Hydromemeteorological characterisation of the flood from the period 14-30 april 2005 in the Timis-Bega River Basin Viorel Alexandru Stănescu, Radu Drobot University of Civil Engineering , Bucharest, Romania 1. Abstract The precipitations which generates the 2005 flood in Timis-Bega River Basin have been analyzed both as spatial distribution (total isohyets map) and as temporal one (time distribution at the meteorological stations). Further on, an analysis of the balance of the flood hydrographs volumes is made. Unusual values of the runoff coefficients have been resulted, as the duration and the quantity of the rainfalls were very high (over 200mm). A mathematical model has been applied aiming to the reconstitution of the flood hydrograph of Timis River at the border of Romania with Serbia- Montenegro. Then the volumes of water penetrating the ruptures in the dikes downstream Sag station have been determined. Thus the very large area that has been flooded has been explained by the huge volume of the flood downstream Sag station (720.106 m3). An analysis of the effects of the storages (permanent and non-permanent ones) led to the conclusion that they were less effective in mitigating the flood crest in the Timis-Bega River Basin. In continuation, an analysis of the hydrological and technical issues of the present embankments in Timis-Bega River Basin is made. Proposals for improving the defence system by gradually implementation of new structural measures deeming and keeping the principle “more space for rivers” as well as the improvement and diversifying the non- structural measures are presented in detail. Key words: Precipitation, maximum discharges, flood volume balance, flood hydrographs, mathematical model, structural measures, ring-shaped embankments, fusible dikes, non-structural measures, informational system, education of population. Meteorological origins of the flood The available observed data refer to the amount of precipitation fallen previously the flood period and to those produced since 14th to 22nd of April 2005 that trigger the occurrence of the flood. In the graphs in Figure 1 show three distinct intervals of times are observed which are separated by no-rainfall periods which vary between 30-45 hours during the 16-17 April period and 9-15 hours in the second period (21 of April). Shorter periods with reduced precipitation amounts continued after 22 of April but they only fed the high discharges without contributing to the increase of the water stages over the alarm levels. The kernel of the rainfalls that had the most abundant quantities in the zones of flood formation ranged between15 to 24 hours. It is worthwhile to emphasise that the period before the flood occurrence (8th to 13th of April) has been characterized by a sudden increase of air temperature which, in the hilly and mountain zones (flood formation areas) ranged between a daily average of 4-6oC. The wormer air and some rainfalls which totalized 10-15 mm during 27 of March-1st of April resulted in increasing with 24-40 mm the soil moisture due to the snowmelt. Nevertheless, the contribution of the snowmelt to the flood formation is poor. Under an elevation of 1000m (corresponding to 96% of the entire area of the river basin) the snowpack did not exist. One can assert that the flood was practically of pluvial nature. The poor snowmelt contribution provided by the small mountainous areas was retained into the reservoirs Poiana Marului (elevation= 650m) and Trei Ape (elevation=870m) and these volumes have not found in the flood hydrographs along middle and downstream sectors of the Timis river. Thus the snowmelt has contributed only to elevated soil moisture which partially explains the high values of the runoff coefficients. According to the data recorded at the pluvial stations the rainfalls that provoked the outstanding April 2005 flood ranged between 60 to 221 mm, the smaller amounts, being recorded only around a restrained area, in the western mountains zone concentrated in two large kernels in the central and north-eastern parts of the catchment where maximum amounts of 221mm and 217 mm were recorded. In the highest mountain zones the quantities were much more modest, they not exceeding 50-70 mm (54 mm at Bucova station - altitude= 591m) and 67 - 69 mm at meteorological stations Cuntu - altitude = 1450m) and Ţarcu -altitude=2180m, respectively). 