The development and application of an autonomous working turbidity measurement network: Assessing the spatial and temporal distribution of suspended particulate matter on tidal flats in the North Frisian Wadden Sea

Abstract

Measuring suspension load (i.e. turbidity) in tide-dominated shallow marine systems like the North-Frisian Wadden Sea forms a major challenge with respect to understanding and modelling sediment origin, transport and dispersal. Sediment transport and deposition in highly dynamic and vulnerable coastal regions becomes especially relevant in terms of the natural adaption capacity of intertidal and supratidal areas to global sea-level rise.We have developed a network of eight autonomously operating turbidity measuring stations in an island-like salt marsh area (Hallig Langeness) in the tidal flats of the North-Frisian Wadden Sea. Using this system we got first insights about the availability of suspended particulate matter (SPM). In order to obtain the best possible estimation of the SPM concentrations in our study area, each measuring station has been individually calibrated considering the local conditions by using sediment collected by self-built sediment accumulators which are installed next to the respective measuring station. With this method we have investigated the availability of SPM and its spatial and temporal distribution during several spring and winter months.The results indicate strong and systematic SPM variability between individual stations ranging on average from 0.04 to 0.56 g/l. Control factors include installation altitude, the proximity to tidal channels, the positive influence on SPM availability by adjacent land reclamation areas or the negative influence on SPM availability due to a shielded position between two spur dykes. Further results show a strong increase of the SPM concentration under harsh weather conditions and inundation events whereby strong winds alone are not sufficient for a significant SPM increase. Instead, the combination of suitable wind direction, its strength and duration play a major role. The increase in SPM concentrations under harsh weather conditions tends to be highest at stations with less protection against the open water. Our new extensive network of turbidity measurements also allows for detecting areas with predominantly low SPM availabilities, the so-called low spots.