Statistical Significance of the Rainfall Intensity That Caused the May 2014 Flood in Serbia

Stevan Prohaska1, Dragan Đukić2, Vladislava Bartoš Divac1, Nikola Božović1

 

 

 

 

1 Jaroslav Černi Institute for the Development of Water Resources, Jaroslava Černog 80, Belgrade, Serbia

2 National Hydrometeorological Service of Serbia

 

 

Abstract

The disastrous flood in mid-May of 2014 caused human casualties and enormous material damage, the greatest on record. It is evident that such a flood was a result of, inter alia, extremely heavy rainfall across most of Serbia, or specifically the area bordered by the Drina, Sava, Zapadna Morava and Mlava rivers. The nature of the flood shows that it was a combination of powerful flash floods in mountainous areas (characterized by a very rapid and destructive front), coinciding flash floods within river basins, and large-scale protracted floods that inundated river valleys, which are typical of lowland rivers. Rainfall events likely to cause such a flood pattern exhibit maximum intensities of different durations of rainfall.

The paper contains a detailed presentation of the maximum rainfall intensities of varying durations, which preceeded the disastrous flood in the designated part of Serbia. The report was compiled using offical hourly rainfall data recorded by the main meteorological stations of the National Hydrometeorological Service of Serbia. The statistical significance of the recorded maximum intensities that led to the flood was assessed using also the most recent analyses of heavy rainfall events recorded by all pluviographs at the main meteorological stations, from the time the pluviographs were placed online to the end of 2008. The spatial distribution of maximum rainfall intensities in Serbia, of different durations, is shown on an isohyet map.

The paper concludes with a statistical analysis of May 2014 precipitation totals at the same locations. These totals were analyzed in parallel with the extremes registered since each of the main meteorological stations was commissioned. The May 2014 precipitation totals were compared to the highest levels on record, then to long-term averages, and finally to the theoretical precipitation levels in May for different probabilities of occurrence. A return period was determined for each recorded May 2014 precipitation total.

Keywords: May 2014 flood, heavy rainfall, rainfall intensity, rainfall duration, probability of occurrence, return period, statistical significance.

 

 

Introduction

The main causes of the major flood in Serbia were heavy rainfall events in most of western and central Serbia. These events, which occurred between midnight (00:00) of 12 May and 3 am on 19 May 2014, affected the Kolubara River Basin, the Lower Drina River Basin, the Zapadna Morava River Basin, the Lower Južna Morava River Basin, the catchment areas of the Velika Morava tributaries, and the Mlava River Basin, as well as the Lower Sava River Basin, from the state frontier to the confluence of the Sava and the Danube at Belgrade. The heavy rainfall that led to the formation of a flood wave on the Sava and its tributaries also affected Croatia and Bosnia and Herzegovina, but these two countries are not included in the present assessment.

A spatial cyclone was formed, developed at all altitudes, as a result of a cold wave of Atlantic air coming via the Alps into the Mediterranean region. It shifted over the Adriatic to the Balkans. On 13 May, the cyclone moved from the Adriatic Sea to the western and central parts of the Balkans, and from 14 to 16 May it strengthened (deepened) at all altitudes and also became localized (moved very slowly).

The focus at the lower end moved from the Gulf of Genoa via the Apennines and the South Adriatic to southern Serbia, Bulgaria and Romania, and then made an elliptical "loop" above southeastern parts of the Pannonian Plain (northern Serbia, eastern and southeastern Hungary and northwestern Romania). On that particular occasion the cyclone deviated from the usual path followed by Genoa cyclones, which is towards the Black Sea, where they generally dissipate. The main zone of clouds and rain was above most of Serbia, particularly western Serbia, then Republika Srpska and Slavonia. It moved very slowly, as did the cyclone itself, and from 14 to 18 May 2014, according to the data collected by the National Meteorological Service of Serbia (RHMZ), delivered extreme amounts of rain, in most places exceeding 200 l/m2, and locally more than 300 l/m2.

This event was preceeded by heavy rainfall between 14 April and 5 May, when most of Serbia received between 120 l/m2 and 170 l/m2 of rain, and southwestern Serbia as much as 250 l/m2 or more All of this together caused disastrous floods, flash floods, erosion and landslides, first along small watercourses (creeks), than medium-size rivers and finally large rivers, particularly the Sava.