LAND USE CHANGE FOR FLOOD PROTECTION - A PROSPECTIVE STUDY FOR THE RESTORATIO N OF THE RIVER JELAŠNICA WATERSHED

Serbia’s hilly-mountainous regions are extremely vulnerable to flooding as a consequence of their natural characteristics and human impacts. Land mismanagement influences the development of erosion processes, and causes soil degradation that significantly reduces the land’s capacity to infiltrate and retain rainwater. Inappropriate land use as well as development activities replace permeable with impervious surfaces in the watershed. This leads to more rapid runoff generation and the more frequent appearance of torrential floods and bed-load deposits on downstream sections. Environmental degradation creates economicsocial problems within local societies which is often followed by depopulation. Restoring watersheds to their optimal hydrologic state would reduce flood discharge and by increasing groundwater recharge would increase both low-flow and average discharges in springs and streams. Best management practices could be developed through the application of specific combinations of biotechnical, technical and administrative measures, and by using the concept of ′′natural reservoirs′′. The design of such practices is explored through a case study of the watershed of the river Jelašnica, southeastern Serbia. Realization of these planned restoration works should help decrease the annual yields of erosive material by 44.1% and the specific annual transport of sediment through hydrographic network by 43.6%. Representative value of the coefficient of erosion will be reduced from Z=0.555 to Z=0.379. The value of maximal discharge Qmax-AMCIII (1%)=54.17 m3·s–1, before restoration, is decreased to Qmax-AMCIII (1%)=41.22 m3·s–1 after restoration, indicating ГЛАСНИК ШУМАРСКОГ ФАКУЛТЕТА, БЕОГРАД, 2011, бр. 103, стр. 115-130 BIBLID: 0353-4537, (2011), 103, p 115-130 UDK: 630*116.6 (497.11-12) (282 Jelašnica)=111 Оригинални научни рад DOI: 10.2298/GSF1103115R Dr. Ratko Ristić, Associate Professor, The University of Belgrade Faculty of Forestry, Belgrade (ratko.ristic@sfb.bg.ac.rs) Boris Radić, Research Associate, The University of Belgrade Faculty of Forestry, Belgrade MSc Nevena Vasiljević, Assistant, The University of Belgrade Faculty of Forestry, Belgrade Dr. Zoran Nikić, Associate Professor, The University of Belgrade Faculty of Forestry, Belgrade Ristić R., Radić B., Vasiljević N., Nikić Z. 2011. Land use change for flood protection a prospective study for the restoration of the river Jelašnica watershed. Bulletin of the Faculty of Forestry 103: 115-130. Ratko Ristić, Boris Radić, Nevena Vasiljević, Zoran Nikić 116 the improvement of hydrological conditions, as a direct consequence of land use changes. Administrative measures are applied through ′′Plans for announcement of erosive regions and protection from torrential floods in the territory of Leskovac municipality′′.


INTRODUCTION
Serbia's hilly-mountainous regions are extremely vulnerable to flooding as a consequence of their natural characteristics and human impacts.. Soil erosion, torrential floods, debris flows, mud flows and landslides have recently caused significant damage all around the world (S k i d m o r e , To y a , 2007, N a p r a d e a n , C h i r a , 2006).Their cause has often been related to the over exploitation or mismanagement of forest and agricultural land, or urbanization.Upper and lower watershed areas both face problems due to the misuse of the soil.Land degradation in mountain watersheds inevitably leads to economic-social problems within affected communities and the consequence is often local depopulation (A n a n d a , H e r a t h , 2003).The restoration of hilly-mountainous regions should become a key priority for Serbia in the XXI century.The task will involve the integration of technical, biotechnical, biological, social, economical, administrative and institutional measures, including political activities.
This paper explains that land use is closely connected to erosion control activities (R i s t i ć , M a c a n , 1997, B a k k e r et al., 2005), protection from torrential floods, drinking water supply, rural development, CO 2 sequestration and biodiversity sustaining (R i s t i ć et al., 2010).It notes that the effectiveness of technical structures in river beds is very limited, and often insufficient to provide protection from torrential floods.So, it argues that a higher degree of protection could be obtained through use of the concept of ″natural reservoirs″ and the application of BMPs (Best Management Practices) on those surfaces that act as ″sources″ of accelerated surface runoff and sediment production.Protection from torrential floods is beside other measures based on land use changes through the concept of ″natural reservoirs″.The essence of this concept is the necessity to retain water in soil on slopes instead of running off as a fast surface runoff, minimize erosion and enable agricultural activities.Each storage component (forest stands; parcels of arable land; pastures; meadows, etc.) represents a kind of reservoir, able to keep and retain a certain volume of water.

