Modeling the Transport and Deposition of Suspended Solids Under Conditions of Low Water and Surge Phenomena in the Don River Estuary Area

An approach is proposed for the joint use of the model implemented in the HEC-RAS software and a balance model to describe the transport and transformation of suspended solids in a river delta. In the river estuary region, hydrological areas are distinguished: channel areas, floodplain areas, flooded during high floods and storm surges from the sea, and the delta front areas. For the hydrological areas, a dynamic model of the balance of water and substances transported by water flow is built. Parameterization of the suspended solids sedimentation processes and their resuspension is introduced depending on the speed of water movement and particle size. Three gradations of suspended solids in size are considered: pelitic fraction (clay), alevrit fraction (silt) and fine sand. The emphasis is on assessing the impact of marine storm surges on the transport of suspended solids into the river delta and their deposition. To describe water flows between areas, movement speeds, level dynamics and floodplain flooding processes, a detailed model based on the HEC-RAS software adapted to the conditions of the Don River estuary area is used. Calculations of the transport and accumulation of suspended solids in the Don River estuary area were carried out for two variants of hydrological conditions – with the water surge from the sea and without it. The spatiotemporal variability of the concentration and granulometric composition of suspended sediment depending on hydrological conditions is considered. It is shown that in the absence of surge phenomena and low water flow rates, suspended solids are mainly deposited in the avandelta outside the sea edge of the delta, and during the surge period they saturate the water and, at the stage of rising its level, enter the delta, partially settling in the branches and in the floodplain areas. At the same time, at the stage of the water level decline, they are carried out of the channel segments beyond the sea edge of the delta, and mostly remain in the floodplain areas. For low-water conditions with the observed frequency of surge events and in the absence of floods, the Don estuary area retains on average 20% of suspended solids entering with the Don River runoff.