Sediment deposition in the fioodplain areas of the Nemunas watershed and its influence on the sedimentation in the Curonian Lagoon

Abstract

Some of the amount is deposited on the soil surfaces gullies and sinks. Less than 5 % is mixed with sediments of river-beds and transported like smallsuspended sediments along the river bed. Amounts of suspended sediments transported and deposited in the valley and floodplains of Lithuanian part of the Nemunas watershed was calculated. It was established that about 15-23 % of this amount can be deposited on the bottom and slopes of drainage channels and rivulets. Approximately 10-17 % is retained in the pond of Kaunas Hydropower Station, and about 15-24 % is deposited in the inundated floodplains of the Nemunas Delta and wetland areas of its main tributaries. Thus, about 100,000-2,150,000 t or 3-38 % are carried out and can be deposited in the Curonian estuary area/year. Some hydraulic sediment retention measures and pollution reduction possibilities are discussed. The sedimentation processes in the Nemunas delta, the Curonian Lagoon and the river watershed based on comprehensive data of some researchers were analysed. High rates of river sediments indicate the sediment deposition intensification process in the Lagoon. It was established that about 80000...2480000 t of sand sediment deposited every year in the Lagoon depending on flood probability extent in the Nemunas river. Only 7-11% of this amount was transported into the Baltic Sea. Therefore, the mouth of the Nemunas River is the greatest source of pollutants with nutrient salts leached from agriculture. In the cases of large sediment transport the pollution of the Curonian Lagoon is quite perceptible. As the modeling of the flood activities covering the period of 1950-1991 had been done (based on the "Delta Mathematical model" elaborated by us), it was established that in the grassland area of flooded delta about 80% suspended flood sediments deposited in whole area of the delta. Consequently the flooded areas are very effective for river water self-purification; therefore the separation of large flood plain areas for agricultural or other needs could increase the water pollution of the Lagoon several times. It is possible to reduce the river's organic sediments (wash load) on the Lagoon by means of new sediment and nutrient trap in the flooding Nemunas delta and the watershed. The sediment deposition in the Nemunas delta can be increased by widening inflow in the fioodplain area. The useful effect of self-purification of watercourses and positive impact of natural floodplains on sediment retention and water quality has been investigated in the watershed of the River Nemunas. Due to the erosion of the soil surface about 4830000 t of ground particles was lost each year in the Lithuanian territory of the Nemunas watershed.