STUDY AREA
The investigation was carried out in the experimental watershed of the river Jelašnica, situated in the southeastern part of Serbia (Figure 1).The total area of the Jelašnica river watershed (up to the confluence into the Južna Morava river) amounts to 30.04 km 2 .Higher parts of the watershed (600-1,055 m above the sea level) are covered with high quality beech forest.The land is principally occupied by agriculture, with significant areas of natural vegetation and complex cultivation patterns.Non-irrigated arable land dominates in the middle and lower parts of the watershed (219-600 m a.s.l.).
The river Jelašnica and its main tributaries are not regulated and torrential floods often endanger houses, roads, bridges and arable land (Figure 2).The flood which occurred in May of 2005 destroyed the local road (about 300 m in length), damaged the foundations of the local bridge and cut 20 households off from the nearest towns (Leskovac and Niš).A similar situation occurred in November of 2007 and November of 2009.Today, river training works are in progress on the 1 km-long section, through village Jelašnica (R i s t i ć et al., 2006).This watershed is dominated by continental climate, mountainous type, with mean annual temperature of air t m =10.7 o C. The mean annual precipitation amounts to P m =649 mm.The current land use and the main hydrographic characteristics of the river Jelašnica are presented in Tables 1 and 2.
The watershed of the river Jelašnica is characterised by its complex geology (Figure 3).It is built from Paleozoic stones in the upper part, neogenic sediments in the middle and lower parts of the watershed and Quarternary sediments in the lowest part of the watershed.The whole Paleozoic complex, where schists dominate, belongs to the group of water impermeable lithologies.Schists are very erodible under atmospheric and anthropogenic influences.Characteristic erosion forms include gullies (0.5 m to 1.5 m deep) and accumulation forms (mainly talus piles of small dimensions).The middle and lower parts of the watershed are built from neogenic sediments.Here the lithology is dominated by sandstones, gravel, and clays.Most of these Neogenic sediments are permeable or lightpermeable.Landlsides appear frequently on the steep banks of river valleys, with the toes of the landslide often resting in the river bed.The biggest landslide is registered on the left bank of the river Jelašnica, (about 290 m long and 150 m wide), 1.5 km from the confluence into the river Južna Morava (Figure 2).Soil genesis was influenced by the geological composition of the watershed.According to the soil map (scale 1:50,000, sections Niš 3 and 4, issued by Institute for Soil, Belgrade, 1982), soil types typical for hilly-mountainous regions (Figure 4) are found in the watershed of the river Jelašnica.These include: eutric vertisol formed on lake sediments (the lower and the middle part of the watershed; 22% of the total area; high content of clay and colloidal fractions, with small water infiltration capacity); vertic cambisol formed on lake sediments (17% of the total area; middle part of the watershed, close to watershed boundary); eutric cambisol formed on schists (the middle and higher parts of the watershed; 21% of the total area; porosity in the surface layer amounts to 55% which decreases with depth); dystric leptosol formed on schists (higher parts of the watershed, close to watershed boundary; 21% of the total area; present at different altitudes and slopes, very erodible); silicate litosol formed on schists (the higher part of the watershed; 11% of the total area; small water infiltration capacity); litosol formed on lake sediment (the lower part of the watershed; 6% of the total area); rigosol formed on lake sediment (2% of the total area).

