Crestline bifurcation and dynamics in fluvially-dominated, tidally-influenced flow

Current understanding of dune dynamics in rivers and estuaries in response to varying flows remains incomplete, particularly with respect to pattern evolution. This study presents observations of dune crestline change during the gradually varied flow caused by the influence of a diurnal tide on the fluvially-dominated Fraser Estuary, British Columbia, Canada. Observations consist of a 1.0 km long and 0.5 km wide patch of a dune field mapped repeatedly during a falling and rising spring tide, on the rising limb of annual hydrograph immediately prior to peak freshet. The fundamental relations between dune geometry, translation rate, sediment flux and the birth, death and piracy of dune crestlines are explored. The data show that the bedform field consists of primary, channel-spanning dunes with a series of interconnected secondary crestlines. The primary crestlines are two-dimensional and experience little change over the tide cycle. The secondary crestlines impart a three-dimensionality to the bedform field that is dynamic with the flow. There is a predominance of dune crestline bifurcation deaths' (disappearances) over births' (appearances) as flow velocity in the channel increases, suggesting that the crestline configuration is moving towards a more two-dimensional configuration. As the flow wanes, bifurcation births outweigh deaths, suggesting that the dune field becomes more complex and three-dimensional. The results suggest that the primary response to variable flow may be a simplification of the dune field towards a two-dimensional configuration as flow increases and added complexity towards a more three-dimensional configuration as the dune flow wanes.