Pluvial risks produced by the exceptional rainfall amounts.

Case study- Harghita County, 23 August 2005

OCTAVIA BOGDAN*, NICOLAE RUSAN**

Key words: pluvial risk, synoptic situation, Harghita county

Pluvial risks produced by extreme rainfalls. A study-case: Harghita Country, August 23, 2005.

The paper makes a brief overview of the mode of manifestation, the genetic conditions rendering extreme precipitation and reconstruction costs. Exceptional rainfalls were triggered by a Mediterranean cyclone extending over the Black Sea while a trough with a high-altitude core centered over the West of Romania gathering the Cumulus clouds with a peak at 11 km altitude. This unleashed heavy rainfall, and strong winds associated with electrical discharges and hail storms, producing considerable damage e.g. eight localities were flooded (water levels in houses raising by 1.2-2 m), 11 people died, 5 disappeared, many dwellings were damaged and the national economy and the environment were seriously affected (table 2).

Introduction

Inter-mountain depressions, through their location in the mountain environment surrounded by almost continuous mountain heights of various altitudes are individual geographic areas, well defined through the environment components, within which the climate holds the largest share.

Scrutinizing the occurrence of exceptional precipitation amounts fallen and accumulated during one year, one month and especially one day is of peculiar practical importance to various sections of the economic activity. Flooding events, in-soil water excess, landslides, land washing-out a.o. are some of the phenomena occurring because of massive precipitation amounts.

The large variability of the annual precipitation pattern is highlighted by the highest monthly amounts that generally accumulate from heavy, showery downpours, of frontal or convective nature.

Large precipitation amounts accumulate anytime throughout the year, yet- most often in the warm period. The exceptional annual precipitation amounts from certain years compared to the mean, multiannual sums show that only rarely do those amounts exceed the multiannual means by 120 – 160%.

Because of Romania’s climate continental pattern, the distribution of precipitation during the year is non-uniform. On the average, Romania receives 640 mm (640 l/m2) of precipitation throughout the year.

The distribution pf precipitation is non-uniform in both time and space. As regards time, the largest precipitation amounts over most of the country’s territory are recorded at spring closing and summer debut (May-June), when our country is under the influence of maritime air masses, due to the Azores High, driving oceanic cyclones at its periphery. It is then that about 50% of the annual precipitation amount falls - on the average.

Here and there, mostly in the mountain areas, significant precipitation amounts may also fall in June-August and, in rare cases, even later in the year. Such a case was recorded on 23 August 2005 in Harghita County, in less than one hour.

Acting manner

On the above-mentioned date, the water amount fallen in less than one hour had a severe impact on the environment and the community. The water engulfed many localities due to the great, numerous high floods rolling down the surrounding hills and mountains and the rivulets swollen by the showery rains that exceeded 100 mm (Table 1).

Table 1

Mean monthly and annual amounts (P) and maximum 24-h precipitation amounts (M) at Odorheiu Secuiesc weather station (1990-2004)

Year/Month / Precipitation amounts (monthly and annual amounts) and the 24-h maximum – as l/m2
I / II / III / IV / V / VI / VII / VIII / IX / X /

