(introductory text...)

Erosion-control measures are designed to reduce runoff rate and its velocity. Runoff management involves safe disposal of excess runoff at low velocities, energy dissipation through drop structures, runoff storage in reservoirs to facilitate sedimentation, etc. Erosion-control measures are used to complement erosion-preventive measures, and should not be installed if erosion can be curtailed adequately by preventive techniques. However, preventive measures based on soil and crop management practices are generally inadequate on steep lands, highly erodible soils, or in regions characterized by intense rains and heavy downpours. In such situations, engineering structures are necessary as backstop measures to cope with the safe disposal of excess runoff. A major objective of all engineering structures is to reduce the shear strength of the overland flow by reducing its amount and velocity, and, therefore, its cutting and carrying capacity. Engineering structures for runoff management are permanent structures, require engineering skills to design and install, and are expensive to establish and maintain. The benefits and returns from engineering structures are observed after a long time. Furthermore, the installation of engineering structures requires additional land, such as that needed for the construction of terraces, drop structures, reservoir construction, etc. Engineering structures require very careful planning, meticulous installation, and regular maintenance. The failure of engineering structures can lead to deep gullying and mass movement. Damage to soil productivity caused by faulty structures, or those inadequately maintained, can be very severe and often expensive, if not impossible, to repair.

There is a wide range of earthworks and mechanical structures (Fig. 15). These structures can be grouped into three categories.

1. Structures to prevent run-on

Run-on prevention from adjacent land is achieved through the installation of a "storm water diversion drain", or "diversion ditch", or "channel" These channels are designed to divert runoff originating off the farmland.

Fig. 15 Erosion control measures based on engineering structures

2. Structures to reduce runoff velocity

Commonly used engineering devices to reduce runoff velocity include terraces and waterways.

(i) Terraces: Long slopes are broken with earthworks installed at right angles to the steepest slope to intercept the surface runoff (Plate 34). An earthwork consists primarily of two parts: an excavated channel, and a bank or ridge on the downhill side formed with the spoil from the excavation. There are different types of terraces based on the design and shape of the channel and the ridge. A terrace with a channel upsweep is called a channel terrace, and the one with a gentle grade in the channel is called a graded channel terrace. A terrace constructed strictly on the contour is called an absorption terrace. The latter may be open-ended or close-ended, depending on whether the terrace is diked at the ends or not. In addition to using earth-moving equipment's terraces can also be constructed by natural sedimentation facilitated by vegetative hedges, and agroforestry systems. Agroforestry systems are discussed in another section.

A lot of research information is available indicating that properly constructed terraces can drastically reduce runoff and soil erosion. The data in Table 24 compare the effects of terracing on runoff and soil erosion on Alfisols in Nigeria. The data show that although graded channels did not decrease the total runoff amount, soil loss was reduced drastically. Another experiment conducted at Ibadan showed that with adequate spacing and regular maintenance, terraces were effective in decreasing runoff and soil erosion. The mean runoff and erosion from an untreated watershed were about 3 and 16 times more than those from the terraced field (Lal, 1982). The data in Table 25 from Sierra Leone show that bench terraces effectively reduced soil loss compared with bundling and no control measures. Similarly, the data in Table 26 from Burundi show that soil erosion from terraced cassava decreased by a factor of 6 to 16 compared with that from unterraced cassava.

Table 24 Runoff and soil loss from terraced and unterraced catchments at Ibadan, Nigeria, from a single rainstorm received on 6 July 1981

Catchement	Runoff (mm)	Soil erosion (Mg/ha)
Terraced	18.1	0.7
Unterraced	18.8	2.3

(Lal, 1982)

Table 25 Comparison of soil erosion losses from various conservation techniques in Sierra Leone

(Mg/ha)
Terraces	Soil loss
	Rice	Cassava
Bench terraces	7.5
Stone bunding	29 5	4.4
Stick bunding	27.3	27.3
Contour bundling	18.0	16.8
No conservation	40.7 54.5	11.2 55.1

