11.3. SELECTION OF SUITABLE CROSS-DRAINAGE STRUCTURE Relative differences in bed and water levels of a canal with those of the crossing stream as well as their discharges are the main factors for deciding the type of cross-drainage structure at a site. By suitably changing the alignment of the canal between offtaking point A and the watershed (Fig. 11.8) the relative difference between the bed levels of the tributaries and the canal at the crossing site can be altered. Consider three possible alignments ABC, ADE, and AFG of a canal taking off from a river at A and intersecting a tributary HBDFI at B, D, and F before mounting the watershed at C, E, and G, respectively (Fig. 11.8). The distances AB, AD, and AF are almost the same and, hence, the canal reaches the crossing site with its bed more or less at the same level. But, the bed levels of the tributary at B, D, and F are significantly different due to the slope of the tributary. Obviously, the bed level of the tributary is the highest at B, and the lowest at F in the reach BDF. Thus, if F is a suitable crossing site for aqueduct, site D may necessitate the construction of a siphon aqueduct or level crossing and site B may require the construction of a siphon or a superpassage. Thus, the type of cross-drainage structure can be changed by suitably altering the crossing site.
