1 

A boundary element model for nonlinear free surface phenomena
A boundary element method has been developed that can be used to calculate twodimensional potential flow phenomena with a free surface. The unsteady Bernoulli equation is applied at the actual position of the free surface. A solution for corners arising in the geometry of the boundary has been found. Comparison of results of the present model with results obtained with other numerical methods shows that reliable results can be obtained with the present model as long as the gap between two adjacent normals at a nodal point is less than about 40 degrees. At this limit the calculations break down. Within this limit, a forward directed jet is well developed in the case of breaking waves. The calculations can proceed if two points at the tip of the jet are considered as corner points. With these special points, the calculations can be continued up to the moment the jet falls down the forward face of the wave.

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2 

Bedforms and undertow in the surf zone; an analysis of the LIP 11Ddata
The present report gives the results of a study on bedforms and undertow in the surf zone. It is the objective of this study to get a better insight into the physical processes in the surf zone. In this study, we make use of the data obtained during the LIP llDexperiments (Arcilla et al. [1994] and Roelvink and Reniers [1994]). We derive the characteristics of bedforms from measured profiles. We relate these bedform characteristics to the hydraulic conditions and analyse if they can be predicted with present prediction methods. Further, we develop an inverse modelling technique, which is based on the mass and momentum balance equations. With this technique we derive values of important physical parameters, like eddy viscosity, shear stresses, friction factors, bed roughness and mass flux. The derived physical parameters are compared with present methods to describe these parameters.

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3 

Land disposal options of contaminated dredged material
The problems associated with contaminated dredged material disposal (basically lack of disposal sites and potential adverse environmental impacts) have become major issues in many industrialized and developing countries. The sediment removed for environmental reasons is obviously contaminated, but in many cases the sediment removed during normal maintenance dredging of waterways and harbours also contains a wide range of potentially toxic substances. In the Netherlands for instance, out of the 50 million m3 of dredged material produced annually during normal maintenance dredging work 20 million m is contaminated to such a degree that its dispersion into the environment without measures to impede contaminant release is unacceptable (Vellinga, 1989). This paper gives a review of the disposal and treatment options currently in use or considered to have the potential for practical use in the near future with an emphasis on land disposal. It briefly discusses the main contaminant release pathways, the governing processes and the stateoftheart methodology, used to assess potential environmental impacts.

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4 

Topic 3. Decay Heat Predictions: Experiments, Methods and Data. Integral validation and decay heat standards.

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5 

Stationary and oscillatory flow through coarse porous media
Measurements in a Utube tunnel were carried out to study flow through coarse granular material. Tests with stationary flow and tests with oscillatory flow were done to study the differences between both. The coefficients from the extended Forchheimer equation, which is supposed to describe nonstationary porous flow, were determined. It appeared that for oscillatory flow the turbulent resistance is larger than under stationary flow conditions. This additional resistance is depending on the flowfield, expressed by the KeuleganCarpenter number. The contribution of the inertial resistance is depending on the flow field as well. Its contribution to the total resistance was rather limited. The influence of the nonstationary flow conditions have been implemented in the expressions for the turbulent resistance and the inertial resistance. Comparisons of the results from the stationary flow tests with other measurements show that the results correspond reasonably well. This is not the case for existing expressions for stationary flow. The existing formulae underpredict the avalues while they overpredict the bvalues. Further research must be concentrated on the influence of parameters such as grading, the aspect ratio and shape. These dependencies can be determined under stationary flow conditions.

