In this article we will discuss about:- 1. Meaning of Terracing 2. Terrace Classification 3. Layout and Survey Procedure 4. Planning.
Meaning of Terrace Classification:
At present, different countries are using different classifications of terraces, depending on their specific criteria.
Few of them are mentioned as under:
i. Arrangement, i.e. symmetrical or asymmetrical
ii. Shape of cross-section
iii. Dyke slope, i.e. inclined or non-inclined
iv. Manner of removing water from the terrace, i.e. without outflow, with surface flow or underground outflows.
Among numerous existing classifications the two basic types of terraces are very distinguished, i.e. ridged terraces and bench terraces. In which, the bench terraces are constructed on the slopes more than 10%.
From last several decades the bench terraces are most commonly used in Europe, Australia and Asia, while in southern USA the ridgeless channel terraces are very common. In late 18th century the terraces were used to construct across the field slope as collector trenches. Later on, various types of terraces have been designed, depending on the requirement of person and condition of the land terrain.
Although, a host of terraces have been designed and classified, but one of the oldest classifications amongst them is made by Czerkasov. In this classification the terraces have been divided into ridged terraces, bench terraces and transitional terraces, as shown in Fig. 13.1.
They are described as below:
Various features of this class terraces are outlined as under:
i. This type of terracing is suitable on the slope ranging from 0 to 0.06%.
ii. Suitable to the land, where ground surface is with reverse slope or horizontal, and with ridge.
iii. The absorptive furrows are under Type I.
iv. Involve lesser earthwork; due to this reason this type of terracing is considered to be economical.
Terraces of this class, i.e. without outflow were formerly constructed with small ridges on the slope 0.05 – 1.1% in Southern United States. The height of ridges was from 0.3 to 0.5 m and base width was 1 m with a small shallow ditch in front of ridge.
This class of terraces have following features:
i. Terraces are constructed on the slopes ranging from 0.06 to 0.13%.
ii. These are ridge terrace with a wide and not very deep trench.
iii. These terraces are commonly called Magnum terraces, named after an American fanner who began using from the year around 1885.
iv. Width of ridge varies from 4.5 to 7.5m; and ridge height from 0.3 to 0.6m.
v. Width of terraces is from 20 to 30m.
vi. The ridges are planted with grass in the first year of their construction.
vii. Terraces may also have an outflow provision. The longitudinal slope of terrace with outflow is the same for 100 m continuous terrace length, but it can be changed for the terrace length more than 100 m.
This class of terraces are used in forested slopes. In Uzbekistan these terraces are very common on hilly slopes under forestation.
i. This class terrace includes deep trench and narrow ridge toe.
ii. Shape of trench is rectangular with inclined bottom.
Important points about this class terraces are as below:
i. The shape of trench is trapezoidal.
ii. Trees are planted on the slope in front of embankment.
iii. The collector trenches are divided into different spaces of 20m widths to prevent large water outflow in case of possibility of damage of the ridge.
Apart from above classification of terraces these are also classified into three main classes as – diversion, retention and bench type terraces (Table 13.1). The primary aim of diversion terrace is to intercept the overland flow, and pass it across the slope to a suitable outlet. Diversion terraces are constructed on a small gradient, usually 1:250 at the contour. The diversion terraces are sub-classified as Magnum, Nichols and Broad base type, described in Table 13.1.
The retention type terraces are preferred to construct, where conservation of surface water by storing on hill side is required. The design of such terraces is done for the return periods of 10-years. The permeable soils with the slope less than 4.5° are suitable for retention type terraces.
In India, the terraces are classified in two major types, given as under:
1. Bench terraces; and
2. Broad base terraces.
Layout and Survey Procedure for Terracing:
In order to construct the terraces in the field, a proper layout of terrace system is very important for controlling the runoff and soil erosion, both. The preparation of layout should be based on the site survey about its physical conditions. Physical condition of the land includes the slope, soil depth, soil texture, erosion, presence of rocks, wetness, vegetative cover and present land use, mainly.
The layout should also include the existing waterways, roads and other farm installations in the area. The farmer’s plan and resources, labour availability and farming methods to be used, should also be included in layout preparation.
