Độ ổn định đá dưới dòng chảy không đều - Luận án TS Nguyen Thanh Hoan, TU Delft

Stone Stability Under Non-uniform Flow Stone Stability Under Non-uniform Flow Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. Fokkema, voorzitter van het College voor Promoties, in het openbaar te verdedigen op maandag 3 november 2008 om 12.30 uur door Nguyen Thanh Hoan civiel ingenieur geboren te Nam Dinh, Vietnam Dit manuscript is goedgekeurd door de promotor: Prof. Stive Copromotor: Ir. Verhagen Samenstelling promotiecommissie: Rector Magnificus voorzitter Prof.

Stive Technische Universiteit Delft, promotor Ir. Verhagen Technische Universiteit Delft, copromotor Prof. Savenije Technische Universiteit Delft Prof. Roelvink UNESCO-IHE Institute for Water Education Prof.

de Rouck Universiteit Gent Dr. Uijttewaal Technische Universiteit Delft Dr. Hofland Deltares Prof. Stelling Technische Universiteit Delft, reservelid Drs.

Booij has provided substantial guidance and support in the preparation of this thesis. This research has been financially supported by the Ministry of Education and Training of Vietnam and Delft University of Technology. Keywords: Stone stability, stone transport, stone entrainment, incipient motion, threshold condition, bed protection, bed damage, non-uniform flow, turbulent flow, decelerating flow. This thesis should be referred to as: Hoan, N.

Stone stability under non- uniform flow. thesis, Delft University of Technology. ISBN 978-90-9023584-4 Copyright c 2008 by Nguyen Thanh Hoan Printed by PrintPartners Ipskamp B. All rights reserved.

No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or me- chanical, including photocopying, recording or by any information storage and retrieval system, without written permission of the author. To my family Contents Summary v Samenvatting ix Tom tat xiii 1 Introduction 1 1.2 Objectives of this study .2 Turbulence and flow properties .1 Uniform open-channel flow over a rough bed .2 Non-uniform open-channel flow .3 Hydrodynamic forces on a single stone .2 The Shields stability parameter .3 The Jongeling et al.4 The Hofland stability parameter .6 Methods for stone stability assessment .1 The stability threshold concept .2 The stone transport concept .3 Comparison and selection of methods. 30 3 Experimental arrangement and data processing methods 33 3.5 Selected time series .6 Data processing methods .1 Velocity and turbulence data .2 Stone entrainment rate data .4 Mean flow velocity .5 The eddy viscosity and mixing length .6 Turbulence intensity data .7 Reynolds shear stress data. 65 5 Stone transport formulae 67 5.2 The proposed stability parameter .3 Final formulation of the proposed stability parameter .4 Evaluation of the available stability parameters .1 The Shields stability parameter .2 The Jongeling et al.3 The Hofland stability parameter .1 Comparison of the stability parameters .2 Sensitivity analysis of key parameters.

84 6 Estimation of stone entrainment using numerical flow modeling 87 6.3 Numerical model set-up .4 Model calibration and verification .5 Estimation of bed damage .6 Conclusions and recommendations. 99 7 Conclusions and recommendations 101 7.2 Velocity and turbulence data. 124 C Numerical flow modeling 129 C.1 Mean-flow equations .2 The two-equation k-ε model .2 Deft input files .3 Typical sequence of an Deft session. 139 List of symbols 141 List of figures 145 List of tables 148 Acknowledgements 151 Curriculum Vitae 153 iv Contents Summary Despite the fact that many studies on the stability of stones in bed protections under flowing water have been conducted, our knowledge is still far from ad- vanced and reliable.

Issues like how to quantify the hydraulic loads exerted on the stones on a bed and how to assess the stability of the stones are central and most challenging in stone stability research. Firstly, it is important that the hydraulic forces exerted on the stones in a bed are adequately quantified. A stability parameter - expressed as a dimensionless relationship between hydraulic loads and bed strength - is often used to quantify the influence of these forces on the bed. As the turbulence fluctuations of the flow are of importance for the stability of stones, their effect has to be taken into account, especially for non-uniform flow.

