Predicted ripple dimensions in relation to the precision of in situ measurements in the southern North Sea

Krämer, Knut

doi:10.5194/os-12-1221-2016

Cited by 20 publications

(24 citation statements)

References 36 publications

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“…Images in which the bed level clearly changed during the sonar rotation, and in which the bed could not be found because of a high amount of sediment in suspension, were removed from the data set. From the remaining images, the ripple height

η_{m}

was calculated from

2 \sqrt{2} σ

(Krämer and Winter, ), with

σ

being the standard deviation of the image. Each image was then rotated in a full circle with steps of 5° and for each rotation the bedform three‐dimensionality (Tb) was calculated using the autocorrelation of the bed elevation data in the

x

and

y

direction according to Núñez‐González et al.…”

Section: Methodsmentioning

confidence: 99%

“…; Wu and Parsons, ) and on continental shelves (e.g. Li and Amos, ; Krämer and Winter, ). In most cases, rather two‐dimensional ripples were observed, but that might be caused by either the constant angle between waves and currents, or by the fact that, although waves and currents were both present, one of the two was still clearly dominant.…”

Section: Introductionmentioning

confidence: 99%

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Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Brakenhoff

Kleinhans

Ruessink

et al. 2020

Earth Surf Processes Landf

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Ebb‐tidal deltas are highly dynamic environments affected by both waves and currents that approach the coast under various angles. Among other bedforms of various scales, these hydrodynamics create small‐scale bedforms (ripples), which increase the bed roughness and will therefore affect hydrodynamics and sediment transport. In morphodynamic models, sediment transport predictions depend on the roughness height, but the accuracy of these predictors has not been tested for field conditions with strongly mixed (wave–current dominated) forcing. In this study, small‐scale bedforms were observed in the field with a 3D Profiling Sonar at five locations on the Ameland ebb‐tidal delta, the Netherlands. Hydrodynamic conditions ranged from wave dominated to current dominated, but were mixed most of the time. Small‐scale ripples were found on all studied parts of the delta, superimposed on megaripples. Even though a large range of hydrodynamic conditions was encountered, the spatio‐temporal variations in small‐scale ripple dimensions were relatively small (height η≈ 0.015 m, length λ≈ 0.11 m). Also, the ripples were always highly three‐dimensional. These small dimensions are probably caused by the fact that the bed consists of relatively fine sediment. Five bedform height predictors were tested, but they all overestimated the ripple heights, partly because they were not created for small grain sizes. Furthermore, the predictors all have a strong dependence on wave‐ and current‐related velocities, whereas the ripple heights measured here were only related to the near‐bed orbital velocity. Therefore, ripple heights and lengths in wave–current‐dominated, fine‐grained coastal areas ( D50<0.25 mm) may be best estimated by constant values rather than values dependent on the hydrodynamics. In the case of the Ameland ebb‐tidal delta, these values were found to be η=0.015 m and λ=0.11 m. ©2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

show abstract

η_{m}

was calculated from

2 \sqrt{2} σ

(Krämer and Winter, ), with

σ

x

and

y

direction according to Núñez‐González et al.…”

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Brakenhoff

Kleinhans

Ruessink

et al. 2020

Earth Surf Processes Landf

View full text Add to dashboard Cite

show abstract

“…Until 2015, 11 ship surveys were carried out, and field data were collected and analysed at different reference sites in the German Bight with sedimentological and morphological characteristics that are representative of large areas of the German EEZ in the North Sea. A combination with other COSYNA seafloor observatories has produced consistent and extensive data sets on various physical and (micro-)biological properties of the domains (Krämer and Winter, 2016). Data are published at http://www.noah-project.de.…”

Section: Lander Sedobsmentioning

confidence: 99%

“…During some parts of the tidal cycle a periodic stratification of the water column has been observed in shallow areas of the German Bight forming distinct layers that move independently with a decoupled tidal ellipticity (Krämer and Winter, 2016;Kwoll et al, 2013Kwoll et al, , 2014Ahmerkap et al, 2017). The difference in sea bed dynamics between fair weather conditions and storms is also investigated in the research area "Seafloor Dynamics" of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Research Center/Cluster of Excellence "The Ocean in the Earth System".…”

Section: Lander Sedobsmentioning

confidence: 99%

The Coastal Observing System for Northern and Arctic Seas (COSYNA)

et al. 2017

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Abstract. The Coastal Observing System for Northern and Arctic Seas (COSYNA) was established in order to better understand the complex interdisciplinary processes of northern seas and the Arctic coasts in a changing environment. Particular focus is given to the German Bight in the North Sea as a prime example of a heavily used coastal area, and Svalbard as an example of an Arctic coast that is under strong pressure due to global change.The COSYNA automated observing and modelling system is designed to monitor real-time conditions and provide short-term forecasts, data, and data products to help assess the impact of anthropogenically induced change. Observations are carried out by combining satellite and radar remote sensing with various in situ platforms. Novel sensors, instruments, and algorithms are developed to further improve the understanding of the interdisciplinary interactions between physics, biogeochemistry, and the ecology of coastal seas. New modelling and data assimilation techniques are used to integrate observations and models in a quasi-operational system providing descriptions and forecasts of key hydrographic variables.

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“…The seafloor sediment consists mainly of fine sands with increasing mud content for areas located in the Elbe palaeovalley (Figge, 1981). Small-scale bedforms (ripples) of 1-2 cm height and around 20 cm length are omnipresent in sandy regions in water depth between 20 and 40 m (Krämer and Winter, 2016;Ahmerkamp et al, 2017). Morphological features at the deeper stations with weak bottom currents and beyond the reach of surface waves are of mostly of biogenic origin.…”

Section: Bathymetry and Sedimentmentioning

confidence: 99%

Relations of physical and biogenic reworking of sandy sediments in the southeastern North Sea

Krämer

Ahmerkamp

Schückel

et al. 2019

Preprint

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Abstract. The reworking of sandy sediments in shallow coastal and shelf seas is mainly driven by physical forcing in the form of wave- and current-induced shear stress. As an important habitat for benthic species seeking shelter and food, the upper seafloor is also marked by intense bioturbation. Although this reworking activity is recognized as an important mechanism for the exchange of particular matter and solutes between sediment and water column, quantifications and assessments of the relative importance of physical and biogenic reworking of subtidal shelf sediments are rare. This work presents in situ measurements of volumetric reworking rates from six different locations in the southeastern North Sea. The investigated sites cover a range of water depths between 23 and 41 m, different magnitudes of physical (wave and current) forcing and sedimentological conditions as well as different habitats and benthic communities. The measured biogenic reworking rates reach up to 14 % of physically driven reworking via bedform migration. Comparisons with physical quantities water depth, median grain size, bottom water temperature and flow velocity reveal good correlations and allow for an approximation of the biogenic reworking rate from a combination of these readily available oceanographic parameters. The diffusive relocation of sediment by benthic fauna also influences the topography of small scale bedforms and may reduce their height by up to 10 % in a few hours during hydrodynamically inactive conditions. The observations show that even in an energetic environment such as the southeastern North Sea, the benthic fauna contributes an important regulating ecosystem service by overturning upper seafloor sediments. This reworking mechanism becomes particularly important in areas and during periods of sub-threshold conditions for physically driven sediment reworking.

show abstract

Predicted ripple dimensions in relation to the precision of in situ measurements in the southern North Sea

Cited by 20 publications

References 36 publications

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

The Coastal Observing System for Northern and Arctic Seas (COSYNA)

Relations of physical and biogenic reworking of sandy sediments in the southeastern North Sea

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