Flow over
Weir-like Obstacles
Shahid Ali
Flow over weir-like obstacles
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op donderdag 22 augustus 2013 om 10.00 uur
door
Shahid ALI
Master of Science in Civil Engineering
University of Engineering and Technology, Taxila, Pakistan geboren te Faisalabad – Pakistan
Dit proefschrift is goedgekeurd door de promotoren: Prof.dr.ir. W.S.J. Uijttewaal Prof.dr.ir. G.S. Stelling Samenstelling promotiecommisie: Rector Magnificus, Prof.dr.ir. W.S.J. Uijttewaal, Prof.dr.ir. G.S. Stelling, Prof.dr.ir. W.H. Hager, Prof.dr.ir. M.J.F. Stive, Prof.dr.ir. A.E. Mynett, Prof.dr.ir. Y.M.A. Zech, Dr.ir. A. Sieben,
Voorzitter
Technische Universiteit Delft, promotor Technische Universiteit Delft, promotor Swiss Federal Institute of Technology Zurich Technische Universiteit Delft
Technische Universiteit Delft/UNESCO-IHE Catholic University of Louvain
Rijkswaterstaat, the Netherlands
This research has been supported by the Higher Education Commission, Pakistan; Minis-try of Transport, Public works and water management (RWS), the Netherlands and Delft University of Technology, Delft, the Netherlands
Cover design by Optima Grafische Communicatie, Rotterdam, the Netherlands Copyright © 2013 by Shahid Ali
Printed by Optima Grafische Communicatie, Rotterdam, the Netherlands ISBN 978-94-6169-415-7
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S
UMMARY
For an accurate prediction of the water levels during flood events, it is important to ac-count for the important processes that contribute to the flow resistance. A significant portion of the energy loss is found in the flow over weir-like obstacles which get sub-merged at high water stages. In addition to the bed friction that is found everywhere, the rapidly varying flow over, for examples groynes, access roads, and summer embank-ments complicates the proper estimation of the conveyance capacity. It becomes even more complicated when the elevated parts of the bed are covered with high vegetation in the form of bushes or trees. The flow that is already accelerated above the crest of the groyne or dyke experience a further acceleration passing between the vegetation. In case of high water the resistance of a river reach is for an important part governed by the bed forms and obstacles in the floodplain. The obstacles can consist of groynes, summer dykes, bridge piers, ditches, bushes etc.
It is especially related during extreme river discharges for which the design height of win-ter dykes is derived and for which costly operations are planned, often based on model (Numerical models) predictions. Measures that will significantly change the flow in a cer-tain river reach can by definition not be calibrated for in advance. Validity of current systems is limited especially where it concerns the underlying physics of rapidly varying non-hydrostatic flows. Yet, extensive calibration has made the systems reasonably accu-rate for current situations, but it is obscuring the deficiencies in modelling approach. These deficiencies are very relevant in cases of high water predictions where the model-ling uncertainties are highest and at the same time the impact is huge. This dangerous combination can be avoided by an extension of knowledge of underlying physical proc-esses and the associated modelling techniques.
The main objective of this study is to quantify the hydraulic resistance caused by the vegetated weir-like obstacles in the floodplain during high water stages. This study fo-cuses on the correct representation of resistance elements in a numerical (depth aver-aged) simulation, as well as consistent behaviour of models towards high water stages. A number of processes related to vegetation, weirs and bed forms are studied in order to come to an improved implementation of the physical processes.
In order to find out in which way the various types of vegetation contribute to the flow resistance, a series of experiments is performed with various schematized types of vegeta-tion placed on top of a standard weir with a hydraulically rough surface.
To estimate and parameterize the form drag caused by the vegetated weir-like obstacles in the flow path during high water stages, a simple expansion loss form drag model is de-rived based on physics. The expansion loss form drag model has been deduced using fun-damental principles of physics, mass, momentum and energy balance. The momentum
viii
balance is applied on the downstream side of the weir where as, the energy balance on the upstream side of the weir. In addition, the mass balance is applied everywhere. For the derivation of the model, it is assumed that the flow is subcritical, steady, frictionless and incompressible. Pressure is considered hydrostatic and the stream lines over the weir crest are straight. These assumptions seem to be reasonable during high water stages. The energy head loss caused by the embankment trapezoidal weir-like structures is mainly due to flow separation with an associated size of recirculation zone behind the weir. The size of the recirculation zone depends on the downstream slope of the struc-ture. In case of a steep slope, the recirculation is large and as the downstream slope is becoming milder the size of the recirculation zone decreases. So the associated energy losses also decreased for gentle slopes. To accommodate the downstream slope effect, we apply the momentum balance in steps. This implies that we use a single step for steep slopes and more than one step for milder slopes.
In the expansion loss form drag model the weir-like obstacles are incorporated as a verti-cal contraction in the flow path and the effect of vegetation is included as a horizontal contraction of the flow path.
In case of the oblique weir-like obstacles, the velocity is decomposed into two compo-nents parallel and perpendicular to the weir crest. It is also clear from the numerical and experimental studies that the parallel velocity component to the weir crest remains al-most constant over the domain and can be neglected for the prediction of energy head losses and the discharge coefficient of the weir-like obstacles. Using this assumption and energy conservation principle, the flow direction over the weir crest can be predicted. Flow direction depends on the Froude number on upstream side of the weir as well as at the weir crest.
The energy head losses caused by the vegetated weir-like obstacles and the discharge co-efficient predicted by the expansion loss form drag model have been compared with the experiments for the submerged vegetated weirs. The expansion loss form drag model ap-pears to provide a good basis for the prediction of the energy head loss for submerged and subcritical flow conditions.
The skin friction for the hydraulically rough bed can be estimated using Colebrook-White formula and the energy head loss due to the skin friction can be simply added to the form drag losses of the weir-like obstacles
As it is assumed for the derivation of the expansion loss form drag model that the pres-sure distribution is hydrostatic and the stream lines in the crest region are parallel there-fore, its application is limited. When the Froude number is greater than 0.6 above the crest of the weir, the flow starts undulating on the downstream slope of the weir. The predicted results start to deviate from the experimental results as the non-hydrostatic pressure effect becomes important (Fr ≥ 0.6).
A series of numerical simulations has been performed to evaluate the performance and limitations of the CFD models. Numerical modelling has been applied using 2DV- RANS mathematical model including free surface modelling for the plain weir-like obstacles and 3D-RANS modelling for the oblique obstacles respectively. A standard and a non-linear k-ε model have been applied for turbulence closure. The numerically predicted results are compared with the experimental data.
ix It is concluded that the standard and non-linear k-ε turbulence models perform well with respect to the prediction of the energy head losses and discharge coefficient of the weir-like obstacles (bulk parameters). Although in the recirculation zone of the obstacles, the flow field is not reproduced very well by the model however the non-linear k-ε turbulence modelling leads to better prediction than the standard k-ε turbulence model. The other flow quantities such as flow velocities, turbulent stresses and kinetics energies are repro-duced reasonably in a qualitative sense but not quantitatively particularly in the recircu-lation zone of an obstacle.