(Millington. 1982)

Table 26 Effect of terracing on soil erosion from cassava plantation steep lands in Burundi

Treatment	Slope (%)	Soil erosion (Mg/ha/yr)
		1981-82	1983- 84
Forest	45-50	0	0
Cassava (terraced)	49	5 2	11.1
Cassava (unterraced)	49	87.4	71.6
Bare plowed	40	441.4	428.2

(Durand 1984)

(ii) Waterways: Excess runoff from channel terraces is discharged into artificial or natural waterways (Plate 35). These waterways are called grassed waterways, sod waterways, or meadow strips, depending upon the nature of the protective material planted to create the desired roughness. These waterways are constructed along the slope and have protective embankments on both sides to contain the runoff. The capacity of waterways is designed on the basis of expected runoff rate and amount.

3. Structures to dissipate runoff energy

Drop structures and other engineering devices are installed to dissipate the energy of concentrated runoff, flood water, or flows of long duration. Some commonly used structures include:

(i) Concrete Structures

There are several permanent structures usually made from concrete. These include:

Drop Structures: These are energy-dissipating devices (Plate 36), and are designed with the following objectives: (i) to stabilize steep waterways and other channels; (ii) to level waterways so that they need not be planted with grass; (iii) to serve as outlets for concentrated flow from sods and drain bed culverts; and (iv) to serve as sediment traps.

Drop structures are generally effective when installed as a pair. The level of toe wall (outlet) of the upstream structure should be the same as the level of the inlet or spillway of the downstream structure so that the flow between the two will be gentle or slow enough to cause sedimentation. Drop structures can fail if the soils have a high swell-shrink capacity and develop large cracks (e.g., Vertisols). Drop structures can also fail if the anchoring wall and toe wall crumble from water washing away the soil, undercutting, and scurrying.

Chutes: These are specially designed spillways that collect flow at one elevation and discharge it down a slope at a lower elevation. The shape of the inlet and outlet is important to stabilize the chutes and prevent failure. Some energy-dissipating structures are necessary at the outlet of the chutes. A box-like device filled with stone is generally suitable for most conditions.

(ii) Porous Barriers

Stream bank erosion is a common problem, especially during periods of high flow. Although the total surface area affected by stream bank erosion is small, land eroded by stream bank erosion is completely and irretrievably lost. Such erosion can also threaten other civil structures (e.g., roads, railway lines, bridges). The risk of stream bank erosion is accentuated if flood plains, because of their usual high fertility and productivity, are also cultivated or grazed.

Two strategies can curtail stream bank erosion. The first is to divert the water away from the susceptible bank or to slow its current down to render it less erosive. The second strategy is to protect the bank against erosive currents by installing some protective devices. Porous structures permit water to seep through them but serve as sediment traps. Both of these strategies are briefly described below.

Check Dams: There are several types of check dams, depending on the objectives. Check dams are constructed to stabilize waterways, store excess water, or trap sediment. Sediment storage (trap) dams are designed to intercept sediment. These dams have a spillway to discharge water slowly and facilitate sediment to settle out. Dams are also designed to stabilize waterways (or prevent gully formation). These dams are constructed at the site of the over fall. If the site of the gullyhead (over fall) is unsuitable, dams can also be located a short distance downstream. If a gully is long, it is advisable to construct a series of dams at frequent intervals. The stabilization of dams is usually more effective when some land forming is done toward the upstream side.

Gabions: Gabions are porous structures comprising pre-fabricated baskets made of heavy-duty wire netting. The basket is placed in position, filled with stones, and covered with a protective wire lid (Plate 37). The dimensions of these baskets are variable, but they can be 4 m long and 1 x 1 m in cross-section. Several baskets can be placed on top of one another. Gabions are flexible to adjust for subsidence or loss of adjacent land due to scurrying. A stream current can also be slowed or deflected by installing permeable spurs made of timber or pre-fabricated metal frames.