AIM OF INVESTIGATION AND STUDY METHODS
The aim of this investigation is to show how land use changes in the watershed could cause strong positive environmental impacts through the improvement of hydrological conditions, erosion control and flood protection.
Land use classification was made on the basis of CORINE methodology (1994.)The consequences of land use changes were analyzed on the basis of field investigations, usage of aerial and satellite photo images, topographic, geological and soil maps.
Area sediment yields and intensity of erosion processes were estimated on the basis of the ″Erosion Potential Method″ (EPM).This method was created, developed and calibrated in Serbia (G a v r i l o v i ć , 1972).It is still in use in all the countries that originated from the former Yugoslavia.The EPM is marked by a high degree of reliability of calculation of sediment yields, transport and reservoir sedimentation.Computation of maximal discharge, in the hydrological conditions before and after restoration of the watershed, was done using a synthetic unit hydrograph and SCS methodology (1979, C h a n g , 2003).In Serbia, this is the most frequently used procedure for the computation of maximal discharges at small, ungauged watersheds.Here the data was enriched by a regional analysis of lag time (R i s t i ć , 2003), internal daily distribution of precipitation (J a n k o v i ć , 1994) and classification of soil hydrologic classes (Đ o r o v i ć , 1984).
The computation was carried out for AMC III (Antecedent Moisture Conditions III-high content of water in soil, and significantly reduced infiltration capacity).Data about maximal daily precipitation were provided from a neighboring rain-gauge station - Leskovac (1948Leskovac ( -2009)).

RESULTS
The results of this investigation are related to the current and planned state of the river Jelašnica watershed following the restoration of degraded surfaces (table 3).Planned restoration works (Figure 5) involve the reforestation of eroded arable land (0.75 km 2 ), establishing of forest protective belts (0.06 km 2 , 1684 m in length, 35.5 m average width) and silt filtering strips (0.04 km 2 , 1668 m in length, 24 m average width).Conservation works are also planned on the existing arable land, which include contour farming and terracing (5.45 km 2 ).The numbers of livestock on grazing surfaces is limited (1-3 pieces per hectare, depending on terrain steepness), in order to reduce negative effects such as compaction of soil surface layer and the reduction of water infiltration capacity.

Erosion and sediment transport
Some characteristic outputs from these sediment yield and transport computations are presented in table 4 along with the representative values of the coefficient of erosion Z, in the conditions before and after restoration of the Jelašnica river watershed (W a -annual yields of erosive material; W asp -specific annual yields of erosive material; W at -annual transport of sediment through hydrographic network; W atsp -specific annual transport of sediment through hydrographic network; W abls -annual amount of bed-load sediment; W ass -annual amount of suspended sediment).

Changes of hydrological conditions
Land use directly affects surface runoff intensity by creating ″losses″ of precipitation, through the processes of interception, depression storage, evaporation, transpiration and infiltration.This is illustrated by hydrological analysis of the Jelašnica river watershed.From May of 2007, the Jelašnica river flooded three times, damaging the local road, the bridge and the houses.
The computed values of maximal discharge (for control profile P 1 , in the village Jelašnica, in the proximity of the confluence into the J. Morava river) are presented in Fig. 6, as hydrographs for different probabilities (p=0.5, 1 and 2%).Comparison of the values of maximal discharge Q max-AMCIII (1%) =41.22 m 3 •s -1 (after restoration) and Q max-AMCIII (5%) = =39.98m 3 •s -1 (before restoration) indicates an increase of the recurrence interval from   20 years (p=5%) to 100 years (p=1%) as a direct consequence of land use changes.At the same time, other significant parameters such as physical characteristics of the watershed (magnitude, mean slope of terrain, mean slope of river bed) and total precipitation remained the same.Some characteristic outputs of these hydrologic computations, in the conditions before and after the restoration, are presented in table 5 (CN -runoff curve number, Pe -effective rain for probabilities 1, 2 and 5%, T p -rising time of hydrograph, T r -recession time of hydrograph, T b -time base of hydrograph).These all indicate more favourable hydrological conditions in the post-restoration environments.

