FUNCTION OF CHECK DAM AGGRADATION IN LOCAL WATER SUPPLY OF MOUNTAINOUS AREAS

Summary: Check dams are built to control erosion processes and torrential floods. In Serbia, legally binding documents, VOS (2002) and PPRS (2010), provide the concept for the water sup ply of the population and industry by regional systems for which water is provided by building high dams and formation of reservoirs. With this concept, it is often not possible to meet the needs of local communities in mountainous areas. In order to contribute to solving the water supply problems of these mostly poor villages, research was conducted on the possibility of using check dam aggradation groundwater for this purpose. Field investigations and analysis of project documentation for numerous check dams and aggradations in Serbia were carried out. Potential quantities and quality of groundwater in the aggradations were analyzed as a resource for the water supply of the population. The results of the research indicate very favorable possibilities of the aggradations for the accumulation of groundwater in the form of unconfined or phreatic aquifer with a free water table, in quantities that can be used for water supply. their traditional role, could be used for local water supply facilities was also highlighted.

Напомена: Истраживање је реализoвано у оквиру пројекта евиденциони број 37008, (Програм ТР), финансираног од стране Министарства за науку и технолошки развој.  (2010), provide the concept for the water supply of the population and industry by regional systems for which water is provided by building high dams and formation of reservoirs. With this concept, it is often not possible to meet the needs of local communities in mountainous areas. In order to contribute to solving the water supply problems of these mostly poor villages, research was conducted on the possibility of using check dam aggradation groundwater for this purpose. Field investigations and analysis of project documentation for numerous check dams and aggradations in Serbia were carried out. Potential quantities and quality of groundwater in the aggradations were analyzed as a resource for the water supply of the population. The results of the research indicate very favorable possibilities of the aggradations for the accumulation of groundwater in the form of unconfined or phreatic aquifer with a free water table, in quantities that can be used for water supply.

FUNCTION OF CHECK DAM AGGRADATION IN LOCAL WATER SUPPLY OF MOUNTAINOUS AREAS
It was stated that: 1) under favorable conditions, with check dams on watercourses with constant flow and large-scale aggradations, there is a possibility of accumulating sufficient quantities of groundwater as a resource for water supply, 2) improving the quality of infiltrated waters by the process of filtration through intergranular porous media of the aggradation, and reaching the level of water supply quality, and 3) favorable economic parameters for opening the source at the aggradation, compared to other types of water sources. The possibility that check dams, in addition to their traditional role, could be used for local water supply facilities was also highlighted.

INTRODUCTION
In Serbia, the water supply of the population and industry is regulated by the following two main documents: Vodoprivredna osnova Republike Srbije (VOS, 2002) and Zakon o prostornom planu Republike Srbije od 2010. do 2020. godine (PPRS, 2010). By these documents it is scheduled for the water supply of the population and industry at the territory of Serbia to be solved by the construction of regional systems composed of high dams and water reservoirs. It is planned that the regional systems, in addition to already built 12 high dams, consist of 33 more high dams and reservoirs, which have yet to be built.
By the legalization of the concept of water supply, water users are integrated into regional systems. There are significant objections to the adopted concept of water supply, including oversized water consumption rates and unrealistic demographic projections from the 1970s and 1980s, which resulted in favoring the construction of surface reservoirs over the other water resources (Dokmanović and Nikić, 2015). VOS and PPRSs concept of building regional water supply systems has put the local communities of mountainous areas at a disadvantage.
It is well known that the villages in the mountainous areas are economically poor, composed mainly of elderly households and characterized by a distinct trend of migration.
There are many reasons for this situation. One of the reasons is the absence or very bad condition of the road, water supply, and electrical infrastructure. Therefore, since they were for years mostly left to themselves, the inhabitants of the mountainous villages started migrating to areas with better living conditions.
As the water supply strategy foreseen by the VOS and the PPRS will not be realized soon, local communities in the mountainous areas must try to solve the problem themselves.
Some settlements with natural resources and economic power have tapped the springs or dug wells and that way conducted water to their households. On the other hand, residents of numerous mountainous areas continue to draw drinking water in the traditional way from nearby springs or wells.
In this paper, based on local experiences and knowledge, an attempt is made to make recommendations that can be of use in planning, designing and solving problems of water supply in rural areas. The initial idea was that the people of mountainous areas would receive drinking water with as little investment as possible.

