The Role of Discharge Variability on Environmental Signal Propagation: An Experimental Study
Field and experimental studies suggest that the time lag between a signal and its manifestation increases with transport distance. However, the Ganges-Brahmaputra system has been documented to transfer tectonic and climatic signals with a very short time lag despite the long transport distance. The unusually high rate of signal propagation suggests that we do not fully understand the signal propagation and preservation processes. This discrepancy leads us to hypothesize that discharge variability can be a dominant control of signal propagation. To test the hypothesis, we set up a flume with uniform grain size on the bed and use constant water and sediment discharge conditions to establish an initial steady state for all the experimental runs. We then adopt different hydrographs with the same mean discharge and sediment supply. The hydrographs represent different discharge variability, and the newly added sediments are colored to represent the signal. The colored sediments include two grain-size groups: one is the same with the uniform bed material, the other group containing coarser grains with the largest size above the flow competence of the constant discharge control group. Once some of the colored sediments have reached the downstream outlet of the flume, the signal they represent has successfully propagated downstream, although just partially. Out preliminary results suggest discharge variability can enhance signal propagation. For the coarser fraction of the signal, those with relatively higher discharge variability is more efficient in transmitting the signal due to their higher flow competence. For a given grain size, the transition from bed load to suspended load primarily depends on flow velocity. For a sediment signal, different transport mode will greatly alter its propagation rate. For the finer fraction of the signal, the variable-discharge hydrographs are more efficient than the constant-discharge hydrograph. However, there is no simple linear relationship between discharge variability and signal propagation efficiency. One possible reason is the changing morphodynamics when flow velocity and Froude number increase. The bedform changes can alter form drag and thus the ability to transport sediments. Currently, we do not know yet when the relationship between discharge variability and signal propagation efficiency will break up. Further studies are needed to address the issue of applicability of the relationship for more broad conditions.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019