Broad Base Terraces and Its Design | Erosion Control | Soil Management

In this article we will discuss about broad base terraces and its design.

Introduction to Broad Base Terraces:

A broad base terrace is the surface channel or embankment type construction, which is formed across the land slope. Broad base terraces may be classified in two types, based on their primary functions, i.e. to retain or remove the runoff water.

These two types of broad base terraces are as:

1. Graded Terrace:

This types of terraces are constructed in the combination of broad channels and embankments. There are constructed at suitable intervals along the graded contours at gentle slope. These terraces are constructed either with variable or uniform grade but should be non-erosive leading to the safe outlet. This type of terraces can be suitably used where ample lands and large-scale mechanical equipments are available for construction.

Different objectives of this type of terraces are mentioned as under:

Objectives:

The important objectives of broad base terraces are given as under:

i. To reduce the slope length and minimize soil erosion.

ii. To intercept the runoff and divert to a safe point.

iii. To utilize the land left between terrace and the ridge for growing the crops or forage.

Locations and Conditions for Use:

These are given as under:

i. In humid regions where water erosion is the problem, and drainage is necessary.

ii. For cultivation on the slope not exceeding 8% in erodible soils; and on the slope not exceeding 12% in erosion resistant soils. However, for establishing grass cover or pasture the slope limit can be as high as 15%.

iii. In the areas with high degree of mechanization.

This terrace is also known as channel-type terrace. The drainage of excess runoff water from the land, is the primary objective of this terrace. Graded terrace also reduces the length of slope and thereby the soil erosion as well as soil loss both are reduced.

Similarly, it also removes the surface runoff with a non- erosive velocity and drains Level terrace into the outlet. The view of graded terrace is shown in Fig. Fig. 13.7. It is constructed by digging a shallow channel on the uphill side and putting the excavated soil to form the embankment, beside the channel. Generally, the side slope of channel as well as embankment is kept as flat as possible to facilitate cultivation activities.

Sometimes, the graded terrace is also used as drainage channel, is referred as drainage terrace.

These terraces are suitable for the areas, where:

(i) Land slope varies from 3 to 10%.

(ii) Permeability of soil is slow.

(iii) Field is dissected by the gullies or depressions.

(iv) Contour cultivation is insufficient to control the surface runoff and soil erosion.

2. Level Terrace:

Level terraces are constructed for conserving the moisture content and controlling the soil erosion. These terraces are most suitable in low to moderate rainfall regions, because they are capable to trap and hold the rain water on their top, which gets infiltrate into the lower soil profile.

Such terraces can also be used on permeable soils for the same purpose. Embankment of this terrace is constructed with the help of excavated soil from the earthwork of channel, by putting both sides of the channel. The height of embankment should be such that the runoff water could not overtop it. This type of terrace is also known as conservation terrace.

The conservation terraces are normally found suitable, where:

i. Rainfall is low.

ii. Rainfall intensities are not excessive.

iii. Land slope is less than 6%.

iv. Infiltration as well as soil permeability are enough to absorb the water.

Design of Graded Terrace:

The design specification of graded terrace involves proper spacing, grade, length and its cross-section. For graded terrace, there should also be a provision of outlet for safe disposal of runoff. The knowledge of soil characteristics, cropping pattern, soil management practices and climatic conditions are most essential pre requisites for design purpose.

Various design parameters are described below:

Terrace Spacing:

Terrace spacing is the vertical distance between the channels of two successive terraces. For the upper most terrace, it is the vertical distance from the top of the hill to the bottom of the channel. The spacing of terrace should be such that it could not cause excessive soil erosion between adjacent terraces. Table 13.4 presents the recommended values of terrace spacing for average condition. This spacing may vary plus or minus by 10 to 20% to suit the local conditions.

