The parallel ditch pattern is suitable for flatter, poorly drained soils that have numerous shallow depressions (Figure 4). Dead furrows are neither desirable nor necessary. Although the ditches must be parallel, they need not be equi-distant. The spacing between them depends upon the permissible length of row drainage for the soil type and upon the amount of earth and the distance it must be moved to provide complete row drainage. The maximum length of the grade draining to a ditch should be 200 m. The success of a parallel pattern depends largely upon proper spacing of the parallel ditches and the smoothing or grading between them. During the grading operation, fill all depressions and remove all barriers. Excavated material from ditches can also be used as fill for establishing grades. 2.9. Shaping the surface.
Land grading (also termed precision land forming) is the reshaping of surface of land with tractors and scrapers to planned grades. Its purpose is to provide excellent surface drainage although the amount of grading will depend upon the soil and costs. To do a good job of land grading, you need a detailed engineering survey and construction layout. To assure adequate surface drainage, eliminate all reverse surface grades that form depressions. The recommended surface grades range from 0.1 to 0.5 per cent and may be uniform or variable. The cross slopes normally should not exceed 0.5 percent. Minimum grade limits should include a construction tolerance that will permit the elimination of all depressions either in original construction or in post-construction touch up.
Figure 4: Parallel drain system.
In fields that can be cultivated up and own slope, parallel field ditches are installed across the slope to break the field into shorter units Of length and make it less susceptible to erosion. The field should be farmed in the direction of the greatest slope.
Reverse grades can be eliminated with relative ease in a field that has "minimum grades of 0.2 per cent. Unusual precision in construction is required to eliminate reverse surface grades in fields that have 0.1 per cent and flatter grades. Land grading is hampered by trash and vegetation. This material should be destroyed or removed before construction and kept under control while the work is being done. The fields should be chiseled before construction if there are hard pans. The field surface should be firm when it is surveyed so that rod readings taken at stakes will reflect true elevation. Do not grade fields when they are wet because working wet soil impairs physical condition of soil.
Land smoothing removes irregularities on the land surface and should be done after land grading and may be useful in other situations. Special equipment such as a land plane or land leveler should be used (Figure 5).
Figure 5 : Smoothing operation.
The purpose of land smoothing is to improve surface drainage. The smoothing operation may ordinarily be directed in the field without detailed surveys or plans, although grid surveys may be needed for some critical parts of the field. A smoothing operation consists of a minimum of three passes with a land leveler.
Make the first two passes on opposite diagonals as noted in Figure 5 and the last pass in the direction of cultivation. Either before or after the final land smoothing operation chisels fields to loosen the cut surfaces and to blend the fill material with the underlying soil. The finished surface should be free from minor depressions so that runoff will flow unobstructed to field or lateral ditches.