Georges Chapalain

Observed and modeled resonantly interacting progressive water-waves

Abstract This study deals with the transformations of long, small waves of finite amplitude propagating in shallow water. The mechanisms of the nonlinear energy transfer between wave components and damping is first investigated by performing a set of laboratory experiments. The results of this experimental investigation are then compared to different models for the propagation of water waves. The models to be tested are the nonlinear potential flow model based on the Mixed Eulerian Lagrangian Method, the Boussinesq model, and lastly the harmonic model based on the concept of nonlinear resonant interaction. All three models are modifield to allow dissipation. The accuracy of the predictions of the modeling approximations is analysed.

Tide, turbulence and suspended sediment modelling in the eastern English Channel

Abstract The present paper is concerned with modelling fluid and suspended sediment dynamics in a tide-dominated environment. The procedure consists of a one-dimensional vertical model driven by an oscillatory horizontal pressure gradient derived from a two-dimensional vertically integrated tidal model. The vertical model includes two linearised momentum equations for the horizontal velocity components and a series of advection–diffusion equations for concentrations of suspended sediment of specific size. Turbulence generated at the seabed is computed with the aid of a two-equation closure describing the time–space evolution of the turbulent kinetic energy k and of the dissipation rate of the turbulent kinetic energy e (standard k−e model). A mixed type bottom boundary condition for the sediment concentration equations is adopted to take into account downward fluxes at times of decelerating flow and slack waters. The model is applied to the eastern part of the English Channel. The tidal currents, turbulent kinetic energy and the total suspended sediment load predicted by the model are compared with field data collected in two sites. The vertical structure of these flow properties is fairly well predicted by the present model. Better results are found at the measuring point located farther from the coastline where advective terms can be reasonably neglected.

Modelling and numerical simulation of turbulence, waves and suspended sediments for pre-operational use in coastal seas

Abstract The role of small-scale processes in models of coastal seas is reviewed, and the respective uses of vertically integrated and vertically resolving models are described. Although applied with heavily tuned empirical parameters to the Holderness coast a vertically integrated model shows the importance of surface waves for predicting suspended particulate matter (SPM) and their distributions. In formulating a generic vertically resolving module, as kernel, the k–e turbulence closure has been selected. On a uniform vertical grid this model gave reasonably accurate results for a neutrally stratified channel flow forced by an M2 tidal wave (Elbe estuary) as well as on a nonuniform grid highly refined in the high-dissipation near-bed region for short-period (8 s) surface waves in a laboratory flume. The model was completed with modules accounting for the effect of waves on the turbulent kinetic energy (TKE) influx at the surface and on the apparent roughness at the bottom. It was finally coupled with different versions of vertically high resolving SPM models. In test applications to the English Channel and to the Sylt–Romo Bight (Germany/Denmark) the generic model versions performed with sufficient accuracy. However, in both cases: (i) fine tuning of erosion and deposition terms was necessary thus underlining the need for further experimental research towards an improved data base on erodible sediments and SPM; (ii) the parameters of the submodel for the TKE injection by surface waves could not be determined consistently and indicate the existence of a further still hidden parameter; (iii) the technical basis for in situ observations of small-scale processes in the coastal zone needs further improvements and consolidation.

Reynolds number variation in oscillatory boundary layers. Part I. Purely oscillatory motion

A numerical model based upon a low Reynolds number turbulence closure is proposed to study Reynolds number variation in reciprocating oscillatory boundary layers. The model is used to compute the boundary layer for flow regimes ranging from smooth laminar to rough turbulent. Criteria for fully developed turbulence are derived for walls of the smooth and rough types. In particular, a new criterion to identify the rough turbulent regime is determined based on the time-averaged turbulence intensity. The reliability of the present model is assessed through comparisons with detailed experimental data collected by other investigators. The model globally improves upon standard high Reynolds number closures. Variation through the wave cycle of the main flow variables (ensemble-averaged velocity, shear stress, turbulent kinetic energy) is remarkably well-predicted for smooth walls. Predictions are satisfactory for rough walls as well. Yet, the turbulence level in the rough turbulent regime is overpredicted in the vicinity of the bed.

About the specification of erosion flux for soft stratified cohesive sediments

The present paper deals with the specification of bed erosion flux that accounts for the effects of sediment-induced stratification in the water column. Owing to difficulties in measuring the bed shear stress τb and the erosive shear strength τs, we suggest a series of methods that combine laboratory and numerical experiments. A simplified turbulent transport model that includes these effects helps to quantify τb and τs. Focusing on soft stratified beds, the present study considers erosion rate formulas of the form ε=εf exp {[Tb-Tsα]β} where β is a model constant (β=1 for Gulartes (1978) formula and β=1/2 for Parchures (1984) formula). First, the bed erosive strength profile τs(Z) is adjusted by “forcing” the turbulent transport model with measured erosion rates. Second, three procedures are suggested to determine the erosion rate formula coefficients εf and α: a global procedure and two different layer-by-layer procedures. Each procedure is applied to an erosion experiment conducted in a rotating annular flume by Villaret and Paulic (1986). The use of the layer-by-layer procedure based on a least squares fitting technique provides a closer fit than the global procedure. The present study points out the complementarity of experimental and numerical approaches and also suggests possible improvements in laboratory test procedures.