Shahid Ali July 2013
xi
S
AMENVATTING
Voor een nauwkeurige voorspelling van de waterstanden tijdens hoogwater is het van be-lang om de dominante processen te kennen die bijdragen aan de stromingsweerstand. Een significant deel van het totale energieverlies wordt veroorzaakt door obstakels, die tijdens hoogwater overstromen. Bovenop de weerstand die veroorzaakt wordt door bodemwrij-ving wordt de afvoercapaciteit beïnvloed door de snel-varierende stroming over de obsta-kels zoals kribben, zomerdijken en toegangswegen. De stroming wordt nog gecompliceer-der als hoger gelegen delen begroeid zijn met struikgewas of bomen. Op de kruin van de kribben of de zomerdijken treedt een extra lokale versnelling op rondom en tussen de ve-getatie. In het geval van hoogwater wordt de weerstand van een rivier in hoge mate be-paald door bodemvormen in de geul en de obstakels in de uiterwaarden.
Dit is vooral van belang tijdens extreme hoogwaterafvoer, waarvoor de ontwerphoogte van de dijken is bepaald. Dure maatregelen zijn gepland voor de vergroting van de af-voercapaciteit op basis van voorspellingen met numerieke modellen. Maatregelen die tot een significante verandering in de stroming leiden in een bepaald riviervak kunnen per definitie niet van te voren gekalibreerd worden. De betrouwbaarheid van de huidige sys-temen is hierdoor beperkt, vooral wat betreft de onderliggende fysica van snelvariërende stroming.
Deze beperkingen zijn vooral relevant in het geval van hoogwaterberekeningen waarvoor de onzekerheden groot zijn en de impact hoog. Deze gevaarlijke combinatie kan gedeelte-lijk weggenomen worden door uitbreiding van de kennis van de onderliggende fysische processen en de gerelateerde modelleringtechnieken.
Het hoofddoel van deze studie is om de hydraulische weerstand te bepalen, die veroor-zaakt wordt door overlaatachtige obstakels met vegetatie, die overstromen gedurende hoogwater in de uiterwaarden. Deze studie richt zich voornamelijk op de correcte weer-gave van ruwheidselementen in een numerieke (diepte gemiddelde) simulatie, als mede op consistent gedrag van de modellen bij hoge waterstanden.
Een aantal processen, dat gerelateerd zijn aan vegetatie, overlaten en bodemvormen zijn onderzocht om te komen tot een verbeterde implementatie van de fysische processen. Een serie experimenten is uitgevoerd met verschillende geschematiseerde vormen van vegeta-tie op de top van een standaard overlaat om te bepalen hoe de verschillende typen vege-tatie de weerstand beïnvloeden.
Een op fysica gebaseerde expansie-verlies formulering is afgeleid om een afschatting en parameterisering te maken van de vormweerstand door overlaten met vegetatie tijdens hoogwater. Deze formulering is gebaseerd op fundamentele fysische balanswetten voor impuls, massa en energie. De impulsbalans is toegepast aan de benedenstroomse kant van de overlaat en de energiebalans aan de bovenstroomse zijde. De massabalans is over de gehele overlaat toegepast. Voor de afleiding is gebruik gemaakt van de aannames dat de stroming subkritisch, stationair en wrijvingsloos is. Bovendien wordt onsamendruk-baarheid aangenomen, wordt de hydrostatische druk aanname gebruikt en zijn de stroomlijnen over de overlaat recht verondersteld. Deze aannames zijn aannemelijk gedu-rende een hoogwatersituatie.
xii
Het energieverlies door de trapeziumvormige overlaten is vooral veroorzaakt door losla-ting van de stroming met een bijbehorende circulatiezone aan de benedenstroomse zijde van de overlaat. De grootte van de recirculatiezone hangt af van de helling van de over-laat aan benedenstroomse zijde. In het geval van een steile helling is een grote recircula-tiezone aanwezig, terwijl voor flauwere hellingen de recircularecircula-tiezone afneemt. Het bijbe-horende energieverlies is daarmee ook kleiner voor flauwere hellingen. Om het effect van de benedenstroomse helling mee te nemen, passen we de impulsbalans toe in verschillen-de stappen. We gebruiken een enkele stap voor steile hellingen en meerverschillen-dere stappen voor flauwe hellingen.
In de expansie-verlies formulering zijn de overlaten meegenomen als een verticale ver-nauwing in de stroming. De effecten van vegetatie worden meegenomen door een horizon-tale vernauwing aan te nemen.
In het geval van de scheve, overlaatachtige obstakels wordt de snelheid ten opzichte van de kruin van de overlaat ontbonden in een parallelle en een loodrechte component. Uit numerieke simulaties en experimenten blijkt dat de parallelle snelheidscomponent con-stant blijft binnen het domein van het obstakel. Er wordt daarom aangenomen dat deze component kan worden verwaarloosd voor de bepaling van het energieverlies en van de afvoercoëfficiënt. Door dit te combineren met de wet van behoud van energie, kan de stromingsrichting over de top van de overlaat worden voorspeld. De stromingsrichting hangt af van het Froude getal aan de bovenstroomse kant en die op de kruin van de overlaat.
In vergelijking met de experimenten met de overlaatachtige obstakels met vegetatie blijkt het expansie-verlies model goed in staat het energieverlies en de debietscoëfficiënt te voorspellen. Dit geldt voor subkritische stroming waarbij de obstakels onder water staan. De bodemwrijving voor de hydraulische ruwe bodem kan worden voorspeld met de Cole-brook-White formule. Het energieverlies door de bodemwrijving kan worden opgeteld bij het energieverlies van de vormweerstand van de overlaatachtige obstakels.
De toepassing van het expansie-verlies model wordt beperkt doordat er een aantal aan-names gedaan is bij de afleiding. In het model is de druk hydrostatisch en lopen de stroomlijnen parallel aan elkaar. Als het Froude getal groter is dan 0.6 dan komen de re-sultaten minder goed overeen met de experimentele data. In dit geval wordt het niet hy-drostatische deel van de druk belangrijk en gaat de stroming golven aan de beneden-stroomse kant van de overlaat.
Een reeks numerieke simulaties is uitgevoerd (CFD) ter vergelijking van de meetresulta-ten met de CFD uitkomsmeetresulta-ten. Om de vlakke overlaatachtige obstakels te modelleren is er een 2DV-RANS mathematisch model gebruikt. Deze neemt de beweging van het vrije oppervlak mee in de simulatie. Voor de scheve overlaten is er een 3D-RANS model ge-bruikt. Voor het turbulentie sluitingsprobleem wordt er een standaard en een niet-lineair k-ε-model gebruikt. De verkregen numerieke resultaten werden vergelijken met de expe-rimentele data.
Zowel het standaard als het niet-lineaire k-ε turbulentie model zijn geschikt om de ener-gieverliezen en de debietscoëfficiënten te voorspellen van de overlaatachtige obstakels. Ondanks dat de stroming in de recirculatiezone van de obstakels niet zo goed wordt voorspeld, geeft het niet lineaire k-ε-model betere voorspellingen wat betreft het
stroom-xiii beeld dan het standaard k-ε-model. Kwalitatief blijken de overige grootheden, zoals snel-heden, turbulentie schuifspanningen en de kinetische energie redelijk goed te worden weergegeven. Daarentegen wijken de waarden kwantitatief af, vooral in de recirculatie zone van een obstakel.
