Nonlinear distribution of sediment at river diversions: Brief history of the bulle effect and its implications

One of the fundamental units in fluvial systems is a bifurcation, where an existing channel splits into two. Diversions are a special class of asymmetric bifurcation, in which one of the channels after bifurcation continues along the original channel. Over the years, engineered diversions have been used for irrigation and navigational purposes. Interest in diversions has also been fueled owing to their similarity with naturally occurring asymmetric bifurcations. Thus, dynamics of the flow and distribution of sediment between the two bifurcating channels at a diversion have been active areas of interest to river engineers for almost 100 years, when a series of experiments showed that the ratio of bed load entering the two channels after a diversion did not mirror the ratio of the water flowing into the two channels. This nonlinear phenomenon is often referred to as the Bulle Effect. Subsequently, other experimental studies corroborated these findings and expanded the parameter space upon which the phenomenon depends. Interest in diversions has been rekindled recently because of their possible use to divert sediment (and water) for mitigating land loss at river deltas. This paper charts the history of studies on river diversions, bringing together studies on the Bulle Effect spanning almost a century. The objective of is to evaluate the state of art in the field and draw some holistic conclusions in order to shed light on the fundamental mechanism driving the nonlinear phenomenon of the Bulle Effect and its implications for the design of sediment and water diversions for reclamation of land in sediment-starved deltas.