Land Forming

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Water Flow

The greatest proportion of broad acre irrigation in Australia is flood irrigation. The water is conveyed in open earth channels. It is then applied to the land through some form of outlet in the channel bank. Once on the land the water flow is controlled by various methods such as contour banks, border check banks or furrows. The control method is determined by the crop to be grown. Regardless of the method used to control the water a slope must always be present on the land to be irrigated. Where natural slopes are used there are nearly always areas that are slightly too high to water and other areas where water will pond and the land will become swampy. In both cases crops will not grow and the land will become degraded, raising water tables and bringing salt to the surface.

Early Landforming

The first type of landforming was done by taking earth from the high spots and placing it in the low spots. This work was performed by machines called land planes, these worked on the same principle as a carpenter’s plane. The land plane had a very long wheelbase with a bucket in the middle. When the machine passed over a mound in the field the blade would cut in removing the high spot. The dirt that was picked up would then be dragged in the bucket until a low spot was found. At this point the earth would fall out of the bucket. In the late 60’s it was not uncommon to see land planes of 100 feet in length and 14-16 feet wide being pulled around irrigation fields. The land plane made a difference to flood irrigation efficiency. It ironed out the bumps and hollows but the irrigation farmer still had to use the natural grades that were present in his field. This meant variation in water infiltration rates, which in turn caused variation in crop growth. Large numbers of channels and control structures were needed because of the contorted nature of the contours of the land. The next stage in the development of landforming was to actually change the natural grades of the land within an irrigated field to a man-made grade that was constant over the whole field. To do this almost every point in the field had to either have dirt removed (cut) or dirt placed (fill).

Grid Survey

The first step is to place survey pegs every say 30-100 metres to form a grid over the whole field. The field is then surveyed to find the height of the ground at each of those pegs. This information is plotted onto a plan and contour lines drawn in. The designer must then settle on a grade to use. The usual idea is to have a constant fall in the direction the water is to flow and have the land level at right angles to that direction. He must also decide on the height of the finished field so that the earth produced by the cuts balances the earth needed for the fills. When designing manually (not using a computer) this is very much a trial and error procedure with only a few shortcut tricks to help. Having decided on a grade and a finished field height the designer must then calculate the volume of cut or fill at every grid point in the field. In doing this he assumes that the height at the peg represents a square the same size as the grid, the peg being at the center of the square. Having made all these calculations they are then added up to give a total of cut and a total of fill. Hopefully these two will balance. If not the field height must be adjusted and the whole procedure repeated. In practice it was found that the volume of cut always needed to be greater than the volume of fill to allow for losses during earth moving operations and a few other more obscure factors. The ratio of cut to fill tends to vary from district to district and soil type to soil type.

Computer Program

The development of a computer program such as Compute-a-Grade, to do landform design, was essential. What used to take hours of calculations can now be done in seconds. This enables many options of water flow direction, grades and field sizes to be examined before deciding on a finished design. It is also possible to do calculations that were too complex to do manually such as the grade of best fit. This is the single grade that can be used which involves the least amount of earth to shift. This is rarely the best design but it gives a very useful starting point. A well known designer always used to say “the cheapest way to shift dirt is on paper.” But at some point we have to take our design and actually move the dirt. At first this was done with elevating scrapers and drag scrapers, more recently drag scrapers have given way to machines that can both drag and carry earth in their buckets. At first machines were controlled manually and levels had to be taken to check that the correct amount of earth was being shifted. When the bulk of the earth had been moved the field was finished with a land plane.

Laser Levelling

During the early 70’s lasers started to be used to control the machines which were shifting the earth. The laser is set up in the middle of the field on a stand about 10 feet high. The laser is a very fine beam of laser light that is spinning around a vertical axis. This beam effectively creates a horizontal disc of light about 10 feet above the field. The machine moving the earth has a sensor mounted on its bucket high enough to intercept the light beam. The sensor is able to detect if it starts to move out of the beam and automatically raises or lowers the bucket so that the sensor stays within the beam. If the laser is tilted so that the light disk is parallel to the finished field it is only a matter of setting the sensor height at one point in the field and the bucket will then follow the finished design surface as the machine moves around the field. The laser removed the necessity to finish off with a land plane. A good carry bucket controlled by a laser can now move large volumes of earth and finish to a high degree of accuracy.

Global Positioning System (GPS)

Nowadays surveys are often done using GPS, which removes the necessity to peg a grid over the field. The position in the field and the elevation of that position are determined using satellites and a ground differential station. The survey is usually transferred to the computer electronically and the designer doesn’t have to type in the data.