A closer look at sedimentation processes in two dam reservoirs

Dam reservoirs are complex depositional systems, which provide barriers to natural sediment transport along river courses. In addition, reservoir sedimentation causes serious problems in their operational management. Given the wide variability of reservoir bottom shapes, total volumes of stored water, river discharges, sediment loads and sediment textures, universal depositional models are difficult to achieve for dam reservoirs. This article focuses on the need to improve current understandings of sedimentation processes, and thorough spatial particle size distribution in dam reservoirs. To achieve this aim, sonar and ground penetrating radar surveys were combined with high-resolution stratigraphic data from 44 short sediment cores in two valley-type reservoirs, Seč and Křižanovice, part of a reservoir cascade in the Chrudimka River, which is a left-hand tributary of the Labe River (Czech Republic). Sonar survey was used to construct bathymetric maps and bottom reflection intensity images. The results showed topography inherited from pre-reservoir times and a good degree of correspondence between bottom reflection intensity and grain size. The sedimentation patterns exhibit distinct proximal-to-distal and transversal changes in sediment facies and grain size. Pre-reservoir and present morphologies have substantially shaped sediment accumulation. Spatial grain size sorting is a function of travel distance within reservoirs, which is conditioned by their bended and narrow geometry, as well as by reservoir management. Two stages of reservoir sedimentation are recognised. The initial stage was characterised by the filling of local depressions, which were overlain by the second stage, typified by continuous and horizontally deposited sediments. Relatively low sediment accumulation rates (from 0.2 to 1.06 cm/year) are the consequence of reservoir characteristics (capacity and associated accommodation space) and factors at a basin scale (low discharge, geology and land-use). Slow reduction of reservoir capacity was observed. The calculated trap efficiency is very high in the Seč Reservoir and much lower in the Křižanovice Reservoir. The main factor controlling trap efficiency is reservoir capacity and shape combined with low inflow. Ultimately, the trap efficiency change over time because of varying reservoir capacity due to seasonal water level fluctuation and varying hydrologic conditions. Due to the slow silting of the reservoir, the trap efficiency has decreased only slightly. Aside from general trends, some specific features exist for every reservoir based on local geometry and reservoir management strategy. This implies that an innovative approach combining map, sonar and geophysical surveys with core data is required to develop better facies models and to optimise sediment management strategies.