Highly urbanized areas constantly need new surfaces for building of commercial, residential or infrastructure objects. Belgrade, the capital of Serbia, is a large regional centre with a population of 2 000 000 dwellers, covering a territory of 3500 km2. The territory of Belgrade is intersected by 187 streams, with watersheds mostly rural in higher parts, urbanized and highly urbanized in lower parts. Torrential floods that once occurred rarely during pre-development period have now become more frequent and destructive due to the transformation of the watershed from rural to urban land uses. Authorities of Belgrade defined a strategy for erosion control and protection from torrential floods, based on the restoration of “blue-green” corridors (residuals of open streams and fragments of forest vegetation). The restoration of “blue-green” corridors helps the establishment of new recreational areas, the preservation of biodiversity and the mitigation of effects of climate change

Malusevic, Ivan

Radic, Boris

Ratko, Ristic

Trivan, Goran

RIMSI

Hydrology in a Changing World: Environmental and Human Dimensions  Proceedings of FRIEND-Water 2014, Montpellier, France, October 2014 (IAHS Publ. 363, 2014).    315 “Blue-green” corridors as a tool for erosion and stream control in highly urbanized areas – case study of Belgrade city  RATKO RISTIĆ1, BORIS RADIĆ1, GORAN TRIVAN2 & IVAN MALUŠEVIĆ1 1 University of Belgrade Faculty of Forestry, Kneza Višeslava 1, Belgrade, Serbia ratko.risticc@gmail.com 2 Secretariat for Environmental Protection of the City of Belgrade, 27.marta 43-45, Belgrade, Serbia  Abstract Highly urbanized areas constantly need new surfaces for building of commercial, residential or infrastructure objects. Belgrade, the capital of Serbia, is a large regional centre with a population of 2 000 000 dwellers, covering a territory of 3500 km2. The territory of Belgrade is intersected by 187 streams, with watersheds mostly rural in higher parts, urbanized and highly urbanized in lower parts. Torrential floods that once occurred rarely during pre-development period have now become more frequent and destructive due to the transformation of the watershed from rural to urban land uses. Authorities of Belgrade defined a strategy for erosion control and protection from torrential floods, based on the restoration of “blue-green” corridors (residuals of open streams and fragments of forest vegetation). The restoration of “blue-green” corridors helps the establishment of new recreational areas, the preservation of biodiversity and the mitigation of effects of climate change. Key words “blue-green” corridors; erosion control; torrential floods; highly urbanized areas  INTRODUCTION  Floods, in all their various forms, are the most frequent natural catastrophic events that occur throughout the world (Berz et al., 2001; Barredo, 2007). Among natural hazards with serious risks for people and their activities, the torrential floods represent the most common hazard in Serbia (Ristić & Nikić, 2007), being the prime cause of losses, causing much damage and loss of human lives, both in urban and rural areas. Land-use changes (deforestation and topsoil removal; inadequate agricultural measures; increase of impervious surfaces) dramatically alter hydrological conditions by reducing the interception and infiltration–retention capacity of the soil (Ristić & Macan, 2002), exposing the soil to the impact of rain, which accelerates erosion and surface runoff. Torrential floods that once occurred rarely during the pre-development period have now become more frequent and destructive due to the transformation of the watershed from rural to urban land uses (Ananda & Herath, 2003; Ristić et al. 2012). Belgrade, the capital of Serbia and a big regional centre with a population of 2 000 000, covers a territory of 3500 km2. The territory of Belgrade has 187 streams with watersheds that are mostly rural in the higher parts and urbanized or highly urbanized in the lower parts (Faculty of Forestry & Institute for Water Resources Management “Jaroslav Černi”, 2005). The authorities of Belgrade defined a strategy for erosion and torrent control based on the restoration of “blue-green” corridors (residuals of open streams and fragments of forest vegetation). The restoration of “blue-green” corridors helps the establishment of new recreational areas, preservation of biodiversity (Saumel & Kowarik, 2010; Ramirez & Zuria, 2011) and urban adaptation to climate change (Kazmierczak & Carter, 2010).  