**3.4.1 Correlation between rainfall and flood levels**

As mentioned above, the primary triggering factors of flash floods are high-intensity convective rainfalls that are often associated with supercells. Below we discuss the spatial and temporal features of the heaviest rainfall event (in the second half of May and early June 2010) in Southern Transdanubia. The attention this event attracted helped us collect the necessary input data for the analyses.

Insurance only covers property damage in Hungary if rainfall events exceed 30 mm daily precipitation, officially confirmed by the Hungarian Meteorological Service (Varannai, 2005). The average of at least one event exceeding 30 mm occurs each year in the study area. The actual number of events in this category is shown in Table 1.

A persistent waving low-pressure system dominated in the central and western part of the Mediterranean and Central and Eastern Europe in mid-May and stayed in this region for three to four days. Similarly, the Carpathian basin was affected by moist air masses generating extensive, prolonged and relatively high-intensity precipitation on 14 to 17 May. The second half of May was characterized by local but intense showers and downpours. The 15 and 16 May flash floods were typical from a hydrological viewpoint, but unusual from a meteorological aspect as typical convective cells were not observed in this period. However, the soils were saturated in the upper and steep portions of the catchments of the Baranya and Hábi Canals and the Bükkösd Stream prior to the event, in early May.


Table 1. Selected rainfall properties of the studied area between May 1 and June 16, 2010

As mentioned above, the primary triggering factors of flash floods are high-intensity convective rainfalls that are often associated with supercells. Below we discuss the spatial and temporal features of the heaviest rainfall event (in the second half of May and early June 2010) in Southern Transdanubia. The attention this event attracted helped us collect the

Insurance only covers property damage in Hungary if rainfall events exceed 30 mm daily precipitation, officially confirmed by the Hungarian Meteorological Service (Varannai, 2005). The average of at least one event exceeding 30 mm occurs each year in the study area.

A persistent waving low-pressure system dominated in the central and western part of the Mediterranean and Central and Eastern Europe in mid-May and stayed in this region for three to four days. Similarly, the Carpathian basin was affected by moist air masses generating extensive, prolonged and relatively high-intensity precipitation on 14 to 17 May. The second half of May was characterized by local but intense showers and downpours. The 15 and 16 May flash floods were typical from a hydrological viewpoint, but unusual from a meteorological aspect as typical convective cells were not observed in this period. However, the soils were saturated in the upper and steep portions of the catchments of the Baranya

> Number of rainy days

Cumulative precipitation (mm)

**3.4 Discussion** 

**3.4.1 Correlation between rainfall and flood levels** 

The actual number of events in this category is shown in Table 1.

and Hábi Canals and the Bükkösd Stream prior to the event, in early May.

days above 30 mm precipitation

Siófok 3 21 257.9 Sellye 4 23 274.5 Sátorhely 2 22 177.3 Sármellék 2 23 204.2 Pécs 3 25 253.6 Árpádtető 5 24 385.0 Nemeskisfalud 3 24 273.2 Nagykanizsa 2 22 251.0 Kisbárapáti 2 25 185.1 Keszthely 3 24 385.5 Kaposvár 3 22 226.7 Iregszemcse 2 26 226.7 Iklódbördőce 4 22 285.6 Homokszentgyörgy 1 22 175.1 Fonyód 3 27 278.3 Bátaapáti 3 25 308.0 Table 1. Selected rainfall properties of the studied area between May 1 and June 16, 2010

Meteorological station Number of rainy

necessary input data for the analyses.

Therefore, the soils acted as an impervious surface triggering extreme surface runoff. Soil moisture content only slightly decreased in the following two-week period, thus the second storm with less cumulative rainfall induced flash floods again on 31 May and 1 June. Over the period of 1 May to 16 June the cumulative number of rainy days reached at least 21 at all rain gauges operated by the Hungarian Meteorological Service in Southwest-Hungary (Table 1 and Fig. 6). Groundwater tables in the observation wells of the area indicated a mean rise of 1 to 1.2 m over the entire region (DDKÖVIZIG, 2010).

Fig. 6. Total cumulative rainfall (a) and number of rainy days (b) in Southern Transdanubia between 1 May and 16 June 2010 (data provided by the Hungarian Meteorological Service)

Table 2 clearly illustrates the extreme precipitation characteristics of the mentioned 47-day period. At many rain gauges in the study area precipitation reached or even exceeded 50% of the mean annual rainfall. The long-term average May precipitation in Pécs is 84 mm, while the cumulative precipitation in May 2010 was nearly threefold higher. The return time of such precipitation is estimated at 400 years.

The extremity of rainfall is also clearly reflected in the actual intensity values. For short periods, intensity values reached 30 mm h-1, while 10-minute intensity was 51.6 mm h-1 at the Keszthely main meteorological station. For small mountainous catchments it is essential to know the areal extent of the rainfall zone. Due to the scarcity of rain gauges, we have to rely on radar images. Convective cells are around 5 to 10 km across, thus radar images of adequate (at present 2 by 2 km) resolution are extremely helpful in the estimation of the areal extent of precipitation for modelling purposes. Heavy rainfall characterized the settlements of Sásd and Csikóstőttős on 15 May 2010 (Fig. 7) and maximum rainfall and intensity were observed basically in the same area on the following day (16 May 2010).