Shahid Ali Juli 2013
Contents
Summary vii
Samenvatting xi
Chapter 1 Introduction 1
1.1 River flows and floods --- 1
1.2 Challenges of flow resistance modelling --- 2
1.3 Floodplain flow resistance during high water levels--- 3
1.3.1 Bed roughness and form drag ...3
1.4 Problem statement--- 4
1.5 Research objective --- 6
1.5.1 Research questions...6
1.5.2 Research approach ...6
1.6 Thesis outline --- 6
Chapter 2 Theoretical background and literature review 9 2.1 Introduction --- 9
2.2 Turbulence and flow properties--- 10
2.2.1 Open channel flow ... 10
2.2.2 Turbulence... 11
2.2.3 Velocity distribution ... 11
2.3 Flow resistance--- 12
2.3.1 Bed and form resistance in the main channel ... 13
2.3.2 Bed and form resistance in the groyne field and floodplain ... 14
Groynes 14 Flow pattern around the single groyne and in the groyne field 18 2.4 Weir-like behaviour of groyne/summer dyke--- 23
2.4.1 Empirical approach... 26
2.4.2 Basic principles... 27
2.5 Special types of weirs --- 29
2.5.1 Broad and sharp crested weirs... 29
Weir discharge formulations 29 2.5.2 Oblique weirs ... 34
2.5.3 Vegetated weir-like obstacles ... 34
2.6 Generalized model for weir flow--- 36
2.7 The groyne/summer dyke as an obstacle --- 37
2.7.1 Yossef’s approach (Yossef, 2005)... 37
2.7.2 Azinfar’s approach ... 38
2.8 Discussion --- 39
xvi
Appendix 2A --- 43
Chapter 3 Flow resistance of vegetated weir-like obstacles during high water stages 47 3.1 Introduction --- 47
3.2 Theoretical frame work --- 49
3.2.1 Case-1; Weir- like structure ... 50
Bed and wall resistance from the measured data 52 Discharge over weirs 53 3.2.2 Case -2; Groynes with Vegetation... 54
Emerged vegetation 54 Submerged vegetation 57 3.3 Experiments --- 59 3.3.1 Experimental Set-up ... 59 3.3.2 Measuring Equipment... 61 3.3.3 Measurements ... 61
Energy head loss measurements 61 Velocity measurements 62 3.4 Results and discussion --- 62
3.4.1 Analysis of the skin friction ... 62
3.4.2 The discharge reduction coefficient (Cd) for submerged conditions... 63
3.4.3 Effect of downstream slope ... 65
3.4.4 Comparison of head loss for different types of pseudo vegetation on the weir 67 3.4.5 Measured vertical profiles of longitudinal velocity around a vegetated weir 70 3.4.6 Analysis of energy head loss using expansion loss form drag model ... 70
3.4.7 Error analysis ... 70
3.5 Conclusions --- 72
Chapter 4 Flow resistance of vegetated oblique weir-like obstacles during high water stages 73 4.1 Introduction --- 73
4.2 To determine discharge over a weir--- 74
4.3 Mathematical formulation (analytical aproach)--- 77
4.3.1 Weir with vegetation ... 82
Emerged vegetation 82 Submerged vegetation 83 4.3.2 Model results ... 83
Flow direction over the weir 83 Energy head loss 84 Comparison with Borghei et al. (2003) 85 4.4 Experiments --- 86
4.4.1 Experimental setup... 86
4.4.2 Measuring equipments ... 87
Point gauges 87 Laser displacement Sensors 87 Acoustic Doppler Velocimeter (ADV) 88 4.4.3 Experimental programme ... 88
xvii
Velocity measurements 88
4.5 Result and discussion --- 89 4.5.1 Non-Vegetated oblique weir ... 89 4.5.2 Comparison of energy head loss and discharge coefficient predicted by the expansion loss form drag model with experimental data for non-vegetated oblique weirs ... 91 4.5.3 Velocity direction on the crest of oblique weirs... 93 4.5.4 Comparison of the energy head loss caused by the vegetated and non-vegetated oblique weirs ... 94 4.5.5 Comparison of energy head loss predicted by the expansion loss form drag model with experimental data for Vegetated oblique weirs ... 95 4.6 Conclusions --- 97 Appendix 4A --- 98
Chapter 5 Flow modelling over weir-like obstacles 99
5.1 Introduction --- 99 5.2 Energy head loss and flow field--- 101 5.2.1 Numerical Modelling... 101
Basic equations 101
The standard and non-linear k-ε model 102 Description of numerical method and boundary conditions 103 5.2.2 Experiments... 105
Experimental set up 105
Numerical experiments 106
5.2.3 Results and discussion ... 107 Energy head loss and discharge coefficient 107 5.2.4 Flow field... 109
Velocity profiles 109
Reynolds stresses 111
Turbulent kinetic energy 113
Recirculation zone 115
Effect of the lee side slope 115
5.3 Undular hydraulic jump --- 117 5.3.1 Theoretical analyses... 117 1-D analytical model for undulations 117
Numerical modelling 121
5.3.2 Experiments (Undular Hydraulic Jump) ... 121 Experimental set up (Undular Hydraulic Jump) 121
Numerical experiments 121
5.3.3 Results and discussion (undular jump)... 123
Analytical approach 123
Numerical approach 126
5.4 Conclusions --- 128 Appendix 5A --- 129
Chapter 6 flow over oblique weir-like obstacles 133
6.1 Introduction --- 133 6.2 Experimental set up --- 135 6.3 Numerical experiments --- 135
xviii
6.4 Energy head loss and discharge coefficient --- 137
6.5 Flow direction--- 140
6.5.1 Velocity vectors and stream lines... 140
Flow near the free surface of flow 140 Flow near the bottom 141 6.5.2 Stream line angle on the weir crest... 142
6.5.3 Variation of two velocity components around the oblique weir... 145
6.6 Flow field--- 147
6.6.1 Flow velocity profiles ... 147
6.6.2 Turbulent kinetic energy... 150
6.6.3 Reynold’s stresses ... 152
6.6.4 Dynamic pressure ... 155
6.6.5 Bed shear stresses ... 156
6.6.6 Recirculation zone and spiral flow ... 158
6.7 Conclusions --- 161
6.7.1 Trapezoidal weirs (angle of obliqueness is 450, upstream and downstream face slope of weir is 1:4) ... 161
6.7.2 Sharp crested weirs (angle of the obliqueness is 450) ... 162
6.7.3 Broad crested weirs (angle of the obliqueness is 450) ... 162
6.7.4 Flow modelling ... 162
Appendix 6A --- 164
Chapter 7 Synthesis and conclusions 165 7.1 Summary --- 165
7.2 Conclusions --- 166
7.2.1 Physical mechanisms for the form drag ... 166
7.2.2 Inferences from the experimental data... 166
7.2.3 The expansion loss form drag model... 167
7.2.4 Numerical modelling of weir-like obstacles ... 169
7.3 Recommendations--- 171 References 173 List of symbols/Nomenclature 186 Roman symbols 186 Greek symbols 189 Acknowledgments 191 List of publications 193
Chapter 1
I
NTRODUCTION
1.1
R
IVER FLOWS AND FLOODSRivers, flowing downhill from their sources to their mouths are vital carriers of water, nutrients, sediments and sometimes ice. For centuries rivers are used as sources for en-ergy and provide important transport routes. Rivers and their fertile floodplains are used for agriculture and drinking water supply. Therefore since ages the surroundings of rivers are popular settling areas and now the river basins are densely populated.