MATERIAL AND METHODS  The concept of restoration of the “blue-green” corridors is presented at two experimental watersheds of the Kaljavi and Jelezovac streams, just a few kilometres from the centre of Belgrade (Fig. 1). The main hydrographic characteristics of the experimental watersheds are presented in Table 1. The experimental watersheds experienced torrential floods (in 1965, 1994, 2002), which endangered buildings, land and roads, when the water levels increased from 0.10–0.15 m to 1.6 m (the Kaljavi stream) and 1.7 m (the Jelezovac stream), as a consequence of severe thunderstorms, with the intensity of precipitation of up to 6 mm min-1.  The consequences of land-use changes have been analysed on the basis of field investigations, the use of aerial and satellite photo images, topographic, geological and soil maps. The land use classification was made on the basis of CORINE methodology (EEA, 1994). The area sediment  Copyright  2014 IAHS Press  Ratko Ristić et al.  316  Fig. 1 Location of the experimental watersheds of Kaljavi and Jelezovac streams.   Table 1 Main hydrographic characteristics of the experimental watersheds. Parameter Mark Unit Kaljavi s. Jelezovac s. Magnitude A km2 1.4 6.22 Perimeter P km 5.76 10.54 Peak point Pp m.a.s.l. 249 308 Confluence point Cp m.a.s.l. 115 112 Mean altitude  Am m.a.s.l. 191.6 184.2 Length of the main stream L km 2.45 4.12 Absolute slope of stream bed Sa % 5.47 4.76 Mean slope of stream bed Sm % 4.74 2.91 Mean slope of terrain  Smt % 7.44 10.64   yields and the intensity of erosion processes have been estimated on the basis of the “Erosion Potential Method” (EPM). This method was created, developed and calibrated in Serbia and it is still in use in all the countries originating from former Yugoslavia (Kostadinov, 2008).  The changes of hydrological conditions were estimated by the comparison of the maximal discharges under current conditions (2013) and after the complete restoration of the “blue-green” corridors (2020). The computations of maximal discharges (Qmax) have been performed using the synthetic unit hydrograph theory and the SCS methodology (SCS, 1979; Chang, 2003), enriched by a regional analysis of lag time, the internal daily distribution of precipitation and the determination of soil hydrologic classes (Ristić et al., 2012). The data concerning maximal daily precipitation have been obtained from the observation system of the Republic Hydrometeorological Office of Serbia (RHOS, 1945–2012). The computation has been performed for AMC III (Antecedent Moisture Conditions III, high content of water in the soil and significantly reduced infiltration capacity). The synthetic triangular unit hydrographs have been transformed to synthetic (computed) curvilinear hydrographs of total discharge, using the computer model “Wave” (Malošević, 2003).  The aim of this investigation was to show how the planned restoration of the “blue-green” corridors, as well as adequate land-use changes, can help the improvement of hydrological conditions in the endangered watersheds, the provision of effective erosion and torrent control, and environmental and social goals.   “Blue-green” corridors as a tool for erosion and stream control in highly urbanized areas   317 RESULTS OF INVESTIGATION Land use changes The land-use changes in the experimental watersheds are presented in Figs 2 and 3, under current (2013) and future conditions (2020). The current area of forest surfaces will be increased from 0.21 km2 (15.0%) to 0.48 km2 (34.3%) in the Kaljavi stream watershed, and from 1.27 km2 (20.4%) to 2.38 km2 (38.3%) in the Jelezovac stream watershed.    