Practical Issues in Hydrologic modeling for flood management of Watercourses Running Through Urban Environments in Greece

The traditional training method of watercourses with closed conduits applied some decades ago is no longer environmentally acceptable. Nowadays all such projects are required to get environmental permit before construction, the natural state of watercourses should be preserved; open cross sections and materials friendly to the environment such as gabions, should be used in most cases. Existing structures and upstream control should be integrated in the final layout. Recently, programs have developed in Europe, e.g. Ecoflood, Daywater, investigating the possibilities of control at the source and restoration of the floodplains. However, this is not always feasible in heavily urbanized areas and especially for high return period floods. Determination of design discharges is an important factor. Design for a given return period is not uniquely defined and may vary considerably depending on the selection of the parameters and methodologies involved. New guidelines require higher return period for the design. In the case of watercourses with ephemeral flow data for calibration are usually not available. Thus, increased urbanization, higher design return period and way of computation result in higher design discharges than before. It is considered necessary to assure that the final design can handle discharges higher than the design discharge with reduced freeboard and that critical points, such as road and highway crossings are designed to handle higher return period floods with safety. Sensitive points of existing closed sections to pressurized flow should be identified and protected. It is proposed that hydrologic models are used that give the time and spatial distribution of peaks along with lag time estimations similar to those proposed by the hydrographs given in the Design of Small Dams.