Nowadays, the main task of the river manager is to take care that the water and sedi-ments in a river system are transported in such a way that, at the same time, people are protected against flooding and attention is paid to the provision of navigation, floodplain agriculture, ecology and recreation. The river system needs to be developed, maintained such that these functions in the area can be accommodated. Therefore the accurate pre-dictions of water level and bed levels in rivers are indispensable. To reduce the flood risk, reliable design of dikes that are high enough to protect surrounding areas is essential during periods of high water levels. Internationally, floods pose one of the most widely distributed natural risks to life. Between 1973 and 1997 an average of 66 million people a year suffered flood damage (IFRC, 1999). This makes flooding the most damaging of all natural disasters. The average annual number of flood victims jumped from 19 million to 131 million during the period 1993 -1997. Between 1985 and 2003, the death toll, ap-proximately was 3,000,000 as a result of river floods (Douben and Ratnayake, 2006). The recent high waters in the Netherlands, such as the floods in 1993 and 1995, have raised a number of matters in river flow and the public concern about the security against flooding. The 1995 flood was highest in the Netherlands since 1926. Approxi-mately, 250,000 people were evacuated. Worldwide, there is an increasing awareness that rivers require more room against the flood safety. From 1996 the policy “ruimte voor de rivier” had become effective. In the past it was standard policy to simply raise the dyke for protection against the flood. This century old policy was abandoned in 2000 in favour of the policy “room for the river”. The objective of this policy is to increase the capacity of the rivers, to decrease the impact of high waters and to lower the flood levels and at the same time improvement of overall environmental conditions. By the year 2015 the river Rhine should be able to safely discharge 16,000 m3/s. In the long term, however,
2 1. Introduction should the design discharge rise to 18,000 m3/s (expected by the end of this century). To achieve these aims the following measures have been identified as shown in figure 1.1(Fokkens, 2007).
1– Lowering the river floodplains. 2– Creation of secondary channels 3– Lowering of the groynes
4– Removal of hydraulic obstacles / summer embankments 5– Deepening of main flow channel
6– Dike strengthening /raising / repositioning 7– Narrowing the summer bed
8– Flood bypass
9– Creation of retention areas 10– Depoldering
11– To Develop the nature in the floodplain
Figure 1.1. Measures identified in the policy “Room for the river”.
1.2
C
HALLENGES OF FLOW RESISTANCE MODELLINGThe fundamental laws describing the flow behaviour are of complex nature, simplifica-tions are needed to describe the flow behaviour in complex large scale river system. Such simplifications limit the general validity of the flow model and range of applicability. Of particular importance in river flow modelling is the understanding of the determining factors of hydraulic resistance, in other words, to understand how topographical bounda-ries of the flow domain and their surface characteristics affect internal flow mixing pat-terns. Mixing patterns feed on kinetic energy from the mean streamwise flow field, thereby decreasing the overall flow velocity and causing higher water level at a certain discharge (Huthoff, 2007).
The hydrodynamic formulation to describe the relevant physical process in a river system is based on (simplified version of) the Navier-Stokes equations. Regarding that de Vriend
1.3 Floodplain flow resistance during high water levels 3 (2006) pointed out that the river flow modelling still lacks the generally applicable de-scriptions of the flow resistance of river reaches. The hydraulic resistance and its parame-terizations play an important role in the river models. The form and surface drag deter-mine the water level and flow velocities in the river reach. Therefore it is important to understand the relevant physical processes to parameterize the flow resistance, and the accuracy of the model predictions can be improved based on better definition of flow re-sistance.
1.3
F
LOODPLAIN FLOW RESISTANCE DURING HIGH WATER LEVELSFloodplains commonly become part of the river flow section during high-discharge condi-tions and the issue of hydraulic resistance of floodplains has always been important in river engineering. In river flow modelling, the determining factors of the hydraulic resis-tance are of great imporresis-tance. One can classify the flow resisresis-tance in a river system dur-ing high water stages as;
1.3.1 Bed roughness and form drag
The hydraulic resistance comprises all the sources of resistance in the flow path as de-scribed above such as; resistance due to obstacles like ditches, groynes, summer dykes, bridge piers, access roads and bed forms (Dunes/ripples), resistance due to vegetation in the floodplain and bed roughness.
Groynes Field
Floodplain
− Bed roughness (Grain friction/skin friction)
− Vegetation • Grass • Bushes/trees − Obstacles • Groynes • Vegetated Groynes Main Channel − Bed roughness (Grain friction/skin friction)
− Obstacles
• Dunes /Ripples
− − Bed roughness
(Grain friction/skin friction)
− Vegetation • Grass • Bushes/trees • − Obstacles • Ditches • Summer dykes
• Vegetated Summer dykes
• Bridge piers
• Access roads
Contributions to flow resistance for river reaches during high water stages
4 1. Introduction Grain friction is a function of the representative grain size of the bed material, however form roughness stems from pressure differences over disturbance to the flow path due to the obstacles and vegetation.
The resistance to the flow due to vegetation has received more attention and studied ex-tensively. The same holds for simple weirs but the combined effect of submerged vege-tated dikes and groynes has not yet been studied in depth. Floodplain features (summer dike, access road and groynes) could be schematized as weir or as drag generating obsta-cles in the flow. Many researchers investigated weir properties; Such as Rehbock (1929) Villemonte (1947), Chow (1959), Henderson (1966), Govida Rao and Muralidhar (1963), Swamee (1988), Lakshmana Rao (1975), Abou-Seida and Quraishi (1976), Chanson (1999), Jain (2001). Yossef (2005) considered a groyne as an obstacle and concluded that to reduce the effect of the spur dikes on flood level, their height should be decreased. Az-infar and Kells (2009) develop a relationship for the drag coefficient of a spur dyke. The effects of submerged and emerged, rigid and flexible vegetation on the flow have been studied by Kouwen et al.(1969), Kouwen and Unny (1973), Li and Shen (1973), Nepf (1999), Kouwen and Fathi-Moghadam (2000), López and García (2001), Järvelä (2004) and Baptist (2005).These studies are providing the information mainly about the drag coefficients of rigid and flexible vegetation, impact of vegetation on depth average flow velocities, effect of flexibility of vegetation on flow characteristics and drag resistance, however the underlying physical processes are not well understood so the application of these methods in general is limited.
1.4
P
ROBLEM STATEMENTFor the proper prediction of the flow through the floodplains, it is important to account for the important processes that contribute to the flow resistance. A significant portion of the energy loss is found in the flow over weir-like structures which get submerged at high water stages. In addition to the bed friction that is found everywhere, the rapidly varying flow over, for examples groynes, access roads, and summer embankments cates the proper estimation of the conveyance capacity. It becomes even more compli-cated when the elevated parts of the bed are covered with high vegetation in the form of bushes or trees. The flow that is already accelerated above the crest of the groyne or dyke experience a further acceleration passing between the vegetation with the interac-tion giving rise to addiinterac-tional fricinterac-tion. In case of high water the resistance of a river reach is for an important part governed by the bed forms and obstacles in the floodplain. The obstacles can consist of plain grass lands, bridge piers, summer dykes, ditches, bushes etc. In the main channel bed forms are subjected to changes as a result of high discharge and associated sediment transport varying from small scale ripples of typical dimensions of 0.1 m dimensions to river dunes with length scale up to tens of meters. Located be-tween the main channel and the floodplain, groynes play a prominent role. They can be considered as a weir when submerged at moderate water levels. At high waters however groynes play a minor role and can effectively considered as a kind of bed roughness. The formulations regarding flow resistance, as they are implemented in current models, ap-pear ambiguous and insufficient. Depending on water level (local Froude number) and
1.4 Problem statement 5 resolution, the modelled bed properties are weir-like and/or roughness like. Additional research is needed in order to come to an approach that has a sound physical and mathematical basis.
