accuracy of coarse grid based morphodynamic simulations, without significantly increasing the computational effort. Over the last years, we have gained experience in hydrodynamic modelling using subgrid based methods [i.e. Defina

(2003), Casulli (2009), Volp et al (2013) ]. These methods combine coarse computational grids with high resolution information. In Volp et al (2013 ) we presented a subgrid based, two-dimensional, depth averaged hydrodynamic model, that is inspired by the method presented by Casulli (2009 ). The model makes use of two grids: a (coarse) computational grid and a high resolution subgrid, see Figure 1. The system of equations is solved at the coarse grid, but high resolution information is taken into account. The water level is assumed to be uniform within a computational cell, but the bed and the roughness are allowed to vary within a cell. Therefore, high resolution effects can be taken into account for the computation of cross-sectional areas, cell volumes, advection and friction. This also implies that cells can be wet, partly wet or dry. The solution based on a coarse computational grid improved significantly, when high resolution effects are taken into account. This result is obtained without a significant increase in computational cost.

50 m/s). A preliminary assessment shows that cross swell, dominant in large regions of hurricanes, allows the roughness under high wind conditions to increase considerably before it reduces to the same low values.","GPS sondes; hurricanes; swell; wave breaking; white caps; wind drag","en","journal article","American Geophysical Union","","","","","","","2013-03-01","Civil Engineering and Geosciences","Hydraulic Engineering","","","","" "uuid:278c4712-82d7-43f9-8506-121952bf889e","http://resolver.tudelft.nl/uuid:278c4712-82d7-43f9-8506-121952bf889e","Mesh generation in archipelagos","Terwisscha van Scheltinga, A.; Myers, P.G.; Pietrzak, J.D.","","2012","A new mesh size field is presented that is specifically designed for efficient meshing of highly irregular oceanic domains: archipelagos. The new approach is based on the standard mesh size field that uses the proximity to the nearest coastline. Here, the proximities to the two nearest coastlines are used to calculate the distance between two islands or the width of a strait through an archipelago. The local value of the mesh size field is taken as the width (or distance between two islands) divided by the number of required elements across the strait (or between the islands). This new mesh size fields are illustrated for three examples: (1) the Aegean Sea, (2) the Indonesian Archipelago, and (3) the Canadian Arctic Archipelago.","unstructured meshes; mesh generation; multi-scale modeling; oceanography","en","journal article","Springer","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","","" "uuid:94e0df5d-c0f5-40a8-9fbc-08d10457184f","http://resolver.tudelft.nl/uuid:94e0df5d-c0f5-40a8-9fbc-08d10457184f","On total turbulent energy and the passive and active role of buoyancy in turbulent momentum and mass transfer","De Nijs, M.A.J.; Pietrzak, J.D.","","2012","","stratified shear flow; total turbulent energy; vertical turbulent kinetic energy; available turbulent potential energy; countergradient buoyancy fluxes; turbulent Prandtl number; energetic turbulent structures; convective motions","en","journal article","Springer","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","","" "uuid:e4329435-b9a7-4132-bce1-a1ee382713b9","http://resolver.tudelft.nl/uuid:e4329435-b9a7-4132-bce1-a1ee382713b9","An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes","De Nijs, M.A.J.; Pietrzak, J.D.","","2011","Measurements of turbulent fluctuations of velocity, salinity, and suspended particulate matter (SPM) are presented. The data show persistent countergradient buoyancy fluxes. These countergradient fluxes are controlled by the ratio of vertical turbulent kinetic energy (VKE) and available potential energy (APE) terms in the buoyancy flux equation. The onset of countergradient fluxes is found to approximately coincide with larger APE than VKE. It is shown here that the ratio of VKE to APE can be written as the square of a vertical Froude number. This number signifies the onset of the dynamical significance of buoyancy in the transport of mass. That is when motions driven by buoyancy begin to actively determine the vertical turbulent transport of mass. Spectral and quadrant analyses show that the occurrence of countergradient fluxes coincides with a change in the relative importance of turbulent energetic structures and buoyancydriven motions in the transport of mass. Furthermore, these analyses show that with increasing salinity-induced Richardson number (Ri), countergradient contributions expand to the larger scales of motions and the relative importance of outward and inward interactions increases. At the smaller scales, at moderate Ri, the countergradient buoyancy fluxes are physically associated with an asymmetry in transport of fluid parcels by energetic turbulent motions. At the large scales, at large Ri, the countergradient buoyancy fluxes are physically associated with convective motions induced by buoyancy of incompletely dispersed fluid parcels which have been transported by energetic motions in the past. Moreover, these convective motions induce restratification and enhanced settling of SPM. The latter is generally the result of salinity-induced convective motions, but SPM-induced buoyancy is also found to play a role.","countergradient buoyancy fluxes; stratified shear flow; available turbulent potential energy; vertical turbulent kinetic energy; energetic coherent