Centre Angling-Steamboat channel bifurcation, Cumberland Marshes, Canada (flow to the right)Bifurcations distribute water, sediment and contaminants over distributaries in deltas. The aim of this study is to develop and test a bifurcation stability model for time-scales of 10¹-10³yr, in which water and sediment is distributed depending on both local (e.g., meandering, bifurcation planform) and regional controls (e.g., climate, base-level). Bifurcations are key elements in delta evolution on time-scales of 10²-10³yr and river flooding mitigation on time-scales of 10¹-10²yr. Bifurcations are often unstable and one of the distributaries then takes over the entire discharge, resulting in channel avulsion. However, some bifurcations, e.g., in the Rhine (The Netherlands), were remarkably stable for >10³yr.

The reasons are unknown, as previous work on bifurcations and avulsions focused either on actual flow and sediment transport processes in braided rivers on time-scales of 1-10¹yr, or on avulsion and delta evolution of ancient rivers on time-scales of 10²yr. Consequently, on intermediate time-scales there is lack of knowledge on the time-integrated effects of local processes (e.g. meandering) versus the impact of regional controls.

Field data of water/sediment distribution and of bifurcation stability in meandering rivers is unavailable so far, so existing models have not been verified. A new hypothesis of local controls on bifurcation stability in meandering rivers will be tested. The data and model will be based on

1. historical maps and flow data since 1400AD of Rhine bifurcations;
2. detailed flow and sediment transport process data at modern Rhine bifurcations;
3. mathematical 1D and 3D-flow modelling of scenario bifurcations including those affected by tidal water level fluctuations.

The model will be nested in a regional delta evolution model to test the effect of local versus regional controls on delta evolution against geological data of the Rhine. The societal benefit lies in the pressing need to understand bifurcation stability in heavily populated deltas which experience increased flooding risks and delta drowning due to present-day climatic change.