Royal Netherlands Institute for Sea Research

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Slope-Induced Tidal Straining

In my PhD, I worked on a process called 'slope-induced tidal straining' (SITS) and investigated its potential to transport sediment. SITS requires three things: a gently sloping sea floor (like it is typical on a continental shelf), a vertical density stratification, as it is common beneath the surface mixed layer, and an oscillatory current, for example tides. These features are quite common, for example on continental shelfs.

Sediment transport by slope-induced tidal straining works in the following way: Imagine a current that is going up- and down a sloping bottom. Friction decelerates the current towards the bottom, and consequently dense water from the deeper part (down the slope) is advected on top of lighter water during the upslope flow phase. As dense water over light water is very unstable, high turbulence (indicated by the big spirals in the sketch below) occurrs during the upslope phase, and this resuspends a lot of sediment and mixes suspended sediment high up in the water column. In the downslope flow phase, everything reverses: light water is pushed over dense water, creating a very stable stratification. A stable stratification suppresses turbulence, and therefore less suspended sediment is mixed up in the water column. If you look at this in the tidal mean (the average over one up- and one downslope flow phase), you can see that more sediment is transported up the slope, and less sediment goes back down.

Sketch of the slope-induced tidal straining mechanism.

In my first publication (see below), the dependance of this 'upslope transport' mechanism on different slope angles, stratification strengths and sediment types is investigated, only based on a numerical model. Later on, a collegue from Japan provided us with observational evidence for slope-induced tidal straining on a continental shelf in the East China Sea. The comparison of this observational data with an extended version of my numerical model is the topic of my second publication. Still, there are no observations of sediment transport induced by SITS, and there are numerous other aspects about SITS that are worth further investigation.

Short CV

since 2018         PostDoc in the ATLAS project (Dick van Oevelen) at NIOZ

2016 - 2018       PostDoc in the MudMotor project (Theo Gerkema) at NIOZ

2013 - 2016       PhD student at the Leibniz Institute for Baltic Sea Research, Warnemünde, Germany.          

Project: 'The Service of Sediments in German Coastal Seas (SECOS)'

2007 - 2013       M.Sc., B.Sc. Applied Mathematics at Technical University Clausthal-Zellerfeld, Germany.

List of Publications

C. van der Boog, M. F. de Jong, M. Scheidat, M. Leopold, S. Geelhoed, K. Schulz, H. Dijkstra, J.
Pietrzak, C. Katsmann. Hydrographic and Biological Survey of a Surface-Intensified Anticyclonic
Eddy in the Caribbean Sea. Submitted to Journal of Geophysical Research: Oceans.

I. Bartl, D. Hellemann, C. Rabouille, K. Schulz, P. Tallberg, S. Hietanen, M. Voss. Particulate
organic matter controls benthic microbial N retention and N removal in contrasting estuaries of the
Baltic Sea. Submitted to Biogeosciences.

K. Schulz, K. Klingbeil, C. Morys, T. Gerkema. The fate of mud nourishment in response to
short-term wind forcing. Submitted to Coastal Engineering.

M. J. Baptist, T. Gerkema, B.C. van Prooijen, D.S. van Maren, M. van Regteren, K. Schulz, I. Colosimo, J. Vroom, T. van Kessel, B. Grasmeijer, P. Willemsen, K. Elschot, A.V. de Groot, J. Cleveringa, E.E. van Eekelen, F. Schuurman, H.J. de Lange, M.E.B. van Puijenbroek (2019). Beneficial use of dredged sediment to enhance salt marsh development applying a Mud Motor. Ecological Engineering, 127, 312-323.

I. Bartl, I. Liskow, K. Schulz, L. Umlauf and M. Voss (2018). River plume and bottom boundary layer–Hotspots for nitrification in a coastal bay? Estuarine, Coastal and Shelf Science, 208, 70-82.

K. Schulz, T. Gerkema (2018). An inversion of the estuarine circulation by sluice water
discharge and its impact on sediment transport. Estuarine, Coastal and Shelf Sciences, 200, 31-40.

Abstract: The Wadden Sea is characterized by a complex topography of branching channels and intertidal flats, in which the interplay between fresh water discharges, wind forcing and the tidal current causes sediment transport rates and direction to be highly variable in space and time. During three field campaigns, indications of a negative estuarine circulation have been found in a channel adjacent to the coast in the Western Dutch Wadden Sea. Contrary to the classical picture of estuarine circulation, a periodic density stratification was observed that builds up during flood and breaks down during ebb. This can be related to a large freshwater source at the mouth of the channel, the sluice in Kornwerderzand. In this study, observations of this phenomenon are presented, and with the help of a numerical model the different drivers for residual suspended matter transport in this area, namely tidal asymmetries in the current velocity and the above mentioned periodic stratification, are investigated. It is found that the residual current in the area of interest points in ebb direction, caused by both the elongated ebb flow phase and the periodic stratification. On the contrary, the stronger flood currents cause a transport of suspended matter in flood direction. This transport is counteracted and therefore diminished by the effects of the sluice discharge.