DISCUSSION
Generally, flood risk assesment has to include the main natural characteristics of the watershed, anthropogenically changed characteristics (land use, urbanization, position of residental and infrastructure objects), as well as all relevant aspects such as costs and benefits, water resources management, spatial planning (Pl a t e , 2002, Pe t t i e r et al., 2005).
The geology of the river Jelašnica's watershed is dominated by water impermeable (schists, clays) or light-permeable stones (quartz sandstones).These lithologies lead to the formation of soil types with low water infiltration capacity (eutric vertisol, vertic cambisol and lithosol formed on lake sediments; eutric cambisol, dystric leptosol, and silicate litosol formed on schists), and high erodibility under both atmospheric and anthropogenic impacts.
Soil erosion on pastures is often more intensive than on arable land (sheet erosion; appearance of gullies).The initial cause is degradation of the vegetation through overgrazing, which leads to removal of ground cover, leaving the soil exposed to erosion.Pasture management has to be supported by restrictions of livestock numbers (optimal number of animals per surface unit) and rotational grazing.Reduction of grazing pressure can lead to the establishment of a better vegetation cover, decreased erosion, increased interception effect, improved water infiltration capacity and reduced surface runoff.
The current state of erosion processes is represented by the coefficient of erosion Z=0.555 (medium erosion), the annual yields of erosive material W a =27631.5 m 3 and a specific annual transport of sediment through hydrographic network of W atsp =397.12 m 3 •km -2 .Restoration works aim to decrease yields of erosive material, increase water infiltration capacity and reduce flood runoff.The future state of erosion processes is represented with the coefficient of erosion Z=0.379 (weak erosion), the annual yields of erosive material W a =15437.5 m 3 and specific annual transport of sediment through hydrographic network W atsp = 224.1 m 3 •km -2 , which suggests a massive improvement.
The effect of the resulting hydrological changes has been estimated by the computation of maximal discharge under the conditions before and after the planned restoration of degraded surfaces in the watershed.At the moment, the Jelašnica river watershed is covered with stable broad-leaved forest on 11.92 km 2 (38.07% of the total surface).After restoration (afforested surfaces, forest belts, silt filtering strips) forest cover will be enlarged by 0.85 km 2 (2.8% of the total surface).Reforestation causes a significant increase in the interception effect (R i s t i ć , M a c a n , 2002), by protecting the soil from direct raindrop impacts and retarding the forming of surface runoff.Simultaneously, runoff curve number would be reduced from CN=88 to CN=83, effective rain from Pe 1% =33.24 mm to Pe 1% =26.70 mm and with prolonged time characteristics of hydrograph.Volume of flood wave will be reduced from W (1%) =0.999•10 6 m 3 to W (1%) =0.802•10 6 m 3 (for AMC III).Realization of these planned restoration works should help decrease the annual yields of erosive material by 44.1% and specific annual transport of sediment through hydrographic network by 43.6%.The representative value of the coefficient of erosion (Z) will be reduced from 0.555 to 0.379.The value of maximal discharge Q max-AMCIII (1%) =54.17 m 3 •s -1 , before restoration, is decreased to Q max-AMCIII (1%) =41.22 m 3 •s -1 after restoration, indicating the improvement of hydrological conditions, as a direct consequence of land use changes.Land use changes will influence time characteristics of the hydrograph (R i s t i ć , 2003, R i s t i ć , 2006).
Administrative measures are applied through ″Plans for announcement of erosive regions and protection from torrential floods in the territory of Leskovac municipality″.The plans put bans on: clear cuttings; cuttings in protective forests; straight row farming down the slope; uncontrolled urbanization; overgrazing (articles 15, 30, Water Law, The Offical Gazette of the Republic of Serbia № 54/96).Land owners have the duty to apply contour farming and terracing of arable land as effective measures of erosion control.

CONCLUSIONS
Protection from torrential floods can be enhanced by planned land use change.This paper calculates the effect of selected restoration measures on the hydological performance of the river Jelašnica, in southeastern Serbia, and shows that modest land use change may have significant impacts.
Forest vegetation significantly decreases the amount and velocity of surface water and intensity of erosion processes, helping the development of soil and its infiltration capacity.Planting stable forest stands on bare lands and steep slopes is an effective anti erosive and flood protective measure.Similarly, positive effects can be provided by properly treating arable land with contour farming and terracing.
The effective revitalization of the Jelašnica river watershed can be based on the application of Best Management Practices including: technical (river trainning works), biotechnical (afforestation; forest protective belts; silt filtering strips) and administrative measures, identification of endangered areas, specifying of land use limits (contour farming and terracing of arable land on slopes; limits for grazing; rotational grazing) and permanent control by experts.

Table 2 .
Main hydrographic characteristics of the river Jelašnica watershed

Table 4 .
Characteristic outputs of sediment yields and transport computations in the conditions before and after restoration works