made of stakes and cuttings forming a wattle fences in the gully. Spacing of live gully breaks depends on the steepness of the channel but ranges from 5 to 10 m between the structures. First the stakes are driven into the gully to form a crescent on the contour, with the outers (wings) higher than the stakes into the centreline of the gully. Then, cuttings are placed behind the stakes for future vegetation establishment. In narrow gullies (on the right in Fig.2) the cuttings are crossed at the back of the gully with the tips higher than the center, while in wider gullies (on the left in Fig. 2 and Fig.1) the structure is more like a "U" shaped wattle fence. The live gully break may be backfilled with local materials to create a small terrace in the gully that will trap additional sediments. This structure of the live gully break is useful to provide the initial protection to the gully while the cuttings and their growth will provide a double effect in the long-term on the soil substrate reinforcement and the entrapment of sediments (Pacas, 1999).
Period of installation: not specified
Materials: live cuttings capable of aventious root growth (e.g. willows, birch), local material, rebar stakes to be driven into the soil (the height of the stakes should be comparable to the gully depth).
Figure 2. On the left: typical live gully break structures along a wide gully. On the rigth: live gully break structure in narrow gully (Polster, 2003).
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 4 | Adapted to entrap debris flowing or sliding down along gullies or scars. It can be realized along erosion gullies that can furthermore experiment erosion. |
| Topple | 2 | |
| Slide | 6 | |
| Spread | 3 | |
| Flow | 8 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 9 | Most suitable for earth soil and for entrapping debris sediments. |
| Debris | 8 | |
| Rock | 2 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | This measure is used when already existing erosion phenomena occurred, to prevent additional surficial sediment transportation along the gully. Not suitable for high depths of movement. |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 2 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 3 | Adequate for contrasting small volumes of extremely slow or very slow moving soil. |
| Slow | 6 | |
| Very slow | 8 | |
| Extremely slow | 8 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 3 | Applicable irrespective of groundwater conditions. Use of adequate vegetal species is recommended. |
| High | 4 | |
| Low | 4 | |
| Absent | 3 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 9 | Applicable to gullies or water courses. Mainly used to control water flow and to prevent the initiation of debris torrenting. It can also prevent debris to be washed away from rainwater erosion. Not really affected by snowmelt. |
| Snowmelt | 7 | |
| Localized | 7 | |
| Stream | 8 | |
| Torrent | 6 | |
| River | 6 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | A well designed and implemented measure can be reliable permanently in preventing recurrent debris torrenting. |
| Feasibility and Manageability | 8 | Easily applicable to both narrow and much larger gullies (scars), difficult to reach the zones high up in the gully. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | The measure is immediately effective in controlling debris torrenting. Depending on the length of the gully to be stabilized it does not require long time for the implementation. However, the effectiveness of the measurement is strengthen with age, once the developed root system will help to hold soils in place and prevent recurrent events. |
| Environmental suitability | 10 | It involves only live cuttings (indigenous plants are preferred), suitable with the surrounding environment. |
| Economic suitability (cost) | 10 | Generally not expensive, it does not require extra equipment for the installation. |
References
- Osman, K. T. (2018). Management of Soil Problems: An Introduction. In Management of Soil Problems (pp. 1-14). Springer, Cham.
- Pacas, C. (1999). Restoration of the Vermilion Wetlands. In Fourth Annual Roads, Rails and Environment Workshop:“Impacts and Solutions for Aquatic Ecosystems” (p. 28).
- Polster, D. F. (2003). Soil bioengineering for slope stabilization and site restoration. Mining and the Environment III, 25-28.
- Stokes, A., Douglas, G. B., Fourcaud, T., Giadrossich, F., Gillies, C., Hubble, T., ... & Mickovski, S. B. (2014). Ecological mitigation of hillslope instability: ten key issues facing researchers and practitioners. Plant and Soil, 377(1-2), 1-23.
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