Modelling transport and deposition of fluvial dispersed seeds

Background/Question/Methods

The structuring of riparian plant communities is linked to the physical processes of hydrogeomorphology and provides insight into the population dynamics along river margins. For instance, fluvial seed dispersal is subjected to the coupled process between the hydrology, river hydraulics, and bedform structures. This study examines the influence of stream flow characteristics on the transportation and deposition of buoyant seeds by applying a one dimensional transport model. Conceptually, the model separates the stream into two components: the main channel and storage zones. The hydrologic processes are governed by an advection-dispersion equation with the inclusion of terms to allow for transient storage and deposition. Additionally, the model allows for flow variation by lateral inflow and outflow terms. The model parameters are based on a bedrock-gravel bed river with pool-riffle morphology where we collected data on a Coastal Northern California stream. The riparian zone of the study reach is inhabited by White Alder (Alnus rhombifolia) that disperses buoyant seeds in late winter/early spring coinciding with the latter part of the wet, Mediterranean climate.  Artificial seeds with similar characteristic traits to White Alder seeds such as buoyancy, density and Bond Number were used to quantify transport times and identify storage areas.

Results/Conclusions

The model output captures a greater number of seeds on the receding end of the hydrograph related to the increase in transient storage and when dispersive flows are dominant. In the field, this is associated with the ends of gravel bars or riffles where flow expansion causes secondary flows.  The results demonstrate the importance of transient storage for depositional processes; however, underline the need for improved measurement techniques, in lieu of empirical coefficients, to advance the mechanistic understanding of the complex hydraulic processes.