Furthermore, the consistent behaviour of predictive models should be guaranteed even in cases where no calibration data is available. It is especially related during extreme river discharges for which the design height of winter dykes is derived and for which costly op-erations are planned, often based on model predictions. Measures that will significantly change the flow in a certain river reach can by definition not be calibrated for in ad-vance. Validity of current systems is limited especially where it concerns the underlying physics of rapidly varying non-hydrostatic flows. Yet, extensive calibration has made the systems reasonably accurate for current situations, but it is obscuring the deficiencies in modelling approach. These deficiencies are very relevant in cases of high water predic-tions where the modelling uncertainties are highest and at the same time the impact is huge. This dangerous combination can be avoided by an extension of knowledge of un-derlying physical processes and the associated modelling techniques.
Most of the measures that are taken in view of flood prevention are evaluated using nu-merical models that cover large river reaches. It is assumed that a model, performing well for a given situation will give also reliable data for a new situation. Currently used models solve for the depth averaged water motion with a parameterization of all proc-esses related to the vertical motion. Typically aspects like bathymetry and small-scale roughness are straightforwardly implemented. Problems arise when the domain contains elements that affect the flow in a more profound way. Flow characteristics can for exam-ple be dramatically affected by the presence of a weir-like structure. The sudden change in bathymetry and associated non-hydrostatic effects contradict the underlying assump-tions of depth averaged flow formulation while also the required resolution for these typi-cally small-scale features in a large scale model can become problematic. Critical flow and excessive energy dissipation may occur locally but with significant consequences for the flow in the whole domain. In most low land rivers, flow over the winter bed is much more complicated than that over the summer bed. There are many obstacles, and land forms which are found in floodplains. An accurate prediction of flow resistance put high demands on modelling systems. Large dimensions of the domain under consideration combined with the restricted computer capacity result in numerical simulations with a coarse resolution where details of the bathometry can not be resolved sufficiently. Unre-solved details like trees, bushes, weirs and ditches can contribute significantly to the flow resistance and can even affect the flow pattern. In current modelling approaches these ef-fects are captured in a formulation with a limited physical basis. These limitations are often obscured due to extensive calibration.
6 1. Introduction
1.5
R
ESEARCH OBJECTIVEThe main objective of this study is to quantify the hydraulic resistance caused by the vegetated weir-like obstacles in the floodplain during high water stages.
1.5.1 Research questions
Inline with this context, the following research questions are identified;
Q1: How to quantify the form drag due to the perpendicular weir-like structures in the floodplain during high water stages?
Q2: How to quantify the form drag due to the oblique weir-like structures in the flood-plain during high water stages?
Q3: How to quantify the flow resistance and form drag due to the vegetated weir-like structures in the floodplain during high water stages?
Q4: To obtain a better understanding of the physical mechanisms and processes, which are relevant to the form drag due to the vegetated weir-like structures in the flood-plain?
Q5: To evaluate the application of a Computational Fluid Dynamics (CFD) model to predict the flow resistance and what are the limitations of these methods and CFD model?
1.5.2 Research approach
This study focuses on the correct representation of resistance elements in a numerical (depth averaged) simulation, as well as consistent behaviour of models towards high wa-ter stages. A number of processes related to vegetation, weirs and bed forms are studied in order to come to an improved implementation of the physical processes. In order to find out in which way the various types of vegetation contribute to the flow resistance, a series of experiments is performed with various schematized types of vegetation placed on top of a standard weir with a hydraulically rough surface.
A series of numerical simulations has been performed to evaluate the performance and limitations of the CFD models. The numerically predicted results are compared with the experimental data.
1.6
T
HESIS OUTLINEIn chapter 2 of this thesis a short literature review about groynes in general and back ground information are presented. The basics of flow in open channels, the ways of repre-sentation of the hydraulic resistance caused by the groynes are discussed. The behaviour of groynes can be considered as a weir-like or as an obstacle during flow over it. The dif-ferent formulations available in the literature are compared and discussed.
1.6 Thesis outline 7 Chapter 3 presents how to quantify the form drag due to perpendicular vegetated weir-like obstacles. In this chapter, the experimental set up and the procedure is explained. Moreover the theoretical formulation has been developed to estimate the form drag and the results are analyses and compared with the experimental data. Chapter 4 focuses on the form drag caused by the oblique vegetated weir-like obstacles. The experimental set up, procedure and theoretical formulation for the oblique vegetated weir-like obstacles are presented and the resulted are analysed.
Thereafter the numerical modelling of the flow field within the vicinity of the weir-like obstacles (non-hydrostatic and non-linear k-ε turbulence modelling approach) is dis-cussed in chapter 5 and 6. In chapter 5 the perpendicular weir-like obstacles are mod-elled using a 2-dimensional model (2DV), subsequently in chapter 6, the oblique weir-like obstacles are modelled using a 3- dimensional model. The numerically predicted energy head losses and the associated flow fields are compared with the experimental data for the perpendicular and the oblique obstacles in chapter 5 and 6 respectively.
Finally chapter 7 reflects on the research questions and the main conclusions of this dis-sertation. The answers to the research questions are summarized and supplemented with recommendations for future research.
Chapter 2
T
HEORETICAL BACKGROUND AND LITERATURE REVIEW
2.1
I
NTRODUCTIONDuring high water stages and floods in rivers the water flows over groyne fields and floodplains. Many hydraulic structures (protection dikes, summer dikes/winter dikes), transportation constructions and other landscape features in the floodplain might be in-undated. Some of these structures are also covered by vegetation and despite of the use-fulness of these structures, affect the flow conveyance capacity of the rivers and the asso-ciated water levels. This situation has consequences for the flood protection such as dikes and ultimately for life, livestock, precious property and industry behind the flood protec-tion structures. Studies show that, over the past century, flood stages for the given dis-charges at various locations along the Middle Mississippi and Lower Missouri rivers have increased by 2 m to 4 m (Criss and Shock, 2001). For the design of flood protection works, the water levels for the design flood are determined by the computer simulations. Mostly these simulations are performed based on 1-dimesional or 2- dimensional com-puter models based on shallow water equations however these models required pre-estimated bed friction coefficient. Normally in practice this requirement is fulfilled based on experience and the empirical formulae which are available in the literature. These formulae are derived from the laboratory and the field data, although the results are use-ful, however, in some cases they can be inaccurate and a possible threat to flood safety. Here the question arises how to estimate the bed resistance in the floodplain and the groyne field caused by these complicated structures. The complexity further increases as effects of vegetation need to be included.
This chapter gives an overview of the available knowledge on the open channel flow, bed forms, groyne field, floodplain and the hydraulic resistance in these flow regions.
10 2. Theoretical background and literature review
2.2
T
URBULENCE AND FLOW PROPERTIES2.2.1 Open channel flow
Figure (2.1) shows the uniform flow in a rectangular horizontal open channel and the forces acting on the fluid element between cross-sections 0 and 1. One can write the force balance per unit of width between the cross-sections 0 and 1 as;
0 1 2 2 1 1 0 2 2 s q u gd gd L ρ ∆ = ρ − ρ −τ (2.1)
where q is the constant specific discharge, ∆u is the mean velocity difference, ρ is the fluid density, g is gravity acceleration, τ0 is the bed shear stress and Ls is the channel length between two cross-sections.
