ABSTRACT
In the existing design methods, consideration is given only on the stochastic process of flood flows imposed on the structure to account for hydrologic risk and the capacity of the structure is treated as deterministic. This thesis presents composite risk models which integrated hydrologic and hydraulic risks in the design of spillway structure. Both static and dynamic risk models were developed and applied to Ogee Type Spillway. While the static model concentrates on the occurrence of single (worse) flood magnitude, the dynamic model takes into account the repeated nature of the random flood magnitude imposed on the structure. Results show that risk evaluated by the conventional method which only considers hydrologic risk underestimates the risk of failure and this becomes significant when return period and safety factors are large. Risk, safety factor, service life, and return period relationships were established and these can be incorporated into optimisation model for determining minimum cost spillway systems.