We propose a new approach to model the dependence structure for aggregating the risk of flood damages from a local level to larger areas, which is based on the structure of the river network of a country and can be calibrated with publicly available data of river discharges. Building upon a suitable adaptation of max-stable processes for a flood-relevant geometry as recently introduced in [1], this enables the assessment of flood risk without the need of a hydrological model, and can easily be adapted for different countries. We illustrate its use for the particular case of Austria. For this purpose, we first develop marginal flood models for individual municipalities by intertwining available HORA risk maps (which represent a detailed zoning of the country into flood risk return level segments) with information about the actual location of buildings. As a second alternative for the marginal modelling, we also advocate an approach based on suitably normalized historical damage claim data of municipalities together with techniques from extreme value statistics. We implement and compare the two alternatives and apply the calibrated dependence structure to each of them, leading to estimates for average flood damage as well as its extreme quantiles on the municipality, state as well as country level. As a by-product, this approach allows to quantify the diversification potential for flood risk on each of these levels, a topic of considerable importance in view of the natural and strong spatial dependence of this particular natural peril. The results of this paper considerably refine earlier respective estimates for Austria.