The probability of a major flood of the Seine River, of a level equal to the great flood of 1910, cannot be overlooked, given the significance and vulnerability of the assets now situated on the floodplains of the Île-de-France. This chapter provides a detailed analysis of the potential impacts of such an event on the public well-being, the operation of critical networks and business activities. The use of a dynamic model provides an exploratory analysis of the macroeconomic impact of such a large-scale shock in terms of gross domestic product, employment and the public finances.