Unraveling the Interactions between Tidal and Fluvial Flood Wave Signals through the Fluvio-Tidal Transition, Chehalis River, USA
Monday, 15 December 2014
Bedload and suspended load transport and deposition within the fluvio-tidal zone is governed by the dynamic interplay between fluvially-derived unidirectional currents with bi-directional tidal flows of varying magnitudes that display significant spatio-temporal variation across a range of scales. These fluctuations in the governing hydraulics are represented directly in the differing characteristics of bar forms, bedforms, and silt/clay interbeds within this region that ultimately control the sedimentary facies observed within the fluvio-tidal transitional zone. In this study, we examined seven water surface elevation gages from both NOAA and the USGS located throughout the full expanse (~ 60 km) of the fluvio-tidal zone on the Chehalis River from the river mouth through the “backwater” reach to the fully-fluvial environment where normal flow prevails. Wavelet analysis coupled with computed water surface energy gradients was conducted for both the tidal (i.e. semi-diurnal and diurnal) and fluvial signals at low and high (i.e. flood wave) fluvial discharge conditions that captured all seasonal fluctuations in both the tidal range and fluvial hydrograph. Spatio-temporal analysis of the fluvio-tidal signal along the “backwater” reach shows that for particular hydrodynamic conditions the tidal flows are significantly altered, if not completely dampened, by upstream originating fluvial flood waves, and also displays where fluvial flood wave signals are absorbed by the tidal flow signal (i.e. tidal prism). Furthermore, the results of the above analysis coupled with: (i) salinity measurements, and (ii) suspended sediment concentrations, illuminates in a spatio-temporal context where/when along the “backwater” reach flocculation of fine-grained sediment is expected. Thus, here we will demonstrate how the large-scale hydrodynamics varies throughout the fluvio-tidal zone and will discuss the implications of these results for the preservation potential of sedimentary facies in this region.