Coupled penetrometer, MBES and ADCP assessments of tidal variations in surface sediment layer characteristics along active subaqueous dunes, Danish Wadden Sea |
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Authors: | Nina Stark Hendrik Hanff Christian Svenson Verner B Ernstsen Alice Lefebvre Christian Winter Achim Kopf |
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Institution: | (1) MARUM-Center for Marine Environmental Sciences, Leobener Str., 28359 Bremen, Germany |
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Abstract: | In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal
inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification
pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below
(quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the
top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared
to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based
on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated
from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion
of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport
and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase
in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers
can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface
layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such
data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes
in highly dynamic coastal environments. |
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