Description
Check dams control measures are used to mitigate the hazard in areas prone to channelized debris flows. They consist in small barriers made of rock, gravel bags, sandbags, fiber rolls, or other proprietary products, placed across a constructed swale or drainage ditch. Check dams are usually constructed in series inside the channel of steep gullies, where the channel gradient is over 25 degrees.
In fact, their purpose is to reduce steep channel gradients locally and to diminish scour process along bed channel and both sides of the stream by reducing flow velocity, because most of the volume of a channelized debris flow is obtained by scouring the channel bed; for this reason, it represents an important point on which should act.
The construction of check dam is very costly, so this type of control measure is exploited only in particular cases such as wildlife habitat or spawning area to protect (Highland et al., 2008).
Check dams serve three purposes when installed in the channels:
- to mitigate the incidence of failure by reducing the gradient in the upper part of the channel
- to reduce the volume of channelstored material by preventing down cutting of the channel with subsequent gully sidewall destabilization and by providing toe support to the gully slopes.
- to store debrisflow sediment, when installed in the lower part of the channel
(Highland et al., 2008).
Figure 1 Check dams in series in a very steep slope
Figure 2 Sketch of plan and lateral views of check dams in series (VanDine, 1996)
Design methods
The most important features to consider for check dams are the likely flow path and the maximum discharge of the channelized debris flow. They are usually designed to withstand dynamic and point impact forces, sliding, overturning, uplift pressures, and foundation and abutment loadings.
Usually, the deposited materials behind the structure are not removed but it is the water flow that tends to do it (Ikeya 1976; Thurber Consultants 1984).
In Austria, Japan, and Switzerland check dams are commonly less than 5 m in height, but can extend up to 15 m. The weirs in the structures are planned to allow the passage of both flood water discharge and channelized debris flow discharge and the same for drainage holes or galleries that are incorporated into the check dam to allow drainage of water inside the debris flow.
Figure 3 - Scheme of check dam structure (Highland et al, 2008)
Check dams can be constructed of reinforced concrete with a maximum height of 8 meters or log crib with a maximum height of 2 m. The spacing of dams depends on channel gradient and dam height. For example, a 2 meters high dam in a 20 degrees channel with 10 degree sloping channel infill will be spaced every 12 m (The Landslide Handbook).
Rock check dams are usually constructed of 8 to 12 inches rock. Log check dams are usually constructed of 4 to 6 inches diameter logs, installed vertically. The logs should be embedded into the soil at least 18 inches. Logs can be bolted or wired to vertical support logs that have been driven or buried into the soil. Gravel bag and sand bag check dams are constructed by stacking bags across the ditch or swale. (California Stormwater BMP Handbook Construction, 2009)
Cost consists of labor costs if materials are readily available (such as gravel on-site). If material must be imported, costs will increase. (California Stormwater BMP Handbook Construction, 2009)
They require extensive maintenance following high velocity flows. In particular, it is recommended that at a minimum, check dams be inspected weekly, prior to forecasted rain events, daily during extended rain events, and after the conclusion of rain events. Replace missing rock, bags, rolls, etc. Replace bags or rolls that have degraded or have become damaged. (California Stormwater BMP Handbook Construction, 2009)
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 2 | Will be updated soon |
| Topple | 3 | |
| Slide | 2 | |
| Spread | 6 | |
| Flow | 4 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Will be updated soon |
| Debris | 9 | |
| Rock | 1 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Will be updated soon |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 4 | |
| Deep (8 to 15 m) | 1 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 8 | Will be updated soon |
| Slow | 6 | |
| Very slow | 3 | |
| Extremely slow | 3 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 8 | Will be updated soon |
| High | 8 | |
| Low | 4 | |
| Absent | 4 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Will be updated soon |
| Snowmelt | 6 | |
| Localized | 6 | |
| Stream | 7 | |
| Torrent | 7 | |
| River | 4 | |
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 | 5 | Will be updated soon |
| Environmental suitability | 5 | will be updated |
| Economic suitability (cost) | 5 | Will be updated soon |
References
- The Landslide Handbook— A Guide to Understanding Landslides – Lynn M. Highland, Peter Bobrowsky. 2008
- Debris Flow Control Structures for Forest Engineering - D.F. VanDine 1- Res. Br., BC Min. For., Victoria, BC, Work. Pap, 8, 1996.
- Ikeya, H. 1976. Introduction to sabo works: the preservation of land against sediment disaster. The Japan Sabo Assoc., Toyko. 168 p.
- Site visit to debris torrent facilities at Charles Creek, Harvey Creek, Magnesia Creek, Alberta Creek, Lions Bay, B.C., Field guide. Thurber Consultants Ltd. and Ker Priestman & Associates Ltd. o date
- California Stormwater BMP Handbook Construction. 2009
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