The energy balance between cross-sections 0 and 1 can be written as (Chanson, 1999);
2 2 0 1 0 2 1 2 0 1 2 2 q q d d gd gd α α + = + +∆H (2.2)
where α0,1 is the kinetic energy correction factor and ∆H is the energy head loss between sections 0 and 1. The specific energy (E) for a particular section is; 22
u g
d+α and the varia-tion of specific energy with respect to the water depth can be expressed as;
2 1 r E F d ∂ = − ∂ (2.3)
here Fr is the Froude number and is an important parameter in the open channel flow which can be defined as; r
u F
gd
= . Depending on the Froude number, the flow can be classified as subcritical (Fr<1), critical (Fr=1) and super critical (Fr>1).
2.2 Turbulence and flow properties 11
2.2.2 Turbulence
Turbulence is the most prominent property of the flow because most flows encountered in the nature and engineering applications are turbulent as with rivers, floodplains and channels. Degree of turbulence of the flow is reflected by the Reynolds number; Re uLk
υ
= ,
Lk is the characteristic length and υ is the kinematic viscosity of fluid.
If the viscous forces dominates the inertial forces, Reynolds number is low (<1000) and the flow is laminar. At high Reynolds numbers (<4000), the inertial forces become domi-nant and the flow becomes turbulent. The turbulence consists of random velocity fluc-tuation so it must be treated statistically. It is important to distinguish between the mean motion (u) and the fluctuating component ( u′) to describe the turbulence and it is called Reynolds velocity decomposition; u u u= + ′ and 1
1 N n N n u u =
=
∑
. In case of the sta-tionary flow, the time average and the ensemble average become equal, so u is the en-semble average. Using this velocity decomposition in the Navier Stokes equations we can obtain the Reynolds averaged Navier Stokes equations. These equations contain extra terms which are called the Reynolds’s stresses (u ui′ ′j). The measure of the magnitude ofthe velocity fluctuations about the mean value is called the intensity of turbulence.
2.2.3 Velocity distribution
In this section some characteristics of open-channel flow over the rough and smooth bed are discussed. Hydraulically rough and smooth flows are characterized by the ratio of Ni-kuradse’s equivalent roughness (ks) and the length scale of the viscous sublayer (υ/u*). here υ is the kinematic viscosity and u* is the shear velocity. Generally ks is related to the grain size and shape. The vertical distribution of streamwise velocity in the turbulent open channel flow is quite complicated, however the logarithmic law is widely used for uniform flow over the full depth for the smooth and the rough bed (Figure 2.1) and it reads 1 * 0
ln
vu
z
u
=
κz
(2.4)where u is the streamwise velocity, 0 *
u = τ ρ is the shear velocity, κv is the von
Kar-man constant (≈ 0.4), z is the vertical coordinate and z0 the zero-velocity level. No defi-nite standard is available for z0, it is an empirical parameter without physical meaning. In case of the hydraulically smooth regimes, z0= 0.11υ/u* and for the hydraulically rough regimes, according to some researches (Grass, 1971, Nakagawa et al., 1975) its values varies from 0.15ks to 0.30ks. van Rijn (1994) approximated it as, z0 = 0.25ks for the sand and gravel bed.
The shear stress at height z in a turbulent steady uniform flow can be described as;
u
u w
z
τ
=
ρυ
∂
−
ρ
′ ′
12 2. Theoretical background and literature review The term u wρ ′ ′ represents the Reynolds’s shear stress, u′is the velocity fluctuating component in longitudinal direction and w′represents the velocity fluctuating component in vertical direction. In most of the open channel flow cases, the viscous shear stress
( u
z
ρυ∂ ∂ ) is much smaller than the Reynolds’s shear stress and can be neglected. There-fore the bed shear stress can be expressed as;
0
u w
τ
=
ρ
′ ′
For the open channel flows, the equation 2.1 can be written as; τ0=ρgdSb
and 2
0 cf u
τ = ρ , where Sb is the channel bed slope and cf is the friction coefficient. The friction coefficient cf is related to the friction factor (f =8cf), (f is related to the skin fric-tion) which in turn is estimated by the Colebrook-White formula (Colebrook, 1939);
10 1 2.51 2.0 log 3.71 s H e k D f R f = − + .
here DH is the hydraulic diameter.
2.3
F
LOW RESISTANCEThe hydraulic roughness is important to predict the flow depth and the sediment trans-port rate in the alluvial channels. It is also the function of flow depth, flow velocity, sediment properties (diameter and gradation) and the bed form types and dimensions. Bed resistance is the total drag force exerted on the flow by the channel bed (Vanoni and Hwang, 1967). According to Einstein and Barbarossa (1952), the bed resistance can be divided into two components:
• Skin friction (grain friction)
• Form drag
The skin friction is a part of the total resistance caused by the individual grains on the river bed. The form drag is the resistance to flow due to the energy loss in the recircula-tion zone on the lee side of bedforms (van der Mark, 2009). Figure (2.2) explains the two components of the flow resistance in a channel. If bedforms are not present and the bed is flat, the flow resistance is only caused by the skin friction otherwise both components contributes to the flow resistance.
2.3 Flow resistance 13
2.3.1 Bed and form resistance in the main channel
To predict the bed and form resistance in channels with sufficient accuracy is quite diffi-cult. Mostly the flow conditions in the channel undergo the changes as a consequence bed configuration also changes. The flow in an alluvial river generates various types of the bed features. These features vary from a few centimetres to several metres height moreover they are quite irregular. Their height and length could only be quantified in a statistical manner. The bedforms can be classified as:
− Small features associated with fine to medium sand are ripples, their height and wave length is considered not to be related to the flow depth. The height is much smaller than the flow depth and the length may be of the order of the flow depth.
− The dunes are large two-dimensional or three-dimensional features. Their dimen-sion depends on the flow depth, the height may be up to the flow depth and the length is much larger. The formation of dunes may be due to the large scale in-stabilities. Due to presence of large scale eddies, there is a decrease and increase in the bed shear stress, so the deposition and the erosion of the bed occur and the dunes formed. During increased stages, particles go into the suspension and the washing of dunes occurs. Ripples can also be present on the top of the dunes.
− The anti-dunes are found in the upper flow regimes. These features are mainly governed by the free surface phenomena. The length of anti dunes is equal to the wave length of the free surface.
− The bedform regimes for the steady flow over sand beds can be roughly classified into three categories. The lower transport regime with flat bed, ripples, dunes, and bars, the transitional regime with wash-out dunes and sand waves and the upper transport regime (Froude number >0.8) with flat mobile bed and anti-dunes.
The values of the friction factor in the sand bed rivers depend primarily on the bedforms configuration which may change from the plane bed to ripples and dunes. During high water stages, the bed form configuration changes are more pronounced. The bed rough-ness due to the bedform could be represented by following two methods, the Nikuradse roughness height formulation and the expansion loss form drag model.
Some empirical formulae for roughness height ks due to bedforms are given below;
Swart (1976) described the roughness height for the bedforms using the following rela-tionship; 25 s l k = δδλ
where δ is dune height and λl is the dune length.
van Rijn (2007) used the superposition principle to get a total roughness height (ks) in the fluvial river bed for the different features such as; grain roughness (k's), Dune rough-ness height (k"sp) and Ripple roughness height(k"ss);
' '' ''
s s sp ss
14 2. Theoretical background and literature review where ' 9 0 s k = d ,
k
sp''1.1
lp(
1
e
25 p)
ξλ
−=
−
, ''1.1
(
1
25 s)
ss lsk
=
λ
−
e
− ξ , ξs is the ratio of the dune height(δ) to the dune length (λl), d90 is the grain diameter for 90% passing per-centage. Subscript “p” for primary features (dunes) and “s” for secondary features (rip-ples).Yalin (1964) and Engelund (1966) considered the effect of bed forms on the flow analo-gous to the sudden expansion in the pipe flow. A simplification of the momentum con-servation equation, the Carnot equation, is proposed by Engelund & Hansen (1967) to predict the form drag caused by the bed forms. An expansion loss form drag model based on fundamental principles, energy and momentum balance has been proposed by van der Mark (2009) for the estimation of the bed form resistance in the alluvial rivers.
