Christensen Drude F.

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Last Name
Christensen
First Name
Drude F.
ORCID
0000-0002-4271-981X

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  • Article
    Surf Zone Turbulence and suspended sediment dynamics-a review
    (MDPI, 2021-11-20) Aagaard, Troels ; Brinkkemper, Joost ; Christensen, Drude F. ; Hughes, Michael G. ; Ruessink, Gerben
    The existence of sandy beaches relies on the onshore transport of sand by waves during post-storm conditions. Most operational sediment transport models employ wave-averaged terms, and/or the instantaneous cross-shore velocity signal, but the models often fail in predictions of the onshore-directed transport rates. An important reason is that they rarely consider the phase relationships between wave orbital velocity and the suspended sediment concentration. This relationship depends on the intra-wave structure of the bed shear stress and hence on the timing and magnitude of turbulence production in the water column. This paper provides an up-to-date review of recent experimental advances on intra-wave turbulence characteristics, sediment mobilization, and suspended sediment transport in laboratory and natural surf zones. Experimental results generally show that peaks in the suspended sediment concentration are shifted forward on the wave phase with increasing turbulence levels and instantaneous near-bed sediment concentration scales with instantaneous turbulent kinetic energy. The magnitude and intra-wave phase of turbulence production and sediment concentration are shown to depend on wave (breaker) type, seabed configuration, and relative wave height, which opens up the possibility of more robust predictions of transport rates for different wave and beach conditions.
  • Article
    The roles of bathymetry and waves in rip‐channel dynamics
    (American Geophysical Union, 2024-01-04) Christensen, Drude F. ; Raubenheimer, Britt ; Elgar, Steve
    The behavior and predictability of rip currents (strong, wave-driven offshore-directed surfzone currents) have been studied for decades. However, few studies have examined the effects of rip channel morphology on the rip generation or have compared morphodynamic models with observations. Here, simulations conducted with the numerical morphodynamic model MIKE21 reproduce observed trends in flows and bathymetric evolution for two channels dredged across a nearshore sandbar and terrace on an ocean beach near Duck, NC, USA. Channel dimensions, wave conditions, and flows differed between the two cases. In one case, a strong rip current was driven by moderate height, near-normally incident waves over an approximately 1-m deep channel with relatively little bathymetric evolution. In the other case, no rip was generated by the large, near-normally incident waves over the shallower (∼0.5 m) channel, and the channel migrated in the direction of the mean flow and eventually filled in. The model simulated the flow directions, the generation (or not) of rip currents, and the morphological evolution of the channels reasonably well. Model simulations were then conducted for different combinations of the two channel geometries and two wave conditions to examine the relative importance of the waves and morphology to the rip current evolution. The different bathymetries were the dominant factor controlling the flow, whereas both the initial morphology and wave conditions were important for channel evolution. In addition, channel dimensions affected the spatial distribution of rip current forcings and the relative importance of terms.