Showing papers by "Jørgen Fredsøe published in 2001"
TL;DR: In this article, an experimental study on the onset of scour below and self-burial of pipelines in currents/waves was conducted, where pressure was measured on the surface of a slightly buried pipe at two points, one at the upstream side and the other at the downstream side of the pipe, both in the sand bed.
189 citations
TL;DR: In this paper, the results of an experimental study on scour around a pile subject to combined waves and current were presented, where irregular waves were used in the experiments, which were carried out both for normal and irregular waves.
Abstract: This paper presents the results of an experimental study on scour around a pile subject to combined waves and current. Irregular waves were used in the experiments, which were carried out both for ...
178 citations
TL;DR: In this article, the results of an experimental study on the steady-streaming flow and the resulting scour process around a large vertical circular cylinder exposed to a progressive wave are presented.
Abstract: This paper presents the results of an experimental study on the steady-streaming flow and the resulting scour process around a large vertical circular cylinder exposed to a progressive wave. The flow is in the unseparated flow regime [the Keulegan-Carpenter number at the bed being KC < O(1)]. The diffraction effect is existent. Two kinds of experiments were made: (1) rigid bed; and (2) actual scour. Detailed velocity measurements were made near the bed in the vicinity of a cylinder (1 m in diameter) in the rigid-bed experiments. These measurements show a strong presence of a 3D steady streaming induced by the waves. No such streaming exists in the absence of the cylinder. The rigid-bed experiments together with the actual scour tests indicate that the waves stir up the sediment and bring it in suspension, and the steady streaming carries the sediment away from the cylinder, leading to scour. The scour is found to depend mainly on KC and the diffraction parameter, the cylinder-diameter-to-the-wave length r...
92 citations
TL;DR: In this article, the authors present the results of an experimental investigation on suction removal of sediment from between armor blocks/stones placed on a loose bed, and they find that the onset of suction is governed by two parameters: the Shields parameter (based on the sediment size); and the ratio of sediment size to stone size, d/D.
Abstract: This paper presents the results of an experimental investigation on suction removal of sediment from between armor blocks/stones placed on a loose bed. The process of suction has been investigated. It was found that the vortices that form in the holes between the armor blocks are key to the process. The sediment swept into these vortices is entrained into the main body of the flow by these same vortices (the suction removal of sediment from between the armor blocks). The critical condition for the onset of suction was determined. It was found that the onset of suction is governed by two parameters: (1) the Shields parameter (based on the sediment size); and (2) the ratio of sediment size to stone size, d/D. The variation of the critical Shields parameter for suction as a function of d/D was determined for a broad range of the parameter d/D, namely, 0.001 & d/D # 1. The timescale of the suction process and the downward displacement of stones (the general lowering of the armor layer) have also been investigated.
58 citations
TL;DR: In this paper, a flow and sediment transport model formulated in curvilinear coordinates is applied to calculate the distribution of suspended sediment concentration, which is relevant when assessing rates of sedimentation of trenches dredged for the purpose of laying out pipelines or cables in the marine environment.
Abstract: By means of detailed mathematical modeling the processes responsible for the morphological evolution of small-scale excavations exposed to wave-induced flow are investigated. The problem is relevant when assessing rates of sedimentation of trenches dredged for the purpose of laying out pipelines or cables in the marine environment. A flow and sediment transport model formulated in curvilinear coordinates is applied. The unsteady Reynolds-averaged Navier-Stokes equations along with a standard k-e turbulence model and the equation of continuity constitute the flow model. The action of waves is simulated by an oscillating pressure gradient imposed on the horizontal momentum equation. The sediment transport is composed of bed and suspended load taking into account the effect of gravity on the former. The convection-diffusion concept is applied to calculate the distribution of suspended sediment concentration. Intraperiod flow and sediment transport processes are resolved, and the effect of these on the backfi...
15 citations
18 May 2001
TL;DR: In this paper, the shape, dimensions and growth rate of a sandy hook is analyzed and the maximum growth rate is found for a finite length scale of the spit and a spit of a finite width is therefore expected to emerge.
Abstract: A theoretical study is made of the shape, dimensions and growth rate of a spit. The spit is growing without changing its shape, being fed by the littoral drift along a long straight coast up drift of it. A one-line model for coastline development is used as basis for a simple analytical model. This is based on the assumption that the local longshore sediment transport is determined by the local orientation of the coastline relative to the incoming waves. The analytical model predicts an exponential shape of the spit. The analytical model gives a growth rate inversely proportional to the length scale and this model predicts an infinitely narrow spit to emerge. The sediment transport around the spit is investigated by two-dimensional models. The shape of the spit is still taken to be exponential and the growth rate is estimated on basis of the longshore sediment transport along the base of the spit. The maximum growth rate is found for a finite length scale of the spit and a spit of a finite width is therefore expected to emerge. INTRODUCTION This study concerns the morphological development at a coast with strong gradients or even interruption in the longshore sediment transport as can be encountered at smaller islands, at abrupt changes in the orientation of the coastline or where the coast is interrupted by an inlet. In such situations a spit or a sandy hook may be formed, which is 1) Ph.D.-student, Environment & Resources (Before 01.01.2001: Department of Hydrodynamics and Water Resources (ISVA)), Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark. dp@isva.dtu.dk 2) Associate Professor, Environment & Resources (Before 01.01.2001: Department of Hydrodynamics and Water Resources (ISVA)), Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark. 3) Professor, Environment & Resources (Before 01.01.2001: Department of Hydrodynamics and Water Resources (ISVA)), Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark,
6 citations
18 May 2001
TL;DR: In this article, the authors evaluate a two-dimensional bottom boundary layer and sediment transport model with field observations of velocity and suspended sediment concentration, showing that the model over- predicts bed shear and underrepresents mid-bottom boundary layer shear.
Abstract: The goal of this research is to evaluate a two--dimensional bottom boundary layer and sediment transport model with field observations of velocity and suspended sediment concentration. The model-data comparisons show several interesting phenomena. First the model over- predicts bed shear and underrepresents mid-bottom boundary layer shear. The Engelund and Fredsoe (1976) bedload model favorably compares with nearbed concentration observations. The model predicted suspended sediment plumes are of similar order magnitude to the observed plumes. These model--data comparisons will help field observation interpretations over complex topography and quantify the strengths and weaknesses in the model's physics.
4 citations
01 Jan 2001
TL;DR: Morphodynamics is a complicated field where the interaction between the flow and an erodible bed is studied Because the bottom changes with time due to erosion or sedimentation, the flow pattern changes correspondingly This means that a theoretical study of the morphological development of a specific case usually requires not only one flow calculation, but a large number of calculations as discussed by the authors.
Abstract: Morphodynamics is a complicated field where the interaction between the flow and an erodible bed is studied Because the bottom changes with time due to erosion or sedimentation, the flow pattern changes correspondingly This means that a theoretical study of the morphological development of a specific case usually requires not only one flow calculation, but a large number of calculations
1 citations