terraces:soil water conservation structure

Terraces

MM HASAN,LECTURER,AIE,HSTU

TERRACING• Terracing is a practice to reduce runoff, soil erosion, and

sediment delivery from upland areas by constructing broad channels across the slope of rolling land.

Reasons for constructing terace If surface runoff is allowed to flow unimpeded down the slope of

arable land these is a danger that its volume or velocity or both may build up to the points where it is not only carries the soil dislodged by the splash erosion but also has a scouring action of its own.

various names given to this techniques are: terraces (U.S.A). ridge or bund (common wealth countries).

Functions

To decrease the length of the hillside slope , thereby reducing sheet and rill erosion.Preventing formation of gullies and retaining runoff in areas of inadequate precipitation.In dry regions such conservation of moisture is important in the control of wind erosion

Types of terraces ( terrace classification)

Classification by AlignmentTerrace alignment can either be non-parallel or parallel.Non-parallel terraces follow the contour of the land regardless of alignment.Parallel terraces are preferred for row crop farming operations.

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Classification by Cross SectionBench terraces (Figure 8.1c) are built on steep (20 to 30%) slopes where labor is cheap or land in short supply. They are more efficient at distributing water under both irrigated and dryland production. However, bench terraces on very steep slopes may be too narrow or too inaccessible for mechanized farming systems.The broadbase shapes include the three-segment section (Figures 8.1a and 8.4), the conservation bench (Figures 8.1b, 8.3, and 8.5b), and the grassed backslope terrace (Figures 8.2 and 8.4c). The three-segment section terrace is more common in mechanized farming systems on moderate slopes (6 to 8%). All slopes on the three-segment section broadbase are sufficiently flat for the operation of farm machinery.


Classification by GradeGraded or channel-type terraces control erosion by reducing the hillside slope length of overland flow, and then by conducting the intercepted runoff to a safe outlet at a nonerosive velocity. Level terraces are constructed to conserve water and control erosion (Figure 8.3). In low to moderate rainfall regions they trap and hold rainfall for infiltration into the soil profile.


Classification by OutletTerrace outlets may be classified as

blocked (all water infiltrates in the terrace channel), permanently vegetated (grassed waterway or a vegetated area), or piped (water is removed through subsurface pipe drains).

Combinations of outlets may be employed to meet specific conditions.

Planning the Terrace System

Selection of Outlets: • or disposal area:• vegetated outlets ( preferable) • Outlet types include

• natural drainage ways, • constructed channels, • permanent pasture or meadow, • stable road ditches, • waste land, • concrete or stabilized channels, • pipe drains, and • stabilized gullies.

Planning the Terrace SystemTerrace location : Factors that influence terrace location include

(1) land slope; (2) soil conditions, such as degree and extent of erosion; (3) proposed land use; (4) boulders, trees, gullies, and other impediments to construction or cultivation; (5) roads and boundaries; (6) fences; (7) row layout; (8) type of terrace; and (9) outlet.

Minimum maintenance, ease of farming, and adequate control of erosion are the criteria for good terrace location.

TERRACE DESIGN • The design of a terrace system includes

• specifying the proper spacing and location of terraces, • the design of a channel with adequate capacity, and • the development of a stable and sometimes farmable cross section.

• For the graded terrace, runoff must be removed at nonerosive velocities in both the channel and the outlet.

• Soil characteristics, cropping and soil management practices, and climatic conditions are the most important considerations in terrace design.

Terrace Intervals

• Spacing is expressed as the vertical distance between the channels of successive terraces.

• For the top terrace, the spacing is the vertical distance from the top of the hill to the bottom of the channel.

• This vertical distance is commonly known as the vertical interval or VI.

• The horizontal interval, HI, is found by dividing VI by the slope (m/m).

Terrace Intervals• Graded• The graded terrace VI is often expressed as a function of land

slope by the empirical formula• VI = Xs + Y (8.1)

• where VI = vertical interval between corresponding points on consecutive terraces or from the top of the slope to the bottom of the first terrace in m,

• X = constant for geographical location (Figure 8.8),• Y = constant for soil erodibility and cover conditions during critical

erosion periods, or = 0.3, 0.6, 0.9, or 1.2, with the low value for highly erodible soils with no surface residue and the high value for erosion resistant soils with conservation tillage (ASAE Standards, 1992),

• s = average land slope above the terrace in percent.

Terrace IntervalsLevelThe horizontal interval for level terraces is a function of channel infiltration and runoff; however, in more humid areas, where erosion control is important, the slope length may limit the spacing. The storage capacity of the terrace should be adequate to prevent overtopping from upslope runoff, and the infiltration rate in the channel should be sufficiently high to prevent serious damage to crops.

