The Annals of Valahia University of Târgovişte, Geographical Series, Tome 9 / 2009 __________________________________________________________________________________________________ REMARKS ON THE FEATURES AND CONSEQUENCES OF THE FLOODS OCCURRED IN THE PETROŞANI DEPRESSION DURING THE LAST THREE DECADES Andra COSTACHE1, Dănuţ TANISLAV1 1Valahia University of Târgovişte Abstract. The paper presents the results obtained by processing and interpreting data on the first two yearly floods, within the interval 1980-2002, recorded at five hydrometrical stations from the upper part of the Jiu River basin. There are emphasized the seasonal and the monthly frequency of the floods, their characteristic elements and the probability of exceeding the safety levels. The impact of the floods was analyzed on the basis of field observation and of several case studies that allowed us to present some recommendations for flood prevention and mitigation. Key words: floods, frequency, characteristic elements, impact, mitigation Introduction. In the Petroşani Depression, the floods and inundations are among the natural hazards with significant impacts on human settlements, economic activities and infrastructure. In order to highlight some characteristics of the floods occurred in this area, which almost completely overlaps to the upper basin of the Jiu river, there were processed and interpreted data on the two most important annual flood (after maximum flow value), within the period 1980-2002. There were analyzed data recorded from five hydrometrical stations (Fig. 1), respectively: Câmpu lui Neag, Iscroni (on Jiul de Vest), Taia (on the Taia creek), Livezeni (on Jiul de Est) and Strâmbuţa. Fig. 1. The Petroşani Depression: hydrographic network, hydrometrical stations and regularization works The impact of some extreme events produced both on the monitored rivers and as a result of the draining on torrents or on unmonitored streams was highlighted through case 97 The Annals of Valahia University of Târgovişte, Geographical Series, Tome 9 / 2009 __________________________________________________________________________________________________ studies. They have allowed the formulation of recommendations on measures aimed to prevent and mitigate the impact of floods in the main areas exposed to this type of hazard. Characteristics of the floods occurred in the Petroşani Depression. Genetically speaking, floods are of rainwater (65% produced in the warm season of the year, following heavy rains or long raining periods) and mixed – produced by rainwater and snow thawing (35%, mainly occurring during spring, but in some cases, and in winter). Regarding the seasonal frequency of floods, most of them are occurring during spring at the stations Câmpu lui Neag, Iscroni and Taia (42-52% of events), while at the stations Strâmbuţa and Livezeni the floods occurred during summer hold the largest share (Fig. 2). 60 50 40 w inter spring 30 (%) s ummer 20 autumn 10 0 Câmpu lui Iscroni Livezeni Taia Strâmbuţa Neag Fig. 2. Seasonal frequency of the floods in the Petroşani Depression Monthly, the highest frequency of floods is recorded in April (22% at Câmpu lui Neag), in May (20% at Câmpu lui Neag, 22% at Livezeni) and June (30% at Taia station and 28% at Strâmbuţa). The autumn and winter months are characterized by the lowest frequency of floods, and floods were absent in January in the range examined (table 1). Line Hydrometric F H Month River no. station (km2) (m) I II III IV V VI VII VIII IX X XI XII 1. Jiu Câmpu lui 155 1346 0 0 2.9 20 20 14.3 8.6 11.4 8.6 2.9 5.7 5.7 Neag 2. Jiu Iscroni 496 1134 0 0 6.7 22.2 17.8 15.6 11.1 8.9 4.4 2.2 2.2 8.9 3. Jiul Livezeni 440 1256 de 0 0 6.7 13.3 22.2 22.2 20 4.4 11.1 0 0 0 Est 4. Taia Taia 135 1206 0 0 4.3 19.6 28.3 30.4 6.5 6.5 2.2 0 2.2 0 5. Jiu Strâmbuţa 1024 1158 0 4.3 2.2 15.2 15.2 28.3 13 13 6.5 0 0 2.2 Table 1. Monthly frequency of floods in the Petroşani Depression The largest floods, that have exceeded over two times the maximum average flow values were recorded in the years: 1981 - Taia (May) and Livezeni (June); 1984 – Câmpu lui Neag (April-May); 1986 - Strâmbuţa (July-August); 1991 - Iscroni (June, July); 1995 - Iscroni (December); 1998 – Strâmbuţa and Taia (June); 1999 - Iscroni (July, August), Strâmbuţa (May), Câmpu lui Neag (July) and 2002 – Câmpu lui Neag (August). The characteristic elements of flood waves (table 2) influence their impact and they are caused by various factors such as length and slope of river, altitude and average slope of the watersheds. In the studied period, the average maximum flow values range between less than 10 m3/s and 100 m3/s and the average total duration of the floods range between 321 98 The Annals of Valahia University