Example $12\mathrm{.}1$ Design guide bunds $($or banks$)$ and launching apron required to be provided for a bridge across a river whose total waterway is $658\mathrm{.}88$ m$.$ The design flood discharge is $13100$ m$^3/$s which may be increased by $20\%$ for the design of launching apron. Mean size of the river bed material is $0\mathrm{.}3$ mm$.$