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6 

Application of sediment transport formulae to sanddike breach erosion
The Technical Advisory Committee on Water Defences in the Netherlands has decided to develop a mathematical model for breach erosion in dunes and dikes, with which it will be possible to predict the growth of the breach and the discharge rate through the breach in case of a dikeburst. An essential part of such a mathematical model is the description of the entrainment of the sediment (sand or clay) and its transport through the breach. The process of breach erosion, especially in the first phases, is characterized by relatively steep slopes and large flow velocities. None of the existing sediment transport formulae has been derived and tested for these circumstances. This report presents the results of an investigation into the applicability of sediment transport formulae to sanddike breach erosion. In view of the steep slopes and the large flow velocities, the following sediment transport conceptions have been included in the study:
formulae for sandwater mixture flows: Wilson (1966), Wilson (1987), Mastbergen and Winterwerp (1987);
formulae for sediment transport in flows on relatively steep slopes: Mizuyama (1977), Smart and Jaeggi (1983), Bathurst et al. (1987), Takahashi (1987), Rickenmann (1991);
formulae for river regimes which have been tested for (relatively) large flow velocities (large shear stress velocities): Engelund and Hansen (1967), Van Rijn (1984a, 1984c);
energeticsbased sediment transport conceptions: Bagnold (1963, 1966), Yang (1979), BagnoldBailard, see Bailard (1981), BagnoldVisser, see Visser (1988), these last two formulae are modifications of the original conception of Bagnold (1963, 1966);
formulae for debris flows: Takahashi (1978, 1980, 1987, 1991). These sediment transport formulae, combined with Galappatti's model (1983) for the pickup of sediment, are compared with the data of two laboratory experiments (Schelde Flume experiments, see Steetzel and Visser, 1992a, 1992b) and the data of a field experiment (Zwin'89 experiment, see Visser et al., 1990). Experimental sediment transport rates have been determined as volumes of sand eroded over a certain period of time. All tests concern supercritical flow (Froude number Fr > 1, i.e. here 2.8<5 Fr<5 4.1), large values for Shields' mobility parameter (10 < theta < 100) and high concentrations (depthaveraged values rising up to about 0.25 by volume). Most of the tested sediment transport formulae predict sand transport rates being much larger than the observed quantities. Only the BagnoldVisser formula, see Visser (1988), predicts sand transport rates within a factor two of the experimental values. With the formulation of Van Rijn (1984a, 1984c) this is possible within about a factor three. All other formulae give larger deviations from the experimental data. These conclusions hold for the three initial phases of the process of breach erosion, when the flow is supercritical, and confirm the good results obtained up to now with the BagnoldVisser formula, see Visser (1988, 1994). Once more it should be emphasized that this formula has not been derived for a situation where the rate of sand entrainment is so large as in the first three phases of the breach erosion process (this applies to both the energeticsbased method and the semiempirical determination of the efficiency factor). The relatively large entrainment of sediment causes a relatively large increase of both the flow rate and the sediment concentration of the sandwater mixture along the inner slope (so that the effect of 'hindered entrainment' is possibly not negligible). Further study is necessary to establish the effects of the large rate of sediment entrainment on the breach erosion process. For the time being it is recommended to apply the formula of BagnoldVisser in a mathematical breach growth model for the description of the first phases (i.e. as long as the flow is supercritical) of the breach erosion process. The present study does not recommend a formula for the important later phases of breach growth (when the flows becomes subcritical), in which most of the breach erosion takes place and in which also the dimensions of the ultimate breach are determined. Probably the data of the recent Zwin'94 field experiment (see Visser et al, 1995) will allow such a recommendation in the near future. For the present the conclusion of Voogt et al. (1991) is still valid, i.e. that the formulae of Engelund and Hansen (1967) and in particular Van Rijn (1984a, 1984c) can also be applied for relatively large current velocities in subcritical flow.

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7 

A multigrid method combined with defect correction for free convection problems at high Rayleigh numbers

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8 

In search of a better sediment mixing coefficient model
Results of sediment transport calculations are often necessary in solving practical coastal engineering problems. (Sediment transport due to waves and currents). Many transport formulae have been proposed in literature in the past. Selection of the proper one while solving a particular problem, is a difficult task for a coastal engineer. In considering sediment transport under wavecurrent conditions it is worthwhile to make a distinction between two situations, viz.: The fluctuations in the orbital motion have to be fully taken into account in order to find the resulting sediment transport (intrawave type of description; often: crossshore sediment transport);  It is sufficient to take timeaveraged effects of the waves into account in order to find the resulting sediment transport rate (intrawave type of description is not required; often: longshore sediment transport). For the longshore sediment transport mode, transport formulae based on timeaveraged velocity distributions and timeaveraged sediment concentration distributions over the water depth can often be used. The present paper is restricted to this type of formula.