Fig. 2 Land use in the watershed of the Kaljavi stream; (a) (2013; 1 Discontinuous and continuous urban fabric, 0.74 km2; 2 Complex cultivation patterns: arable land, orchards, gardens, 0.14km2;  3 Grasslands, 0.31 km2; 4 Mixed forests and forest belts, 0.21 km2); (b) (2020; 1 Discontinuous and continuous urban fabric, 0.79 km2; 2 Complex cultivation patterns: orchards, gardens, 0.03 km2;  3 Grasslands, 0.10 km2; 4 Mixed forests and forest belts, 0.48 km2    Fig. 3 Land use in the watershed of the Jelezovac stream; (a) (2013; 1 Discontinuous and continuous urban fabric, 2.49 km2; 2 Complex cultivation patterns: arable land, orchards, gardens, 1.90 km2;  3 Grasslands, 0.56 km2; 4 Mixed forests and forest belts, 1.27 km2); (b) (2020; 1 Discontinuous and continuous urban fabric, 2.73 km2; 2 Complex cultivation patterns: orchards, gardens, 0.48 km2;  3 Grasslands, 0.63 km2; 4 Mixed forests and forest belts, 2.38 km2.    The traditional agricultural production will be transformed into organic food production, with a significant reduction of agricultural surfaces from 0.14 km2 (10.0%) to 0.03 km2 (2.1%) in the Kaljavi stream watershed, and from 1.9 km2 (30.6%) to 0.48 km2 (7.7%) in the Jelezovac stream watershed.  (a) (b) (a) (b) Ratko Ristić et al.  318  Restoration works will be performed in the 2014–2020 period, on the basis of erosion and stream control demands, as well as environmental and social requests. The following biological and soil-bioengineering activities are planned: the afforestation of degraded arable land on steep slopes (1.15 km2; 1500–2000 seedlings per ha, 2- to 3-years old) with planting along the contours on the previously prepared bench terraces; re-grassing of the degraded meadows, 0.73 km2; the establishment of orchards on terraces, with grassing between terraces (mostly apple, plum trees and currant; 0.2 km2) and gardens (cherry tomato, red peppers, basil; 0.31 km2) for organic food production, instead of abandoned plough land; the establishment of protective forest belts along the stream beds; and walking and cycling paths (0.23 km2). Also, some administrative measures (bans) are planned, including clear cuttings, cuttings on steep slopes, straight row farming down the slope and uncontrolled urbanization. Land owners have (with financial support from the authorities) the duty to apply contour farming and terracing of agricultural land (orchards and gardens) as effective measures of erosion control. In addition, 10 km of sealed walking and cycling paths, 1.7 km of unsealed forest paths, six open gyms and seven rest areas are planned.   Erosion and sediment transport Some characteristic outputs of the computations of sediment yields and transport are presented in Table 2, along with the representative values of the coefficient of erosion Z, in current conditions (2013) and after the complete restoration of the “blue-green” corridors (2020), in the experimental watersheds (Wa, annual yields of erosive material; Wasp, specific annual yields of erosive material; Wat, annual transport of sediment through hydrographic network; Watsp, specific annual transport of sediment through hydrographic network; Wabls, annual amount of bed-load sediment; Wass, annual amount of suspended sediment).  Table 2 Characteristic outputs of computations of sediment yields and transport under current conditions (2013) and after restoration (2020). Parameter Current conditions (2013) After restoration (2020)  Kaljavi s. Jelezovac s. Kaljavi s. Jelezovac s. Wа (m3) 336.9 1730.3 187.9 1084.1 Wаsp (m3km-2year-1) 240.6 278.2 134.2 174.3 Wаt (m3) 71.8 427.4 40.0 267.8 Watsp (m3km-2year-1) 51.3 68.7 28.6 43.1 Wabls (m3year-1) 7.0 32.6 1.9 14.9 Wass (m3year-1) 64.8 394.8 38.1 252.9 Z 0.217 0.239 0.147 0.175    Fig. 4 Computed hydrographs of maximal discharges into the Kaljavi and Jelezovac streams (under conditions before and after restoration of the “blue-green” corridors). “Blue-green” corridors as a tool for erosion and stream control in highly urbanized areas   319 Changes in hydrological conditions The effects of hydrological changes were estimated by determining the maximal discharges in current conditions (2013) and after the complete restoration of the “blue-green” corridors (2020). The computed values of maximal discharges (for the control profiles P1 and P2, at the Kaljavi and the Jelezovac streams, Figs 2, 3) are presented in Fig. 4, as hydrographs for probability p = 1%.  The values of computed maximal discharges under current conditions (Q1%_Jel-2013 =  22.4 m3 s-1; Q1%_Kalj-2013 = 7.0 m3 s-1) and after the planned restoration works (Q1%_Jel-2020 =  11.4 m3 s-1; Q1%_Kalj-2020 = 3.5 m3 s-1), will be significantly reduced, as well as the volumes of a direct runoff (W1%_Jel-2013 = 0.226 × 106 m3; W1%_Kalj-2013 = 0.071 × 106 m3; W1%_Jel-2020 = 0.136 × 106 m3; W1%_Kalj-2020 = 0.043.