Stabilizing Structures for Gullies and Embankments

The mechanics of gully erosion can be reduced to two main processes [24]: down-cutting and headcutting. Downcutting of the gully bottom leads to gully deepening and widening. Headcutting extends the channel into ungullied headwater areas and increases the stream net and its density by developing tributaries. Thus, effective gully control must stabilize both the channel gradient and the channel headcuts. Where an effective vegetation cover will grow, gradients can be controlled by the establishment of plants without supplemental mechanical measures. If growing conditions do not permit the direct establishment of an effective vegetation cover, engineering measures will be required at the critical locations where the erosion processes take place. The main objective of these structures is to stabilize the gully gradient. Once the gully gradient is stabilized, vegetation can establish and gully banks can stabilize. Because of sediment accumulation in the gully above the structures, the storage capacity of the channels increase, channel gradients decrease, and thus peak flows decrease.

In practice, the control of gully erosion is difficult and expensive. Therefore, the decision whether control structures are justified depends on other objectives, such as the prevention a storage dam downstream from being silted up by sedimentation.

The structures consist in small dams and weirs built across gullies to trap sediment and thereby reduce channel depth and slope. These works can provide temporary or permanent stability and are normally used in association with agronomic treatment of the surrounding land where grasses, trees, and shrubs are well established.

The spacing of the dams can be obtained from the following formula [24]:

where DS = dam spacing (m); HE = dam height, measured from the crest of the spillway to the gully floor (m); and K = constant coefficient that depends on local conditions. The equation is based on the assumption that the gradient of the sediment deposits is (1 — K) tan0, with 0 = slope angle (degree). In Colorado, for example, the estimated values for K are 0.3 for tan 0 < 0.2 and 0.5 for tan 0 > 0.2.

The height not only influences structural spacing but also volume of sediment deposits. The structures are provided with spillways to deal with overtopping during storms. The spillway or notch must be big enough to pass the whole of the flood; otherwise, the water will pass over the whole width of the structure and erode the vulnerable banks on either side of the structure. At the foot of the structure, where the flood water falling over the structure strikes the ground with a destructive force, an adequate protection must be provided to prevent scour and undercutting of the structure.

The danger of scouring and tunneling around the check dam can be minimized by a key in such way that lateral seepage around the end of the dam is prevented. Therefore the route of seepage must be considerably lengthened. The dam stability will be greatly increased by keying the dam into the sides and floor of the gully by digging a trench, usually 0.6 m deep and 6 m wide. Where excessive instability is demonstrated by large amounts of loose materials on the lower part of the channel side slopes or by large cracks and fissures in the bank walls, the depth of the trench should be increased to 1.2 or 1.8 m [24]. The trench is filled with loose rock in such way that no large voids will remain in the key. If available, use a mixture of pebbles with 80% smaller than 14 cm. When log dams are used, the bottom layer should be sunk below ground surface or long posts driven deep into firm soil.

Based on the cost of construction, the type of dams best suited for different depths of the gully are listed in Table 4.13. The dividing line between temporary and permanent structures is quite arbitrary. Many works could have a variable life depending on how they are constructed and maintained, and what pressure of use they had to withstand. As a general rule, low cost and simplicity are often principal objectives. The materials and

Table 4.13. Types of dams for gully stabilization

Dam Type

Recommended Height (m)

Gully Depth (m)

Loose rock

<0.45

<1.2

Single fence

0.45 ^ 0.75

1.2 ^ 1.5

Double fence

0.75 ^ 1.70

1.5 ^ 2.1

Source: Modified from [11].

Source: Modified from [11].

Figure 4.32. Types of small stabilizing structures for gully erosion control. Source: [1].

the design must be chosen ad hoc to suit the individual conditions of each site. Some simple designs are shown in Fig. 4.32.

The shape that gives the best strength/weight ratio is the arch weir. However, in the situation of a rock that runs across the bed of the gully, a straight gravity wall also is indicated. More detailed information on structure designs and design criteria can be found in the literature [1, 23-31].

Stabilization structures are also used for erosion control on steep slopes. A special case is represented by soil bioengineering measures where stabilization is achieved primarily by imbedding and arranging in the ground plants and plant parts [32]. For example, driving live willow stakes into the soil along the contour to a depth of 0.2 m may guarantee stability for short time periods, to allow growing of a denser vegetation cover. Stakes should be placed approximately 1 m apart and form rows with a distance from 10 to 20 m, according to the slope steepness.

Stabilization structures as a component of biotechnical slope protection may be necessary in areas of potential mass movement. A structure placed at the foot of a slope helps to stabilize the slope against mass movement and protects the toe and face against scour and erosion. Selection of a suitable retaining structure entails a variety of choices. Several basic types are available, each with its particular advantages, requirements, and limitations. Selection of a suitable retaining wall will depend upon such considerations as site constraints, availability of materials, appearance of the wall, ease of construction, opportunities for incorporation of vegetation into structure, and cost [28]. Figure 4.33 shows the two basic types: the toe wall and the toe-bench structure. More detailed information on structure designs and design criteria can be found in the literature [1, 28].

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