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9 

Book review: L. Dorn. Driver behaviour and training

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10 

Flying is the safest way to travel: How aviation was a pioneer in independent accident investigation

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11 

Behavioural effects of advanced cruise control use: a metaanalytic approach

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12 

Numerical study of a multigrid method with four smoothing methods for the incompressible NavierStokes equations in general coordinates

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13 

An entrainment model for fluid mud
An entrainment model for fluid mud is derived by integrating the equation for turbulent kinetic energy across the mixed layer and introducing some modelling assumptions. The resulting entrainment model is similar to models of mixedlayer deepening in lakes and reservoirs, but in addition accounts for the work needed to entrain bed material. Two basically different flow conditions are considered: (1) flow in the water layer but no flow in the fluidmud layer, and (2) flow in both layers driven by a tideinduced streamwise pressure gradient. In the first case, which applies to laboratory experiments in an annular flume, for example, the water layer is the turbulent mixed layer that erodes the quiescent fluidmud layer. In the second case the fluidmud layer is the mixed layer, which deepens because of entrainment of water from the overlying water layer. The water layer then is the quiescent layer. The viscous drag of the quiescent layer due to the flow in the mixed layer, which effect can play a part in laboratory experiments, is accounted for. Empirical model coefficients are obtained from the literature.

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14 

Life cycle cost analysis for managing rail infrastructure

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15 

The impact of institutional structures on transport infrastructure performance: a crossnational comparison on various indicators

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16 

Energy dissipation in breaking solitary and periodic waves
It is well known (see Divoky et al., 1970, for a review) that on gentle slopes (slope S < 1:30, say) the wave height after breaking does not decay in proportion to the mean depth. The curve H/Hb vs h/hb is concave upwards (for plane bottom). The concavity increases with decreasing S and with increasing Ho/Lo (Nakamura et al., 1966). The oftenused hypothesis H(x) = gamma h(x), gamma = const. for given (S, Ho/Lo)' does not incorporate the effects mentioned above. Its success in the prediction of setup can perhaps be ascribed Co the fact that it has been tested mainly on plane and relatively steep slopes, where indeed it is in reasonable agreement with the data. The hypothesis H = gamma h is not applicable in regions where the depth is constant or increasing in the propagation direction, such as in a bartrough profile. Visual observations of the latter situation cannot fail but to give the impression that the immediate postbreaking behavior is governed primarily by the characteristics at breaking, with its own imposed length scale. The bottom slope is believed to affect this behavior only if it is sufficiently steep, so that the rate of change of wave height due to changes in depth ("shoaling" with either increasing or decreasing depth) is comparable to that due to breaking.

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17 

Book review: Urban transport development. A complex issue (Gunilla Jönson and Emin Tengström (eds.))

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18 

On accurate discretization of turbulence transport equations in general coordinates

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19 

Onedimensional models for mountainriver morphology
In this report, some classical and new simplifications in mathematical and numerical models for river morphology are compared for conditions representing rivers in mountainous areas (high values of Froude numbers and relatively large values of sediment transport rates). Options for simplification are summarized based on time scale of hydrographs and length scales of river geometry. This results in concepts based on quasisteady and/or quasiuniform flow assumptions. Additionally, the behaviour of frictionless, critical flow with a mobile bed is considered. The nonlinear interaction between changing flow and morphology is investigated for different values of the Froude number. Neglecting this interaction in numerical solution procedures appears to affect the solution. Also, mass and momentum contributions of sediment in transport on the mixture of water and sediment are analyzed. It is shown that errors due to simplification in numerical models for river morphology vary with the different up or downstream propagating waves that are part of the solution. Conclusions further refer to the importance of wave nonuniformity (wave length, dominance of friction), Froude number and bed mobility on the error made when using simplified modelling concepts. Application of simplified modelling concepts based on subcritical lowland rivers in the modelling of transcritical and supercritical flows can result in significant errors.

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20 

ADAS impact assessment by microsimulation

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