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Effects of Urbanization On Appearance of Torrential Floods 1) Ratko Ristic, 2)Zoran Gavrilovic, 3)Milutin Stefanovic, 4)Ivan Malusevic, 5)Irina Milovanovic 1) Forestry Faculty of Belgrade University, Belgrade, SERBIA&MONTENEGRO 2), 3), 4), 5) Institute for Water Resources Management, Belgrade, SERBIA&MONTENEGRO e-mail: email@example.com Abstract Process of urbanization decreases permeable surfaces in the catchment, and simultaneously increases impervious surfaces, with next consequences: faster forming of surface runoff, more frequent appearance of flood waves. Urbanization influences on development of erosion processes, land degradation with significant reduction of soil water capacity. Problem was analyzed on small catchment on Belgrade territory. Key words: urbanization, land use, floods, surface runoff Introduction Sudden appearance of high water level in river bed, with high concentration of hard phase and enormous destructive energy is named as ″torrential flood″. Two-phase fluid (water and sediment) contents particles with diameter from several centimeters up to a few meters.Torrential catchment is specific geomorphological unit with main stream and tributaries, surfaces under erosion processes of different forms and intensities. Storm rainfall, snow melting or their coincidence cause appearance of fast surface runoff, transport of water and erosive material from the slopes to hydrographic network and forming of torrential flood wave. Attribut ″torrential″ belongs to each catchment with described characteristics, regardless to magnitude. Torrential floods are the most frequent phenomenon in the arsenal of natural disasters (floods, droughts, earthquakes, landslide) in Serbia (Ristić, R.; Stefanović, M., 2005). Frequency of event, intensity and diffusion, in the whole territory, make them as permanent threat with consequences in ecology, economic and social sphere. Climate, specific characteristics of relief, distinctions of soil and vegetation cover, social-economic conditions have done that the occurrence of torrential flood waves is one of the resulting forms of existing erosion processes. Decreasing the surfaces under forest vegetation, urbanization and unadequate agricultural measures are some negative aspects of human work which cause floods, so that former discharges with recurrence interval of 100 years, become events with recurrence interval of 20 years (Framji, K.K.; Garg, B.C., 1976). Land use directly affects on surface runoff intensity by ″losses″ of precipitation through the processes of interception, depression storage, evaporation, transpiration and infiltration (Ristic, R.; Macan, G., 1995). Amount of effective rain (runoff) depends on land use, characteristics of vegetation cover, air and water capacity of soil (Ristic, R.; Macan, G; Malusevic, I., 2005). Presence of vegetation increases surface roughness, reduces velocity of surface runoff and prolongs time characteristics of hydrograph (time of rising; time of recession; lag time; concentration time). Storage capacities are different for agricultural land, forest land and urbanized areas. Process of urbanization decreases permeable surfaces in the catchment, and simultaneously increases impervious surfaces, with next consequences: faster forming of surface runoff, more frequent appearance of flood waves. Figure 1. Location of investigated area Description of site studied General conditions The area of research is catchment of Kaludjericka river, 15 km from the centre of Belgrade (figure 1). Climate characteristics of the catchment • continental climate, moderate type; • mean annual precipitation, Pm=670 mm; • continental pluviometric regime (56.5% of mean annual precipitation in period IV-IX month); • mean annual temperature of air, tm=11.2°C; • mean annual humidity of air Rm=69%. Hydrographic characteristics of the catchment magnitude, A=11.39 km2; length of the catchment, L=7.26 km; the distance from the point in the river bed, nearest to the centroid of the catchment area, to the outlet profile, Lc=3.6 km; mean width, Bm=1.57 km; peak point, Pp=268 m; confluence point, Pc=85.1 m; mean slope of terrain, St=16.7%; absolute slope of river bed, Sa=2.52%; mean slope of river bed, Sm=1.15%. Land use Table 1. Structure of surfaces on the Kaludjericka river catchment in year 1953. Surafce Magnitude [km2] Percentage of total area Urbanized area 0.65 5.71 Plow fields 10.34 90.78 Forest 0.40 3.51 Total area 11.39 100 Table 2. Structure of surfaces on the Kaludjericka river catchment in year 2005. Surafce Magnitude [km2] Percentage of total area Urbanized area 4.425 38.85 Plow fields 5.99 52.59 Degraded forest 0.42 3.69 Meadows 0.06 0.53 Orchards 0.055 0.48 Forest 0.44 3.86 Total area 11.39 100 Figure 2. Land use map (year 1953.) Materials and method Computation of maximal discharge was done by usage of combine method: sinthetic unit hydrograph (maximal ordinate of unit runoff, qmax) and SCS methodology (deriving effective rainfall Pe from total precipitation Pb). Data about maximal daily precipitation were provided from rain-gauge station Vracar-Belgrade (75 years of observations). Kombine method was adopted for Serbia, using regional characteristics for computation lag time (Ristic, R., 2000), internal daily distribution of precipitation (Jankovic, D., 1994) and classification of soil hydrologic groups for determination CN-runoff curve number (Djorovic, M., 1975). Computation was carried out for AMC II (Antecedent Moisture Conditions) and AMC III (AMC II – average value of soil infiltration capacity; AMC III – high content of water in soil, and