3. Results and discussion

This section presents the results of hydrological and hydraulic model calibration and validation and some results of the investigation of the influence of the proposed flood safety protection measures in the Savinja River catchment.

#### 3.1. Hydrological model and analysis

The hydrological model was calibrated based on the flood event that occurred in September 2007 and caused large damage in different parts of Slovenia [2]. The average value of the Nash-Sutcliffe coefficients for the calibration of the model for the 21 sub-catchments (with available discharge data) was 0.85. Figure 9 shows an example of the calibration results for the location of the Laško gauging station on the Savinja River with the Nash-Sutcliffe coefficient as 0.93.

Figure 9. Hydrological model calibration results using the data from year 2007 for the station Laško on the Savinja River (in the lower figure with red and blue is simulated and observed discharge, respectively).

The validation of the model was performed using the data from floods that occurred in years 1990 and 1998 and also caused large damage in the Savinja River catchment [3, 4]. For the 1990 event, the average value of the Nash-Sutcliffe coefficients for nine stations with available data was 0.85. Using the calibrated and validated hydrological model, we were able to reconstruct the hydrological situation in the Savinja catchment also for the locations where discharge data were not available (either no gauging station or station was damaged during the flood) for floods that occurred in years 1990, 1998 and 2007. Table 1 shows calibration results for the 2007 flood event for 19 sub-catchments where measured discharge data were available in order to perform evaluation of the hydrological model. Moreover, Table 2 shows hydrological model validation results for the 1990 flood event for gauging stations with available measured discharge data. The number of gauging stations in the 1990 was smaller than in the case of 2007 because gauging network was extended in the recent decades and several gauging stations were damaged during the 1990 flood event.

properties table based on the underlying digital terrain model used (1 m resolution). The HEC-RAS pre-processor computes the elevation-volume relationship and other geometric characteristics crucial for hydraulic calculations for each cell face [16]. Figure 8 shows the main rivers that were included in the hydraulic model from the confluence of the Savinja and Dreta Rivers to the confluence of the Savinja and Sava Rivers. It should be noted that due to the improved 2D modelling algorithm that is implemented in the HEC-RAS version 5 [16], the entire 135 km of the river network with multiple flood areas was modelled as one model. Moreover, the total

This section presents the results of hydrological and hydraulic model calibration and validation and some results of the investigation of the influence of the proposed flood safety protec-

The hydrological model was calibrated based on the flood event that occurred in September 2007 and caused large damage in different parts of Slovenia [2]. The average value of the Nash-Sutcliffe coefficients for the calibration of the model for the 21 sub-catchments (with available discharge data) was 0.85. Figure 9 shows an example of the calibration results for the location of the Laško gauging station on the Savinja River with the Nash-Sutcliffe coefficient as 0.93.

Figure 9. Hydrological model calibration results using the data from year 2007 for the station Laško on the Savinja River

(in the lower figure with red and blue is simulated and observed discharge, respectively).

computational time did not exceed 2.5 h.

76 Achievements and Challenges of Integrated River Basin Management

tion measures in the Savinja River catchment.

3.1. Hydrological model and analysis

3. Results and discussion


Note that computational period to calculate discharge sum was from 1.3.2007 to 14.12.2007.

Table 1. Hydrological model calibration results for the 2007 flood event for the 19 sub-catchments where measured discharge data were available.


Table 2. Hydrological model validation results for the 1990 flood event for the sub-catchments where measured discharge data were available.

In order to define the design hydrographs, the flood frequency analysis was also performed. The annual maximum method was used for sample definition and log-Pearson type III distribution was applied to define the relationship between design discharge and return period.

#### 3.2. Hydraulic model and analysis

The calibration and validation of the hydraulic model were also performed using the data from 1990, 1998 and 2007 floods. Besides discharge data, information about water level was also used (rating curves were used to transform water level data to discharge). Comparison between the measured maximum flood extent on the floodplain areas and computed inundation extent was also carried out. Figure 10 shows an example of the calibration results for the gauging station Celje on the Savinja River in the year 1990. Similar results were also obtained for some other gauging stations in the Savinja catchment for the 1990, 1998 and 2007 events. Model evaluation was performed on rivers Dreta, Ložnica, Voglajna, Hudinja and Savinja. Figure 11 shows calibration results for the large natural floodplain area before the Celje city for the 1990 event. Similar graphical comparison was also carried out for other flooding areas.

