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Continuous monitoring bed-level dynamics on an intertidal flat: Introducing novel, stand-alone high-resolution SED-sensors
Hu, Z.; Lenting, W.; Van der Wal, D.; Bouma, T.J. (2015). Continuous monitoring bed-level dynamics on an intertidal flat: Introducing novel, stand-alone high-resolution SED-sensors. Geomorphology (Amst.) 245: 223-230. http://hdl.handle.net/10.1016/j.geomorph.2015.05.027
In: Geomorphology. Elsevier: Amsterdam; New York; Oxford; Tokyo. ISSN 0169-555X, more
Peer reviewed article  

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Keywords
    Marine; Brackish water
Author keywords
    Sediment dynamics; Spatiotemporal scales; Hydrodynamic forcing; Measurement validation

Authors  Top 
  • Hu, Z., more
  • Lenting, W.
  • Van der Wal, D., more
  • Bouma, T.J., more

Abstract
    Tidal flat morphology is continuously shaped by hydrodynamic forces, resulting in a highly dynamic bed surface. The knowledge of short-term bed-level changes is important both for assessing sediment transport processes as well as for understanding critical ecological processes, such as vegetation recruitment on tidal flats. High frequency bed-level measurements with a high vertical resolution are generally needed for hypothesis testing and numerical model validation. However, conventional manual bed-elevation measurements tend to have a coarse temporal resolution (weeks to months) due to the labor involved. Existing automated methods for continuous monitoring of bed-level changes either lack a high vertical resolution or are very expensive and therefore limited in spatial application. In light of this, we developed a novel instrument called SED (Surface Elevation Dynamics) sensor for continuous monitoring with a high vertical resolution (2 mm). This sensor makes use of light sensitive cells (i.e. phototransistors) and operates stand-alone. The unit cost and the labor in deployments are reduced, facilitating spatial application with a number of units. In this study, a group of SED-sensors is tested on a tidal flat in the Westerschelde Estuary, The Netherlands. The obtained bed-level changes are compared with the data obtained with precise manual measurements using traditional Sedimentation Erosion Bars (SEBs). An excellent agreement between the two methods was obtained, confirming the accuracy and precision of the SED-sensors. Furthermore, to demonstrate how the SED-sensors can be used for measuring short-term bed-level dynamics, two SED-sensors were deployed at two sites with contrasting wave exposure. Daily bed-level changes were obtained including a severe storm event. The difference in observed bed-level dynamics at both sites was statistically explained by their different hydrodynamic conditions. Thus, the stand-alone SED-sensor can be applied to monitor sediment surface dynamics with high vertical and temporal resolution, which provides opportunities to pinpoint morphological responses to various forces in intertidal environment. We expect that this sensor can also be applied in other morphological environments, such as rivers, salt-marsh, beaches and dunes, but the actual applicability remains to be tested. Furthermore, the SED-sensors may also offer opportunities for ground-truthing remote sensing techniques aimed at describing morphological changes. Finally, further improvements in future SED-sensors are discussed, including an inclination and compass sensor, wireless data retrieving function and additional IR-light bar for measurements with poor light availabilities.

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