Terrace spacing can be calculated by using the following empirical relationship –

V.I. = aS + b … (13.45)

Where,

VI = vertical distance from top of the slope to the bottom of the first terrace (feet)

a = constant depends on geographical location

b = constant depends on erodibility and cover condition

S = average land slope in percent

The vertical spacing or vertical interval of broad base terrace depends on the climatic condition and type of soil.

For different climatic regions and soil types the suitable formulae for VI are outlined as under:

Where,

Cd = depth of cut (cm)

h = depth of channel, which is the sum of depth of flow and freeboard (cm)

s = original land slope (%)

W = width of side slope (cm)

Width of Side Slope:

The front and back slopes of broad base terrace should be gentle, so that the cultural operations can be performed by using machines. Generally, for mechanized cultivation the parallel terrace system in straight array is found suitable. For this case, the terrace width is recommended as 4m or more; and it’s gradient 5:1 or less.

On the other hand, when terrace is to be used for pasture/rangeland, or for non-mechanized cultivation purposes, then width can be reduced, and side slope could be increased. The width of side slope is determined on the basis of machine width, if the channel depth (h) and depth of cut (Cd) are known. The value of side slope can be calculated, using the following formula –

Where,

S_f = front slope

S_c = cut slope

S_b = back slope

C_d = depth of soil cut

h = depth of channel

W = width of channel slope

Fig. 13.8 illustrates various notations of broad base terrace, used in above formulae.

Terrace Slope:

It should be such that the runoff may be safely disposed from the area. Generally, the terrace channels are constructed either with a uniform or variable grades.

The graded terrace with uniform grade is referred as uniform graded terrace or channel, while the graded terrace with variable grades is called variable graded terrace. In uniform graded terrace, the slope of channel remains constant throughout its whole length. Uniform grade is suitable for the terraces of shorter length having drainage facility.

For uniform-graded terrace, the grade from minimum 0.1% to maximum 0.4% is generally recommended. The variable grades provide better alignment of terraces by making easy to link them with the field. The size of variable graded terrace is kept more towards the outlet because the runoff amount gets increase as channel approaches towards outlet. The recommended variable grades are given in Table 13.5.

Terrace Length:

The terrace length is dependent on the size and shape of the field, outlet position, rate of runoff, infiltration rate and channel capacity. In general, 400 to 500m is the maximum limit for a continuous terrace length, provided that the terrace carries the runoff in only one direction.

However, shorter terraces are also desirable; it should be used if convenient. Length of graded terrace may be greater in permeable soil than the impermeable soil. The terrace length should also be such that the flow velocity may not be erosive. Large cross-section of terrace should also not be used. However, the length of terrace per hectare can be computed by using following formula, reported by singh et at (1991) –

L = 10,000/HD … (13.52)

In which, L is the terrace length per ha, (m) and HD is the horizontal distance between two terraces, which can be estimated by using the equation 13.43.

If suitable outlets are available at either ends of terrace, then a longer terrace may be constructed, as it can drain the runoff from its both the ends.

Terrace Cross-Section:

The cross-section of graded terrace is defined by the height of channel’s ridge, side slope and channel width. The terrace cross-section should be such that the sufficient amount of runoff can be drained through that. The slope of channel as well as ridge should be kept flat to facilitate the farming operations, smoothly. Generally, the side slope of channel and ridge is used as 4:1 and 5:1, respectively. Channel width and ridge height derive the channel capacity.

It is recommended that the cross-sectional area of channel should not be less than 0.75 sqm. In addition, there must also be given settlement allowance for newly constructed terrace system. The settlement of any earthwork depends on the soil, moisture content and types of machine used for construction. Usually, 10 to 20% as settlement allowance is used for the loose fill condition. The settlement allowance can also be calculated, using the equation (15.3).

Design of Broad Base Level Terrace:

Selection Criterion:

The selection of broad base level terrace is done on the basis of following criterion:

i. The area should be large.

ii. The area of poor population density.

Objectives:

Various objectives of construction of broad base level type terrace are outlined as under:

i. To reduce the slope length.

ii. To intercept the runoff; and facilitate infiltration in to the soil.

