Interplay between hyporheic exchange and bedform morphodynamics in mobile clay-sand beds

Fine particles (0.1-100 microns) are ubiquitous within the water column of rivers and estuarine systems. Clogging of sediment beds resulting from the deposition and storage of fine particles has been shown to inhibit hyporheic flow, altering nutrient cycling and the retention of contaminants. However, available observations of the interactions between fine particles and hyporheic flow are limited to immobile sediment beds. It is hypothesized that clay deposition within the mobile bedform layer due to mixing and hyporheic flow may result in morphodynamic change from cohesive effects. To understand this coupling in marine, estuarine, and freshwater systems, we performed laboratory experiments to observe changes in bedform morphodynamics and hyporheic flow following additions of kaolinite clay. Cores of the sediment bed combined with time-lapse images show clay accumulation within and below the mobile bedform layer, with the peak accumulation coincident with the most frequent bedform scour depth. Bed form height, celerity, and flux decreased linearly with increasing clay accumulation in the bed, while bed form length showed no appreciable change. For the largest observed clay/sand mass ratio of 0.4 under freshwater conditions (salinity of 0.25 ppt), we found that clay reduced bedform height, celerity and sediment flux by 14%, 22% and 29%, respectively. Further, the hyporheic exchange flux decreased by a factor of two after the initial addition of clay to the system and did not decrease further following introduction of additional clay. These results show that the dynamics of bed sediment morphodynamics, fine particle deposition, and hyporheic exchange are tightly coupled.