|Sustainable Management of Soil Resources in the Humid Tropics (UNU, 1995, 146 pages)|
|VII. Runoff management and erosion control|
|B. control measures: engineering structures|
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.