The effects of bedform roughness on hydrodynamics and sediment transport in Delft3D

To contribute to solving scientific and practical questions, numerical morphodynamic models like Delft3D are often used to predict the hydrodynamics, sediment transport processes and morphological development of coastal systems. In such models, many of the processes are parameterized based on a variety of assumptions. One of the parameterized variables is the bedform-related hydraulic roughness ks, which is often assumed to be related to the ripple height. This roughness affects the magnitude and vertical structure of the flow and, consequently, the magnitude of the sediment transport. Yet, their sensitivity to ks is not well understood

Alongshore bed load transport on the shoreface and inner shelf

Bed load transport rates on the shoreface and shelf are determined by tidal currents, wave-current interaction and grain size. There is, however, a strong lack of field data and validated models because bed load transport under waves cannot be measured in the field, while bed load transport by currents without waves commonly is barely measurable in spring tidal conditions. Herein, bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13-18 m in fine and medium sands at 2 to 8.5 km offshore the Dutch coast. Near-bed flow velocity was recorded at 2 Hz. The measurements are used to derive an empirical bed load model, in which transports are normalized by grain size and density. The model produces bed load transports that are at least a factor 5 smaller than predicted by existing models. However, they agree with a large laboratory data set of sand and gravel transport in currents near incipient motion. Cohesion of sediment due to mud in-mixing or biological activity was excluded. Including turbulence probabilistically in bed load models strongly improves predictions near incipient motion, and predict 20% more alongshore transport annually for currents only. The effect of wave-current interaction is predicted to be twice as large, and the combined effect results in 100% larger transports. The effect of wave stirring is gives much larger flood and ebb transports but the net transport is the same as for the combined wave-current interaction and turbulence case. An overestimation of the current velocity leads to much larger transports than any of the model combinations. Concluding, the effects of turbulence, wave-current interaction and wave stirring are of secondary importance compared to the choice of empirical or existing bedload predictor and the representation of the current climat

Dynamiek van het strand bij Noordwijk aan Zee en Egmond aan Zee en het effect van suppleties

DEZE ENTRY BEVAT ZOWEL DE ORIGINELE RAPPORTAGE (T0-RAPPORT, 2006) ALS HET VERVOLG DAAROP (T1-RAPPORT, 2007). In deze studie beschrijven en analyseren we maandelijks hoogtemetingen uitgevoerd op twee intergetijdestranden langs de Hollandse kust. Naast de wetenschappelijk vragen bij het seizoensgebonden gedrag van het intergetijdestrand richten we ons op vragen omtrent kustbeheer. De kennis over de effecten van strand- en onderwatersuppleties op het intergetijdestrand is beperkt. Het doel van deze studie is daarom meer inzicht te verschaffen in het seizoensgebonden en langjarig gedrag van het intergetijdestrand en het effect van suppleties op deze zone. De stranden hier beschreven zijn dat bij Noordwijk (81.250-82.750), waar in 1998 een onderwatersuppletie is uitgevoerd, en het strand ten zuiden van Egmond (ruwweg 40.100-41.100), waar in de subgetijdezone en op het intergetijdestrand geen ingrepen hebben plaatsgevonden. Van beide intergetijdestranden wordt het seizoensgebonden gedrag bepaald aan de hand van duidelijk gedefinieerde strandparameters. Naast de maandelijkse hoogtemetingen hebben wij gebruik gemaakt van de jaarlijkse JARKUS raaien (raaien loodrecht in zee waarlangs de bodemdiepte wordt gemeten). CONCLUSIES BETREFFENDE HET EFFECT VAN SUPPLETIES Analyse van de hoogtemetingen op het strand en onderwateroever bij Egmond aan Zee tonen aan dat 2 jaar na aanleg van een serie van 8 strandsuppleties en 1 relatief kleine onderwatersuppletie < 1.000.000 m3) het strandvolume toeneemt tot +25^3/m boven het voorspelde niveau zonder suppleties. Ongeveer 5 jaar na aanleg van de onderwatersuppletie (de laatste van de serie) is het strandvolume echter weer op het niveau zoals voorspeld zonder suppleties. De combinatie van 1 strandsuppletie en 1 relatief grote onderwatersuppletie lijkt een vergelijkbaar effect te hebben als de 8 strandsuppletie en 1 kleine onderwatersuppletie. De suppleties (op het strand en onder water) bij Egmond lijken geen eenduidig positieve invloed te hebben op het binnenste brekerzonevolume (tussen -3 m en -0.76 m NAP). De huidige analyse van hoogtemetingen bij Egmond aan Zee toont aan dat het strand geen meetbaar effect ondervindt van suppleties welke ongeveer 1 km noordelijker zijn geplaatst. Analyse van de hoogtemetingen op het strand en onderwateroever bij Noordwijk aan Zee tonen aan dat drie jaar na plaatsing van een relatief grote onderwatersuppletie het strandvolume is toegenomen met +19 m^3/m. Dit is ongeveer 5% van het suppletievolume. Na deze drie jaar neemt het strandvolume niet verder toe. Ongeveer 7 jaar na het aanbrengen van de onderwatersuppletie is het strand volume teruggekeerd naar de voorspelde waarde zonder suppleties. Naast het strandvolume ondervindt ook het binnenste brekerzonevolume bij Noordwijk een positief effect van de onderwatersuppletie. Drie jaar na het aanbrengen van de onderwatersuppletie is het brekerzonevolume ongeveer 16 m^3/m meer dan zonder onderwatersuppletie. Ongeveer 8 jaar na het aanbrengen is het brekerzonevolume teruggekeerd naar de voorspelde waarde zonder suppleties. Onderwatersuppleties hebben vooral een beschermende invloed op het strand en in mindere mate, maar wel aanwezig, een voedende. Dit effect is zichtbaar gedurende 5 tot 7 jaar na aanbrengen van de onderwatersuppletie. In tegenstelling tot het positieve effect bij Noordwijk hebben de (onderwater) suppleties bij Egmond geen effect gehad op het binnenste brekerzonevolume (tussen -3 m en -0.76 m NAP). Redenen voor dit verschil zouden verband kunnen houden met verschillen in suppletie-ontwerp (grootte, vorm, locatie). Nader onderzoek aan de hand van een modelstudie waarin het effect van verschillende suppletie-ontwerpen wordt onderzocht zou hierover meer helderheid kunnen geven. Het effect van onderwatersuppleties op het strand is beperkter dan dat op het MKL-volume.RKZ-166

