**Design Rainfall in Engineering Applications with Focus on the Design Discharge on the Design Discharge**

**Design Rainfall in Engineering Applications with Focus** 

DOI: 10.5772/intechopen.70319

Nejc Bezak, Mojca Šraj and Matjaž Mikoš Nejc Bezak, Mojca Šraj and Matjaž Mikoš Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70319

#### **Abstract**

Design hyetograph or design storm definition is one of the most important parts of the design discharge determination in case of ungauged catchments. Design hyetograph duration and temporal rainfall distribution can have large impact on the peak discharge values and the shape of the runoff hydrograph. The influence of these two factors on the design runoff values is presented in the case study of the Glinščica River catchment that covers 16.85 km<sup>2</sup> and it is located in central part of Slovenia, Europe. A combination of Huff and intensity-duration-frequency (IDF) curves is used to construct the design hyetograph for the presented case study. The duration of the design storm is determined by the catchment time of concentration. The results are compared to the Natural Resources Conservation Service (NRCS) curves and the so-called frequency storm method. The hydrological modeling result that was carried out using the hydrologic modeling system (HEC-HMS) software indicates that differences among different methods should not be neglected. For the 10-year return period, differences in the peak discharge values can be larger than 10%, while even larger differences can be expected for longer return periods. Some studies showed that these can be larger than 50%. Therefore, the guidelines on how to construct the design hyetograph are presented.

**Keywords:** design rainfall, design discharge, modeling, rainfall, hydrologic engineering

## **1. Introduction**

Design peak discharge values or in some cases, even the complete design hydrographs are needed for the design, planning, and construction of different hydraulic structures such as dams, water retention reservoirs, and levees that can be used to improve the flood safety. These design values or design hydrographs (sometimes also called design floods) can be determined using various approaches. In case of gauged catchments or when plenty of discharge data

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

are available, the most commonly used approach is to perform the flood frequency analysis (FFA). Most often univariate approach is selected where usually only peak discharge values are considered in the analysis (e.g., [1]). Alternatively, multivariate approach, where besides peak discharge, also hydrograph volume and (or) hydrograph duration are selected, can be carried out. Copula functions can be used to perform the multivariate flood frequency analysis (e.g., [2–4]). Using the FFA approach, the relationship between the design discharge and the return period is estimated (e.g., [1, 5, 6]). This relationship can then be used for the design of, for example, different engineering structures or river channels. The adequate return period is selected according to acceptable risk or estimated flood damage. On the other hand, in some cases, complete design hydrograph or design flood is needed. For example, unsteady hydraulic analysis of different engineering structures, such as bridges or culverts, requires complete design hydrograph. This can be determined with the combination of the FFA results and the analysis of past measured extreme events in order to determine the shape of the design hydrograph.

analyzed. For example, Bonta [14] indicated that different Huff curves should be derived for different seasons and Bonta and Shahalam [15] showed that a sample of 110–140 rainfall

Design Rainfall in Engineering Applications with Focus on the Design Discharge

http://dx.doi.org/10.5772/intechopen.70319

3

The main aims of this chapter are as follows: (i) to make an overview of the procedures available for the definition of the design rainfall events (e.g., Huff curves, frequency storm method, NRCS curves); (ii) to describe the procedure for the definition of the Huff curves that can be used to define the temporal rainfall distribution; and (iii) to analyze the influence of the tem-

In order to investigate the impact of the temporal rainfall distribution and design rainfall duration on the design discharge and design hydrograph values, we used a case study from one of the experimental catchments in Slovenia [16, 17]. The Glinščica catchment is part of the Gradaščica catchment [16] and it is located in the central part of Slovenia (**Figure 1**). Part of the Glinščica catchment is also located in the urban area of the Ljubljana city; therefore, the orographic catchment boundary does not represent the actual catchment area [16, 18]. Thus,

**Figure 1.** The Glinščica catchment with the location of the Ljubljana-Bežigrad rainfall station and the water gauging station.

events (storms) is large enough to derive a stable set of Huff curves.

**2. Data and methods**

poral rainfall distribution and rainfall duration on the design discharge values.

In cases when no measured discharge data are available, a procedure suitable for the ungauged catchments should be selected (e.g., [7]). Ungauged catchments are those where very little or no discharge data are available. Among a set of possible procedures with different complexity for the definition of the design discharge values in case of ungauged catchments (e.g., regional flood frequency analysis), one can also use design rainfall events (also named design hyetographs or design storms) in combination with hydrological model to determine the design peak discharges and complete design hydrographs (e.g., [8]). In most cases, a nearby rainfall gauging station can be used to determine the design rainfall events. These design hydrographs can then be used for unsteady hydraulic analysis and modeling. However, appropriate rainfall properties should be used to construct the design rainfall events because in case of ungauged catchments no discharge data are available and the uncertainty in the determined design peak discharge values and complete design hydrograph depends on the model parameters and selected design storm. In addition to the intensity-duration-frequency (IDF) curves [4], temporal rainfall distribution within rainfall event also named internal storm structure (can be described with Huff curves) is important part of this procedure (i.e., design rainfall determination) and can have significant influence on the hydrological model results [9]. For example, if most of the rainfall occurs in the second part of the rainfall event, this situation is more critical from the surface runoff point of view than the case where most of the rainfall occurs in the first part of the event due to the lower antecedent wetness in this latter situation (e.g., [9]). In case that limited discharge data (e.g., some rainfall-runoff events) are available for the investigated catchment, this information should be used for model calibration.

Different procedures are possible for the determination of the design rainfall events such as Natural Resources Conservation Service (NRCS) rainfall characteristics also named rainfall profiles known as Types I, IA, II, and III (e.g., [10]) that correspond to a specific region in the United States. Moreover, also other methodologies can be found in literature (e.g., [11–13]). Huff curves [11] connect dimensionless rainfall duration with dimensionless rainfall depth and can be derived based on the high-frequency measured rainfall data. Different Huff curves can be constructed depending on the rainfall event duration (e.g., [11]). From 1967, when the Huff curves were proposed by Huff [11], several different aspects of these curves have been analyzed. For example, Bonta [14] indicated that different Huff curves should be derived for different seasons and Bonta and Shahalam [15] showed that a sample of 110–140 rainfall events (storms) is large enough to derive a stable set of Huff curves.

The main aims of this chapter are as follows: (i) to make an overview of the procedures available for the definition of the design rainfall events (e.g., Huff curves, frequency storm method, NRCS curves); (ii) to describe the procedure for the definition of the Huff curves that can be used to define the temporal rainfall distribution; and (iii) to analyze the influence of the temporal rainfall distribution and rainfall duration on the design discharge values.
