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.