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Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium Evolution of secondary cellular circulation flow above submarine bedforms imaged by remote sensing techniques I. Hennings (1), D. Herbers (1) 1. GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, Germany - [email protected] Abstract Normalized radar cross section (NRCS) modulation and acoustic Doppler current profiler (ADCP) measurements above submarine sand ribbons and sand waves are presented. The two study areas are located in the Southern Bight of the North Sea at the Birkenfels wreck and in the sand wave field of the Lister Tief in the German Bight of the North Sea. These measurements reveal the developments of secondary cellular circulations in tidally induced coastal sea areas. Secondary circulation cells can develop perpendicular as well as parallel to the direction of the dominant tidal current flow. Circulation cells developed perpendicular to the direction of the dominant tidal current flow are associated with marine sand ribbons manifested near an underwater wreck. Secondary circulation cells within the water column observed parallel to the direction of the dominant tidal current flow have been initiated during flood and ebb tidal current phases associated with submarine sand waves. These two types of cellular circulations must obey the Hamiltonian principle of classical mechanics. The current–short surface wave interaction is described by the action balance or radiation balance-equation based on weak hydrodynamic interaction theory. The calculated current gradient or strain rate of the applied imaging theory has the same order of magnitude for both bedforms such as marine sand ribbons and sand waves, respectively. 1. INTRODUCTION U0 are associated with marine sand ribbons manifested near an underwater wreck in the During the last three decades it turned out that the Southern Bight of the North Sea (Hennings & wave-current-bedform interaction mechanism is a Herbers 2010). In contrast, secondary circulation rather complex process. Marine remote sensing cells related parallel to the dominant direction of data gave a certain impulse for the explanation of the tidal flow U0 are associated with marine sand the different imaging mechanisms in the visible, waves in the tidal inlet of the Lister Tief in the infrared and microwave parts of the German Bight of the North Sea (Hennings & electromagnetic spectrum as well as in the sound Herbers 2006). waves spectrum. Normalized radar cross section A similar kind of secondary circulation arising in (NRCS) modulation and acoustic Doppler current the water is the vortex pair generated by the ship profiler (ADCP) measurements above submarine hull. Lateral current gradients behind the ship have sand ribbons and sand waves reveal the been measured (Marmorino & Trump 1996). development of secondary cellular circulation in Surface strain rates associated with the divergent tidally induced coastal sea areas. Mathematically flow had a value of ∂u/∂x = 0.03 s-1. This strain and physically such kind of associated rate is of the same order of magnitude as strain hydrodynamics obeys the Hamiltonian principle of rates associated with internal waves, fronts and sea classical mechanics (Neumann 1946, Neumann bottom topography in littoral waters (Gasparovic 1949, Alpers & Hasselmann 1978). & Apel 1988, Marmorino & Trump 1994, Secondary circulation cells can develop Hennings & Herbers 2010). perpendicular as well as parallel to the direction of The motivation for this study is the mathematical the dominant tidal current flow U0. In this paper and physical formulation of the imaging both cases will be presented. Secondary circulation mechanism of radar signatures at the sea surface cells developed perpendicular to the direction of caused by submerged wrecks as well as ADCP XXX Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium measurements above marine sand waves. First, the formation of wreck marks like sand ribbons at the sea bed and the associated manifestation of radar signatures at the water surface caused by a vortex pair or helical flow cells triggered by unidirectional tidal current flow interacting with a wreck are summarized. Second, the ADCP measurements and the characteristic flow related to Figure 1. Standing cellular wave dynamics published by marine sand waves in a tidal channel are shown. In Neumann (1946). section 2, a brief background of the theory is presented. Secondary circulation flow associated with sand ribbons and sand waves are described in k (2b) section 3. Finally, section 4 contains the discussion x and conclusions. where x,k,t 'k k Ux,t (3) 2. THEORY denotes the angular wave frequency in the moving medium with