× 106 m3). In addition, planned land use changes will cause a decrease in the values of runoff curve numbers CN, under the conditions before (CNJel-2013 = 70; CNKalj-2013 = 70) and after the restoration of the “blue-green” corridors (CNJel-2020 = 55; CNKalj-2020 = 57).  DISCUSSION The restoration works were planned to ensure minimal impacts of the surrounding environment built on the restoration areas (Hostetler et al., 2011), maximize connectivity between “blue-green” areas and minimize development at the watershed scale (Ives et al., 2011), taking into account ecological, recreational, natural and cultural characteristics of the local areas (Asakawa et al., 2004; Briffett et al., 2004). Biological and soil-bioengineering works, as well as the application of administrative measures, will alter and improve the hydrological conditions by increasing the interception and infiltration-retention capacity of the soil, protecting the soil from the impact of rain, decreasing erosion and surface runoff. In this way, the experimental watersheds of the Kaljavi and Jelezovac streams will become less responsive to extreme events such as with high rainfall intensities.  The restoration of “blue-green” corridors in the experimental watersheds of the Kaljavi and Jelezovac streams will decrease the values of maximal discharges (p = 1%) by about 50%, and the volumes of direct runoff by about 40%. Erosive material production and transport will be    Fig. 5 C1 (Microscale connections; intra-watershed level); C2 (Mesoscale connections; inter-watershed level); C3 (Macroscale connections; trans-watershed level); Ks (Kaljavi stream); Js (Jelezovac stream); P1 (outlet control profile at the Kaljavi stream watershed); P2 (outlet control profile at the Jelezovac stream watershed). Ratko Ristić et al.  320 decreased by about 44% in the Kaljavi stream watershed, and 37% in the Jelezovac stream watershed. Ten kilometres of sealed walking and cycling paths, 1.7 km of unsealed forest paths, six open gyms and seven rest areas will enrich the potential of this area for sports and recreation. In addition, the restoration will help the protection and controlled usage of the natural and cultural values in the area, including a very rare object of geodiversity (phonollyte rocks), the section for ornitofauna (40 bird species) observation and a neolite settlement. The restoration of the “blue-green” corridors enables their connection at different spatial levels (Fig. 5), including the following: at the intra watershed level (C1-microscale connections), the inter-watershed level (C2-mesoscale connections) and the trans-watershed level (C3-macroscale connections). The final goal is the creation of a network of “blue-green” corridors in the territory of Belgrade city, which provides effective erosion and stream control, and environmental and social services.  Acknowledgements The Serbian Ministry of Science supported this study within project 43007 (Investigation of climate changes and their influence on the environment: monitoring, adaptation and mitigation; Subproject: Frequency of torrential floods and degradation of soil and water as a consequence of global changes).  REFERENCES Ananda, J. & Herath, G. (2003) Soil erosion in developing countries: a socio-economic appraisal. J. Environ. Manage. 68,  343–353. Asakawa, S., Yoshida, K. & Yabe, K. (2004) Perceptions of urban stream corridors within the greenway system of Sapporo, Japan, Landscape and Urban Planning 68, 167–182. Barredo, J. I. (2007) Major flood disasters in Europe: 1950–2005. Natural Hazards (Springer) 42, 125–148. Berz, G., Kron, W., Loster, T., Rauch, E., Schimtschek, J., Schmieder, J., Siebert, A., Smolka, A. & Wirtz, A. (2001) World map of natural hazards – a global view of the distribution and intensity of significant exposures. 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"Blue-green" corridors as a tool for erosion and stream control in highly urbanized areas - case study of Belgrade city

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"Blue-green" corridors as a tool for erosion and stream control in highly urbanized areas - case study of Belgrade city

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