Tunnels and cofferdams serve as the diversion of river water in a dam construction. One may choose a very large diameter tunnel capable of carrying the largest flood predicted or one may design a very high upstream cofferdam height, only enough to form a barrier to the river. Between these two extremes, there lie numerous possible combinations of tunnel diameter and cofferdam height, from which the best practical combination entailing the least cost should be sought. Given the fact that the cost of diversion works could be as high as 10-20 of the total dam construction cost, due attention should be paid to optimum design of the diversion works. Through various mathematical calculations and design considerations, it was found out that when the valley is narrow, a higher cofferdam is proffered. For wide valleys, larger tunnels are preferred because an increase in height(H) leads to significant overall cost increase. However, if the valley is very wide (width(W) greater than 600-700m) the growth of diameter(D) stops at a limit and the curves become horizontal. Longer tunnels are generally associated with smaller optimum diameter (Dopt regardless of diversion discharge (Q) and W and the support type. When the tunnel is very long, an overly large tunnel must be avoided and heightening of the cofferdam should be examined instead. Another option is to use multiple diversion tunnels because large cross-section tunnels are associated with higher tunnels stability risk and complicated excavation or support equipment.