Description
Two types of hydraulic control works are normally used: stream channel linings and check dams (Figure 1).
Channel linings are effective both in maintaining channel alignement and in reducing the frequency and volume of debris flows. They are most effective if applied over the entire reach of an unstable channel.
Channel linings are usually made by masonry or stone pitching with high-quality concrete, preferably reinforced by steel fiber to resist abrasion; protruding boulder are set in the concrete to dissipate the energy of waterflow. Where required, boulders may be tied together by C chaped steel bars drilled and grourted ito adjacent boulders.
Check dams are small sediment-storage dams built across the channels of steep gullies to slow down the flow, dissipating part of the energy, to stabilize the channel bed, thus preventing or mitigating landslides caused by basal erosion. They are also used to control the frequency and volume of channelized debris-flows and/or to control ravelling and shallow slides in the source area of debris-slides. Channelized debris flows are associated with channel gradients over 25° and obtain most of their volume by scouring the channel bed.
Check dams serve three purposes when installed in the channels (Chatwin et al., 1994):
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To mitigate the incidence of failure by reducing the channel gradient in the upper channel;
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To reduce the volume of channel-stored material by preventing down cutting of the channel with subsequent gully sidewall destabilization and by providing toe support to the gully slopes;
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To store debris-flow sediment when installed in the lower part of the channel.
Check dams can be constructed of timber cribs (Figure 2) or concrete cribs, concrete mortared rock or plain or stone faced reinforced concrete (Figure 3). Concrete mortared rock dams do not usually exceeed 8 m in height, whereas concrete or timber crib dams do not exceed 2 m.
(photo: G. Vaciago, SGI-MI)
(photo: G. Vaciago, SGI-MI)
The spacing of check dams along the channel depends on the natural and infill gradient of the channel infill and the dam height; as an example, a 2 m high dam in a 20° degree channel with 10° sloping channel infill will be spaced every 12 m. (Highlands and Bobrowsky, 2008).
Reference may be made to Popescu M.E., Sasahara K. (2009) for further discussion and examples of check dams for the mitigation of debris flows.
Channel linings are usually less expensive than check dams, especially if a long reach is to be stabilized; check dams are preferable, however, if the banks are very unstable because a dam can be keyed into the bank, providing toe support, enhancing stability. Check dams are expensive to contruct and therefore are usually built only where necessary to protect vulnerable elements downstream.
Design methods
Channel linings need to be designed to have adequate stability against disturbance by the current; current velocity and bank slope angle govern the minimum and median block size in rip rap and stone linings. The local stability of the lining will also need to be verified with respect to static equilibrium under various groundwater conditions. Where concrete slabs or equivalent systems are used, special care will need to be paid to relieving water pressures at the contact with the underlying soil, especially where the lining obstructs free drainage towards the channel.
Lateral stream erosion and scour by spillway water are the main drawbacks of check dams. To prevent check dam failure the following recommendations apply:
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During construction the wingwalls must be tied into the gully walls and the streambed to withstand backfill pressures and lateral scour; wingwalls should slope about 70% and extend a minimum of 1÷2 m into the banks;
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The foundation of the dam should have a minimum width of 1/3 the total height of the dam and be deeper than any scour holes likely to develop;
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Downstream aprons (Figure 3b) or stilling basins should be provided, where feasible;
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The dynamic equilibrium of the whole reach should be considered, remembering that sediment accumulated by check dams tends to be replaced by increased streambed erosion downstream.
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Backfilling the dam, rather than allowing it to fill naturally, reduces the dynamic loading on the structure and results in a more stable design. The slope of the backfill should be less than 1/2 the channel gradient.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Most suited to rotational or pseudo-rotational slides; may be useful to reduce toppling hazard in certain conditions. |
| Topple | 0 | |
| Slide | 8 | |
| Spread | 0 | |
| Flow | 8 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 7 | Mainly applicable to landsliding involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode. |
| Debris | 7 | |
| Rock | 0 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 8 | Typically applicable to relatively small and/or shallow landslides. The implications of large scale filling and procurement typically make this technique impractical for deep and very deep slides. On the other hand, it may be the only suitable technique in very large landslides, besides drainage. |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 8 | |
| Deep (8 to 15 m) | 5 | |
| Very deep (> 15 m) | 3 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Can be carried out only when the rate of movement is extremely slow or at most very slow (maximum 1.5 m/year). |
| Slow | 1 | |
| Very slow | 5 | |
| Extremely slow | 7 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 5 | Applicable in all groundwater conditions. Adequate drainage must be provided at the back of impervious linings, especially where artesian or high ground water levels exist. |
| High | 0 | |
| Low | 5 | |
| Absent | 8 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 5 | Applicable to water courses. Most useful in high energy environments. Unaffected by and ineffectual with respect to rain and snowmelt. |
| Snowmelt | 5 | |
| Localized | 8 | |
| Stream | 7 | |
| Torrent | 10 | |
| River | 8 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the technique depends on the reliability of the evaluation of the demand in terms of hydraulic and/or debris flows. |
| Feasibility and Manageability | 8 | Well established technique. Potential benefits and limits of applicability are well understood. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | May be complex in permanent water courses. Requires heavy construction equipment which may have access restrictions. |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 5 | Moderate to high, depending on access conditions and availability of materials. |
References
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Chatwin S.C., Howes D.E., Schwab J.W., Swanston D.N. (1994). “A guide for menagement of landslide-prone terrain in the Pacific Northwest” 2nd edition, Ministry of Forests, British Columbia, 200 p. http://www.for.gov.bc.ca/HFD/Pubs/Docs/Lmh/Lmh18.htm
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Highland L.M., Bobrowski P. (2008). “The Landslide Handbook – A Guide to Understanding Landslides”. Circular 1325, U.S. Geological Survey.
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Popescu M.E., Sasahara K. (2009). “Engineering measures for Landslide disaster mitigation”. In: Landslide Risk reduction”, K. Sassa, P.Canuti (eds.), Spinger-Verlag, Berlin, 609-631.