Channel closure in large sand-bed braided rivers

Abstract

In large braided rivers, river training is often required to protect the banks against erosion, to improve navigability and for land reclamation. Closing one of the channels is a promising option to achieve these goals. However, there is a lack of systematic research on channel closures and no guidelines for their use exist. In the few documented cases, the river reopened the closed branch by eroding a channel across the island that separated the channels. The goal of this study is to analyse the consequences of closure, find the variables that affect channel reopening and provide guidelines for channel closures. The problem is studied with a numerical Delft3D model. A simplified reference case is set up, with two branches separated by an island. It is based on typical reaches of large braided rivers. Initially, one of the channels is closed with a combination of a weir in the channel and a short embankment on the island. A bypass channel is eroded around the weir in the first weeks of the wet season. A sensitivity analysis of different physical properties is performed in order to determine how general the simplified case is. Quantitative results vary, but the qualitative morphological response to closure remains similar. The largest part of the study consists of analysing simulations with various interventions. Combinations of a weir in the channel and embankment on the island, multiple weirs, roughness elements, vegetation and partial closures with bandals are tested in different positions along the channel. Analysis is extended by including simulations with higher water levels, four additional geometries and some longer-term simulations. Finally, aggregated results are analysed to find connections between variables. Channel reopening due to formation of channels on the island is the main cause of reduction in closure effectiveness. Bypass channels of varied depth form around the intervention in most cases. Channels across the island mostly occur when longer embankments are used. Their extent is less predictable and increases in time, so they should be avoided. The type of channel on the island that will most likely develop can be predicted from the initial hydrodynamic conditions after closure. Correlation between erosion of channels on the island and the initial hydrodynamic conditions is found. The water level gradient is the most important parameter that determines the location and degree of channel erosion. Water depth on the island plays a role when water levels are relatively low. Higher water levels cause more erosion, which can be mostly prevented with the use of submerged weirs. Flow patterns on the island determine the exact position of the eroded channels and directly contribute to erosion. Sediment supply to the newly formed channels reduces their growth. Deposition in the closed branch improves conditions over time, especially with interventions that do not block the channel fully. Length and width of the island play a role in the extent of erosion on the island, whereas channel and bifurcation asymmetry mostly do not. The best overall solution is found to be a combination of a weir with a long embankment or roughness elements. Further details, such as position of the interventions and weir crest height, depend on goals of closure and water levels in the system.