Live/rock check dams (NBS)

Category: MODIFYING THE SURFACE WATER REGIME – Surface drainage

Description

Live/Rock check dams are small sediment-storage dams built across steep gullies usually following the contour. These form a strong barrier to slow down the flow, dissipating part of the energy, to stabilize the channel bed, and trap material moving downwards. In the long term, a small step will develop in the floor of the gully. The function of these measures is almost the same of the check dams made out of concrete (see measure 3.6). The choice of material depends on the availability on site and whether or not vegetation is desired as permanent solution (Fay et al., 2012). In fact, live check dams are generally used on gullies to catch debris, and to armour, and reinforce the gully floor, where large woody cuttings are planted across a gully, usually following the contour.

 

Live check dams (also called Brushwood check dams) are preferable when additional reinforcement of the bed of the gully is needed through the roots effect, whereas rock dams are more effective in ditches where the velocity of runoff is expected to be high. In fact, live check dams are affected by degradation of the material in the long term (Previati et al., 2012) and they are not suitable for gullies subject to high rates of small scale sumpling (Howell, 1999). B

 

Advantages:

  • Suitable for a wide range of gully sites and for moderately steep slopes;

  • Generally little or no maintenance is needed (except for replacing failed sections or thinning established vegetation for live check dams);

Disadvantages:

  • Not fully effective for large and very active gullies, which usually require stronger measures;

  • Especially for live check dams the failure of the check dams may result in severe disasters during catastrophic flood events

 

Figure 1. On the left picture farmers building live check dams along the Himalayan slopes (Thomson Reuters Foundation). On the right picture rock check dams in a roadside ditch (USDCM, 2010)

 


Design methods

Live check dams (from Hollow, 1999)

The spacing between check dams depends on the steepness of the gully slope and the profile of the gully floor. Usually the interval between two live check dams is between 3 and 5 meters. Within the check dams, cuttings should be about 30 to 50 mm apart in steep slopes in order to provide an adequate protection, while for gentle slopes a wider gap is acceptable. If a double, offset line is planted, it will give a much stronger check dam.

Once the best location for the live check dam is chosen in terms of maximum stabilization that can be achieved, the holes deep and big enough to insert vertical hardwood cuttings of the largest size available are made. Then the vertical cuttings are inserted by carefully pushing them into the hole and firming the soil around them. They should protrude about 300 mm above the ground surface. Place fascines or long hardwood cuttings on the uphill side of the vertical stakes. Key these horizontal members into the wall of the gully. Backfill around the check dam and compact the soil with foot pressure.

Figure 2. Components of a live check dam (Howell, 1999)

 

Rock check dams

Rock check dams are mainly constructed of riprap and an erosion control geotextile should be used at the bottom or base of the rock check dam to prevent undercutting and to provide a stable foundation and for easier removal (Minnesota Stormwater Manual). In some instances, erosion control geotextile should also be installed on the overflow portion of the dam to prevent erosion. For the construction is always better to start from the lowest check dam. As for live check dams, rock check dams should be placed at regularly spaced intervals along the gullies or ditches. The sides of the check dam must be higher than the centre in order to make sure that the possible water flowing downwards is always directed over the centre of the dam. The maximum height of the check dam at its centre should be 1 m and the outer edges at least 0.2 m higher than the centre (Erosion Control Practices, ARC). The spacing of the check dams depends on the slope of the channel. Some manuals help in the selection of of the spacing between check dams and their heights depending on the slope steepness (Figure 3).

In some cases, live cuttings can be installed inside the pore spaces of the rock check dams in order to create a mixed rock-live check dams, (live sediment trap) (Figure 3).

Figure 3. Standard rock check dam design (Erosion Control Practices, ARC).

 

Period of installation: when using living cuttings and plants, the construction can only be undertaken during the dormant season. Any specific period requested for rock check dams.

Materials: for live check dams: large cuttings (2 meters long and 20-50 mm diameter) made form woody material, pointed planting bars or crowbars to make the holes for planting. For rock check dams: well graded stones consisting in a mixture of rock sizes, mainly rip-rap or river rock; geotextile to be used at the bottom of the rock dam to prevent undercutting.

 


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 4 Most suited to rotational or pseudo-rotational slides; may be useful to reduce toppling hazard in certain conditions.

Use of adequate vegetal species is recommended
Topple 4
Slide 7
Spread 1
Flow 7

Material type

Descriptor Rating Notes
Earth 9 will be Mainly applicable to landsliding involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode.
Debris 9
Rock 2

Depth of movement

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) 5
Deep (8 to 15 m) 2
Very deep (> 15 m) 1

Rate of movement

Descriptor Rating Notes
Moderate to fast 2 Can be carried out only when the rate of movement is extremely slow or at most very slow (maximum 1.5 m/year).
Slow 5
Very slow 7
Extremely slow 8

Ground water conditions

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 7
Low 7
Absent 4

Surface water

Descriptor Rating Notes
Rain 9 Applicable to water courses. Most useful in high energy environments. Unaffected by and ineffectual with respect to rain and snowmelt.
Snowmelt 9
Localized 7
Stream 8
Torrent 8
River 3

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 8 Alternative to the check dams, it only involves the use of natural materials, with very limited impact to the surrounding environment.
Economic suitability (cost) 7 Depending on access conditions and availability of materials, it can be less costly than the check dams, because of the materials involved

References

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