Locations and Conditions:

The level type broad base terraces are suitably used at the locations, where:

i. Land slope is less than 3%.

ii. Rainfall is less.

iii. Water erosion is the problem, but drainage is not necessary because of good soil permeability.

Design Specification:

Various design specifications of level broad base terraces are outlined as under:

1. Terrace Length:

Its length is comparatively much longer than the channel type terraces. It can be up to 1000m. In North America 1000m length is commonly used for this type of terrace system.

2. Channel Gradient:

In this terrace the channel gradient is absolutely level. Also, there is freedom to keep the end portion open or close permanently or seasonally closed, depending on the fanner’s requirement.

3. Width:

Its width is generally kept same to the channel type broad base terrace.

4. Ridge:

The ridge top is kept level. However, the area immediately above the ridge can also be levelled to store extra runoff. The proportion of level ground of overall space depends on several factors like soil, cover and rainfall.

5. Vertical Interval:

The vertical interval is generally same to the channel type terrace. However, using following formula the VI of this type of terrace can also be computed –

VI = (S/10) + 2 … (13.53)

In which, S is the slope in percent, more than 3.

6. Horizontal Spacing, Side Slopes and Terrace Length:

These parameters can be calculated in the same way as in case of graded terrace.

7. Layout and Surveying Procedure:

Layout procedure is the same to the graded type terrace, except that they are staked on level ground surface. The centre line of ridge is preferred for staking, because this line of stake remains during construction work.

8. Construction Procedure:

The main construction component of this type of terrace is the embankment. The construction of embankment is done with the help of soil taken from both the sides of ridge. For providing a greater safety, an allowance is given to the embankment height, called settlement allowance.

And to create a greater runoff storage area the land immediately above the ridge should be levelled. The terrace end is either kept close or open to the protected area. Construction is performed in the same way as the graded terrace.

9. Operation and Management:

Operation of this type of terrace is done as per below:

i. If the seasonal closing of terrace ends is under planning then this work should be done at appropriate time, each year.

ii. The other points regarding operation and management are the same to the graded terraces.

10. Protection and Maintenance:

It is done as per below:

i. Special attention is given for protection and maintenance of ridges.

ii. The overtopping or leakage caused by the trapped water should be checked, immediately.

iii. All tillage operations should be done with great care, so that there should not be any damage in the terrace edge.

iv. When cultivating the land between the terraces, then it should be on contour because contour furrows act as an auxiliary measure to harvest the surplus runoff.

11. Runoff from Terraced Area:

The design of graded terrace is similar to the drainage channel or waterways, which is totally dependent on the runoff rate. The peak runoff rate can be computed by using the rational method. The runoff computation should be based on 10-years recurrence interval.

12. Terrace Channel Capacity:

The channel capacity of graded terrace must be sufficient so that the runoff at most severe conditions can be disposed very safely. If channel grade and its dimensions are known, then channel’s capacity can be calculated by using the Manning’s formula, given as under –

in which, Q is the design runoff rate; A is the cross-sectional area of the channel; V is the flow velocity; R is the mean hydraulic radius (A/P); P is the wetted perimeter; S is the channel grade and n is the Manning’s roughness coefficient. The value of Manning’s roughness coefficient is usually taken as 0.04 for the design purpose. A free board of about 20% of design depth should also be added to the channel depth, after including the settlement height of the ridge.

The capacity of channel should be sufficient to handle the runoff from the land, u/s of terraced area. The design of channel of broad base terrace is done on the basis of 10-years return period. The channel cross- sectional area is fixed based on the slope steepness of the land. For example – in North America the minimum cross sectional area of channel of at least 30 to 45cm depth and for different slope gradients is given in Table 13.6.

However, the width of side slope, depth of cut and gradient of side slope also decide the cross-sectional area of channel. After determining the cross-sectional area of channel, the volume of earthwork per ha can be computed by multiplying the terrace length and its cross-sectional area, together.