In the few studies available, no sta- bility parameters have proven to be adequate in quantifying the hydraulic loads exerted on the bed for non-uniform flow. Secondly, the method with which the stability of stones is assessed also plays an important role. Available stability formulae used to determine the required stone sizes and weights are mainly based on the concept of incipient motion of bed material. Due to the stochastic nature of bed material movement, a robust flow condition at which the stones begin to move does not exist.

Therefore, the threshold of movement is a rather subjective matter and the stone stability assess- ment method based on it often yields inconsistent design criteria. In contrast, the stability assessment method based on the stone transport concept leads to a re- sult with a cause-and-effect relationship between flow parameters and the bed response. Such a relationship provides consistent and more reliable design crite- ria and allows an estimate of the cumulative damage over time which is impor- tant for making decisions regarding maintenance frequency and lifetime analysis of hydraulic structures. Surprisingly, most of the previous studies on stone sta- bility are restricted to the stability threshold concept and few have attempted to derive stone transport formulae.

As a result, no physical relationship between the hydraulic load and the bed response is available for non-uniform flow. These two challenging issues are dealt with in this thesis. The objectives of the study are (i) to increase insight into the effect of hydraulic parameters, such as the velocity and the turbulence fluctuations, on the stability of stones in bed protections, (ii) to establish a physical relationship between the hydraulic param- v vi Summary eters and the bed damage (i., stone transport formulae) for non-uniform flow to obtain a reliable estimate of bed damage, and (iii) to evaluate the use of the outputs of numerical flow modeling to predict bed damage. Experimental work is central in this study.

A detailed set of measurements was carried out in a laboratory flume. The program comprised the measurement of the flow in gradually expanding open-channels and of the induced damage to the bottom. This flow configuration was chosen because in such a flow the turbu- lence intensity is high. Three experimental configurations with different expan- sion rates were used to create different combinations of velocity and turbulence.

The bed response (quantified by a dimensionless entrainment rate) and the flow field (quantified by velocity and turbulence intensity distributions) were mea- sured. The subsequent analysis has been directed towards the understanding of the effect of hydraulic parameters on stone stability and the cause-and-effect relationship between the flow and its induced damage to the bottom. Based on our data, the various ways of quantifying the hydraulic loads ex- erted on the stones on a bed have been extensively reviewed, verified and ex- tended. The physical reasoning behind this is that if a stability parameter prop- erly describes the hydraulic loads exerted on a bed, it should correlate well with the bed response (i., the dimensionless entrainment rate).

The correlation analysis has yielded quantitative confirmation of earlier find- ings on the inappropriateness of using the bed shear stress alone to represent the hydraulic loads exerted on a bed in non-uniform flow. An approach that uses a combination of velocity and turbulence distributions to quantify the flow forces has been verified for the first time since it was proposed by Jongeling et al. Inspired by this approach, a new stability parameter has been proposed to bet- ter quantify the hydraulic loads exerted on the stones. The formulation of the newly-proposed stability parameter has physically explained and quantitatively described the hydraulic loads exerted on the stones in bed protections.

This pro- vides valuable insight into the understanding of the influence of the different flow characteristics such as velocity and turbulence distributions on stone stabil- ity. Based on the physical analysis and practical considerations, a final expression for the new stability parameter was formulated. For the first time, the physical relationship between flow parameters and the bed damage - expressed as stone transport formulae - has been established for non-uniform flow. Since a good collapse of the data is obtained for a variety of stone densities (varying from 1320 to 1970 kg/m3 ), the influence of stone density is well incorporated into the formulae.

Therefore, the newly-developed stone transport formulae are likely to be valid for other bed materials with different densities, including natural stones. The newly-developed stone transport formulae can be used together with the outputs of numerical flow modeling to estimate bed damage. This was evaluated by comparing the measured and the calculated damage using the outputs of nu- merical flow modeling. The analysis has shown a good agreement between the Summary vii measurements and calculations.