1 6 Meteorological Station Tarcu (total 72mm) 4 29 mm 26 mm 17 mm 2 h(mm) 0 1 4 7 10131619222528313437404346495255586164677073 Interval of 3 hours 14 Meteorological Station Caransebes (total 168 mm) 12 10 45mm 88 mm 35 mm 8 6 h(mm) 4 2 0 1 4 7 10131619222528313437404346495255586164677073 Interval of 3 hours 14 Meteorological Station Lugoj (total 175mm) 12 10 56 mm 73 mm 30 mm 8 6 h(mm) 4 2 0 1 4 7 10131619222528313437404346495255586164677073 Interval of 3 ore 20 Meteorological Station Resita (total 159mm) 18 16 14 12 38 mm 103 mm 34 mm 10 h(mm) 8 6 4 2 0 1 4 7 10131619222528313437404346495255586164677073 Interval of 3 hours 10 Meteorological Station Semenic (total 114mm) 8 6 36 mm 62 mm 16 mm 4 h(mm) 2 0 1 4 7 101316192225283134374043464952555861646770 Interval of 3 hours Figure 1. Timiş-Bega River basin: Temporal distribution of the rainfalls during the 14-22 April 2005 2 As it is seen in Figure 2, where the isohyets map is represented, the highest quantities have As seen in Figure 1, the highest amount of rainfall was in 18-20 April, which, as a matter of fact, generated the flood peak. In the history of the greatest floods produced in Timis-Bega River catchment those from Mai 1912, July 1966 and April 2000 have also been reported together with the 2005 flood. The flood from 25-30 May 1912 has been the highest one, the maximum discharge has been reconstructed as about 1600m3/s. This flood has been provoked by a rainfall that covered the entire basin area and exceeded 200mm in the mountainous areas of Semenic and Poiana Rusca. Recas Surduc Timisoara Buzias Cebza 0 Cadar Duboz 5 1 1 Graniceri 50 Bucova Tarcu Soceni Ohabita Gataia Cuntu Moniom Resita150 150 Semenic Figure 2. Isohyets of the precipitation fallen 14-22 April 2005. The flood from April 2000 has been provoked by the combined effect of the important rainfalls and the water resulted from snowmelt. Having a probability of about 2% the April 2000 flood produced the rupture dikes from the left-side bank of Timis River near the border with Serbia and Montenegro. Hydrological analysis of 2005 flood The analysis of April 2005 flood has been performed on the basis of the records of the discharges at the 20 gauging stations. First the volume balance at the main nodes of confluence has been considered. This analysis has been performed in order to check the degree of reliability of the data concerning the gauged and rating curve – extrapolation discharges. The agreement of the volumes of computed floods obtained by addition of the upstream wave hydrographs (recorded and evaluated over the ungauged basins (RB-rest of basins)) in a node and the recorded ones is quite good. In the Table 1 a selected part of this checking is provided as an example. So, the data can be considered to have an acceptable precision for further statisticall computations. The check has been made for the total runoff (htotal) of the hydrograph, but also for the surface runoff (hs) and base runoff (hbase). Unusual high estimates of the runoff coefficients α (surface runoff versus rainfall) should to be pointed out in table 1. 3 Table 1. The balance of the surface volumes (Ws), base volumes (Wsub) total volumes (Wtotal), the corresponding depths of runoffs, the rainfall (hp) and runoff coefficient α during the flood period in Timis – Bega River Basin. Basin area Wtotal 2 6 F(km ) (10 htotal Wbase Hbase Wsub hs 3 6 3 6 3 River Station m ) (mm) (10 m ) (mm) (10 m ) (mm) hp(mm) α Timis Teregova 167 24.9 149.19.49 56.8 15.4 92.2 120 0.77 Rece Rusca 163 28.05 172.08 9.26 56.83 18.79 115.25 150 Fenes Fenes 125 19.59 156.71 7.10 56.8312.49 99.88 130 Sum F 455 78.29172.08 25.86 56.8352.44 115.25 150 RB 105 18.07 172.08 5.97 56.83 12.10 115.25 150 0.75 Total 96.36 31.8 64.54 Timis Sadova 560 96.84 173 24.9 44.5 71.94 128.5 175 0.73 Golet Golet 41 7.91 192 3.07 74.9 4.84 118.1 165 0.72 Sebes T. Ruieni 122 21.44 5.42 44.4616.01 131.25 175 0.75 RB 349 61.31 175.67 36.18 103.67 25.13 72 120 0.60 Sum F 723 Total 187.5 69.6 117.9 90 Timis Caransebes 1072 185.9 173.4 69.8 65.1 116.1 77.2 Bistra Obreja 659 87.5 101.4 28.59 43.4 58.92 68.28 130 0.53 Nadrag Nadrag 35 7.97 227 2.65 75.7 5.32 152 200 0.76 CHE Turnu Ruieni 14.87 14.87 Suma F 1766 Without basin area of reservoir RB Poiana Mărului 736 141.06 191.66 36.18 43.38 104.88 142.5 190 0.75 Total 437.3 152.1 285.2 Timis Lugoj 2706 458.0 169 159.0 58.8 298.7 110.4 150 0.74 Hydrographs of the flood waves 14-30 April 2005 R. Timis River - Teregova-Lugoj Sector 1400 R.