A study of sedimentation problems in the lower reaches of the river Österdalälven

Abstract: The river  Österdalälven  deposits  large  amounts  of sediment  when it  passes through  the city of Mora. The sediment  deposition  risks  clogging  the inlet  to the lake  Siljan,  hampers navigation,  and creates a risk  of the river  forming  new channels.  This study  has addressed the problem  by creating  a numerical  2D depth-averaged combined hydrodynamic  and sediment  transport  model  of the reach. The study focused on the mechanisms  behind  the sedimentation  and erosion  patterns. River  training  structures in  the form of groynes  were added to the model  to investigate  whether  mitigation  of the problem  by physical  structures was possible.  Because  of the lack  of field  data, some of the flow  and sediment  transport parameters  had to be estimated. Sensitivity  analyses were performed to analyse the model’s response to the choice  of boundary  conditions,  input parameters, and auxiliary  models. The study concluded that erosion  occurs in areas where  the shear stress or flow  velocity  is  high  and sedimentation  in  areas with flow circulation and lower flow  velocity.  The sediment yield at the problem  area, i.e. at the mouth in Siljan  was flow-dependent  and increased  with  larger  flow discharges. The yearly  sediment  yield was low  compared  to stations  downstream. The model  was sensitive  to the choice of boundary conditions,  Manning’s roughness  coefficient,  and sediment  transport  mode and transport  capacity  formula.  The main  conclusion was that it  is  crucial  to collect  the relevant  field  data to obtain more reliable  result  for further  studies. It was further concluded  that physical  structures  in  the form of groynes  could  decrease the amount  of sediment  that deposits at the mouth of Österdalälven in  Siljan. The study has shown that it  is  possible  to create a working  numerical  river  model  based on the physical  understanding  of the flow  despite  the lack of field  data.