Climate change contributes to local weather modifications such as storm events and heavy rainfall, and in particular, changes in the frequency and intensity of extreme storm and rainfall events. Observations have indicated that rainfall-runoff processes, like sewage overflow may induce fecal contamination in coastal water. Pathogens present in runoff water could therefore contaminate shellfish and be responsible for food-borne disease outbreaks. Mathematical models for this were used to predict the effect of climate modifications on shellfish quality. Using a hydrodynamic model, simulations of water quality conditions under a variety of storm event scenarios were run and are presented here for a harvesting area in France. Major storm events occurring in the watersheds were selected to estimate the critical fecal loads which could be due to overflow. Escherichia coli fluxes were calculated for these scenarios. Norovirus fluxes were estimated from data in the literature. The hydrodynamic model was then used to calculate the impact of hydraulic load and wastewater inputs on seawater quality. Our results indicated that major rainfall events generated relatively high runoff volumes and high pathogen loadings to the sea which led to shellfish contamination. After an extreme contamination event, the time needed to recover good water quality as calculated using the model, depended on the sectors in question.