Modeling the interaction of wave hydrodynamics with flexible aquatic vegetation

123 Pages·2015·2.21 MB·English

Modeling the interaction of
wave hydrodynamics with
flexible aquatic vegetation
“Processing experimental data and development
and validation of the 1DV-Dynveg model.”
Joost W. Do¨bken
June 2015
M.Sc. Thesis Committee
Delft University of Technology Prof.dr.ir. Wim Uijttewaal,
Faculty of Civil Engineering & Geosciences Delft University of Technology
Section Environmental Fluid Mechanics Dr.ir. Rob Uittenbogaard, Deltares
Dr.ir. Niels Jacobsen, Deltares
Dr. Zhan Hu, Delft Univesity of Technology
Ir. Arnold van Rooijen, Deltares &
Delft University of Technology

Abstract
Seagrasseshavethecapabilitytodampwave,increasetheuptakeofnutrientsfromthewatercolumn,
provide a habitat for valuable ecosystems and result in sedimentation and stabilization of the bed.
Although there is understanding of how stationary flow is affected by vegetation and vice versa, the
interaction between wave hydrodynamics and (flexible) aquatic vegetation is hardly studied. Most
literature focuses directly on the attenuation of the wave height, but these methods lack a physical
basis since the stem scale processes are not considered.
The main research objective of this thesis is to strive to a more fundamental approach of the in-
teraction between wave-hydrodynamics and flexible vegetation by determination and description of
physical phenomena and processes. The interaction is divided into two one-way couplings: the wave-
hydrodynamicsaffectedbyanarrayofrigidcylindersandthewave-inducedmotionofasingleflexible
stem. The interaction is studied by wave flume experiments and with the development of the 1DV-
Dynveg model for simulating the motion of flexible vegetation in periodic wave conditions.
Two unique flume experiments are performed to gain insight in the physical phenomena of the two
one-waycouplings. Velocitiesinanarrayofsubmergedrigidcylindersaremeasuredinwaveconditions
by Hu et al. (2014) to determine wave hydrodynamics. The wave-induced motion of flexible plastic
stems, and forces on the base of the stems, is obtained from another wave flume experiment.
The measurements are used to develop and validate the 1DV-Dynveg model. The model consist
of a wave model for wave–current–turbulence interactions of periodic non-breaking surface waves
propagating over a flat horizontal bed, equivalent to the model presented by Uittenbogaard et al.
(2001)andUittenbogaard&Klopman(2001),andadynamicvegetationmodelsimulatingthebending
oftheplants,similartothemodelpresentedbyDijkstra&Uittenbogaard(2010)forflexiblevegetation
in stationary flow. The model is further developed with inertia forces on the momentum equations
for orbital motion, implementing orthogonal canopy permeability, description of periodic non-linear
waves, including orbital advection in the two-equations of the applied k-ε model and the dynamic
pressure profile, derived with wave–vegetation interaction forces.
Measurements and model results of wave hydrodynamics in and over rigid vegetation reveal reduced
in-canopy velocities with phase leads compared to the surface elevation. A mixing layer is generated,
moving in and out the canopy, with strong velocity gradients. The height and motion of the mixing
layervarieswithcanopycharacteristicsandwaveconditions. Periodaveragedstreaminginthemixing
layer is observed in the wave propagation direction with a large contribution of the Stokes’ drift. In
order to compensate the Stokes’ drift, a return current is observed above the canopy.
The wave-induced motion and the base forces on and of single flexible stems are determined by both
flume experiments and the 1DV-Dynveg model. The test cases of the experiment are derived from
natural conditions by stem dimensions and wave conditions. Forces on rigid cylinders are in general
proportional to the squared flow velocities. Due to lower relative velocities between the stem and the
flow, lower base forces are observed with flexible vegetation. Streamlined positions are observed with
larger orbital velocities, reducing the base forces, even with relative stiff stems. Moreover, lateral
motionandtwisting ofveryflexiblestems isobservedinnon-linearwaves, whereonlyasmall portion
of the total stem length, the effective stem length, contributes to the base force.
4
Preface
This report is the result of my thesis of the master in hydraulic engineering at the Delft University of
Technology. Here I did research on the interaction of wave hydrodynamics with flexible vegetation,
such as sea grasses.
I am very enthusiastic about recent solutions in hydraulic engineering, in which natural processes are
used in order to achieve coastal safety and durability. These solutions are the elegant alternative to
hardengineeringstructuresthatcanhaveanegativeimpactontheirenvironmentandareoftencostly
to maintain. Insight in the interaction between vegetation, currents, waves, and sediment transport
contributes to the application of vegetation for coastal protection.
This thesis would not be possible without the extensive support from my graduation committee.
I would like to thank Prof. Wim Uijttewaal for the valuable comments and useful discussions
bothinandoutsidethecommitteemeetingsandforprovidingtheopportunitytoconductexperiments
in the wave flume of the Laboratory of Fluid Mechanics.
Rob Uittenbogaard and Niels Jacobsen supported me on a daily basis at Deltares in Delft. I
want to thank them for giving me the opportunity to graduate on this subject, for their critical and
striking remarks and considerable contribution to this thesis.
I would also like thank Zhan Hu and Arnold van Rooijen for the advice on writing and the
delimitation of the thesis.
Furthermore, I would like to thank Wouter Kranenburg and Jasper Dijkstra from Deltares for the
great help for staying on track and keeping the right focus during our frequent update meetings.
Wout Bakker did an excellent job on performing the wave flume experiments with flexible mimics
during his 3-month internship. In my opinion we had a lot of fun during this period with the exper-
iments and discussing our common interests. I want to thank him in particular for his contribution,
which has been very valuable for this thesis.