Terrace GradesThe gradient in the channel must be sufficient to provide adequate drainage while removing the runoff at nonerosive velocities. The minimum slope is desirable from the standpoint of soil loss. Grades may be uniform or variable.In the uniform-graded terrace, the slope remains constant throughout its entire length. A grade of 0.4% is common in many regions; however, grades may range from 0.1 to 0.6%, depending on soil and climatic factors. Generally, the steeper grades are recommended for impervious soils and short terraces.

Terrace GradesThe variable-graded terrace is more effective because the capacity increases toward the outlet with a corresponding increase in runoff.

The grade usually varies from a minimum at the upper portion to a maximum at the outlet end.

Terrace Length Size and shape of the field, outlet possibilities, rate of runoff as affected by rainfall and soil infiltration, and channel capacity are factors that influence terrace length. The number of outlets should be a minimum consistent with good layout and design.The length should be such that erosive velocities and large cross sections are not required. The maximum length for graded terraces generally ranges from about 300 to 500 m, depending on local conditions

Terrace Cross SectionThe cross section of a broadbase terrace can be considered a triangular channel as shown in Figure 8.4a. The flow depth d is the height h to the top of the ridge minus about 0.08 m for freeboard. After smoothing, the ridge and bottom widths will be about 1 m, which will give a cross section that approximates the shape of a terrace after 10 years of farming (Figure 8.4b).

Terrace Cross SectionIn designing the cross section, the frontslope width Wf is specified to be equal to the machinery width ordinarily used for farming operations. The depth of flow is determined from the runoff rate for a 10-year return period storm or for the required runoff volume for storage-type terraces. When the side slope widths are equal (Wc = Wf = Wb= W), cuts and fills from the geometry are

c + f = h + s × W (8.2)where c = cut (L),f = fill (L),h = depth of channel including freeboard (L),s = original land slope (L/L),W = width of side slope (L).

Pipe OutletsPipe outlets allow straightening the terrace at natural channels with an earth fill, so it is easier to make terraces parallel. The pipe outlet as shown in Figure 8.9b has an orifice plate to restrict the outflow. This restriction ensures that the subsurface drains are not overloaded, and that sediment in the runoff has time to settle in the terrace channel, improving the quality of the runoff water. The outlet pipe should not be perforated, and should be able to withstand static soil loads and dynamic construction and farm machinery loads.

Figure 8.9–(a) Grassed backslope and pipe outlet graded terraces and (b) details of controlled-flow pipe intake. (Redrawn from SCS, 1979.)

Terrace Channel CapacityWith graded terraces, the rate of runoff is more important than total runoff, whereas both rate and total runoff influence the design of level, pipe outlet, and conservation bench terraces. Graded terraces are designed as drainage channels or waterways, and level terraces function as storage reservoirs.The maximum design velocity will vary with the erodibility of the soil but should rarely exceed 0.6 m/s for soil devoid of vegetation. The design peak runoff rate should be based on a 24-h, 10-yr storm.The 24-hr design storm will be removed in 24 hours for most crops or within 48 hours for flood-tolerant crops like corn

Terrace Channel CapacityTo restrict the flow in to the tile drain, the orifice opening is determined from the orifice equation:

where q = flow (L3/T),C = orifice coefficient = 0.5,A = orifice area (L2),g = acceleration due to gravity (L/T2),h = head of water above orifice (L).

Layout ProcedureWhen available, topographic maps or high resolution digital elevation models (DEMs) are used to develop an initial plan.The plan should specify the approximate locations of terraces, drainage structures, and other important features. After the plan is developed, a surveying level and chain or tape are used to set out the terrace location. Field modifications to the plan may be necessary to address topographic features not shown on the contour map.

• Construction Equipment• A variety of equipment is available for terrace construction,

including bulldozers, scrapers, motor graders, and hydraulic excavators (Figure 8.10).

• Smaller equipment, such as moldboard and disk plows, is suitable for slopes of less than about 8%, but the rate of construction is much slower than with heavier machines.

• Soil and crop conditions are likely to be most suitable for construction in the spring and fall.

Terrace Construction

• Settlement of Terrace Ridges• The amount of settlement in a newly constructed

terrace ridge depends largely on • soil and water conditions, • type of equipment, • construction procedure, and • amount of vegetation or crop residue.

• The settlement based on unsettled height will vary from 5% or less for a motor grader to 10 to 20% for a bulldozer.

Terrace Construction

Terrace MaintenanceProper maintenance is as important as the original construction of the terrace. However, it need not be expensive since normal farming operations will usually suffice. Any breakovers should be repaired as soon as possible. The terrace should be watched more carefully during the first year after construction, and any excessive set-tlement, failures, or cracking repaired. Channels may occasionally need to be cleared of deposited sediment or ridges rebuilt.


Terrace MaintenanceTillage Practices

In a terraced field, all farming operations should be carried out as nearly parallel to the terrace as possible (Figures 8.2 and 8.3). The most evident effect of tillage operations after several years is the increase in the base width of the terrace. Reversible plows can be used to increase ridge heights or to redistribute soil that has accumulated in the channel.


terraces:soil water conservation structure

Engineering