significantly reduced infiltration capacity). Control profile at Kaludjericka river is on the catchment outlet, before the confluence with Bolecica river. Maximal discharge was computed for land use conditions in 1953. and 2005. (based on aerial photo images and field investigations). Quality hydrologic analysis is the base for calculation of maximal discharge Qmax(%), which is the main input data for designing longitudinal and transversal structures in torrent beds. Combine method is the most frequent procedure in Serbia used for computations of maximal discharges on unstudied catchments. t p = 0.751 (L ⋅ Lc )0.336 [h]................................................................................................... (1) Im B a ⋅ T 1440 ⋅ A + 1 H (T , P ) = ⋅ ⋅ H ( 24 h , P ) [mm]......................................................................(2) 1440 A ⋅ T + 1 H (T , P ) - designed rainfall (D – duration; P – probability) a ≈ 1.0 (constant) A = 0.3 (constant) B = 0.805 (regional value for Belgrade territory) T - duration of runoff [min] H ( 24 h , P ) - maximal daily precipitation (on the basis of data processing from rain-gauge station Vracar- Beograd, and application of Log-Pearson Type III distribution) [mm] Figure 3. Land use map (year 2005.) Results of investigation Maximal discharge Computed values of maximal discharge for AMC III are presented on diagram (figure 4), as hydrographs for different probabilities (p=1, 2 and 5%). Value of maximal discharge Qmax-AMCIII (1%, 3. -1 3. -1 3. -1 2005)=50.6 m s is much higher than Qmax-AMCIII (1%, 1953)=33.8 m s . Qmax-AMCIII (5%,2005)=34.86 m s is 3. -1 very close to Qmax-AMC III (1%,1953)=33.8 m s . 60 50 p=1% (2005.) p=2% (2005.) p=5% (2005.) p=1% (1953.) p=2% (1953.) 40 p=5% (1953.) Qmax (m3/s) 30 20 10 0 0 5 10 15 20 time (hours) Figure 4. Maximal discharges for AMC III (in 1953. and 2005.) Specific maximal discharge qmax Values of specific maximal discharges qmax(1%) on Kaludjericka river were plotted on diagram with anvelopes of specific maximal discharges for Serbian rivers (Jankovic, D., 1994.). Values of qmax(1%) were presented with four points: point 1 – AMC II, qmax(1%)=1.46 m3.s-1.km-2 (year 1953.); point 2 – AMC III, qmax(1%)=2.97 m3.s-1.km-2 (year 1953.); point 3 – AMC II, qmax(1%)=3.12 m3.s-1.km-2 (year 2005.); point 4 – AMC III, qmax(1%)=4.44 m3.s-1.km-2 (year 2005.); Discussion Changes in land use in period 1953-2005 affected significant differences in values of maximal discharges on Kaludjericka river. On the beginning of 1953. Kaludjerica river catchment was very rare inhabited (about one hundred residential objects). But in 2005. situation was quite different: 4000-5000 residental objects, buisness centres, with dense network of streets. Consequence is very often appearance of torrential floods, with damages in the lowest parts of Kaludjerica river valley. 100 Tr=50 god. q [m3*s-1*km-2] Tr=100 god. Tr=1000 god. Tr=10000 god. 10 4 3 2 1 1 2 A [km ] 0.1 1 10 100 1000 10000 Figure 5. Specific maximal discharges qmax(1%) on Kaludjericka river On figures 6. and 7. are presented aerial photo images (1953. and 2005.) of the upper part of Kaludjerica river catchment. This area is the most urbanized part of the catchment. Urbanized surfaces enlarged from 0.65 km2 in 1953. up to 4.425 km2 in 2005. Figure 6. Aerial photo image of the upper part of Kaludjerica river catchment (1953.) Figure 7. Aerial photo image of the upper part of Kaludjerica river catchment (2005.) Conclusions Significant changes in land use affected on increasing of maximal discharges on Kaludjerica river. Urbanized areas (mostly with impervious surfaces) were enlarged in period 1953.-2005. more than 6 times. Process of urbanization has destroyed or destructed natural drainage network, so that forming of surface runoff is very fast, especially on impervious surfaces and degraded forest or agricultural land. Urbanization and degradation of forest and agricultural land (unadequate agricultural measures) influenced that former discharges with recurrence interval of 100 years (Qmax-AMC III (1%,1953)=33.8 m3.s-1) became very close to events with recurrence interval of 20 years (Qmax-AMCIII (5%,2005)=34.86 m3.s-1). Uncontrolled urbanization is very dangerous from aspect of appearance of floods, especially increment of impervious surfaces. All levels of plannning (spatial, urbanistic, regulation) have to considere different aspects of land use in the catchments (erosion processes; favorable conditions for surface runoff forming; flood zones). References Framji, K.K.; Garg, B.C., 1976: Flood control in the world. ICID, New York. Djorovic, M., 1984: Determination of soil Hydrologic class. Journal for Water Resources Management, Vol. 87, pg.57-60, Belgrade, Serbia&Montenegro. Jankovic, D., 1994: Characteristics of intensive rainfall for territory of Serbia. Civil Egineering Manual, Belgrade, Serbia&Montenegro. Ristic, R.; Macan, G., 1995: Influence of the forest ecosystems on runoff process as the part of the global hydrologic cycle. The Second International Study Conference on GEWEX in Asia and GAME, Proceedings, pg.327-329, Pattaya, Thailand. Ristić, R.; Macan, G; Malušević, I., 2005: Influence of forest ecosystems on runoff process on micro- catchments. International Conference on Forest Impact on Hydrological Processes and Soil Erosion, Proceedings, pg. 30-35, Yundola, Bulgaria. Ristić, R.; Stefanović, M., 2005: Extreme discharges on torrential catchments in Serbia. International Conference on Forest Impact on Hydrological Processes and Soil Erosion, Proceedings, pg. 280-286, Yundola, Bulgaria.
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