Movable Bed Models Research Articles

3D Physical Modeling of an Artificial Beach Nourishment: Laboratory Procedures and Nourishment Performance

Beach nourishment represents a type of coastal defense intervention, keeping the beach as a natural coastal defense system. Altering the cross-shore profile geometry, due to the introduction of new sediments, induces a non-equilibrium situation regarding the local wave dynamics. This work aims to increase our knowledge concerning 3D movable bed physical modeling and beach nourishment impacts on the hydrodynamics, sediment transport, and morphodynamics. A set of experiments with an artificial beach nourishment movable bed model was prepared. Hydrodynamic, sediment transport, and morphological variations and impacts due to the presence of the nourishment were monitored with specific equipment. Special attention was given to the number and positioning of the monitoring equipment and the inherent constraints of 3D movable beds laboratory tests. The nourishment induced changes in the beach dynamics, leading to an increase in the flow velocities range and suspended sediment concentration, and effectively increasing the emerged beach width. Predicting and anticipating the morphological evolution of the modeled beach has a major impact on data accuracy, since it might influence the monitoring equipment’s correct position. Laboratory results and constraints were characterized to help better define future laboratory procedures and strategies for increasing movable bed models’ accuracy and performance.

Applicability of the De Marchi Hypothesis for Side Weir Flow in the Case of Movable Beds

Side weirs are widely used hydraulic structures typically designed and studied in the case of fixed bed conditions. In the case of subcritical flows, the hydraulics of side weirs can be modelled by using the classical De Marchi hypothesis. In the present work, a generalization of this hypothesis is developed for the case of side weirs over movable beds. Experiments showing the effects and feedbacks between the spilling discharge and the bed morphodynamics are presented. The application of the experimental observations to the generalized De Marchi hypothesis clearly show that the functioning of side weirs on a movable bed can be modelled by using this hypothesis. These findings could be instrumental for the design and verification of these structures. DOI: 10.1061/(ASCE)HY.1943-7900.0000566. © 2012 American Society of Civil Engineers. CE Database subject headings: Weirs; Experimentation; Channel flow; Movable bed models. Author keywords: Weirs; Subcritical flow; Experimentation; Channel flow.

Applicability of CADMAS-SURF/3D to local wave scour around the pile-supported wave absorbing breakwater

This study describes applicability of a numerical method to local scour by wave action around the pile-supported wave absorbing breakwater. CADMAS-SURF/3D(C-S/3D) was applied to the investigation which was performed by movable-bed models. In the present paper, basic application of CADMAS-SURF/3D was focused, since bottom shape was handled as fixed bed. The verification was achieved by comparing the velocity field of the calculation and the experiment. In the result, simulated value of the mean velocity was good agreement with experimental values, but simulated value of the maximum velocity was greater than experimental values. Moreover, it was not need to take a contracting flow effect by slit consideration in C-S/3D.

"Barbs" for river bend bank protection: application of a three-dimensional numerical model

A three-dimensional numerical model was used to examine the turbulent flow field and associated sediment transport due to a series of barbs (submerged groynes) in a channel bend. Model results were in good agreement with measured laboratory data and adequately simulated the important features of sediment transport. Statistical comparison of the predicted and measured equilibrium bed geometry found average regression coefficients of determination of 0.77 and 0.72 for the 90° and 135° channels, respectively. The predicted velocity data followed expected trends. The capability of a three-dimensional numerical model to simulate sediment transport through bend sections of a channel containing barbs was verified. This included the simulation of the effects of different arrangements of barb groups and an analysis of the data to determine the relation of the flow field to associated scour and deposition in a complex fluvial environment. These novel results are useful for improved analyses of the bank-protection capabilities of these structures and for the development and improvement of design guidelines.Key words: three-dimensional models, numerical models, movable bed models, channel bends, turbulence, secondary flow, scour, barbs, groynes.

Evaluation of the Micromodel: An Extremely Small-Scale Movable Bed Model

The micromodel is an extremely small physical river model having a movable bed, varying discharge, and numerous innovations to achieve quick answers to river engineering problems. In addition to its size being as small as 4cm in channel width, the vertical scale distortion up to 20, Froude number exaggeration up to 3.7, and no correspondence of stage in model and prototype, place the micromodel in a category by itself. The writer was assigned to evaluate the micromodel’s capabilities and limitations to ensure proper application. A portion of this evaluation documents the deviation of the micromodel from similarity considerations used in previous movable bed models. The primary basis for this evaluation is the comparison of the micromodel to the prototype. The writer looked for comparisons that had (1) a reasonable calibration of the micromodel and (2) about the same river engineering structures constructed in the prototype that were tested in the micromodel and (3) a prediction by the micromodel of the approximate trends in the prototype. Evaluation of these comparisons shows a lack of predictive capability by the micromodel. Differences in micromodel and prototype likely result from uncertainty in prototype data and the large relaxations in similitude. Based on the lack of predictive evidence, the micromodel should be limited to demonstration, education, and communication for which it has been useful and should be of value to the profession.

