ABSTRACT
Surface ‘drainage’ problems occur in nearly flat areas, uneven land surfaces with depression or ridges preventing natural runoff and in areas without outlet. Soils with low infiltration rates are susceptible to surface drainage problems. It is also important in both rainfed and vegetated areas. Surface drainage is intended for safe removal of excess water from the land surface through land shaping and channel construction. It uses the head difference that exists due to land elevation to provide necessary hydraulic gradient for the movement of water. Functionally the system may be considered as-(i) Collection system, (ii) Conveying system, or (iii) Outlets. Water from the individual field is collected and is then removed through the conveying system to the outlet. Four types of surface drainage system are used in flat areas having a slope less than two per cent for drainage of agricultural lands. They are-(i) random drain system, (ii) parallel field drain system, (iii) parallel open ditch system, and (iv) bedding system. In sloping areas with a slope greater than two per cent the system consists of-(i) bench terracing, (ii) graded bunding, and (iii) cross-slope ditch system or intercepting system. Generally open ditches are commonly adopted. Further surface drainage is required for: (a) the removal of storm rainfall where the subsurface drainage is not economically feasible, (b) the collection and disposal of surface irrigation runoff, and (c) the collection and disposal of drainage in deltaic areas. The first situation is commonly found in heavy soil situation in the tropical area. Generally, wet season rains of high intensity fall on soils with low infiltration rates and surface drainage
is required to remove the excess. Therefore, estimation of excess storm runoff is necessary in order to design a field surface drain which will shed it without erosion and to design disposal by the channel having adequate capacity. The third situation arises because the land elevation of the lower estuarine region is nearly the same or sometimes lower than the tidal high water level of the adjacent estuary or tidal creeks. The river water levels frequently rise above the levels of the land surface which slope away from the river margins and there is the liability to frequent and excessive submergence with corresponding risk of serious damage to occupied areas and cultivation by the intrusion of saline water. Great difficulties are experienced in disposing of and drainage of this water and also of the rain falling in the area. In earlier days, these difficulties were not accentuated because only the elevated grounds were occupied and the great extent of unoccupied low-lying lands in depressions gave storage capacity into which drainage was disposed off without inconvenience. Since the land in this area is very fertile, cultivation extended and the lands become more valuable, blocks lying at lower and lower levels were occupied, drainage difficulty appeared and became more and more accentuated. Attempts have always been made to save the areas from inundation by constructing embankments on the tidal creeks and estuaries and by excavating a network of drainage channels. The outflow of these drainage channels is obstructed by the continuous variation of tidal levels at the outfall. Therefore, lands up to a certain elevation can only be drained by gravity flow with suitable sluice at the outfall. Below this level, the gravity drainage through such channels becomes more and more difficult and sometimes, it becomes necessary to install large pumps near the outfall. At this lower level, it is not economically feasible to secure adequate drainage, and cultivation of the land must be attended with risk of great losses of crop from submergence after any abnormally heavy rainfall. In the upper region of the estuary, the drainage problem is not so acute as the ground elevation is in general higher than the tidal high water. Drainage channels in this region may not be sluiced at the outfall and the tide water is allowed to play freely in such a channel. Thus the selection of the type of drainage channel in deltaic areas is mainly governed by the following factors: (i) Relative elevation of land surface to be drained with respect to tidal levels at the outfall. (ii) Volume of water to be drained.
