Systematic approach for ungaged basins’ discharge determination in Western Peloponnese, Greece
Determining peak flows for ungaged areas is difficult and involves high uncertainty. Advanced computer capabilities provided by geographical information systems and hydrologic modeling software are used for detailed determination of the parameters involved, both for simple methods as the rational method and more detailed ones as the application of synthetic hydrographs. For the determination of the runoff coefficients and other runoff parameters, digital elevation models were combined with geologic and land use maps in order to extract morphologic and other distributed parameters with the use of GIS for computation of watershed runoff. The discharges were determined by the rational method, the Fuller’s formula and a hydrologic model using synthetic hydrographs according to SCS. A dependable estimate of the design discharge is necessary both for demarcation studies of watercourses and the assessment and management of flood risks according to the guidelines of Directive 2007/60/EC. In this paper a case study in Western Peloponnese is presented. Area specific fitted equations relating the drainage area to the peak flows were developed, that can be used in adjacent regions. The results of the hydrologic studies performed for many geographical units of the new Patra - Pyrgos - Tsakona highway, presently under construction, are analyzed and presented. Results of the rational method and modeling with the HEC-HMS system using SCS hydrographs were compared. Transverse drainage works, where the motorway crosses watercourses, were designed with the 50- yr design discharges determined by the hydrologic model with the use of synthetic hydrographs, lag time and CN, depending on land use, geology and soil cover, determined according to SCS. In the case of bridges with piers the corresponding 100-yr values were used. Differences between the rational method and the hydrologic model are not significant in most cases. Pronounced differences were noticed only in the combined presence of high water transmissivity soils and forest cover. Regional equations were developed for use. In the basins used there are no areas with major urban development or basins with control structures. The equations refer to the 50-yr design flood and can be used in any further computation that may be required in the vicinity without further hydrologic analysis. Comparison with other studies shows good agreement and with similar equations from other projects resemblance.