Systematic investigation of time scale in movable‐bed model

The objective of this study is to derive the time scale to be applied to bed‐load transport in movable‐bed experiments. Based on the model similitude law, the hydraulic time scale and the sedimentation time scales, which were derived from nine selected empirical formulas, were compared in detail. The hydraulic time scale is found to usually be smaller than the sedimentation time scale in a length‐scale distorted model. The time‐scale distortion problem can only be solved using a model that employs heavy (more dense) sediment, which is contrary to the general movablebed models that employ light (less dense) sediment. The analytical results show that the deviation between these two time scales increases as the model length‐scale distortion ratio increases. The time‐scale distortion problem may only be negligible when the model has a small length‐scale distortion ratio.

Unified modeling guidance based on a sedimentation parameter for beach changes

Review of four available proposals for designing coastal movable-bed models demonstrates that each suggested model-scaling law contradicts all the others. Accurate laboratory models providing the separate empirical bases, however, all appear fairly consistent with the new design guidance developed here. This formal development considers sediment excursions under wave action by means of a parameter including bed slope and the threshold of sediment movement. The consequent relationship for sedimentation similitude between model and prototype indicates that appropriate model sediment and horizontal scale depend on the selected vertical scale along with wave period and prototype sand characteristics. Explicit forms derived from the general design guidance recover two expressions resembling those previously reported by E.K. Noda (1972) and by P. Vellinga (1982); the route towards Noda's result indicates that guidance is not appropriate for small models of large situations, but the present development yields an independent basis and a needed refinement for Vellinga's empirical results. Considerations of sedimentation similitude are also shown to be supported by successful new laboratory models using sand profiles developed in a large wave tank as prototypes. Spatial distortion seems to be the advisable strategy for small models but proper distortion is rather modest with prototype materials, according to present results.

Numerical movable-bed models for practical engineering

Numerical modelling of sediment transport phenomena is an ambitious aim, which can only be attained in the long term by first formulating simplified models. On the basis of practical experience, these may be subsequently refined to give better results. This paper describes some simplifications of the full set of governing equations associated with sedimentation processes under time-dependent flow conditions, such as those in tidal rivers. A numerical movable-bed model is presented which, although economical in its computer requirements, nevertheless simulates some of the complex three-dimensional physical effects. Results from practical applications are given and compared with both laboratory experiments and field investigations.

ADDED EVIDENCE ON NEW SCALE LAW FOR COASTAL MODELS

Further positive results on appropriate scaling of movable bed models support an analytical expression developed in 1983. That development examined a simple parameter for a profile view of nearshore sedimentation.

SHORE PROTECTION AT VENICE: A CASE STUDY

The lagoon of Venice (N. Adriatic) is the most important in Italy (Fig. 1). Between 1840 and 1933 the tifial entrances facing the lagoon were controlled by six large jetties, after the construction of which more evident erosion phenomena appeared in some tourist resort beaches near the inlets. Detailed historical, movable bed models, and field studies have recently been conducted with the aim of defining a proper defence system for these beaches. The results of these studies are reported here.

SCALE RELATIONS FOR EQUILIBRIUM BEACH PROFILES

There are many practical problems related to coastal engineering, which may be solved by using movable bed scale models of the coastal zone. The scale model must obey the laws of sediment transport in order to obtain satisfactory results. However, sediment transport in the coastal zones is so complex that the mechanism is not fully understood. Some authors have attempted to find a model law relationship for equilibrium beach profiles. Yalin-'-^ derived a scale law for the offshore zone using a bed velocity based on laminar boundary conditions. Brebner, Karaphuis and Paul^' have performed extensive experimental tests on movable-bed models using light weight sediment, and Le Mehaute^) presented a scaling law for coastal movable-bed models in the breaker zones. The purpose of this study is finally to determine the scale law relationship for coastal movable-bed model. As first step, this paper concerns itself with the derivation of proper scale laws for modeling of equilibrium beaches.

MOVABLE BED MODEL TESTS ON DUNE EROSION

The primary sea defence system of the Netherlands consists for a large part of sandy beaches and dunes. The row of dunes, however, is rather narrow in some places, due to long-term erosion, and reinforcement has become necessary (Figure 1 and 2). In this connection a special governmental committee requested the co-operation of the Delft Hydraulics Laboratory to developed a design criterion for a dune sea defence system that could withstand a storm surge with a frequency of occurrence of once in 10,000 years (Figure 3). For that purpose all available field observations on dune erosion were analysed and a provisional, empirical, guide-line was developed in 1972 \_\J , but because of the limited amount of field data and the complexity of a theoretical approach it was decided to check the validity of this guide-line by means of a model investigation. As no adequate scaling relationships are available for movable bed models with waves, the tests were set up in the form of a scale series. A large number of two-dimensional tests with various geometric scales using two types of sand (D50 = 225 um and D50 = 150 urn) was carried out in 1975.

FIELD AND MODEL STUDIES FOR VISAKHAPATNAM HARBOR

Model studies and analysis of oceanographic and littoral drift data were undertaken to advise Howe India (Private) Ltd. on littoral drift, siltation and shore erosion problems to be encountered during and after the construction of Visakhapatnam Outer Harbor Project. Distorted fixed-bed and movable-bed models with a horizontal scale of 1:300 and a vertical scale of 1:80 were calibrated to reproduce the integrated net effect of an average southwest and northeast monsoon season. Experiments were conducted to assess and predict seasonal changes resulting from the construction of the system of breakwaters under normal and extreme conditions. Recommendations were made concerning breakwater and sand trap location, shore protection, dredging and disposal of dredged material.