Preliminary assessment of applying permeable groynes in the Lower Yellow river

River training in the Lower Yellow River (LYR) has a history of over 300 years. River training works that wereintended for channel regulation were started in 1950. The main engineering structures of the river training works in the LYR are groynes. In the past halfcentury, more than8,000 groynes have been constructed in the LYR. As a result,the main channel has been trained in terms of channel migration. However, the flow entering into the LYR has reduced substantially since 1986 because of the operation of upstream reservoirs, the decrease of rainfall over the catchment and the increase of water demand for agriculture and industry. The water volume is decreased to 56% of the previous long-term average value and the sediment supply is reduced to 49%. The changing water and sediment conditions have disturbed the interaction between the flow processes and channel characteristics in LYR and the river training works. Subsequently, the river training works have partly lost control of the river morphology and somebank erosion occurred and some abnormal river bends were formed. This increases the risk of flooding and it results in damage and the need for expensive repair of the river training works. Inspired by the Dutch experiences in particular in the Jamuna River in Bangladesh, in this study permeable groynes are proposed as a supplementary measure to restrain the flow from swinging and to keep the channel pattern in the LYR stable. Physical model tests were used to assess the impacts of permeable groynes. The physical model includes four river bends and it was designed following the sediment similarity laws usually applied for Yellow River mobile bed models. Simulation of morphological processes in models is quite complex because it requires the correct simulation of water flow, sediment motion, bed topography and planform changes at the same time.Conflicts between the various scale conditions and the impossibility to fulfil them all at the sametime will result in so-called scale effects. Scale effects in the applied model were studied and it was found that by the use of coal ash asbed material in the model, the scale effects are limited. They affect to a limited extent the planform dimensions and bank erosion rates; other aspects like as sediment transport, bed deformation time, are less subject to scale effects. One test without permeable groynes and two tests with permeable groynes were carried out in the model. The experiment results show that in a test with permeable groynes on one side, the bank erosion area was reduced to 35 %; the main flow swing distance reduced to 60%; the sediment transport capacity was increased by about 10 % and substantial bed degradation happened. In the test with permeable groynes on both sides, the bank erosion area was reduced to 7 %; the main flow swing distance was reduced to 30%; the sediment transport capacity increased by 38 %; and bed degradation increased even more. Some of the observed bed degradation however mightbe due to the specific operation of the model during the tests. The resultsshow that implementing permeable groynes in the crossings in between the river bends can reduce the bank erosion area, increase the sediment transport capacity, and decrease the flood level. All these impacts are positive for the LYR