Necessity of Cross Drainage Works

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.