Therefore, with the availability of the newly- developed stone transport formulae and more reliable turbulence models, the bed damage level can be more accurately computed for arbitrary flow conditions. viii Summary Samenvatting Ondanks het feit dat er veel studie is gedaan naar de stabiliteit van stenen in bodemverdedigingen in stromend water, is onze kennis nog onvoldoende. As- pecten zoals het kwantificeren van de hydraulische belasting op de stenen in de bodem en hoe de stabiliteit van de stenen te bepalen staan centraal en zijn vooral uitdagend in steenstabiliteitsonderzoek. Ten eerste is het belangrijk dat de hydraulische krachten op de stenen op de bodem goed worden gekwantificeerd.

Een stabiliteitsparameter - uitgedrukt als een dimensieloze relatie tussen hydraulische belasting en bodemsterkte - wordt vaak gebruikt om de invloed van deze krachten op de bodem te kwantificeren. Omdat de turbulente fluctuaties van de stroming van belang zijn voor de sta- biliteit van de stenen, moet dat effect ook in beschouwing genomen worden, vooral bij niet-uniforme stroming. In de weinige beschikbare studies, heeft geen van de stabiliteitsparameters bewezen een adequate kwantificering van de hy- draulische belastingen van niet-uniforme stroming op de bodem te kunnen geven. Ten tweede, de methode waarmee de stabiliteit van stenen wordt beoordeeld speelt ook een belangrijke rol.

Beschikbare stabiliteitsformules om benodigde steengrootte en gewicht te bepalen zijn vooral gebaseerd op het concept van be- ginnend bewegen van bodem materiaal. Door het stochastische karakter van bodem materiaal beweging bestaat er geen eenduidige stromingsconditie waar- bij de stenen beginnen te bewegen. Daarom is de grens van bewegen tamelijk subjectief en steenstabiliteitbeoordeling hierop gebaseerd leidt vaak tot inconsis- tente ontwerpcriteria. De stabiliteit beoordelingsmethode gebaseerd op het steen transport concept, daarentegen, leidt tot een resultaat met een causaal verband tussen stromingsparameters en bodemrespons.

Zo’n verband draagt bij aan con- sistente en betrouwbaardere ontwerp criteria en biedt de mogelijkheid cumu- latieve schade in de tijd te schatten. Dit is belangrijk voor besluitvorming betref- fende de onderhoudsfrequentie en levensduur analyse van waterbouwkundige constructies. Het is daarom opmerkelijk dat de meeste eerdere studies over steenstabiliteit, beperkt waren tot het stabiliteitsgrens concept en enkelen een poging tot het afleiden van een steen transport formule beschrijven. Daarom is er geen fysische relatie tussen de hydraulische belasting en de bodem respons beschikbaar voor niet-uniforme stroming.

Deze twee uitdagende aspecten komen aan de orde in dit proefschrift. De ix x Samenvatting doelen van de studie zijn (i) inzicht verbeteren in het effect van hydraulische pa- rameters zoals de stroomsnelheid en turbulente fluctuaties, op de stabiliteit van stenen in bodem verdedigingen, (ii) vaststellen van een fysische relatie tussen de hydraulische parameters en de bodemschade (d., steentransportformules) voor niet-uniforme stroming voor het verkrijgen van een betrouwbare schatting van de bodemschade, en (iii) evaluatie van het gebruik van de resultaten van numerieke stromingsmodellering om bodemschade te voorspellen. Experimenteel werk staat centraal in deze studie. Een gedetailleerde set van metingen is uitgevoerd in een laboratorium.

Het programma behelsde metingen van vrije oppervlakte stroming in een geleidelijk breder wordende goot en van de veroorzaakte schade aan de bodem. Deze stromingsconfiguratie is gekozen om- dat hierbij de turbulente intensiteit hoog is. Drie experimentele configuraties met verschillende mate van verbreding zijn toegepast om verschillende combinaties van snelheid en turbulentie te creëren.