It is carried out as under:
The equipments usually required for conducting the survey work for preparation of bench terrace layout, are mentioned as under:
1. Dumpy level, abney level or A-level.
2. Measuring tape and rod.
3. Soil auger.
It is done as per below:
i. For Level Terraces – Contouring or leveling technique.
ii. For Upland Bench Terraces – Graded-contouring technique.
iii. Setting of Base Line – Setting of up-and-down base line along representative slope.
This method is recommended for constructing the terraces, manually. If specification of bench terraces is not available then vertical interval (VI) can be determined in the field by using survey method. For which a level is used to determine and stake the VI of terraces along the base line. It should be followed by graded contouring or leveling survey depending on the type of terrace to be constructed.
After completing the staking work for all the contours or graded contours, one more line of marked stakes is added between them. This additional line serves as the bottom line for upper terrace and top line for lower one. A top line should also be added to the first terrace on the upward slope; and a bottom line to the last terrace on the downward slope.
This method is recommended for terrace construction using mechanical or animal power. In this method, the base line is staked considering the width of terrace (W) with the help of tape. The contour or graded contour line is fixed from each stake for the entire area.
The contour or graded lines serve as the bottom lines for the upper terraces, and as the top lines for succeeding terraces. The stakes should be removed from the sharp turn and narrow bottleneck, because they interfere the operations. The extra cuts or fills should also be removed from the site.
Planning of Terrace System:
The terrace system is the “series of terraces consisting of suitable slope with adequate discharging capacity of the runoff into one or more outlet channels” or in other words it is a coordinated system having complete water disposal system for the area and providing adequate consideration for proper land use. A typical layout of terrace system is shown in Fig. 13.15.
Planning of terrace system involves to locate the followings:
1. Land use
3. Terrace locations
4. Terrace direction
5. Farm road locations, etc.
Terrace system should be planned on watershed basis; and should also include all those terraces that have to be constructed in future. If possible, the adjoining areas having the same drainage basin should be planned as a joint terrace system. For this purpose the topographical map of the area is most essential.
Selection of outlet is the first step of planning of the terrace system. The terrace outlet may be any channel or place into which flow from one or more terraces can be diverted to discharge. The natural depressions, manually constructed channels, sod flumes, permanent pastures, road ditches, waste lands and stabilized gullies etc. can be used as outlet.
The natural depressions are most suitable and economical sites for outlet. But where such depressions do not permit adequate capacity, the grassed waterway along the field boundary can be constructed as the outlet.
The location of manually constructed outlet should have least interference for tillage operations. This type of outlet should also be fully protected by establishing the grasses. The wastelands or permanent pastures are also feasible to use as outlet, provided that the erosion is fully controlled from there.
As for as possible the sod flumes and concrete channels as outlet should be avoided as they involve heavy expenditure of money to construct. The road ditches and gullies should be used with great care; there must be a provision for preventing the soil scouring and their enlargement.
After deciding the location of outlet, the next step of terrace planning system is to fix the locations of terraces in proposed area. This involves the laying out of terrace, first from the outlet end. Always, the top terrace should be located at the proper place and in proper size, so that it cannot he overtopped by runoff and also not damage to the other terraces, constructed below.
The location of top terrace should be done based on following points:
1. The top terrace should have the drainage area not more than 1.2 hectares.
2. If the top of the hill comes as one of the points under consideration, then vertical interval of terrace should be kept 1½ times the regular interval.
3. On long ridges, where terraces are approximately parallel to the ridge, a regular interval should be used to locate the top terrace.
4. If there occurs some short abrupt change in the slope, then location of terrace should be fixed just above the break point.
The direction of terrace should be such that it can be safely drained to the waterway. Crop rows should be maintained parallel to the terraces. This reduces the silting problem caused by water flow across the terrace channel.
Terrace layout is prepared on the basis of surveying technique along with a long experience on this particular work. The terrace layout procedure consists of determining the predominant land slope above the terrace, first, and then suitable vertical interval to follow. In laying of the terrace system the terrace is staked first from the outlet end. However, few exceptions can also be made; such as if it is desired to locate a definite terrace at a definite place, then this terrace should be located first.
After the upper stake is over, then the other terraces are also staked in similar way. There should be made some adjustments at sharp corners of the field so that the farming operations may not be hindered. At last, if the final check of the terrace and outlet shows satisfactory, then locations of the terraces should be marked in the field by plough furrow for construction.