#### 3.3. Flood safety

The calibrated and validated hydrological and hydraulic models of the Savinja River catchment were used to investigate the impact of the proposed flood protection measures on the flood safety. The main suggested flood protection measures are dry retention (flood-control) reservoirs that are planned to be built at several locations in the Savinja catchment. Eight floodcontrol reservoirs are to be constructed in the location of the large natural flood area before the Celje city (Figure 11). Relatively sophisticated and complex hydro-technical equipment is selected to operate these reservoirs with the total volume of approximately 8<sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> . Figure 12 shows comparison between three different situations, namely natural-actual conditions during the 1990 event, full operation of the proposed flood-control reservoirs with increased volume

(retention of 10106 m3

floodplain inundation).

increase in peak discharge for approximately 100 m3

) and proposed flood-control reservoirs that failed to operate. We can

/s due to the exclusion of large natural

/s;

conclude that proposed flood-control reservoirs reduce the peak discharge for about 150 m<sup>3</sup>

however, potential technical problems with hydro-technical equipment would lead to an

Figure 11. Calibration results for the largest natural floodplain area before the Celje city for the 1990 event (light blue is modelled extent of floodplain inundation by combined 1D/2D model and grey with pink outline is measured extent of

Figure 10. Calibration results for the gauging station Celje on the Savinja River for the 1990 event (blue is modelled water

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level and red is measured water level by the Slovenian Environment Agency).

floodplain area (Figure 12). It can be seen that the construction of the reservoirs would lead to about 15% decrease in the peak discharge compared to the natural conditions during the 1990 event. This means that the flood risk downstream of the Celje city would decrease in case of

operation of reservoirs without any problems and according to the procedure.

Historical, Hydrological and Hydraulics Studies for Sustainable Flood Management http://dx.doi.org/10.5772/intechopen.74432 79

Figure 10. Calibration results for the gauging station Celje on the Savinja River for the 1990 event (blue is modelled water level and red is measured water level by the Slovenian Environment Agency).

In order to define the design hydrographs, the flood frequency analysis was also performed. The annual maximum method was used for sample definition and log-Pearson type III distribution was applied to define the relationship between design discharge and return period.

Table 2. Hydrological model validation results for the 1990 flood event for the sub-catchments where measured

Measured discharge sum

Nash-Sutcliffe R2

[mm/period]

1-Savinja1-Luče 2562 3010 0.85 0.89 4-Dreta-Kraše 4442 4901 0.90 0.92 5-Savinja3-Letuš 4285 3692 0.59 0.89 12-Ložnica-Levec 1453 1845 0.94 0.96 13-Savinja5-Celje\_brv 2970 3111 0.97 0.98 15-Hudinja2-Škofja\_vas 1278 1382 0.79 0.83 17-Voglajna2-Celje 1011 1478 0.79 0.87 19-Savinja6-Laško 2444 2582 0.97 0.97 21-Savinja7-Veliko Širje 2320 1301 0.84 0.91

The calibration and validation of the hydraulic model were also performed using the data from 1990, 1998 and 2007 floods. Besides discharge data, information about water level was also used (rating curves were used to transform water level data to discharge). Comparison between the measured maximum flood extent on the floodplain areas and computed inundation extent was also carried out. Figure 10 shows an example of the calibration results for the gauging station Celje on the Savinja River in the year 1990. Similar results were also obtained for some other gauging stations in the Savinja catchment for the 1990, 1998 and 2007 events. Model evaluation was performed on rivers Dreta, Ložnica, Voglajna, Hudinja and Savinja. Figure 11 shows calibration results for the large natural floodplain area before the Celje city for the 1990 event. Similar graphical comparison was also carried out for other flooding areas.

The calibrated and validated hydrological and hydraulic models of the Savinja River catchment were used to investigate the impact of the proposed flood protection measures on the flood safety. The main suggested flood protection measures are dry retention (flood-control) reservoirs that are planned to be built at several locations in the Savinja catchment. Eight floodcontrol reservoirs are to be constructed in the location of the large natural flood area before the Celje city (Figure 11). Relatively sophisticated and complex hydro-technical equipment is

shows comparison between three different situations, namely natural-actual conditions during the 1990 event, full operation of the proposed flood-control reservoirs with increased volume

. Figure 12

selected to operate these reservoirs with the total volume of approximately 8<sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup>

3.2. Hydraulic model and analysis

discharge data were available.

Sub-catchment Model discharge sum

[mm/period]

78 Achievements and Challenges of Integrated River Basin Management

3.3. Flood safety

Figure 11. Calibration results for the largest natural floodplain area before the Celje city for the 1990 event (light blue is modelled extent of floodplain inundation by combined 1D/2D model and grey with pink outline is measured extent of floodplain inundation).

