Waterways

Water that overflows from terraces and contoured fields or requires draining must be conveyed in a non-erosive manner for overall soil conservation. The runoff must be discharged into either natural streams, watercourses, or artificial excavated waterways. Their purpose is to convey runoff to a suitable outlet. Normally, their dimensions must provide sufficient capacity to cope with peak runoff rates with a 10-year return period. There are three types of waterways (Fig. 4.31):

• diversion channels,

• terrace channels,

• artificial watercourses, (grass) waterways.

The characteristics and aims are described in Table 4.12. More detailed information on design criteria can be found in the literature [16-22]. Waterways are surface drainage devices. Therefore, for design criteria, reference should also be made to Chapter 5, Section 5.6, "Drainage."

Artificial watercourses may need special attention, depending on the topographic conditions and the equipment available. The major types of waterways are

Type

Table 4.12. Types and functions of waterways

Function

Diversion ditches

Terrace channels

Grass waterways

Placed upslope of areas where protection is required to intercept water from top of hillside; built across slope at a slight grade so as to convey the intercepted runoff to a suitable outlet.

Placed upslope of terrace bank to collect runoff from interterraced area; built across slope at a slight grade so as to convey the runoff to a suitable outlet.

Used as the outlet for diversions and terrace channels; run downslope, at grade of the sloping surface; empty into river system or other outlet; located in natural depressions on hillside.

Figure 4.31. Layout of waterways. Source: [11].

Figure 4.31. Layout of waterways. Source: [11].

Grass waterway, a parabolic-shaped channel planted with low and rhizome-type grasses. The channel should be shaped as uniform cross section and as consistent in gradient as possible. It is the most inexpensive type of waterway on gentle slopes and its maintenance is easy.

Grass waterway with drop structures. On moderately steep slopes or in a discontinuous type of channel, small drop structures and check dams can be used in conjunction with grass to take care of the steep sections.

• Ballasted waterway. On moderately steep slopes where large quantities of head-size stones are available, ballasting the parabolic channel with stones keyed in the ground can provide good protection.

• Prefabricated concrete waterway. On very steep slopes and where it rains so frequently that normal construction is hampered, these prefabricated structures, parabolic or V-notch type, can be used readily to protect the center part of a waterway and leave two sides protected by grass. They also can be used in these channels where there are constant small flows due to seepage or groundwater.

Maintenance procedures prevent the failure of waterways due to insufficient capacity, excessive water velocity, or inadequate vegetation cover. The solution to the first two of these problems is largely a matter of design. The condition of the vegetation is affected by both the construction criteria and the subsequent management procedures. With reference to this issue, grass waterways should be mowed and raked several times a season to stimulate new growth and control weeds. Annual application of manure and fertilizer maintains a dense sod, any break of which should be repaired. When land adjacent to the waterway is being plowed, the ends of furrows abutting against the vegetated strip should be staggered to prevent flow concentration down the edges of the water course. Sometimes, underground outlets are necessary to take runoff from low points in terrace or diversion channels and carry it through a pipeline to a place of safe discharge. This procedure normally is used where the slope is too steep for non-erosive water flow.

Good conservation practice at the watershed level is, however, the most effective means of waterway maintenance. As a matter of fact, accumulated sediments smother vegetation and restrict the watercourse capacity. Extending vegetal cover well up the side slopes of the work and into the outlets of terrace channels helps to prevent sediment from being deposited in the waterway. Control of vegetation also reduces the accumulation of sediment. Often the opportunity to provide protective cover by natural revegetation is overlooked, and unnecessary expenditures are made for structures and planting.

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