Morgan (1986) has also described the procedure for laying out of terrace system, which is accomplished under following steps:
Using aerial photograph or topographic map of the area and making reconnaissance survey, determine the initial location of grassed waterway to use it as terrace outlet. Normally, the natural depressions, drainage lines etc. are preferred to use as the grassed waterway.
Indicate all the main breaks of slope and badly eroded or gullied areas, if any. The terrace bank, wherever possible, should be located to incorporate in the slope breaks and should be placed at upslope of the eroded lands.
Find out the terrace spacing, using any suitable method. The spacing can be deviated by 25 to 30% to allow the adjustment in position of the terraces, to conform with slope breaks and avoiding eroded areas.
Determine the length of terrace. The length should be in such a limit that, the problem of runoff accumulation should not be there, and the provision of large cross-sectional area of terrace channel should also be avoided.
If necessary, adjust the position of grassed water way to avoid the excessive length of terrace.
Also, locate the position of paths or farm roads in terrace system. Usually, these are located along the divide line, between separate terraces and waterway system. The use of crest locations for farm path, minimizes the catchment area contributing the runoff to the terraced area. However, the runoff of that area can also be directed to the surrounding lands, without use of side drains.
Plan the layout of terrace system with the help of contour map and aerial photograph of the area.
Locate the position of grassed water way and diversion channels on the contour map or aerial photograph. Also, demarcate the position of top terrace first and proceed for others in relation to top terrace. All terraces should be located in accordance with designed spacing, length and grade.
Verify the layout whether it is practical for all agricultural operations or not. For this purpose, the inter-terrace area is checked for its sufficiency in respect of movement of machineries, adoption of contour cultivation etc. If inter-terrace area is not wide for the movement of farm machineries then terrace position should be modified to suit the requirement. Normally, the terrace width is adjusted to the nearest multiple of the equipment’s width, which is to be used for the operations.
On irregular land the terraces are converged on steeper sloping parts which results into formation of several rows during farming operations. This phenomena causes inconvenience. However, it can be minimized by establishing parallel terrace system. In parallel layout system, the terrace spacing is maintained at 0.67 VI.
As usual, the upper most terrace is located first, and others are located with reference to the first, using the interval 0.67 VI, in parallel sequence. The constructed terrace should also be verified for their down-hill grade. The grade should be non-erosive.
Determine the design dimension of the grassed waterways, diversion channels and terrace channels.
It should be carried out by accounting the following points:
i. Design should be performed for different sections, to adjust the variations of slope inclination.
ii. At the points, where slope exceeds 11°, there should be made the provision of drop structure in the waterway, to prevent the wideness of waterway at the top of the slope, especially where runoff is less.
Construct the waterway as per design specifications, obtained in step (11).
Stake the positions of diversion and terrace channels at appropriate vertical interval along the outlet or grassed waterway. At these positions, the terraces are pegged across the slope as per grade, selected.
Construct the diversion channel first followed by the top and other terraces so on at the given intervals. During construction, if top soil is not sufficient, then it should be scrapped and stored, and then spread over the inter-terrace area after completing the terrace construction.
Construction of terrace consists of cutting the earth in such a way as to make the channel and ridge that can be easily crossed by the farm machineries. There are various types of machines for constructing the terrace.
The selection of suitable machines depends on the following points:
i. It can displace the cut soil to the desired place of ridge construction.
ii. It can construct the terrace with rapid speed.
iii. The machine could easily be used on steep slopes ranging from 15 to 20%.
iv. The operating cost of machine should also be minimum.
The upper terrace should be constructed first and then moved down for next terraces. The first top terrace should be constructed with great care regarding its stability and design specifications, because safety of lower terraces depends on it. The dead furrows and ditches should be filled before starting the construction. Surplus vegetations should also be thoroughly removed from the field.
The success of terrace system depends on its proper maintenance. Terraces should be inspected after their construction. The damage occurred in the terrace section should be repaired and the channel as well as ridge grade should also be maintained in proper form.
Similarly, the outlets should also be watched sincerely so that necessary measures can be followed for making their repair, if any. In terraced field the farming operations should be done parallel to the terrace, as possible. The farming operation develops several depressions and ridges between the terraces, which is beneficial for moisture conservation and erosion control, both.
A good terrace section can be achieved by using the proper ploughing practices. Sometimes, the strip cropping combined with terracing, provides best control of soil erosion. In combination of these two practices, the strips of inter-tilled and close-growing crops should be kept, nearly in uniform width.