SUBJECT OF STUDY
In the mountainous areas of Serbia, in a period between 1907 and 2018, numerous check dams were constructed to control erosion processes and torrential floods. Torrential check dams are objects with a total height of 5 to 15 m, constructed transversely in a torrential stream waterbed, with a range of ten to tens of meters (Ristic et al., 2012). Their main purpose is the retention of sediments, stabilization of banks, the elevation of the longitudinal profile of a torrent bed and mitigation of the destructiveness of torrential floods.
Over time, at many check dams, the space provided to retain sediments is completely filled with deposited material. That way, aggradation is formed upstream of the check dam. The torrential aggradations represent the artificial accumulation of different grain size and lithology material, brought by waters from the watershed area and deposited upstream of the check dam (Ko stad i n ov, 2008). The size of the body (mass) of torrential aggradation depends on the construction characteristics of the check dam and the morphological characteristics of the waterbed, upstream of the check dam. In the case of a completely filled reservoir space, the water from the watershed area is flowing over the spillway or through the weir overflow of the check dam. In this case, under favorable conditions, groundwater accumulates at the aggradation.
The quantities of groundwater that can be accumulated depend on the technical characteristics of the check dam, the size, and characteristics of the aggradation, and the hydrological condition of the watercourse.
Groundwaters accumulated at the aggradation are considered as a potential water resource that can be used for the water supply of the mountainous villages' population.
The subject of this paper is the original idea (which has not been presented or analyzed so far), that the problem of providing drinking water to the population of mountainous areas can be solved by using groundwater from torrential aggradation formed by the construction of a check dam in the torrential watercourse. This way, under favorable conditions, already constructed, or about to be constructed, torrential check dams, in addition to their primary role, acquire another significant function in the field of water supply.

MATERIALS AND METHODS
An analysis of the characteristics of aggradations formed upstream of check dams in torrential watercourses was conducted in the territory of Serbia. Attention was paid to the check dams with completely filled reservoir spaces. Characteristics of the aggradation, lithological composition, and grain size of particles were analyzed, as well as their spatial distribution in aggradation as the medium in which the filtration and accumulation of groundwaters take place. At the check dams, the condition of weir overflows and their function during different hydrological periods were examined, as well as the role of the watercourses, on which the aggradation was formed, the aquifer recharge during the high waters and in the recession period. Check dam construction projects were analyzed, with a focus on the morphology of torrential beds and reservoirs, as well as the granulometric characteristics of the material at the aggradation.
Reconnaissance in the field made it possible to get acquainted with the condition of the check dams, the size of the aggradations, the characteristics of fractions at the aggradations, the flow of the watercourses through the aggradations and more.
The analysis of the collected field data was performed. A synthesis of the obtained results was carried out, on the basis of which certain conclusions and recommendations were made. They relate to the possibilities of aggradation groundwater use for the needs of water supply to a number of users in mountainous villages.

