Necessity of Cross Drainage Works
A cross drainage work (also called CD work) is a structure built on a canal where it is crosses a natural drainage, such as a stream or a river. Sometimes, a cross-drainage work is required.when the canal crosses another canal. The cross-drainage work is required to dispose of the drainage water so that the canal supply remains uninterrupted. A cross-drainage work is also called as drainage crossing. The canal at a cross-drainage work is generally taken either over or below the drainage. However, it can also be at the same level as the drainage.
The canals are, preferably, aligned on the watershed so that there are no drainage crossings. However, it is not possible to avoid the drainages in the initial reach of a main canal because it takes off from a diversion headworks (or storage works) located on a river which is a valley. The canal, therefore, requires a certain distance before it can mount the watershed (or ridge). In this initial reach, the canal is usually a contour canal and it intercepts a number of natural drainages flowing from the watershed to the river. After the canal has mounted the watershed, no cross-drainage work will normally be required, because all the drainage originate from the watershed and flow away from it. However, in some cases, it may be necessary for the canal to leave the watershed and flow away from it. It may be necessary for the canal to leave the watershed for a short distance where the watershed takes a sudden small loop and it is not possible to align the canal along the loop. In that case, the canal intercepts the drainages which carry the water of the pocket between the canal and the watershed and hence the cross-drainage works are required.
A cross-drainage work is an expensive structure and should be avoided as far as possible. The number of cross-drainage works can be reduced to some extent by changing the alignment of the canal. However, it may increase the length and hence the cost of the canal.Sometimes it is possible to reduce the number of cross-drainage works by diverting the small drainages into large drainages or by constructing the cross-drainages work below the confluence of two drainages by shifting the alignment. However, the suitability of the site for the construction of the structure should also be considered while deciding the location of the cross-drainage works.