Turbulence structure in bottom layer of a tidal estuary

River hydrodynamicsTurbulent open channel flow and transport phenomen

Application of Acoustic Tomography for Gaging Discharge of Atidally Dominated River

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

An innovative methodology/technology for streamflow observation

River engineeringInnovative field and laboratory instrumentatio

New acoustic system for continuous measurement of river discharge and water temperature

AbstractIn many cases, river discharge is indirectly estimated from water level or streamflow velocity near the water surface. However, these methods have limited applicability. In this study, an innovative system, the fluvial acoustic tomography system (FATS), was used for continuous discharge measurement. Transducers with a central frequency of 30 kHz were installed diagonally across the river. The system's significant functions include accurate measurement of the travel time of the transmission signal using a GPS clock and the attainment of a high signal-to-noise ratio as a result of modulation of the signal by the 10th order M-sequence. In addition, FATS is small and lightweight, and its power consumption is low. Operating in unsteady streamflow, FATS successfully measured the cross-sectional average velocity. The agreement between FATS and acoustic Doppler current profilers (ADCPs) on water discharge was satisfactory. Moreover, the temporal variation of the cross-sectional average temperature deduced from the sound speed of FATS was similar to that measured by a temperature sensor near the bank

Variability in salt flux and water circulation in Ota River Estuary, Japan

In this study the sub-tidal and intra-tidal variations of salt fluxes in the upstream section of a shallow estuary (with a water depth of less than 3 m) were investigated. The salt fluxes were estimated based on the cross-sectional average salinity and velocity measured by the fluvial acoustic tomography system (FATS). The results indicate that the magnitude of seaward fluxes is approximately two times greater than that of landward fluxes under normal conditions. The results of short-term observation in the study area indicate that there is a phase lag of the bottom and surface salinities between the regions with the largest and smallest depths. The vertical shear flux with a peak value of −0.7 m2/s during the ebb tide indicated an important contribution to the total salt flux compared with the advective flux. A phase lag occurred between the vertical shear terms in the regions with the largest and smallest depths, which resulted from the correlation between the vertical variations of the salinity and velocity and the existence of transversal velocity circulations

Suspended particulate matter concentration in response to tidal hydrodynamics in a long mesotidal floodway

Analyses of seasonal data obtained using acoustic Doppler current profilers provide an understanding of the behavior of suspended particulate matter concentration (SPMC) toward different forcings in tide-controlled floodways. In this work, the relative contributions of external forcings on SPMC variability were quantified in a tidal river system using singular spectrum analysis (SSA). The main environmental features affecting SPMC were identified as i) spring-neap tidal oscillation, ii) the ebb/flood velocities, and iii) tidal straining. Large SPMC fluctuations occurred under strong mixing states and were directly related to the sediment resuspension stirred up by spring-neap tidal cycles (73.6%–81.9%) and ebb/flood velocities (9.6%–19.5%). On the seasonal scale, river discharge is the key variable explaining the downstream flushing and promoting the occurrence of a convergence zone at the floodway. Upstream from the floodway (greater than 4.8 km from the river mouth), the spring-neap tidal oscillation dominated the suspended particulate matter (SPM) mobility under low river discharge. Two interesting findings were revealed in this work: (i) the SPMC/SPM transport variation responses to tidal forcing (tidal asymmetry) were dominated and modified by river discharge and (ii) the effect of river discharge on the SPMC/SPM transport did not result in a uniform state along the floodway. It is believed that these findings provide further understanding of the dynamics of suspended sediments in shallow tidal systems.This work was supported by JSPS KAKENHI grant number JP17H03313

Characteristics of Tidal Discharge and Phase Difference at a Tidal Channel Junction Investigated Using the Fluvial Acoustic Tomography System

This study investigates the tidal discharge division and phase difference at branches connected to a channel junction. The tidal discharge at three branches (eastern, western, and northern branches) was continuously collected using the fluvial acoustic tomography system (FATS). The discharge asymmetry index was used to quantify the flow division between two seaward branches (eastern and western branches). The cross-wavelet method was applied to calculate the phase difference between the tidal discharge and water level. The discharge asymmetry index shows that the inequality of flow division is obviously prominent during the spring tide duration, where the eastern branch has the capability to deliver greater amounts of subtidal discharge, approximately 55–63%, compared with the western branch. However, the equality of flow division between the eastern and western channels can be observed clearly during the neap tide period. The wavelet analysis shows that the phase difference at the western branch is higher than at the eastern branch, because the geometry of the western branch is more convergent than that of the eastern branch. Accordingly, the amplitude of the tidal wave at the western branch is more magnified compared with that at the eastern branch. Moreover, the phase difference at the northern branch is greater than at the two seaward branches, implying that the phase difference is slightly increased after passing through the junction into the northern branch