Sediment transport in the Schelde-estuary: a comparison between measurements, transport formula and numerical models

ABSTRACT The Schelde-estuary serves different estuarine functions and therefore faces managers with multiple challenges: increasing tidal propagation vs. safety against flooding; sedimentation in the navigation channel vs. port accessability; changing dynamics vs. ecology. Within the Flemish-Dutch Long Term Vision for the Schelde-estuary, a 4 year (2014)(2015)(2016)(2017) research programme was defined, in which 8 topics will be dealt with (e.g. tidal penetration, risk for regime shift, sediment strategies, valueing ecology). Two fundamental tools will be crucial in answering the different questions towards the future management of the estuary: expertise/system understanding and numerical models. At this moment (first year), several projects are ongoing trying to increase the system understanding and improving the state-of-the-art numerical models. Where the numerical models reproduce the hydrodynamics reasonably well, sediment transport and the resulting morphological changes is still a big challenge. Therefore an extensive monitoring campaign was performed in 2014, during which both hydrodynamic and sediment transport measurement were performed in the Schelde-estuary. At more than 10 locations, from the up-estuarine part (Boven-Zeeschelde) to the mouth area (Vlakte van de Raan), measurements were executed over a full tidal cycle (13h). Currents were measured using ADCP, while sediment transport was measured using both direct (Delft Bottle and pump samples) and indirect (OBS, ABS) techniques. This extensive dataset allows an in-depth analysis of the sediment transport processes occuring in the estuary. A comparison will be made with several transport formula (e.g. Bagnold, Engelund-Hansen, Van Rijn, ...). The data will also be used to validate the existing numerical models, allowing a better assesment of the possibilities and limitations of the present numerical models. KEY WORDS Sediment transport, measurements, numerical modelling, estuary THE SCHELDE-ESTUARY The Schelde-estuary is a macro-tidal estuary with a length of 180 km in Flanders and the southern part of the Netherlands The estuary is characterised by semi-diurnal tides, causing ebb and flood currents with important sediment transports of both cohesive as non-cohesive sediments. The Schelde-estuary serves different estuarine functions and therefore faces managers with multiple challenges: increasing tidal propagation vs. safety against flooding; sedimentation in the navigation channel vs. port accessability; changing dynamics vs. ecology. Within the Flemish-Dutch Long Term Vision for the Schelde-estuary, a 4 year (2014-2017) research programme was defined, in which 8 topics will be dealt with (e.g. tidal penetration, risk for regime shift, sediment strategies, valueing ecology). METHODOLOGY In order to supply managers with adequate answers, research tools (both expertise/system understanding and numerical model

Uncertainties associated with in situ high-frequency long-term observations of suspended particulate matter concentration using optical and acoustic sensors

Measurement of suspended particulate matter concentration (SPMC) spanning large time and geographical scales have become a matter of growing importance in recent decades. At many places worldwide, complex observation platforms have been installed to capture temporal and spatial variability over scales ranging from cm (turbulent regimes) to whole basins. Long-term in situ measurements of SPMC involve one or more optical and acoustical sensors and, as the ground truth reference, gravimetric measurements of filtered water samples. The estimation of SPMC from optical and acoustical proxies generally results from the combination of a number of independent calibration measurements, as well as regression or inverse models. Direct or indirect measurements of SPMC are inherently associated with a number of uncertainties along the whole operation chain, the autonomous field deployment, to the analyses necessary for converting the observed proxy values of optical and acoustical signals to SPMC. Controlling uncertainties will become an important issue when the observational input comprises systems of sensors spanning large spatial and temporal scales. This will be especially relevant for detecting trends in the data with unambiguous statistical significance, separating anthropogenic impact from natural variations, or evaluating numerical models over a broad ensemble of different conditions using validated field data. The aim of the study is to present and discuss the benefits and limitations of using optical and acoustical backscatter sensors to acquire long-term observations of SPMC. Additionally, this study will formulate recommendations on how to best acquire quality-assured SPMC data sets, based on the challenges and uncertainties associated with those long-term observations. The main sources of error as well as the means to quantify and reduce the uncertainties associated with SPMC measurements are also illustrated