structures; convective motions","en","journal article","Springer","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","","" "uuid:a542586f-c23a-46aa-893c-21fd359ccbd0","http://resolver.tudelft.nl/uuid:a542586f-c23a-46aa-893c-21fd359ccbd0","Advection of the salt wedge and evolution of the internal flow structure in the Rotterdam Waterway","De Nijs, M.A.J.; Pietrzak, J.D.; Winterwerp, J.C.","","2011","An analysis of field measurements recorded over a tidal cycle in the Rotterdam Waterway is presented. These measurements are the first to elucidate the processes influencing the along-channel current structure and the excursion of the salt wedge in this estuary. The salt wedge structure remained stable throughout the measuring period. The velocity measurements indicate decoupling effects between the layers and that bed-generated turbulence is confined below the pycnocline. The barotropic M4 overtide structure is imposed at the mouth of the estuary, and the generation of M4 overtides within the estuary is found to be relatively small. Internal tidal asymmetry does not make a significant contribution to the M4 velocity frequency band. Instead, the combination of barotropic and baroclinic forcing, in conjunction with the suppression of turbulence at the interface, provides the main explanation for the time dependence and mean structure of the flow in the Rotterdam Waterway. This gives rise to the observed differences in the length of the flood and ebb, in the magnitudes of the flood and ebb velocities, in the length of the slack water periods, and in the timing of the onset of slack water at the surface and near the bed. It results in the formation of distinct exchange flow profiles at the head of the salt wedge around slack water and the creation of maximal velocities at the pycnocline during flood. Advection governs the displacement and structure of the salt wedge since turbulent mixing is suppressed. The tidal displacement of the salt wedge controls the height of the pycnocline above the bed at a particular site. Hence, it controls the height to which bed-generated turbulence can protrude into the water column. Consequently, the authors find asymmetries in the structure of the internal flow, turbulent mixing, and bed stresses that are not related to classical internal tidal asymmetry.","estuaries; salinity; tides; Cchannel flows; currents; mixing","en","journal article","American Meteorological Society","","","","","","","2011-07-31","Civil Engineering and Geosciences","Hydraulic Engineering","","","","" "uuid:0f2da991-8c23-4547-886b-4baf14fc66f8","http://resolver.tudelft.nl/uuid:0f2da991-8c23-4547-886b-4baf14fc66f8","The effects of the internal flow structure on SPM entrapment in the Rotterdam Waterway","De Nijs, M.A.J.; Winterwerp, J.C.; Pietrzak, J.D.","","2010","Field measurements are presented, which are the first to quantify the processes influencing the entrapment of suspended particulate matter (SPM) at the limit of saltwater intrusion in the Rotterdam Waterway. The estuarine turbidity maximum (ETM) is shown to be maintained by the trapping of fluvial SPM at the head of the salt wedge. The trapping process is associated with the raining out of fluvial SPM from the upper, fresher part of the water column, into the layer below the pycnocline. The dominant mechanisms responsible are baroclinic shear flows and the abrupt change in turbulent mixing characteristics due to damping of turbulence at the pycnocline. This view contrasts with the assumption of landward transport of marine SPM by asymmetries in bed stress. The SPM transport capacity of the tidal flow is not fully utilized in the ETM, and the ETM is independent of a bed-based supply of mud. This is explained by regular exchange of part of the ETM with harbor basins, which act as efficient sinks, and that the Rotterdam Waterway is not a complete fluvial SPM trap. The supply of SPM by the freshwater discharge ensures that the ETM is maintained over time. Hence, theETMis an advective phenomenon. Relative motion between SPM and saltwater occurs because of lags introduced by resuspension. Moreover,SPM that lags behind the salt wedge after high water slack (HWS) is eventually recollected at the head. Hence, SPM follows complex transport pathways and the mechanisms involved in trapping and transport of SPM are inherently three-dimensional.","","en","journal article","American Meteorological Society","","","","","","","2011-06-30","Civil Engineering and Geosciences","Hydraulic Engineering","","","","" "uuid:881bec0b-2d64-4e5c-b118-9c0c5b7cd5e2","http://resolver.tudelft.nl/uuid:881bec0b-2d64-4e5c-b118-9c0c5b7cd5e2","An accurate momentum advection scheme for a z-level coordinate models","Kleptsova, O.; Stelling, G.S.; Pietrzak, J.D.","","2010","In this paper, we focus on a conservative momentum advection discretisation in the presence of zlayers. While in the 2Dcase conservation ofmomentum is achieved automatically for an Eulerian advection scheme, special attention is required in the multi-layer case. We show here that an artificial vertical structure of the flow can be introduced solely by the presence of the z-layers, which we refer to as the staircase problem. To avoid this staircase problem, the z-layers have to be remapped in a specific way. The remapping procedure also deals with the case of an uneven number of layers adjacent to a column side, thus allowing one to simulate flooding and drying phenomena in a 3D model.","shallow water equations; advection; flooding and drying; momentum conservation; z-layer; staircase problem","en","journal article","Springer Verlag","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""