Finally, I want to thank my family and friends for their support during my study.
Joost W. D¨obken
Delft, May 2015
6
Contents
Abstract 3
Preface 5
1 Introduction 9
1.1 Problem description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Scope and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Report outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 Theoretical background 13
2.1 Aquatic vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Flow through and over a canopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Forces on cylinders in oscillating flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4 Flexible vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Model equations 21
3.1 Dynamic vegetation model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Momentum equations for orbital motion . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.3 Dynamic pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.4 Turbulence closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.5 Momentum equation for wave-averaged flow . . . . . . . . . . . . . . . . . . . . . . . . 31
7
4 Wave hydrodynamics with rigid submerged cylinders 33
4.1 Model validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2 Steady state results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5 Wave-induced motion of flexible vegetation 47
5.1 Model validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.2 Vegetation-induced wave dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.3 Parameter sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6 Conclusions and recommendations 61
6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Appendix A Additional theory 67
Appendix B Discretization of the 1DV-Dynveg model equations 75
Appendix C Sensitivity to numerical settings 83
Appendix D Figures to Chapter 4 87
Appendix E Figures to Chapter 5 95
Appendix F Time series corrections 111
Appendix G Data structuring 115
References 120
List of symbols 121
8
Chapter 1
Introduction
1.1 Problem description
Aquatic vegetation, found in coastal waters and inter-tidal areas, has the capability to damp waves
(e.g. Dean & Bender, 2006; Mendez & Losada, 2004), increase the uptake of nutrients from the water
column(e.g.Costanza&Voinov,2001;Nepf&Koch,1999),provideahabitatforavarietyofvaluable
organisms (e.g. Bouma et al., 2005; Peralta et al., 2008) and result in sedimentation and stabilization
of the bed (e.g. Scoffin, 1970; Gacia & Duarte, 2001). These features result in a positive feedback on
the development of aquatic ecosystems (e.g. Sunda et al., 2006; van der Heide et al., 2011). These
features are also important for solutions in hydraulic engineering, water management and coastal
zone management, regarding bed and shoreline protection and water quality. Since the interaction
processesbetweenaquaticvegetationandtheambienthydrodynamicsplayakeyroleintheprocesses,
there is an increasing demand for insight in the mechanisms and improved model approaches.
There is some understanding of how currents affect aquatic vegetation and vice versa; less is known
about interactions between waves and aquatic vegetation. According to the report by Koch et al.
(2006), the hydrodynamics are complex, since the interaction of waves with vegetation is highly
coupled and non-linear. Several publications describe the streaming effects found in and above the
vegetation canopy (Luhar et al., 2010; Ma et al., 2013; Pujol et al., 2013a). Understanding the
interactionbetweenwavehydrodynamicsandthemotionofaquaticvegetationisessentialforareliable
modeling of the physical phenomena.
The common techniques to model the interaction between waves and aquatic vegetation are mostly
based on direct determination of the attenuation of the wave height. The wave-dissipated energy is
subsequently related to the work done by the vegetation on the waves, assuming linear wave theory
9
(e.g. Dalrympleet al.,1984; Kobayashi etal., 1993;Mendez & Losada,2004). These approacheshave
proventhemselvestobeapplicable. However,theylackaphysicalbasis: theydependmostlyonrough
assumptions (such as linear wave theory) and empirical parameters and give little information about
the hydrodynamic structure in and above the canopy.
For sediment transport, this conventional method is subsequently applied to determine the reduced
bed-shear-stress, τ , bysubtractingthestressonthevegetation, τ , fromthetotalbed-shear-stressin
b v
the non-vegetated case:
τ =τ τ (1.1)
b t v
−
Using the bed-shear-stress to model the bed-load sediment transport is a common method (e.g. In-
fantes et al., 2012; Rasel et al., 2013), but with a lacking enhancement of physics, this method is
unjustified. Moreover, the contribution of suspended sediment transport is influenced by a change in
turbulent mixing.
Only few publications focus on the motion of flexible aquatic vegetation. It is indisputable that
vegetation flexibility and buoyancy influences the interaction with a steady flow (e.g. Dijkstra &
Uittenbogaard, 2010; Mullarney & Henderson, 2010). In the reviewed literature, only Pujol et al.
(2013b) and Luhar et al. (2010) investigate wave hydrodynamics with flexible vegetation in flume
experiments, but no generic solution is found.
1.2 Scope and objectives
The main objective of this study is to develop a detailed understanding of the interaction processes of
wave hydrodynamics with flexible aquatic vegetation by processing experimental data and development
and validation of a numerical 1DV model. This thesis discusses the present knowledge, investigates
thephysicalprocessesandintroducesthe1DV-Dynveg model. The1DV-Dynvegmodelconsistsofan
one-directional vertical wave model, equivalent to the model presented by Uittenbogaard & Klopman
(2001), and a dynamic vegetation model, equivalent to the Dynveg model for studying steady flow
over flexible vegetation (Uittenbogaard, 2003; Dijkstra & Uittenbogaard, 2010).
The strategy of this report, is to divide the interaction between wave hydrodynamics with flexible
vegetation into two one-way couplings:
1. wave hydrodynamics over an array of rigid cylinders: the wave hydrodynamics are affected and
the motion of the cylinders is negligible; and
2. wave-inducedmotionofasingleflexiblestem: influenceonthewavehydrodynamicsisnegligible.
10