K. Schulz, T. Endoh and L. Umlauf (2017). Slope-induced tidal straining: Analysis of rotational
effects. Journal of Geophysical Research: Oceans, 122(3), 2069–2089.

Abstract: Tidal straining is known to be an important factor for the generation of residual currents and transports of suspended matter in the coastal ocean. Recent modeling studies and field experiments have revealed a new type of ‘‘slope-induced’’ tidal straining, in which the horizontal density gradient required for this process is induced by the presence of a slope rather than by river runoff (as in classical tidal straining). Slope-induced tidal straining is investigated here with the help of an idealized numerical model, and results are compared to a recent data set from the East China Sea providing first direct observational evidence. The focus of this study is on the effect of rotation that was ignored in previous investigations. The model is shown to reproduce the key features of the observations, in particular the strain-induced generation of unstable stratification in the bottom boundary layer during periods of upslope flow. Rotation effects are found to significantly reduce the upslope tidal pumping of suspended material and also give rise to a newly identified pumping mechanism that results in a vigorous transport of suspended material along the slope. It is shown that slope-induced tidal straining is likely to be relevant for a wide range of oceanic slopes exposed to tidal motions.

K. Schulz, and L. Umlauf (2016). Residual transport of suspended material by tidal straining near
sloping topography. Journal of Physical Oceanography, 46(7), 2083 – 2102.

Abstract: Tidal straining is known to have an important impact on the generation of residual currents and thetransport of suspended material in estuaries and the coastal ocean. Essential for this process is an externally imposed horizontal density gradient, typically resulting from either freshwater runoff or differential heating. Here, it is shown that near sloping topography, tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role. A combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process. The main finding of this study is that, for a wide range of conditions, suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping. Downslope transport may also occur, however, only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle larger than a critical threshold. The effective residual velocity at which suspended material is transported across isobaths is a significant fraction of the tidal velocity amplitude (up to 40% in some cases), suggesting that suspended material may be transported over large distances during a single tidal cycle.

Selected Presentations

K. Schulz, K. Klingbeil, C. Morys, T. Gerkema. Hydrodynamics and sediment transport in the
Kimstergat channel. Mini Symposium Slibmotor Koehoal, Leeuwarden, The Netherlands, 06.
December 2018. Oral presentation.
K. Schulz, K. Klingbeil, T. Gerkema. The effect of tides, wind, and fresh water on sediment
transport in the Wadden Sea. Physics of Estuaries and Coastal Seas Conference, Galveston, Texas,
U.S.A, 15.–19. October 2018. Oral presentation.
K. Schulz. Slope-induce tidal straining – a missing link for transport in the ocean? Technical
University Eindhoven, Eindhoven
, The Netherlands, 01. October 2018. Invited talk.
K. Schulz, K. Klingbeil, T. Gerkema. Unraveling the decisive factors for the transport of sediment as a supply for salt marsh growth. NCK days, Haarlem, The Netherlands, 21.-23. March 2018. Oral presentation.

K. Schulz, T. Endoh and L. Umlauf. Slope-induced tidal straining: Analysis of rotational effects.
Liège Colloquium - Marine Turbulence Re3-visited, Liège, Belgium, 22.–26. May 2017. Oral
K. Schulz, T. Endoh and L. Umlauf. Slope-induced tidal straining: Analysis of rotational effects.
Physics of Estuaries and Coastal Seas Conference, Scheveningen, The Netherlands, 9.–14. October
2016. Oral presentation.
K. Schulz, L. Umlauf. Residual Transport of Suspended Sediment by Tidal Straining near Sloping
Topography. Ocean Sciences Meeting, New Orleans, Lousiana, U.S.A., 22.- 26. February 2016.
Oral presentation.
K. Schulz, L. Umlauf. Residual Transport of Suspended Sediment by Tidal Straining near Sloping
Topography. Warnemünder Turbulence Days, Konferenz, Vilm, Germany, 31. October - 03.
September 2015. Oral presentation.
K. Schulz, L. Umlauf. Residual Transport of Suspended Sediment by Tidal Straining near Sloping
Topography. Gordon Kenan Research Seminar, Biddefort, Maine, U.S.A., 05.- 07. June 2015. Oral