**Acknowledgements**

**4. Conclusions**

12 Engineering and Mathematical Topics in Rainfall

peak discharge values.

This chapter presents hydrological modeling results using various types of design hyetographs and their influence on the design discharge values. The presented methods can be used for the design peak discharge value or even complete design flood hydrograph definition in case of ungauged basins or in cases where very little discharge data are available. The HEC-HMS model and the unit hydrograph theory were used in the modeling process. This model is frequently used in the hydrologic engineering practice around the world. Surface runoff was calculated using various design hyetographs where the focus was arbitrarily set on the 10-year return period. Based on the presented results, we can propose general guidelines on how to determine

i. Use at least 20 years of high-frequency data from the closest rainfall station to determine

ii. Based on the time of concentration of the investigated area (i.e., catchment), determine the design storm duration. Based on the modeling and design aim, select appropriate return

iii. Transform the dimensionless Huff curves into the design storm hyetograph using the appropriate total rainfall amount and duration. Select the median Huff curve as the representative one and two others (e.g., 20th and 80th percentile curves, depending on the

Using aforementioned procedure of design hyetograph determination and selected hydrological model, one can calculate either the complete design flood hydrograph or just design

**a.** Huff curves represent the actual rainfall distribution within rainfall event and should

**b.** Differences among available methods for the design storm definition were relatively large which was shown for the investigated Glinščica catchment and selected 10-year return period. Furthermore, even larger differences can be expected for longer return periods. Thus, one should avoid assuming and specifically using constant rainfall intensity during rainfall event in order to construct design hyetograph because this can result in the underestimation of the design discharge values and leads to the higher uncertainty in the design flood estimation because differences between constant (uniform) and temporal rainfall distribution determined by the Huff curves should not be neglected. In the case that data from nearby rainfall station are available, one should preferably use Huff curve and the

procedure is described in this chapter to determine the design discharge values.

**c.** In case that measured discharge data are available, it is necessary to include these data in the process of the design flood hydrograph definition (either for statistical analysis or

) catchments:

the design hyetograph in case of ungauged (small size, i.e., less than 20 km<sup>2</sup>

the Huff curves and the intensity-duration-frequency (IDF) curves [22].

period, and from the IDF curves, determine the total rainfall amount.

Furthermore, next conclusions can be made based on the presented results:

design purpose) to calculate the confidence intervals.

therefore be used in practical hydrologic applications.

hydrological model calibration and validation).

The results of the study are part of the Slovenian national research project J2-7322: "Modelling hydrologic response of nonhomogeneous catchments" and research program P2-0180: "Water Science and Technology, and geotechnical engineering: Tools and methods for process analyses and Simulations, and development of technologies" that are financed by the Slovenian Research Agency (ARRS). We wish to thank the Slovenian Environment Agency for data provision.
