Hydrologic modeling for the determination of design discharges in ungauged basins

Abstract

All stormwater management projects in Greece are required to get environmental permit
 before construction. The design return period is often determined by the environmental
 permit. Determination of design discharges is an important parameter for the design.
 Design discharge for a given return period is not uniquely defined and may vary
 considerably depending on the selection of the parameters and methodologies involved.
 Using a rainfall height distribution that tends to maximize the peak discharge (worst profile
 distribution) in essence corresponds to a lower probability of occurrence that is not
 quantified at present. There are publications, based on data from the US and UK that
 show that center-loaded storms are appropriate for design of stormwater systems.
 In this paper a test case study is presented. Possible variation of the estimated flood
 peak that can result from variation of rainfall distribution of given total height and duration
 is shown. Comparisons are shown between the rational method, hydrographs based on
 the ones given in Design of Small Dams and also with modeling with the HEC-HMS
 system using SCS hydrographs. Results from the HEC-HMS modeling include variation
 of the rainfall peak location, use of two different idf curves and SCS - Type I storm. It is
 seen that the same discharge value can be derived from different storm durations or
 different return periods depending on the rainfall distribution. Another interesting
 unresolved matter is the size of sub basins that should be included in modeling of
 hydrographs as the processes involved are not linear and the computed results may vary.
 It is proposed that hydrologic modeling is used including a considerable part of the
 drainage system of a basin and different “scenarios” of rainfall distribution and rainfall
 duration are used to get a feeling of the possible variations of the T-year return flood that
 the environmental permit describes, before more strict guidelines are adopted.