Current roughness over small bedforms and waves

Sand grains, bedforms and the wave boundary layer cause roughness for tidal currents. This paper reports roughness and current shear stress in calm weather and storms derived from 1900 hours of detailed flow measurements on a sandy shoreface, 2 km off Noordwijk, The Netherlands. Two methods are employed: fitting logarithmical velocity profiles to data of 3 to 7 sensors, and the inertial subrange method from the spectra collected at 2 Hz. The results are compared to observed bedform dimensions. Various technical problems are discussed. The roughness decreases for increasing current velocity in Hm0 waves <2 m, but increases for larger waves. Individual events show contrasting trends that are probably related to bedform development. Recommendations for future instrumentation and for further analysis of this data are give

Sediment concentrations and Sediment transport in case of Irregular breaking waves

Coastal changes occur mostly as a result of changes in sediment transport along the coast. If at cross-section A, the sediment transport is for any reason larger (or smaller) than at cross-section B, accretion (or erosion) will take place in between the two cross-sections. For prediction of coast-lines in the future, the prediction of the net sediment transport is therefore essential. Various models, such as that of Bijker, Van Rijn, Nielsen, Engelund & Hansen and Ackers & White are available to predict the sediment transport by knowledge of wave height and current strength. The reliability of these models is unknown because data under field conditions are scarce. Only few relations between sediment transport, current velocity and wave height are known. For these reasons a laboratory study was carried out to extend the knowledge of the basic phenomena in morphological processes. The study contains experiments in which sediment concentrations and fluid velocities have been measured in case of irregular breaking waves alone and in combination with a current. Chapter 2 deals with the sediment transport basics. Two types of sediment transport, the longshore and the cross-shore sediment transport are discussed and the objectives of the present experiments are presented. In Chapter 3 the experimental set up is described. The measured parameters, methods and instruments are discussed. The experimental programme of the series A and the series B I and B2 are presented. Chapter 4 covers the experimental results from test series A. The wave characteristics, fluid velocities and the influence of these parameters on the sediment concentration, sediment load and sediment transport are studied. Chapter 5 deals with the experimental results from test series B I and B2. The distribution of the sediment concentrations, fluid velocities, sediment loads and sediment transport rates over a sand bar are studied. In Chapter 6 a comparison is made between the measurements and the sediment transport models by Van Rijn and Bijker. Transport rates, concentration profiles and velocity profiles are compared. In Chapter 7 a list of conclusions and recommendations is presented.Hydraulic EngineeringCivil Engineering and Geoscience

Measurement errors of instruments for velocity, wave heigt, sand concentration and bed levels in field conditions

Field measurements of hydrodynamics (fluid velocities and wave height), sediment dynamics (sand concentrations) and morphodynamics (bar behaviour) as performed during the COAST3D campaigns at the Egmond site in 1998 and at the Teignmouth site in 1999 inevitably involve the problem of the accuracy of the measured variables. The measurement errors are related to: \u95 the physical size of the instrument including supports, cables, housing for electronics, etc.; \u95 the measurement principle including electronic instability, drift, offset, calibration procedure, sampling size and applicability and validity ranges of the instrument concerned; \u95 the conversion principle including assumptions of applied theories (for example: conversion from fluid pressure to wave height; errors in position of pressure sensor above bed). Information of the measurement errors involved can be obtained by comparing instruments based on different measurement principles under controlled conditions. Recently several studies focussing on hydrodynamics and sand transport in the large scale wave tanks of Delft Hydraulics (The Netherlands) and of the \u91Forschungszentrum Küste\u92 in Hannover (Germany) have been carried out. Various types of instruments have been used to measure fluid velocities, wave heights and sand concentrations during the experiments in the wave tanks. In addition data sets from various field experiments are used to evaluate the performance of the instruments considered. In this note some results of instrument intercomparisons are presented. Furthermore, information of instrument characteristics are given.Sandpit - Coast3