of Târgovişte, Geographical Series, Tome 9 / 2009 __________________________________________________________________________________________________ hours at Strâmbuţa and more than 450 hours at Livezeni. The maximum total duration of the floods has reached 1115 hours at Câmpu lui Neag (March-May 1986), 1715 hours at Iscroni (March-May 1986) and 2003 hours at Livezeni (March-June 1985). Line River Hydrometric F H Q max. Q max. T T T total q max. no. station (km2) med average abs. increasing decreasing average average (m) (m3/s) (m3/s) average average (hours) (l/s·km2) (hours) (hours) 1. Jiul Câmpu lui 155 1346 35,6 189 98 225 323 de Neag (12.07.1999) Vest 229,7 2. Jiul Iscroni 496 1134 100,9 817 101 280 381 de (27.12.1995) Vest 203,4 3. Jiul Livezeni 440 1256 52,5 137 154 307 461 de (19.06.1981) Est 119,3 4. Taia Taia 135 1206 9,01 33,5 111 294 405 (19.06.1998) 66,7 5. Jiu Strâmbuţa 1024 1158 8,5 22,5 98 223 321 (12.06.1998) 8,3 Table 2. Characteristic elements of the floods The mean propagation time in the river-bed is influenced by slope, depth and shape of the river-bed and roughness coefficient; during floods occurred in the spring of 2001, they were of 4.5 hours between Câmpu lui Neag and Iscroni, of 2.5 hours between Taia and Livezeni and of 2 hours between Livezeni and Strâmbuţa. Line Hydrometric Reference Heights (cm) River Height „0” no. stations system attention inundation danger 1. Jiul de Vest Câmpu lui Neag 767,31 MB 300 330 400 2. Jiul de Vest Iscroni 552,74 MB 200 250 300 3. Jiul de Est Livezeni 578,88 MB 300 350 500 4. Taia Taia 683,51 MB 230 300 330 5. Jiu Strâmbuţa 539,13 MB 100 200 300 Table 3. Safety levels at the hydrometric stations in the Petroşani Depression Of the total events analyzed, in 52% of cases there were recorded values above the safety levels (Table 3); the stations Taia and Strâmbuţa are distinguished by the highest frequency of such events (31% and 26% of above mentioned cases were recorded at these stations). At all stations, the floods that have exceeded the level of attention have the highest frequency (Table 4), except the Câmpu lui Neag station, where the values that exceeded the inundation level predominated (18% of cases). Only in four cases there were recorded values above the danger level, at the stations Câmpu lui Neag (July 1999), Iscroni (July 1999, August 1999), and Taia (June 1998). During floods, the rivers could maintain over the safety levels for different periods of time, which depend on the station's position along the rivers and on relief characteristics. For example, during the flood of July 1999, the waters of Jiul de Vest River maintained over the attention level for 138 hours, over the inundation level for more than 45 hours, and the danger levels were exceeded for 8 hours at Câmpu lui Neag and for 4.5 hours at Iscroni. The probability of exceeding the safety levels is particularly significant for the assessment of the human exposure to floods in the Petrosani Depression. The highest probability of exceeding the inundation levels was recorded at Câmpu lui Neag and Taia stations (20%), but the level of exposure to flooding for the downstream settlements is different: higher in the middle sector of Jiul de Vest river (where the hydrographical works, 99 The Annals of Valahia University of Târgovişte, Geographical Series, Tome 9 / 2009 __________________________________________________________________________________________________ such as dams and flood control works are lacking or less effective), and lower on the Jiul de Est river, where major investments have been made for flood prevention works. The probability of exceeding the danger levels and the potential impact of these cases are also higher in the catchment of the Jiul de Vest river. Hydrometric Events that Events that exceed Events that exceed Probability of exceeding the station exceed the the inundation levels the danger levels safety levels (%) attention levels No. cases % No. cases % No. cases % Attention Inundati Danger on Câmpu lui Neag 4 9 8 18 1 3 28 20 2 Iscroni 12 27 5 11 2 5 41 15 4 Taia 28 60 8 18 1 3 80 20 2 Livezeni 17 38 2 5 0 0 41 4 0 Strâmbuţa 29 63 0 0 0 0 63 0 0 Table 4. The frequency and the probability of exceeding the safety levels Areas exposed to floods. Two categories of areas exposed to frequent flooding can be identified in the Petroşani Depression, depending on their causes (Fig. 3): • Areas exposed to floods due to rivers overflowing (the lower sectors of Taia and Buta valleys, upstream of confluences with Jiul de Est and Jiul de Vest respectively); • Areas exposed to floods due to torrential draining, such as the catchments of Morii Creek, Sterminos, Crevedia (Gruniului Valley), Aninoasa, Merişoara, Tusu and Slătinioara.