Structural Soil and Water Conservation Measures

Introduction

Degraded areas, denuded gullies, and steep slopes contribute greatly to the sedimentation and siltation of stream channels. Huge amount of soil particles that are washed away settle in farm areas in the lowlands through surface runoff thereby destroying agricultural crops. Some are discharged to the coastal areas which create pollution and damage to the ecosystem. Most of the fish resources die due to suffocation. Coral reefs and sea grasses are rendered useless and spawning grounds are damaged. This condition generally results to the decrease in the production of agricultural farms and coastal zones.

To address this problem, rehabilitation measures should be undertaken to provide immediate ecological stability of the areas. Structural rehabilitation measures such as checkdams, riprap or stone walls, bench terracing, and gabions are considered very effective as rehabilitation measures.

Targets for rehabilitation using these measures are those areas exhibiting advanced soil erosion and degradation. Other areas are those that are critically and biophysically degraded sites which adversely affect population centers in terms of flood occurrence. Specifically on very steep slopes, solid structures as mentioned above may provide the immediate protection and desired ecological stability.

a. Loose rock or stone checkdam
b. Pole or log checkdam
c. Brushwood checkdam
d. Riprap or drystone walls
e. Gabions
f. Riprap interplanted with cuttings
g. Slope stabilization with worn-out tires

Loose Rock or Stone Checkdam

Rocks or stones are the primary materials used in the construction of the dam. Cement and gravel may be applied to fill-in the gaps or crevices between the rocks or stones.

Stone checkdams are commonly used in gully control and are generally constructed as dry stone wall (Figure 1). In constructing the checkdam, the slope gradient of the gully banks is first reduced to 45 degrees; then a foundation of 30-50 cm deep is dug. The soil is dug. The soil is piled upstream to be used later for the refill. The largest stones should be placed at the bottom layer.

On top or of the stone checkdam is the spillway or the overflow section, which must be wide enough to cope with peak runoff. Generally, a length of half the span of the dam and a depth of 20-30 cm is appropriate. Large flat stones that could not be easily washed away shall be reserved to construct the notch.

When large quantities of runoff are expected, it is advisable to use concrete for the notch at the crown of the dam, or cover everything with a wire netting. Also, an apron has to be constructed with stones below the dam for added support. On the upstream side, the dam has to get an earth fill for greater strength. Finally, the structure can be supplemented by planting seedlings and cuttings of suitable species with dense and widespread root systems like banaba, tibig, or creepers like kudzu, kikuyo or centrosema along the banks.

Pole or Log Checkdam

Where other materials are scarce, checkdams may be constructed using poles or strong branches. The posts are driven into the ground upright or in vertical position across the gully in rows to keep the horizontal poles in place. The middle or center posts are driven down to notch at spillway levels, while the others reach the height of the checkdam. Some of the poles are placed lengthwise below to form the apron supporting the dam (Figure 2).

The horizontal logs/poles are then nailed or tied to the upright/vertical poles with galvanized wire. The filling materials to be used are soils, stones and boulders. A structure of this type may last for several years. If bamboos are used as posts, it is recommended that the sprouting ones with developing root systems be used to finally control the expansion of the gully.

Brushwood checkdams

The primary materials used are brushwood, posts, or pegs and the filling materials are soils and stones. This can be used on small gully heads for stabilization. Posts or pegs to be used could be sprout-producing tree species available in the area.

First, a foundation extending into the banks is dug. The brushwood is then placed between two rows of pegs driven in 40 cm apart across the gully bed. The distance between the rows is from 80-100 cm for gullies with about 5 m in width. The brushwood is then packed firmly and the two rows of pegs are tied together with wire. A notch or spillway of about half of the span and the top of the dam is then spared.

On the lower side of the dam; brushwood is placed lengthwise to provide an apron that will prevent scouring by overflow (Figure 3). It is very important that these temporary structures are supplemented by cuttings and seedlings, which will replace the brushwoods when decayed.

Riprap or Drystone walls

Sometimes called stone terraces. This structure is usually used for the stabilization of road-cuts, riverbanks, or slides by piling up stones or rocks of more or less uniform sizes and form. Sometimes, cement mortar is applied to make the stones/rocks stick together to provide more strength and stability.

Similarly, riprap is common and a traditional technique to stabilize terraces and steep slopes which are not too high. Sometimes, it serves more as a revetment than as a retaining wall. A special drainage is not required, since riprap walls are self-draining.

In its construction, the points could be broken like in ordinary masonry such that it serves as drainage or outlet of water. The largest stones are reserved for the foundation at the lower portion of the wall. It is important that each layer of stones are set, soil is filled behind and firmly compacted to minimize sudden saturation of rainwater. Otherwise, the wall would collapse after a heavy rainfall.

Gabions

Gabion is the name given for large rectangular wire crates that are filled with stones and used as an erosion control measure. This was developed in Italy. This is similar in shape with loose rock checkdam structure., only the pile rocks are enclosed with wire mesh to reinforce the structure (Figure 4).

The flexibility of the wire mesh is sufficient to permit adjustment in the structural slope. Similarly, the wire mesh should be resistant to erosion and sufficiently strong to withstand pressure exerted by the structure. Common mesh wires used are gauge No. 12 or No. 16 for smaller gabions.

Gabions are becoming popular in the country for slope stabilization purposes. It can be used as retaining walls along road shoulders, and riverbanks, in eroded areas, and in degraded or gullied areas for stabilization. For this purpose, the gabion elements can be combined in various ways.

It is possible to stabilize the base of a slope with only one row of gabion, or to arrange them in terrace formation for higher slope stabilization. When used in the former, the gabion should be slightly inclined towards the slope. For structures with height of two or more decks, the gabions must be solidly connected by wire.

Some significant advantages of gabions are: they are flexible because they can bend without breaking or cracking; these structures are permeable and do not need extra drainage system; and these are cheaper than other engineering structures when stones are available in sufficient quantity.

Riprap interplanted with cuttings

Riprap retaining walls and revetments can be reinforced by interplanting suitable plant species in the joints or crevices between stones. The length of the cuttings usually depend on the thickness of the riprap (Figure 5).

The cuttings must be planted deep enough to avoid drying up. Only 2-3 cm of the cuttings should be exposed outside the riprap walls. In the stabilization of riverbanks, grasses can be planted in between the joints or crevices.

Slope stabilization with worn-out tires

For environmental considerations, the disposal of worn-out tires is a common problem. Using old tires for slope stabilization may be a good way of disposal, while helping in soil and water conservation without impairing the environment and violating aesthetic values too much. Seedlings or cuttings can be planted inside or in the space between tires (Figure 6).

Worn-out tires can be used effectively for retaining walls. They are constructed such that they are piled one over the other in a stairway form along the slope.

Another way is to cover the slope with tires parallel to the surface. Afterwhich, seedlings or cuttings are planted inside the tires. Another line of tires at the base are set vertically to prevent the tires from sliding downhill. For additional strength, the tires can be connected by wires or fastened with pegs.

Source: DENR-ERDS-CAR, 1994. Technology Transfer-Tips, Vol. 1(2).