XI

/

XII

/ Multiannual mean
1990 / P / 23.2 / 31.9 / 16.1 / 49.1 / 38.0 / 64.4 / 59.9 / 42.5 / 27.3 / 60.1 / 20.3 / 28.4 / 461.2
M / 15.1 / 6.2 / 5-4 / 180 / 7.7 / 26.2 / 22.5 / 21.1 / 13.3 / 27.9 / 5.6 / 6.5 / 27.9/21X
1991 / P / 11.8 / 25.7 / 6.4 / 32.3 / 150.4 / 109.3 / 107.4 / 45.5 / 28.4 / 61.7 / 18.2 / 28.9 / 625.7
M / 5.3 / 14.3 / 6.0 / 6.1 / 35.0 / 29.1 / 26.6 / 11.5 / 10.2 / 21.0 / 9.8 / 10.3 / 35.0/18V
1992 / P / 17.3 / 19.3 / 8.1 / 55.9 / 23.1 / 136.8 / 95.8 / 19.2 / 54.7 / 80.3 / 30.4 / 12.6 / 553.5
M / 6.5 / 4.6 / 2.5 / 13.2 / 8.4 / 20.9 / 33.1 / 11.5 / 32.8 / 15.2 / 8.7 / 4.2 / 33.1/25VI
1993 / P / 19.1 / 17.2 / 50.7 / 56.6 / 30.4 / 48.0 / 74.5 / 60.1 / 88.4 / 28.2 / 37.1 / 64.6 / 574.9
M / 9.4 / 3.9 / 19.2 / 14.2 / 8.4 / 11.2 / 32.0 / 11.9 / 39.6 / 11.0 / 20.1 / 18.5 / 39.6/11IX
1994 / P / 28.5 / 7.8 / 49.6 / 43.9 / 103.6 / 67.5 / 116.2 / 43.2 / 33.0 / 58.0 / 29.1 / 32.6 / 613.0
M / 5.1 / 2.5 / 15.8 / 11.9 / 21.9 / 18.8 / 48.6 / 9.7 / 18.9 / 19.9 / 7.9 / 8.0 / 48.6/2VII
1995 / P / 23.3 / 19.2 / 30.4 / 19.6 / 91.3 / 136.1 / 49.7 / 92.3 / 102.4 / 4.5 / 41.4 / 78.7 / 688.9
M / 5.0 / 8.4 / 11.2 / 3.4 / 12.7 / 40.3 / 30.2 / 23.8 / 23.5 / 3.0 / 9.9 / 35.9 / 40.3/24/VI
1996 / P / 12.9 / 40.1 / 15.0 / 21.2 / 102.0 / 53.0 / 30.5 / 44.8 / 110.9 / 20.8 / 28.6 / 47.0 / 526.8
M / 3.3 / 10.0 / 6.0 / 8.2 / 33.6 / 25.0 / 11.3 / 16.8 / 22.7 / 8.6 / 16.3 / 12.8 / 33.6/14V
1997 / P / 11.7 / 38.6 / 19.6 / 60.2 / 35.0 / 74.4 / 141.0 / 93.2 / 49.3 / 43.8 / 19.5 / 70.3 / 656.6
M / 6.9 / 9.1 / 6.9 / 11.8 / 12.3 / 23.8 / 19.6 / 18.1 / 18.0 / 10.6 / 4.8 / 15.6 / 23.8/20VI
1998 / P / 37.0 / 18.8 / 32.3 / 34.4 / 90.9 / 173.2 / 119.6 / 100.1 / 56.0 / 76.9 / 53.5 / 17.8 / 810.5
M / 10.5 / 5.4 / 11.0 / 15.8 / 12.6 / 37.5 / 34.3 / 34.5 / 10.2 / 29.3 / 15.7 / 6.4 / 37.5/30VI
1999 / P / 11.1 / 50.3 / 10.9 / 115.7 / 52.1 / 131.1 / 31.7 / 43.4 / 25.7 / 67.7 / 19.6 / 73.4 / 632.7
M / 5.2 / 11.1 / 3.8 / 39.8 / 11.1 / 28.5 / 13.6 / 13.4 / 10.1 / 17.5 / 12.0 / 18.8 / 39.8/19IV
2000 / P / 29.1 / 42.9 / 45.4 / 32.7 / 43.7 / 58.6 / 59.4 / 39.4 / 48.7 / 1.8 / 2.3 / 31.7 / 435.7
M / 7.1 / 16.3 / 10.2 / 9.3 / 14.6 / 21.7 / 29.9 / 29.0 / 15.2 / 1.7 / 0.8 / 12.6 / 29.9/13VII
2001 / P / 25.6 / 38.9 / 46.1 / 42.6 / 66.2 / 105.4 / 73.0 / 43.7 / 104.8 / 41.3 / 68.3 / 35.5 / 691.4
M / 12.2 / 11.5 / 12.5 / 18.0 / 13.1 / 21.1 / 20.6 / 29.8 / 27.0 / 22.5 / 14.0 / 13.3 / 29.8/12VIII
2002 / P / 21.0 / 10.7 / 22.8 / 23.1 / 45.8 / 102.1 / 112.1 / 101.6 / 80.3 / 61.2 / 41.2 / 11.1 / 633.0
M / 6.8 / 2.7 / 13.2 / 5.1 / 30.2 / 29.5 / 24.3 / 21.0 / 19.4 / 9.2 / 9.7 / 2.5 / 30.2/25V
2003 / P / 44.3 / 16.7 / 20.1 / 34.9 / 14.1 / 33.4 / 97.3 / 23.6 / 51.2 / 80.7 / 24.6 / 13.6 / 454.5
M / 11.2 / 5.0 / 4.7 / 8.0 / 4.6 / 21.7 / 21.2 / 10.6 / 28.8 / 15.4 / 9.3 / 8.7 / 28.8/X
2004 / P / 48.4 / 47.4 / 38.3 / 63.4 / 39.8 / 28.5 / 86.2 / 82.2 / 40.4 / 25.6 / 42.4 / 14.1 / 556.7
M / 11.6 / 7.4 / 17.6 / 24.3 / 19.5 / 7.0 / 24.3 / 14.8 / 9.8 / 13.4 / 15.4 / 6.0 / 24.3/VII
Annual Amount / 23.5 / 29.6 / 27.1 / 50.1 / 62.7 / 92.6 / 81.7 / 58.4 / 60.0 / 48.0 / 31.7 / 40.3 / 605.8
2005 / P / 49.3 / 22.2 / 26.0 / 46.6 / 130.0 / 98.7 / 96.1 / 211.5
M / 14.3 / 5.8 / 7.1 / 16.4 / 24.0 / 18.6 / 27.4 / 42.0

The results from the above table that during the summer months, 100 – 170 mm rainfalls are likely in Harghita County. As can be noticed, in no other year than 2005 had such abundant water amounts fallen – 211.5 mm – so much the less in August, usually a droughty month. The mentioned amount exceeded the mean of the month by almost 4.5 times. The maximum precipitation amount fallen in 24 hrs. was 42.0 mm.