variable velocity U(x,t) and ω’(k) is The equations of Hamiltonian flow in phase space the intrinsic angular wave frequency. connect position x with momentum px through the The relationship between the wave height Hamiltonian H. Volume in phase space is spectrum Ψ(k), the wave-energy density spectrum conserved under Hamiltonian flow, a property F(k) and the wave action density spectrum N(k) = known today as Liouville’s theorem (Nolte 2010). F(k)(ω’(k))-1 is defined by (Holliday et al. 1986) Neumann (1946) already concluded that a cellular '(k)2 current must obey the Hamiltonian principle of F(k) '(k)N(k) (k) (4) classical mechanics. The so called standing k cellular wave published by Neumann (1946) is After transforming the mathematical formulations shown in Figure 1. Standing cellular circulation like described by Alpers & Hennings (1984) have implies that volume or mass is not only be been carried out then the expression for the conserved by an acceleration or deceleration of the modulation of the first order perturbed wave flow associated with submarine bedforms but that energy density spectrum δF/F0 = (F - F0)/F0, with up- and downwelling of the three-dimensional F0 as the unperturbed and F as the local wave current field is also important (see section 3). energy density spectrum, respectively, is derived According to this fundamental background the as a first approximation F current–short surface wave interaction can be described by the action balance or radiation F0 balance equation (Alpers & Hasselmann 1978) (5) u 1 dN perp 1 x k N (4 ) cg U 0 dt t x k (1) x perp L Sx,k,t where γ is the relation between the group velocity c and the phase velocity of the short waves, where N is the wave action density spectrum, x is g ∂u /∂x is the gradient or strain rate of the the space variable, t is the time variable, k is the perp perp current velocity component of the secondary wave number and S(x,k,t) is a source function. The circulation perpendicular to the bedform, U is the waves propagate along trajectories in phase space 0 mean dominant tidal current flow of the which are given by the ray equations undisturbed sea area, L denotes the spatial scale of x (2a) the flow cell in xperp-direction, and µ is the k relaxation rate parameter. Equation (5) describes that the modulation of the first-order perturbed wave energy density spectrum of capillary and short gravity waves is proportional to the current XXX Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium gradient of the secondary circulation caused by marine bedforms such as sand ribbons (Kenyon 1970) and sand waves (Hennings & Herbers 2010). However, equation (5) is only valid if the one- dimensional model for describing the sea bottom topography-current interaction is applicable. Otherwise, a full three-dimensional approach is required. If it is assumed that the radar is a linear imaging system then the relative radar image intensity modulation δI/I0 is equal to the relative normalized radar cross section (NRCS) modulation δσ/σ0 based on quasi-specular scattering. It follows that δσ/σ0 is then proportional to δF/F0 defined in equation (5) (Hennings & Herbers 2010). Figure 2. Schematic representation of the relationship 3. SECONDARY CIRCULATION between secondary circulations of the boundary layer, zones of convergence and divergence on the sea bed, Transverse secondary circulation of flow over a sand ribbons, and sedimentary furrows in the model of rough sea bed is equivalent to Langmuir furrow maintenance according to Viekman et al. (1992). Copyright 1992 by the American Society of Limnology circulation at the water surface due to wind stress. and Oceanography, Inc. Circulation patterns associated with Langmuir super cells off the coast of New Jersey of the velocity. It is directed upward above the crests and eastern U.S. coast have been identified by Gargett downward towards the troughs. These residual et al. (2004). The dominant scale of such flow components cause a residual circulation circulation may be responsible for low-relief sand which favours the growth of the bedform, as ribbons in the presence of strong tidal currents. outlined in Fig. 2. In situ measurements verify up- Karl (1980) and Viekman et al. (1992) assumed and downwelling phenomena of the three- that helical secondary-flow circulation within a dimensional current velocity field contributing boundary layer is responsible for the lineation significantly to the interaction between marine development of sand ribbons and sedimentary sand waves and the tidal flow, which has not been furrows. Secondary circulations in the sea bottom known before in detail (see section 3.2, Hennings boundary layer concentrate large, low-density et al. 2004, Hennings & Herbers 2006).