Subject: Irrigation Engineering, Class: B.Tech IInd YearCourse teacher: Dr.S.K.Sharma

Title: Land GradingLand grading is reshaping of land surface to a planned grade. It is necessary in making a

field surface to control the flow of water, to check soil erosion and provide for surface drainage.

Usually field is not graded to a level surface, but a gentle uniform slope is maintained to meet the

requirements of irrigation and drainage.

Irrigated areas benefit greatly from land grading since the ground in its natural state is

seldom suited for the efficient application of water. When an uneven land is irrigated the high

spots are watered too little and the low spots too much. This results in uneven crop, reduced

yields and loss of water. A properly graded land surface ensures unobstructed smoth flow of

water into the land, without eroding the soil and ensures uniform distribution of water

throughtout the field.

Criteria for Land Levelling or Grading

Criteria for land grading are influenced by the characteristics of the soil profile,

prevailing land slope, rainfall characteristics, cropping pattern, method of irrigation and other

considerations.

1. Soil profile condition: A soil survey of the area to be levelled is necessary before undertaking

the leveling work. The soil survey map will show the nature of the top soil, the subsoil and the

material under the subsoil. It will show the depth to sand, hard pans, rock or other material that

might limit the depth of cut as well as the extent of such areas. Alkali spots will be outlined and

depth to water level shown. The information will aid in planning the best possible layout for land

leveling. Soils that are shallow have limitations in the depth of cut that is allowable. Deep cuts in

such cases may expose inert material which may not grow cops. In deep soils, however, cuts into

subsoil may not be harmful because organic matter can be added t o build up its fertility quickly.

Soil survey will furnish information relative to infiltration and hydraulic conductivity. If

the sub soil and substratum are sand or sandy loam and the infiltration rates are high the

irrigation runs must be shorter than if there is clay loam or clay underlying the surface.

2. Land slope: If the land slope is very steep and undulating, and the soils are shallow, it may not

be possible to shape the surface to uniform slopes on good irrigation grade. Such areas should be

left without any major land grading operations being done and should be kept in pasture as much

of time as possible in order to prevent severe soil erosion.

Recommended safe limits of land slope for efficient

Type of Soil Longitudinal slope in %

Heavy (Clay) soil 0.05 to 0.25

Medium (Loamy) soil 0.20 to 0.40

Light (sandy) soil 0.25 to 0.65

A good land grade designed in accordance with the infiltration rates of soil, the size of

the irrigation stream available the crop to be grown and the erosion hazard from rainfall, permits

uniform distribution and high irrigation efficiency.

3. Rainfall characteristics: The rainfall characteristics of an area are important in determining the

maximum and minimum grades allowable for a field. Minimum grade must meet the drainage

requirements which are substantially influenced by the amount of rainfall and its intensity,

frequency and season occurrence. Likewise, the minimum permissible grade for a given soil is

often determined by the maximum grade which is non erosive under the prevailing rainfall

conditions.

4. Cropping pattern: The kind of crops to be grown should be considered in selecting the

irrigation or drainage method and the resulting land grading criteria. A high value crop or a crop

with high labour requirement may justify a high degree of leveling to reduce labour and

production cost. Growing of vegetables may justify the high leveling cost.

5. Irrigation methods: Each method of irrigation has its own limitations as to permissible cross

slope and downfield slope. When several methods of irrigation are to be used on the same field

the requirement of the most restrictive method must be adapted.

6. Other considerations: Field subdivision based on natural topographical boundaries should be

considered for the entire farm in the initial plan itself, even though only a part of the farm may be

leveled in first year. Often it may be possible to subdivide the farm area into relatively narrow

strips on the approximate contours to reduce the slope to acceptable limits.

Land Levelling design methods

1. Plane method

2. Profile method

3. Plan inspection method.

4. Contour adjustment method

Plane method:

(i) Determine centriod of the field- The centriod of a rectangular field is located at the point of

intersection of its diagonal The centriod of a triangular field is located at the intersection of the

lines drawn from its corners to the mid point of the opposite sides. To determine the centriod of

irregular fields, the area is divided into the rectangles and right angled triangles. The centriod is

located by computing moments about two references lines at right angles to each other. The

distance of the centriod of the field from any of line of reference is equal to the sum of the

products obtained by multiplying the area of each part times the distance from the line of

reference to its centriod divided by the area of the entire field. By computing the distance to the

centriod from two lines of reference perpendicular to each other, the exact point of the centriod

can be determined.

(ii) Determine the average elevation of the field- This is obtained by adding the elevation of all

the grid points in the field and dividing the sum by the numbers of grid points.

(iii) Compute the slope of the plane of best fit- The slope of any line x or y direction on the plane

which fits the natural ground surface can be determined by the least square method. In a

rectangular field this can be represented by the following equation

∑(DH) - (∑D)(∑H)/n S=------------------------------- ∑(D2) – (∑D)2 /n

In which

S= Slope of a line in a plane

D= Distance from the reference line,

H= elevation of the grid point

N= number of grid points

(iv) Compute the formation level, cuts and fills- With the elevation of the centriod determined

the formation level of any point (the elevation which the point should attain after land grading

operation) may be determined using the computed or assumed Sx and Sy

(v) Determine the cut fill ratio- Experience in land grading with modern earthmoving equipments

has shown that the cut-fill ratio should be greater than one. This means that a greater volume of

cut than fill must be allowed. If grid existing elevation is more than the formation level, it means

the grid point will have cut and vice-versa. So sumup all the cuts and fills separately then

estimate the cut fill ratio.

Other methods of land levelling, some problems related to land levelling and Earth work

calculations method are given in PDF files

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