It is noteworthy that the mentioned amount accumulated in just 50 minutes, which caused vast flooding and extensive damage. It happened so also because the sloppy relief favoured the discharge and on the other hand because the soil oversaturated with water could no longer absorb it.

Genetic circumstances

In the initial stage of the creation of conditions favourable to the occurrence of the phenomenon (figure 1) Europe was under the influence of more pressure fields; on the north-western side the Icelandic Low was stationing in its birth place, with a 995 hPa nucleus closed at the center; over the north-east of the continent there was a continental high, with a nucleus 1020 hPa in its center; over the west of the continent the Azores High sent a ridge descending markedly southwards to the Mediterranean Sea, which made the Mediterranean Cyclone formed there be pushed across the Balkans, to the south-east of the continent, towards the Black Sea.

Fig. 1 Ground-level synoptic situation, superposed to the altitude one, at the level of 500 hPa, on 23 August 2005: a) at 3:00 a.m. LT; b) at 3 p.m. LT

(Karten Archiv – www2.wetter3.de/Archiv)

At the same time another low pressure nucleus, detached from the same mentioned Mediterranean Cyclone, stationing during the night over the west of the continent, with a 1010 hPa isobar merged with the one acting over the Black Sea, thus becoming stronger.

In the above-described situation the ground-level air circulation over Romania was southerly, which drove a warm, moist air mass and the pressure was 1008 hPa, the 1010 mb isobar expanding north-westwards, to the borders of our country.

The fact that the high precipitation amounts prevailed in the central-southern half and partly the eastern half of the country ascertains that the collision of a south-western circulation, induced by Mediterranean cyclones with an anticyclone eastern or northern one favours short-lived massive rainfalls (figure 2). On the date of our scrutiny, in southern and Western Europe, i.e. in Romania also the relative humidity was 90%, high enough to produce precipitation.

In the altitude a low-pressure trough was acting on all levels: 700 hPa (about 3500 m) and 500 hPa (about 5500m), with a nucleus centered over the western part of Romania with an isohypse of 568gpm (geopotential meters). In the context of such a synoptic situation, deep cumulization episodes occurred, leading to significant rainfall events, with thunder, wind gusts and sparse hail (see figure 1).

Figure 2. The relative moisture distribution on the European Continent

on 23.08.2005, at the level of 700 hPa (3500m), at 3 p. m LT

(Karten Archiv – www2.wetter3.de/Archiv)

The air temperature was 12° - 13°C at the height of 850 hPa (approximately 1500 m) and 28°C at ground level, which meant a 16°C - vertical thermal contrast and a 41°C – thermal amplitude (figure 3).

Figure 3. The air temperature distribution on the European Continent on 23.08.2005

(Karten Archiv – www2.wetter3.de/Archiv)

In the observation messages issued at 14:00 UTC (17:00 hrs. LT), Odorheiu Secuiesc weather station transmitted ongoing precipitation and accompanying phenomena: hail, thunder and wind gusts. Also, the now-casting meteorologists (those who elaborate meteorological forecasts and very short term warnings, with a lead time of 0-3 hrs. on the grounds of radar images) issued a warning advisory valid for the whole area where those phenomena were expected to act, based on images and on the elements basic to the occurrence of the phenomena in question: reflectivity (the answer-signal received by the radar after releasing electromagnetic waves) was 65 dBz. It is to be mentioned that the higher the reflectivity, the more severe the phenomenon and the more imminent the probability of its occurrence (figure 4).

Figure 4. Radar image – reflectivity at 17:26 LT

The echo-top is the height of the cloud top. In the studied case the cloud top reached 11 km in height, which tell that the cumulization process was very intense (figure 5). Hour Precipitation is the radar image showing the amount of precipitation that might fall on the ground at a certain moment. There resulted from the image that 99 mm of water could fall on the ground at that moment (figure 6).

Figure 5. Radar image – Echo-top at 17:20 LT

Figure 6. Radar image – 1-hour precipitation at 17:4 LT

The National Meteorological Administration in Bucharest issued warnings concerning hazardous phenomena with a high likelihood of occurrence. In turn, the Regional Meteorological Center South-Transylvania, the Weather Forecasting Service in Sibiu, after the synoptic analysis and having studied every model and map, issued warning bulletins, specifying heavy rainfalls in short intervals, torrential and showery, and the other accompanying phenomena: hail, thunder and wind- hour days prior to their occurrence.

Consequences

Despite the warnings and information about the weather evolution and the phenomena that were going to occur, necessary measures were minimal or not taken at all, such that consequences were very severe (disastrous), taking into account the life losses and the damage recorded. We mention that a rainfall yielding only 15 mm is capable of inducing intense erosion processes and flooding. The rainfall of 23 August 2005 was 2.5 times more abundant than that and the synoptic situation favoured hail occurrence too, with thunder and wind gusts that enhanced the intensity of the disaster.