(retention of 10106 m3 ) and proposed flood-control reservoirs that failed to operate. We can conclude that proposed flood-control reservoirs reduce the peak discharge for about 150 m<sup>3</sup> /s; however, potential technical problems with hydro-technical equipment would lead to an increase in peak discharge for approximately 100 m3 /s due to the exclusion of large natural floodplain area (Figure 12). It can be seen that the construction of the reservoirs would lead to about 15% decrease in the peak discharge compared to the natural conditions during the 1990 event. This means that the flood risk downstream of the Celje city would decrease in case of operation of reservoirs without any problems and according to the procedure.

influence on the situation in the lower Sava River. This kind of local measures mostly have minor impact on the global situation in the larger catchment such as the Savinja River catchment but can lead to improved situation locally. Similar conclusions were also made for the

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Furthermore, several other aspects of the flood safety such as the impact of high waters at the river confluences on the downstream flood safety were also investigated but are not discussed

Using the calibrated and validated combined hydrological (HBV-light) and hydraulic (HEC-RAS 5) models, we investigated the influence of the proposed flood protection measures (e.g. several flood-control reservoirs are to be built in the large natural flood area before the Celje city) on the flood safety. Moreover, using the hydraulic model HEC-RAS that is presented in Section 3.2, we also investigated the backwater effect on different tributaries in the Savinja catchment. Figure 14 shows an example of the backwater effect on the Ložnica River. It can be seen that due to the increased peak discharge on the Savinja River, the maximum water on the Ložnica River also increases. This increase is the largest for the cross section located near the

decreases for upstream river station. Moreover, the backwater effect is detected for the cross section that is located 1.5 km upstream of the confluence of the Savinja and Ložnica Rivers. Similar analysis was performed for other rivers (e.g. Hudinja and Voglajna; Voglajna and

of analysis can be very useful also for the policy makers because it is essential to understand

Figure 14. The influence of the Savinja River on the Ložnica River (backwater effect) when the Ložnica input hydrograph is constant during different hydraulic model runs. Different coloured lines represent different cross sections on the Ložnica River where the number indicates river station from the confluence with the Savinja River upstream [m].

/s) and generally,

/s. This kind

rivers' confluence (about 0.6 m for peak discharge increase at 400 m<sup>3</sup>

Savinja). The backwater effect can be up to 0.25 m for a peak discharge of 1000 m3

case study of the alpine Inn River in Austria [17].

in this chapter.

3.4. Backwater effect

Figure 12. Impact of the proposed flood-control reservoirs with increased total volume (10106 m3 ) on the situation at the Savinja outlet during the 1990 flood (dark blue), exclusion of large natural flood area before the Celje city (situation when proposed flood-control reservoirs fail to operate, purple) and actual situation during the 1990 flood (light blue).

Moreover, several smaller flood protection measures (e.g. channel widening at critical cross sections, river banks' reconstruction, local level construction) are also proposed in the Savinja catchment (mostly on rivers Ložnica, Hudinja and Voglajna). The analyses of these measures showed that they mostly positively influence the flood situation at the confluence of Savinja and Sava Rivers. Flood protection measures mostly fasten the hydrograph propagation but often do not significantly influence the peak discharge values (the decrease in the peak discharge is, in most cases, smaller than 1 or 2%). The analysis of catastrophic past flood events demonstrated that the peak discharge on the Savinja River mostly occurs before the peak discharge on the Sava River (Figure 13). Thus, faster hydrograph propagation has a positive

Figure 13. Analysis of time differences between peak discharge values at the confluence of the Savinja and Sava Rivers. Positive values indicate that peak discharge of the Savinja River occurs before the peak discharge of the Sava River.

influence on the situation in the lower Sava River. This kind of local measures mostly have minor impact on the global situation in the larger catchment such as the Savinja River catchment but can lead to improved situation locally. Similar conclusions were also made for the case study of the alpine Inn River in Austria [17].

Furthermore, several other aspects of the flood safety such as the impact of high waters at the river confluences on the downstream flood safety were also investigated but are not discussed in this chapter.