Modeling the Tide-Induced Modulation of Wave Height in the Outer Seine Estuary

Abstract GUILLOU, N. and CHAPALAIN, G., 2012. Modeling the tide-induced modulation of wave height in the outer Seine estuary. During the last decades, numerous models have been implemented to investigate the effects of tide on wind-generated surface-gravity waves. The present study analyzes the influences of the tide-induced time-varying water depths and currents on the wave height in the outer Seine estuary (France, English Channel). Two modeling systems based on the coupling of circulation and wave propagation modules are applied at the scales of (i) the English Channel and the North Sea and (ii) the Bay of Seine and the harbor of Le Havre, respectively. Numerical results are compared with field data collected with two current meters and two wave buoys in the access channel to Port 2000 (Le Havre). Predictions exhibit a local increase of 30% of the wave height induced by current refraction at slack tide. On the basis of this local comparison of numerical results with measurements, three mappings of the wave height modified by the (i) tide, (ii) water levels alone, and (iii) currents alone are established at the scale of the outer Seine estuary. The currents have a major influence on the wave height modulating at a tidal frequency the direction of incident waves through refraction. This effect is exhibited along the southern breakwater of the harbor of Le Havre and the vicinity of coastal topographic features of the outer Seine estuary.

Sea surface temperature modelling in the Sea of Iroise: assessment of boundary conditions

The present study investigates the sensitivity of the COupled Hydrodynamical–Ecological model for REgioNal and Shelf seas (COHERENS) to predict sea surface temperature (SST) patterns in the Sea of Iroise (western end of French Brittany) in relation to the spatial and temporal resolutions of open boundary conditions (OBCs). Two sources of daily operational OBCs of temperature are considered, derived from (1) the Mercator Global Ocean and (2) the Iberian Biscay Irish analysis and forecasting systems delivering predictions at spatial resolutions of 1/12° and 1/36°, respectively. Coastal model performance is evaluated by comparing SST predictions with recently available field data collected (1) along the route of a vessel travelling between the coast and the isle of Ushant and (2) at two offshore stations. The comparison is extended to SST spatial distribution derived from remote-sensing observations. The influence of OBC spatial resolution is exhibited in the north-eastern area of the Sea of Iroise in relation to the intrusion of cold surface waters. OBC temporal resolution is found to have a lower impact advocating for the implementation of climatological temperature forcings to predict major SST patterns in the Sea of Iroise.

Estimates of wave decay rates in the presence of turbulent currents

A full-depth numerical model solving the free surface flow induced by linear water waves propagating with collinear vertically sheared turbulent currents is presented. The model is used to estimate the wave amplitude decay rate in combined wave current flows. The decay rates are compared with data collected in wave flumes by Kemp and Simons [J Fluid Mech, 116 (1982) 227; 130 (1983) 73] and Mathisen and Madsen [J Geophys Res, 101 (C7) (1996) 16,533]. We confirm the main experimental finding of Kemp and Simons that waves propagating downstream are less damped, and waves propagating upstream significantly more damped than waves on fluid at rest. A satisfactory quantitative agreement is found for the decay rates of waves propagating upstream, whereas not more than a qualitative agreement has been observed for waves propagating downstream. Finally, some wave decay rates in the presence of favourable and adverse currents are provided in typical field conditions.

Effets d’une houle de tempête sur les mises en suspension des sédiments de fond dans le détroit du Pas-de-Calais. Comparaison de la modélisation avec les observations

ABSTRACT A three-dimensional modelling of suspended multicomponent sediment transport based on the coupling of the circulation model COHERENS and the wave propagation model SWAN is applied in the Dover Strait and its coastal adjacent areas during a storm. The model reproduces measurements of waves parameters, tidal currents and the total suspended sediment concentration off Merlimont beach. The wave effects on these parameters and the concentration of the different classes of suspended sedimentary particles are highlighted.ABSTRACT A three-dimensional modelling of suspended multicomponent sediment transport based on the coupling of the circulation model COHERENS and the wave propagation model SWAN is applied in the Dover Strait and its coastal adjacent areas during a storm. The model reproduces measurements of waves parameters, tidal currents and the total suspended sediment concentration off Merlimont beach. The wave effects on these parameters and the concentration of the different classes of suspended sedimentary particles are highlighted.

Bed Friction in Combined Wave-Current Flows

The accurate prediction of shear stresses at the seabed forms an essential element in modelling of the coastal environment. Flows which induce bed friction generally include a mean component (from wind-induced currents or tides) and an oscillatory component (from waves), and this paper describes an investigation into seabed friction under such combined wave-current flow. In particular, it presents new laboratory measurements of the shear stress exerted by waves or wave-like flows on a rough boundary and how that oscillatory component is modified by the addition of a current. The effect of the waves on the current is reported, and the results compared with predictions from wave-current models, including some being developed as part of the present project.