Groundwater in the aggradation
There are several thousand check dams in Serbia. They were built commonly in the mountainous areas (upper and middle portion of the basins), on perennial and intermittent streams characterized by the erratic -torrential character of the hydrological regime (Ko stad i n ov, 2008).
The construction of the check dam disrupts the dynamic equilibrium of the stream (N i k i ć , 2008). Since torrential streams are characterized by considerable sediment transport (R i st i ć and M a l o š e v i ć , 2011), the construction of the check dam leads to the deposition of the material upstream from it. The main consequence of the check dam construction and formation of aggradation is the reduction of the stream velocity and kinetic energy, and consequently, its capacity for sediment transport. Sometime after check dam construction, the upstream accumulation area will be filled with sediment material. The rate of deposition and material properties depend on numerous drainage basin features (level of forest cover, climatic, hydrological, geomorphological, geological, anthropogenic factors).
Upstream of the check dam, the suspended load and bedload form the aggradation from the bottom to the overflow, whose height corresponds to the check dam height. The amount of material and its features will depend on the check dam dimensions, the bed morphology, the basin lithology, the amount and intensity of precipitation, the frequency of torrential floods, the presence of vegetation, the dimensions of the weir overflow and other elements. In general, the aggregations are elongated and relatively narrow shaped, rarely exceeding a height of 10 m in the zone of check dam (or in some local river bed depression) (Figure 1). Since there are no general rules for the deposition, considering the size of the particles several types of deposits may be formed. The largest particles of deposited material in the form of blocks and debris canreach the size of several meters. These are commonly deposited in the upstream portion of the aggradation, although depending on the nature of the torrent, basin features and the shape of the accumulation area, the blocks can be deposited within the entire aggradation, up to the zone of the check dam. The finer sediment particles are transported furthest along the aggradation, filling the space and the pores between larger particles within the entire aggradation up to the check dam itself. The aggradation material is unconsolidated, angular-shaped, poorly abraded with sharp edges. The grain size of the aggradation material ranges from blocks and debris to sand and clay particles.
The deposited material originates from the entire catchment area upstream of the check dam. The sediment particles can originate from igneous, sedimentary and metamorphic rocks, different stratigraphy (Paleozoic to Quaternary) and geochemical character (acidic to ultrabasic) (N i k i ć , 2008). The geochemical or mineralogical composition of the material ranges from uniform to heterogeneous, as a consequence of the lithology of the catchment area. The diversity of sediment properties is a consequence of the complex geological function of the stream: the erosion of different lithological units, transport of weathered material and its deposition in the aggradation (N i k i ć and Pavl ovi ć, 2012).
The aggradation is an unconsolidated rock mass with intergranular porosity of a super-capillary type. There is no classification of material by the size. The different-sized material, from the blocks to the finest particles can be deposited at the same place. Due to the kinetic energy of high waters, the blocks are often deposited over finer sediments. In addition, very fine sediments can be deposited over larger ones.
The aggradation as an intergranular porous media is a rock mass with pores filled with water or air. The less porous and permeable material is usually located at the bottom and flank of the aggradation. The infiltration of surface water through the aggradation occurs during the entire year. Due to gravity, the infiltration of surface water occurs through pores towards the deeper parts of the aggradation. Within the cross-section of the aggradation, the water fills the pores from the bottom to the land surface. Thus, a groundwater body, an unconfined or phreatic aquifer with a free water table is formed (Ni ki ć and Pavl ovi ć, 2012).
From the hydrogeological standpoint, the aggradation cross-section consists of a saturated and unsaturated zone. The vertical distribution of these zones depends on the hydrological season, the amount of infiltrated water, the geometry of aggradation, and the technical properties of the check dam, especially the weir overflow. The water table is the highest during the wet period and the lowest during the recession period. The aquifer geometry corresponds to the geometry of aggradation (from the check dam to its most upstream section). The aggradation represents a hydrogeological system, which can be schematized as a single-layer porous media with impermeable bottom and flank boundaries. The main source of recharge is the flow of surface water above the aggradation, and the recharge from the precipitation is secondary. The amount of water in the stream varies over time and depends on the season. As a consequence of the hydrogeological properties of the aggradation, the stream and aquifer are directly hydraulically connected (Nikić and Pavlović, 2012). The aquifer drains through the weir overflow or overflow above the check dam, while the impact of evapotranspiration is negligible.
Hydrogeological parameters that govern groundwater filtration and transport are porosity, hydraulic conductivity, and geometry of porous media (Josipović, 1985). As a consequence of conditions of deposition, spatial distribution of different grain size particles is considerably nonuniform. Thus, the porosity and permeability of sediments in the aggradation are very heterogeneous. The hydraulically interconnected super-capillary pores can be considered as crucial for the movement of groundwater due to gravity. The wide particle size ranges of deposited material results in a diverse size of pores. Therefore, a laminar and turbulent flow regime can occur. The loss of energy on the friction forces during the movement of groundwater through porous media, expressed by the hydraulic conductivity, ranges considerably. The hydraulic conductivity values are considerably heterogeneous both horizontally and vertically (Kx ≠ Ky ≠ Kz). The accumulation of groundwater formed within aggradation is an unconfined or free water table aquifer. The groundwater flow movement is a function of the water table gradient towards the check dam, while the variations of the water table are spatially diverse. Since the aquifer recharge conditions and the material properties are diverse, the flow velocity changes both spatially and temporally causing the nonstationary groundwater flow (J o s i p ovi ć, 1985). The amount of water accumulated in the aggradation (x, y, z direction) that equals the difference between the inflow and outflow of water over time (∆t) is described by the following continuity equation (1): where П is a piezometric head (L), T is a coefficient of transmissibility (L 2 T -1 ); ε is a specific yield (-), q is a water inflow per unit area (LT -1 ); t is time (T).
The aggradation as an intergranular natural filter contributes to the improvement of the quality of infiltrated water. The groundwater movement through porous media is influenced by different processes of "mass transfer" and "mass transport". The processes which, due to the filtration through porous media, contribute to the improvement of water quality are numerous. Thus, these processes are: solute transport (convection, dispersion, diffusion), mass transfer from solid phase to solution (desorption, dissolution of fractions into solution), transfer from solution to aquifer matrix (physical sorption, chemical sorption, precipitation), decomposition of matter (biodegradation, radioactive decay, oxidation-reduction processes excluded from the biological degradation), evaporation (Filipović and Vujasinović, 1982). However, depending on the sediment geochemical composition, the dissolution of different chemical elements (e.g. iron, manganese, and others) may lead to the deterioration of the groundwater quality.

