Development of methodology for establishing protection zones in the proximity of water reservoirs from erosion and sediment transport aspect

Abstract

Construction of reservoirs is a priority for providing sustainability of the water resources. In the Republic of Macedonia there are 23 big and over 120 small water reservoirs. On the other hand, erosion and filling the reservoirs with sediment is one of the main problems of reducing the lifespan of the reservoir. The total annual accumulated sediment in all the reservoirs in the Republic of Macedonia is 3.000.000 m3 (Erosion map of RM, 1993). Because of this, the main prerequisite for protecting the reservoirs from filling up with sediment is their sustainable use. In the world literature there is no exact and wide-spread methodology for zoning of the reservoirs. There are only some empirical approaches which are not based on scientific facts but on expert judgment. In this study there are two case studies taken, from the catchment of the river Bregalnica: “Kalimanci” reservoir (large catchment) and “Gradce” reservoir (small catchment). At first, the land use in the catchment is examined in order to assess the dynamics of the land use over the time. The change of the land use is connected with the demography, cultivated land, the area covered with pastures, the number of livestock in the region and the forested areas. In comparison of the period when the reservoirs were built and today, there are considerable differences. The inhabitants migrated from the villages to the towns, then the agriculture land is diminished and the areas under forest and pastures are increased. Further on, the amount of accumulated sediment in the reservoirs over the time is discussed. The amount of accumulated sediment is needed to validate the erosion model. There can be observed two periods. The first lasts to the middle of the 1980’s, when the amount of accumulated sediment in the reservoirs is more than double than the second period which lasts till present. Next, the methodology for calculating the produced and transported sediment is discussed. The main method used is the EPM Gavrilovic method and the validity is assessed with the accumulated sediment in the reservoirs. Then the critical subcatchments from erosion point of view are delineated and the source of the sediment in the reservoir is discussed and where it is accumulated. Most of the critical subcatchments are situated around the reservoir “Kalimanci”, the middle of the catchment is with medium severity and the highest parts of the river Bregalnica is the least critical. The largest part of the sediment transported by the river Bregalnica is deposited in the valleys, almost 75%. Next, there were several models created if the land use in the catchment was changed and it was concluded that changes in the land use largely affects the erosion processes and phenomena. With the aggregation of the previous steps, the zones of the reservoir “Kalimanci” from erosion point of view were created and also the procedure for development of the zoning was explained.