2.3.2 Bed and form resistance in the groyne field and floodplain
Groynes
Groynes are hydraulic structures (small dams) which are constructed at an angle to the flow generally protruding from the bank into the river. They are made of stones, gravels, rocks, earth or piles, beginning at the riverbank with a root and ending at the regulation line with a head. They perform one or more functions such as to prevent the river bank erosion, to maintain and train the desired channel for the purpose of flood control and navigation and to improve the ecological environment and the scenery. According to
Beckstead (1975), (as reported by Przedwojski et al. 1995 and Yossef, 2002) the groynes
can be classified according to;
Methods and material of construction:
The groynes may be permeable or impermeable. The standard groyne as it is used in the large European rivers is typically an impermeable structure made of rubble mound ori-ented almost perpendicular to the river axis. The distance between the successive groy-nes varies between one and three times its length. The area between the groygroy-nes, the groyne field, is partially filled with sediment. The height of the groyne crest is chosen close to or slightly above the yearly averaged water level. An overly high crest can lead to an unnecessarily high flow resistance at high water levels whereas a low crest de-creases its capacity of confining the flow at lower stages (Uijttewaal, 2005).
The permeable groynes are constructed from piles (bamboo or timbers) however the im-permeable groynes are constructed from rocks, gravels and gabions.
Classification according to the submergence:
The groynes may be designed according to the nominal flow condition and can be sub-merged or esub-merged hydraulic structures. Usually the impermeable groynes are designed as emerged whereas submerged groynes are sometimes permeable to avoid the strong ac-celeration of the flow over the top which enhances erosion.
2.3 Flow resistance 15 Groynes may attract, repel or deflect the steam flow depending on their orientation rela-tive to the main flow direction. The attracting flow groynes point downstream and serve to attract the flow towards them. The repelling flow groynes point upstream and serve to repel the flow away from them. Deflecting groynes are generally short and used for local protection (Figure 2.3).
Appearance plan view:
These may be of various shapes, straight groynes, T-head, L-head, hockey typed, in-verted hockey stick groynes, straight groynes with pier head, wing or tail groynes. The straight groyne is set at an angle from the bank and has a rounded head to provide the extra volume and area for scour protection at outer end. The T-head groyne is normally set at a right angle from the bank and it has a straight shank with a rectangular guide vane at the outer end. L-head, wing or tail groynes provide great protection to the banks and are more effective to channelization for navigation. Hockey-shaped groynes have scour holes that are more extensive in area than the T-head groynes (Figure 2.3 & 2.4). The length of groynes can be established by determining the channel width and depth desired. The working length is usually kept between the lower and upper limits of the mean depth and a quarter of the mean width of the free surface respectively. In order to obtain a well defined deep channel navigation, to keep a spacing of 1.5 to 2 times the groyne length is recommended, where as for bank protection the ratio of spacing to groyne length is less and distances from 2 to 6 times the groyne length are generally used, although there exists successful examples of bank protection with short groynes spaced apart 10 to 100 times their length where the banks are protected with riprap or vegetation (Yossef, 2002).The formation of scour holes should be taken into consideration for the design of the base depth of the groynes
Several terms are used for these structures in the literature, such as groynes (or groins), spur dikes, wing dams, transverse dikes, cross dikes, cross dams, navigational dikes, jet-ties, bend way weirs and barbs etc. (e.g., USACE, 1980; Rajaratnam and Nwachukwu, 1983; Richardson and Simons, 1984; Ouillon and Dartus, 1997; Pinter et al., 2001) how-ever in the Netherlands usually the term groynes is used but the most popular term is the spur dikes. Figure 2.5 shows some examples of the river groynes fields in the Nether-lands and some other countries.
16 2. Theoretical background and literature review
T-headed
L-headed Hockey stick hockey stickInverted
Backward inclined Straight Forward inclined Forward kinked Backward kinked Flow φ Inclination angle (φ)
Figure 2.3. Different types of groynes in practice (Przedwojski et al. 1995).
b).
a).
Figure 2.4. (a) Extended groyne heads near Sint Andries (River waal), (b) Extended groyne heads (Pannerdensch canal), (Talstra, 2011).
2.3 Flow resistance 17
Figure 2.5. Groynes fields; Waal River, the Netherlands (Upper), (http://en.wikipedia.org); Mis-souri River, North of Saint Louis (Lower Left), ( Azinfar, 2010); Odra River in Poland ( Lower Right), (Criss, 2002).
18 2. Theoretical background and literature review
Flow pattern around the single groyne and in the groyne field
An understanding of the flow field and the flow structures in the groyne field is useful for explaining the flow conveyance capacity and the flow hindrance caused by the groyne or other such type of structures in the floodplain. Based on the flow condition, a groyne may operate in either a submerged condition or an emerged condition. During a normal flow situation in the river, the groyne is either submerged or emerged while during high water stages the groyne field is submerged. For emerged conditions, the flow only passes around the groyne however for submerged conditions the flow passes over the groyne as well as around the structure.
The flow field for the emerged groyne is mostly approximated as two-dimensional flow while for the submerged groyne case the flow is fully three-dimensional. In the groyne field, the effect and interaction of the groynes on each other can not be ignored but that also depends on the spacing between the groynes.
The characteristics of the flow field around a groyne of emerged type have been investi-gated by many researchers, e.g. Ishii et al. (1983), Chen and Ikeda (1997), Ouillon and Dartus (1997),Uijttewaal (2005), and Yossef (2005). Such studies gave an impression about the geometry of the separation region downstream a groyne in a rectangular chan-nel. The flow field can be divided into the four sub regions such as; main flow zone, re-turn flow zone, shear layer and reattachment zone (Figure 2.6). The flow in the main channel from the tip of the groyne on one bank to the tip of the groyne on the opposite bank is called the main flow zone and in this zone the flow is accelerated because of the contraction of the main channel width. High velocity in the main channel causes channel degradation due to scour and as a result an increase in the thalweg depth. The return flow zone is located at the downstream side of the groyne. The flow inside the groyne fields shows the circulation pattern with two large eddies as reported by Uijttewaal et al. (2001). The primary eddy forms in the downstream part of the groyne and covers nearly 2/3 of its spacing. The secondary eddy is also there, driven by the primary eddy and with an opposite sense of rotation and with a much smaller flow velocity. The dynamic eddy that sheds regularly from the tip of the upstream groyne migrates in downstream direction and merges with the primary one, which in return changes in size due to the in-teraction with migrating eddy. After this eddy has travelled enough distance from tip of the groyne another eddy has room to form. This whole circulation pattern is driven by the velocity difference between main stream and groyne field via momentum exchange through the mixing layer which is found between the separation point at the groyne tip and the reattachment point (Figure 2.6(a)).
The reattachment point is defined as a point at which the separation stream line reat-taches to the channel boundary. The reattachment point fluctuates instantaneously due to the fluctuations in the shear layer and the pressure gradient between the different flow zones. Chen & Ikeda (1997) observed that the length of the reattachment zone is almost constant and cover a range from 11 to 17 times the length of the groyne. Klingeman et al. (1984) reported six types of flow patterns that develop between the groynes as the spacing/length ratio between them increases (Figure 2.7 (b)).