#### 3.4. Backwater effect

Moreover, several smaller flood protection measures (e.g. channel widening at critical cross sections, river banks' reconstruction, local level construction) are also proposed in the Savinja catchment (mostly on rivers Ložnica, Hudinja and Voglajna). The analyses of these measures showed that they mostly positively influence the flood situation at the confluence of Savinja and Sava Rivers. Flood protection measures mostly fasten the hydrograph propagation but often do not significantly influence the peak discharge values (the decrease in the peak discharge is, in most cases, smaller than 1 or 2%). The analysis of catastrophic past flood events demonstrated that the peak discharge on the Savinja River mostly occurs before the peak discharge on the Sava River (Figure 13). Thus, faster hydrograph propagation has a positive

Figure 13. Analysis of time differences between peak discharge values at the confluence of the Savinja and Sava Rivers. Positive values indicate that peak discharge of the Savinja River occurs before the peak discharge of the Sava River.

Savinja outlet during the 1990 flood (dark blue), exclusion of large natural flood area before the Celje city (situation when proposed flood-control reservoirs fail to operate, purple) and actual situation during the 1990 flood (light blue).

) on the situation at the

Figure 12. Impact of the proposed flood-control reservoirs with increased total volume (10106 m3

80 Achievements and Challenges of Integrated River Basin Management

Using the calibrated and validated combined hydrological (HBV-light) and hydraulic (HEC-RAS 5) models, we investigated the influence of the proposed flood protection measures (e.g. several flood-control reservoirs are to be built in the large natural flood area before the Celje city) on the flood safety. Moreover, using the hydraulic model HEC-RAS that is presented in Section 3.2, we also investigated the backwater effect on different tributaries in the Savinja catchment. Figure 14 shows an example of the backwater effect on the Ložnica River. It can be seen that due to the increased peak discharge on the Savinja River, the maximum water on the Ložnica River also increases. This increase is the largest for the cross section located near the rivers' confluence (about 0.6 m for peak discharge increase at 400 m<sup>3</sup> /s) and generally, decreases for upstream river station. Moreover, the backwater effect is detected for the cross section that is located 1.5 km upstream of the confluence of the Savinja and Ložnica Rivers. Similar analysis was performed for other rivers (e.g. Hudinja and Voglajna; Voglajna and Savinja). The backwater effect can be up to 0.25 m for a peak discharge of 1000 m3 /s. This kind of analysis can be very useful also for the policy makers because it is essential to understand

Figure 14. The influence of the Savinja River on the Ložnica River (backwater effect) when the Ložnica input hydrograph is constant during different hydraulic model runs. Different coloured lines represent different cross sections on the Ložnica River where the number indicates river station from the confluence with the Savinja River upstream [m].

that some local measure can also have significant impact on the upstream flood conditions and also on the flood situation at the upstream tributary.

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#### 4. Conclusions

In this chapter, combined hydrological and hydraulic modelling was performed in order to investigate the influence of the proposed flood protection measures on the flood safety in the Savinja catchment and in the lower Sava River catchment in Slovenia. The main conclusions are: (1) some of the proposed flood protection measures have positive influence on the flood situation in the Savinja catchment and also at the confluence with the Sava River (either faster hydrograph propagation or peak discharge maximum water level reduction); (2) the main flood protection measures (several flood-control reservoirs) are to be built in the natural large floodplain area before the Celje city and potential problems with operation (or some other problems such as increased sediment transport at the reservoirs inflow) of these reservoirs would lead to the flood safety decrease; and (3) backwater effect in the Savinja River catchment can have a large impact on the flood safety, for example, the backwater effect at the confluence of Savinja and Ložnica Rivers can be up to 0.25 m at the 1000 m3 /s peak discharge of the Savinja River. These conclusions indicate that (small) local measures do not really play an important role in the global flood situation at the catchment and that some local measure can even worsen the flood situation upstream of the measure location. Therefore, complex models (hydrological and hydraulic) of the entire catchment are needed in order to really understand the flood behaviour and to select the most suitable measure that will have positive impacts on the flood safety. Moreover, the selection of the flood measure should also be in-line with the sustainable flood risk management, which means that environmental, social and economic conditions that are mutually connected should be investigated.

## Acknowledgements

The results of the study are part of the Slovenian national research Programme P2–0180: "Water Science and Technology, and Geotechnical Engineering: Tools and Methods for Process Analyses and Simulations, and Development of Technologies" that is financed by the Slovenian Research Agency (ARRS). We wish to thank the Slovenian Environment Agency for data provision.

#### Author details

Mitja Brilly\*, Andrej Kryžanowski, Mojca Šraj, Nejc Bezak, Klaudija Sapač, Andrej Vidmar and Simon Rusjan

\*Address all correspondence to: mbrilly@fgg.uni-lj.si

Faculty of Civil and Geodetic Engineering, University of Ljubljana, Ljubljana, Slovenia
