Estimating Tidal Parameters from Cross-sectional ADCP Data in Estuarine Environments

Human engineering can strongly impact the hydrodynamics and salinity concentrations of coastal areas such as estuaries and tidal rivers. Given the geometric complexity of these altered regions, it is often necessary to have a detailed picture of three-dimensional tidal flow velocities and salinity concentrations in order to explain observed system sensitivities with respect to changing conditions. To estimate the dynamics in such areas (such as the Rotterdam harbor area), flow velocities are often estimated using moving-boat Acoustic Doppler Current Profiler (ADCP) sensors, and salinity concentrations using Conductivity, Temperature, Depth (CTD) sensors. These raw data must be analyzed and fitted to human-intelligible functions in space and time, after which further conclusions can be drawn about flow and salinity characteristics.

In the present work, we present a unified fitting procedure for three-dimensional ADCP and CTD data. It extends the method of Vermeulen et al. (2014) to reduce the dependence of estimated flow patterns on spatial homogeneity assumptions for mesh-based flow velocity estimation, and uses available information on tidal motion to reduce assumptions on temporal homogeneity when fitting ADCP or CTD data that were collected at different times within the tidal cycle by projecting the data on harmonic constituents within the cross-section. The method allows for directly converting raw data to amplitudes and relative phase differences of different tidal constituents that constitute flow velocities and salinity concentrations in both Cartesian and curvilinear coordinates (in particular, sigma coordinates). Estimated parameters are projected on a cross-sectional grid, on which vertical and lateral flow and salinity shear can be analyzed in terms of tidal constituents.

The method is applied to a 13-hour dataset consisting of repeat transect data in the Rhine-Meuse delta of both ADCP and CTD data. A sensitivity analysis is performed to investigate the reliability of obtained parameters. The estimated flows show considerable vertical and lateral phase differences of tidal motion. Additionally, terms in the continuity and momentum equations are estimated on the cross-sectional domain, which are a posteriori used to improve the estimated flow velocities and salinity concentrations.