The ADDRESS of a grain: Sediment particle tracking as an approach to assessing ecosystem quality in dammed reservoirs

The study integrates hydrodynamic modelling and geospatial analysis methods to describe the dynamic transport and sedimentation conditions of suspended solids (SS) in large lowland reservoirs. The method was tested on a dam reservoir on the Vistula River (Poland). Stochastic particle tracking was used with a Lagrangian approach, then geostatistical analyses to interpret the simulation results. It is shown that, excluding the backwater, SS sedimentation is greatest in distal parts of the reservoir's lacustrine and river sections. The reservoir retains an average of 79 % of inflowing SS. The primary process affecting the sedimentation of SS is a dam-wards increasing fineness of settling grains. Hydrodynamic simulation results showed deposition of clay fraction to also be possible in the reservoir's upper part, even during floods. Moreover, sand grains entered the dam even during very low flows. Interpretation of Particle Tracking Model (PTM) simulation results and geostatistical modelling reveals that the submerged bed of the pre-dam river still has a transport function, while fine SS accumulates in slow-flowing areas outside it. The anastomosing pre-dam riverbed still influences flow conditions and SS deposition in the upper and middle reservoir. PTM modelling and geostatistical analyses determined the influence of hydrodynamic conditions on the spatial variability of SS sedimentation in terms of the quantity and grain-size distribution of sedimenting matter. An original indicator (ADDRESS [A]; Accumulation or Discharge of Dam REservoir Suspended Solids) was developed determining the ability of polymictic reservoirs to either retain SS of different diameters or allow them through the reservoir to be discharged through the dam. The indicator quickly and simply determines particle behaviour under given hydrological conditions. Information on whether all or only some of grains of specific diameters will flow through the dam is important for explaining many natural processes affecting the reservoir ecosystem and the river ecosystem below the dam.