Type one: This type of groynes pattern has small ratio. Main flow is deflected outside the groyne field and well developed eddy is appears between the groynes.
2.3 Flow resistance 19 Type two: This type of groynes pattern also has small ratio. In this type, a second eddy also appears. The main flow is still deflected outside the groyne field
Type three: The spacing between the groynes increases; the main flow is directed into the groyne field creating a stronger eddy near the upstream groyne and greater turbu-lence along the upstream face.
Type four: The spacing between the groynes further increases and the upstream eddy is washed out and a single strong reverse current arises.
Type five: the flow that is directed by the upstream groyne is directed to the bank in the groyne field. Eddies form on both side of this flow providing some protection to the bank.
Type six: The spacing between groynes is the largest. The flow attacks the bank di-rectly, as the downstream eddy no longer provides protection to the bank.
Figure 2.7(a) illustrates the influence of the geometry on the mean flow pattern and shows that how the primary and secondary gyres are varying in size for different geome-tries. Comparing the case 5-6 to case 2A, shows that the mean flow pattern not substan-tially influenced by the presence of a single upstream splitter plate. The steady gyre pat-tern shifts in downstream direction, over a distance approximately equal to the splitter length. In cases 7-8, the total reattachment length shortens compare to reference case 2A. Cases 9 and 10 compared to Case 2A show the impact on the steady gyre pattern when the recirculation area is confined in streamwise direction by a single groyne. Many other researchers investigated the separation zone and its characteristics such as; Ishii et al. (1983), Rajaratnam and Nwachukwu (1983),Brolsma (1988),Tingsanchali & Maheswarn (1990),Termes et al. (1991), Przedwojski et al. (1995), Ouillon & Dartus (1997), Uijttewaal (1999), Sukhodolov et al.(2002), Haltigin et al. (2007), Marson et al. (2003), Miller et al. (2003), Koken and Constantinescu (2008), Weitbrecht et al. (2008) and Talstra (2011).
a). b).
Figure 2.6. (a) Plan view of an emerged groyne, showing different flow zones (Azinfar, 2010), (b) Cross- section of a submerged groyne.
20 2. Theoretical background and literature review In case of the submerged groynes, the flow does not show the recirculation pattern as in case of emerged groynes but there is a low velocity region in the groyne fields. The veloc-ity in the groyne field is less than the velocveloc-ity in the main channel, so due to the velocveloc-ity difference there is a mixing layer in between main channel flow and groyne field flow. There are a lot of variations in velocity patterns in the groyne field with different sub-mergence levels.
In case of the submerged groynes the flow is complex due to the three-dimensional na-ture. Although limited studies have been reported in this regards some studies are re-ported in the literature; Aya et al. (1997), Peng et al. (1997), Krebs et al. (1999),Tominaga et al. (2001), McCoy et al. (2007)and Abad et al. (2008). Aya et al. (1997), mentioned a sharp drop in the water level between the upstream and downstream sides of the groynes, this means that the water surface slope between two adjacent groy-nes is less than the slope in the main channel. Tominaga et al. (2001) reported the two kinds of vortex namely vertical axis and the transverse axis vortex in submerged groyne flow conditions. Peng et al. (1997) compared three-dimensional numerical results with experimental results and observed the variation of the size of the vertical vortex over the depth and reattachment length. At the upstream face of the groyne, the flow shows an upward motion because of the blockage effect of the groyne. Krebs et al. (1999) reported the same features such as the secondary flow structure in case of submerged groyne. Abad et al. (2008) discuss the suppression effects of the jet on the vertical axis vortex in the groyne field. McCoy et al. (2007) numerically investigated the flow pattern and the turbulent flow characteristics in the submerged groyne field (Figure 2.7 (c)). Peng et al. (1997) discuss the effect of the spacing between the groynes on the reattachment zone and the bed shear stress to the flow.
The hydraulic resistance due to groynes in the groyne field and other engineering struc-tures in the floodplain such as summer dykes and access roads, during high water stages (submerged flow conditions) could be represented in two ways;
− The groyne/ summer dyke are considered as a weirs (Sieben, 2010)
2.3 Flow resistance 21
Figure 2.7(a). Simulated flow patterns for different configurations of emerged groyne fields (Tal-stra, 2011).
22 2. Theoretical background and literature review
Figure 2.7(b). Different types of flow pattern in an emerged groyne field (Klingeman et al., 1984).
Figure 2.7(c). Flow pattern and vertical axis vortices in a submerged groyne field (McCoy et al., 2007).
2.4 Weir-like behaviour of groyne/summer dyke 23
2.4
W
EIR-
LIKE BEHAVIOUR OF GROYNE/
SUMMER DYKEA weir is one of the most common and simple hydraulic structures that has been used for centuries by hydraulic engineers. They can be used for various purposes like energy dissipation, flow measurement and diversion, regulation of flow depth and many others. Many obstacles in a floodplain can act as a weir e.g. summer dykes, groynes etc. The groyne field could be considered as a channel with a groyne over the full width of the channel. In case of a submerged groyne, it is like a river with a weir. A weir also leads to increase of the upstream water level like a groyne.
Kolkman(1989) classified the three flow regimes over a weir as follows;
• Free flow regime (perfect flow conditions): The flow is independent of the down-stream water depth and there is a hydraulic jump downdown-stream of the weir.
• Submerged flow regime: The water surface is almost horizontal and there is a re-circulation zone behind the weir.
• Transition flow regime: The water surface is undulating downstream of the weir and also a recirculation zone is present.
Following figure (2.8) shows the different flow regimes
Figure 2.8. Flow regimes over the weir, Free flow regime (top), Submerged flow regime (middle) and Transition flow regime (bottom).
24 2. Theoretical background and literature review Escande(1939) classified the flow regimes into following four types;
− Hydraulic jump; with a clear bore located downstream of the weir structure. The flow is not influenced by the downstream water depth.
− Plunging jump; with main stream following the downstream slope of the weir and with a clear surface roller. The flow is almost independent from the downstream water depth.
− Surface wave flow; with main stream following the free and wavy surface and a recirculation zone near the bottom.
− Surface jet flow; flow depth is larger. Surface is nearly horizontal and smooth. The flow strongly depends on the downstream water depth.
The surface wave flow regime also called the transition flow regime; this also has the re-verse flow behind the weir (recirculation). This wavy flow is called undular hydraulic jump.
The energy loss in the flow over the weir is due to incomplete conversion of the kinetic energy into the potential energy. Upstream of the weir, in the acceleration zone, part of the potential energy of the flow is converted into kinetic energy. While the water is flow-ing, it is continuously losing energy against the surface resistance but this loss is small especially in comparison with the form resistance loss at the downstream side of the weir. The mechanism of the form resistance is that part of the kinetic energy of the flow is converted into the energy of vortices of all scales and turbulent fluctuations. Subse-quently this supply of energy will eventually be dissipated into heat via viscosity. The following figure (2.9) illustrates different zones in the flow over a weir according to Hoff-mans (1992).
The discharge values can be used to calculate the water depth and the velocity of flow. Knowing the water depth and the velocity of flow up- and downstream of the weir, the decrease in energy height can be calculated. The discharge itself strongly depends on the downstream water level (for submerged flow regime) and the weir geometry.
One of the most important parameter for the flow over the weir is the energy head (H) upstream and downstream of the weir. The energy head loss (∆H) caused by the weir is the difference between the upstream and downstream energy head of the weir.