The advantages of the utilization of groundwater accumulated in the aggradation for the water supply
The groundwater sources in the aggradation are very similar to the sources in the alluvial sediments, and the sources with infiltration basins.
The important features of the source are the quality and amount of tapped water, and their long-term stability during the exploitation (L u ki ć, 2014). The capacity of a source is determined considering its minimum yield, during the recession period (F i l i p ov i ć , 1980). The variation between the minimum and maximum yield for the wells and springs tapped for water supply may be up to 1:10 or greater. The stability of the groundwater sources in the aggradation should also be considered in this manner.
The capacity of the groundwater source in the aggradation is characterized by the size of the aggradation, the amount of flowing water, local hydrogeological conditions, properties of the material, the water tapping object types, the legally obligated minimum environmental flow downstream of the check dam. The perennial streams with extensive and deep aggradations are considerably more favorable for the formation of groundwater sources. The streams that dry up in the recession period provide modest possibilities for the formation of groundwater sources.
Settlements and industries are not usually located upstream of the aggradation. Therefore, the quality of the aggradation inflow water is relatively favorable. This is considerably different compared to the alluvial sources since settlements and industries are commonly located along rivers. The agricultural production is intensive on alluvial plains, while the alluvial streams are common recipients of municipal and industrial wastewater.
The quality of water used for the population water supply needs to meet the regulatory criteria. In the case of the water from the aggradation, it depends on the quality of the inflow water and the hydrogeological properties of the aggradation. Compared to the surface water, the filtration through the aggradation leads to the improvement of groundwater quality. Unlike the alluvial aquifers, the variations of the quality characteristics of groundwater from the aggradation are commonly low. These variations can be emphasized during high waters, with a duration of up to several days. At the end of the flood period, the quality of water returns to its normal level. The impact of high waters on the quality of groundwater from the aggradation can be addressed using the adequate type of water tapping object. It can be expected that disinfection is the only technological treatment required before the use of groundwater from aggradation for the water supply.
Compared to other types of groundwater sources, the formation of sources in the aggradation is considerably economically advantageous. This advantage can be evaluated by comparing the investment and the operating costs for the source. The water as a resource for the water supply has a value that is difficult to express financially. Compared to other types of sources, the economic advantages of the aggradation sources are lower costs of its opening, exploitation, and maintenance. The additional advantage is that water is tapped from the underground reservoir with narrow sanitary protection zones and transported to consumers by gravity, from the existing check dam and aggradation. The areas upstream of the aggradation are commonly not threatened by anthropogenic pollution, and therefore the technological treatment of water is minimum.
Due to the formation of sources in the aggradation, the negative effects on check dams should not be expected. The construction of appropriate types of water tapping objects contributes to the lowering of the water table, and thus to the reduction of hydrostatic pressure on the check dam. The water tapping objects in the aggradation can have the same function as the weir overflow.

CONCLUSIONS
The regulation plans for the public water supply in Serbia envisage the construction of expensive regional water supply systems. However, it is not realistic that all planned systems will be built soon. The communities in the mountainous areas are particularly affected by the fact that many villages are still without appropriate water supply. This situation probably contributes to the migration of the population from mountainous areas. Considering this "water reality", inhabitants of these areas are forced to seek alternative solutions.
The check dams on torrential streams are constructed in higher parts of the terrain, to protect people and their property in the downstream parts of the basin. Upstream of the check dam, the material deposited within the aggradation forms intergranular porous media with an unconfined or free water table aquifer. The groundwater accumulation formed in the aggradation has commonly favorable qualitative characteristics for the water supply of the population.
The tapping of groundwater from the aggradation is suggested for the water supply of the population in the mountainous areas with water scarcity and where there are conditions for the tapping of groundwater. Thus, in addition to their main role in sediment retention, check dams can have another function for water supply by groundwater from the aggradation. The negative effects on the check dam, due to the construction of water tapping objects in the aggradation, should not be expected.
The advantages of the utilization of groundwater from the aggradation for the water supply are the following: -The financial resources needed for establishing this type of water supply system are relatively modest. -The potential of groundwater natural purification by filtration through the aggradation is considerable and contributes to the improvement of water quality (from the inflow to more favorable outflow). -Relatively narrow sanitary protection zones are required, since the catchment area is usually outside the anthropogenic and industrial impact, with the groundwater body below the surface of the terrain within the intergranular porous media. -Check dams and aggradation are located in the relative vicinity of potential users of water. Groundwater from the aggradation as a resource for the water supply of local communities can be used in the following cases: -the technical analysis has confirmed that the function of the check dam will not be threatened by the construction of a water tapping object and the use of groundwater from the aggradation, -there is no other water resource which can be used for water supply, -the feasibility analysis has confirmed that this is more economically advantageous compared to other water supply options