On 15 May 86 mm of rain fell on the upper catchments of the Baranya Canal, where Tc is shortest within the catchment, with similar flood stages. As a consequence, rapidly rising flood stages were just slightly off from the previous records (Fig. 8). South of the divide, in the mountainous Bükkösd Stream catchment, the rainfall was much more prolonged and high water stages persisted longer at the Szentlőrinc stream gauge than at the gauges upstream (Fig. 9).

Flash Flood Hazards 43

Fig. 8. Stages of selected Southern Transdanubian watercourses between 15 and 18 May 2010, in percentage of the highest stage observed to date. Crucial settlements are marked

Fig. 9. Discharge and water level curves of the Bükkösd Stream at the Szentlőrinc and Hetvehely stream gauges, showing cumulative rainfall amounts from 15 to 22 May 2010 (data from Institute of Hydrology, Research Institute for Environmental Protection and

Water Management [VITUKI])


Table 2. Cumulative rainfall amounts of selected settlements in Southwest-Hungary between May 1 and June 16, 2010, compared to the long-term annual average

Fig. 7. Total rainfall (mm) (a) and maximum daily rainfall intensities (mm h-1) ( b) triggering floods on 16 May in Sásd and Csikóstőttős

Total precipitation in % of the mean of 1941– 1970

Annual mean cumulative precipitation, 1961–1990 (mm)

Total precipitation in % of the mean of 1961– 1990

Annual mean precipitation, 1941–1970 (mm)

Bátaapáti 308.0 741 41.57 593.0 51.94 Fonyód 278.3 730 38.12 561.2 49.59 Homokszentgyörgy 175.1 773 22.65 648.2 27.01 Iklódbördőce 285.6 … … 688.0 41.51 Iregszemcse 226.7 640 35.42 617.0 36.74 Kaposvár 225.8 746 30.27 578.6 39.02 Keszthely 385.5 664 58.06 526.9 73.16 Kisbárapáti 185.1 688 26.9 559.3 33.09 Nagykanizsa 251.0 743 33.78 726.0 34.57 Nemeskisfalud 273.2 ... … 648.8 42.11 Pécs. Ifjúság u. 6. 331.6 741 44.75 … … Pécs Pogány 253.6 666 38.08 620.0 40.90 Pécs, Árpádtető 385.0 839 45.9 729.6 52.77 Sármellék 204.2 … … 585.3 34.89 Mohács, Sátorhely 177.3 631 28.10 588.0 30.15 Sellye 274.5 725 37.86 695.6 39.46 Siófok 257.9 615 41.93 577.0 44.70 Table 2. Cumulative rainfall amounts of selected settlements in Southwest-Hungary between May 1 and June 16, 2010, compared to the long-term annual average

Fig. 7. Total rainfall (mm) (a) and maximum daily rainfall intensities (mm h-1) ( b) triggering

floods on 16 May in Sásd and Csikóstőttős

Meteorological station Total

precipitation in the study period (mm)

Fig. 8. Stages of selected Southern Transdanubian watercourses between 15 and 18 May 2010, in percentage of the highest stage observed to date. Crucial settlements are marked

Fig. 9. Discharge and water level curves of the Bükkösd Stream at the Szentlőrinc and Hetvehely stream gauges, showing cumulative rainfall amounts from 15 to 22 May 2010 (data from Institute of Hydrology, Research Institute for Environmental Protection and Water Management [VITUKI])

Flash Flood Hazards 45

The basically static approach of GIS-based modelling (focusing on passive factors of inundation risk) is supplemented by hydrodynamic modelling, which expresses basic physical and hydrological relationships with mathematical equations (Maddox et al. 1979). Runoff is represented in critical flow or stage value, which is further analyzed with a flood transformation model. If appropriate data of sufficient spatial resolution are available, the HEC software environment is also suitable for the estimation of the extention of potentially

Firstly, the HEC-HMS model determines the actual discharge responding to critical rainfall for the catchment under study. However, the output data verification will only be feasible if stream gauge data are available for the catchment. If the simulation is carried out on an unexplored catchment, total runoff (flow) has to be estimated by empirically based

Threshold precipitation values, i.e. those that trigger floods with a given return period are determined for various flood levels. In our investigations, based on observed rainfall, a 400 year return period (during which probably a series of undocumented flash flood events occurred) had to be taken into consideration. In this case, in addition to the actual rainfall values, we have to acquire comprehensive knowledge on the entire hydrological cycle, including information on elements like the hydraulic conductivity and infiltration rate of soils, canopy and surface storage. The numerical models also involve topographical analyses, but they are focused on the study of cross-sections. Valley cross-sections are established at predetermined spacing and analyzed along the whole length of the watercourse (Fig. 11). The actual width of the cross-section is designed with regard to the critical flood level above the valley floor or the mean long-term water stage. River flow or

Fig. 11. Cross-sections across the Bükkösd Stream valley (left) and water levels at a sample cross-section for floods of a given probability computed by the HEC-RAS model (right)

inundated areas. Thus, it can also fulfil a verification function.

stage values are then determined for each cross-section (Fig. 11).

equations (Koris, 2002).
