Dissertation Title:

The Effects of Relative Submergence on the





Hydrodynamics of an Open Vegetated Channel





Student no.:


200909858



Dissertation supervisor:

Dr Stuart McLelland



Submission date:


25/04/2012



Word count:


10571



By submitting this dissertation the author is declaring that the work contains no section
copied in whole or in part from any other source unless it is explicitly identified by
means of quotation marks. The author declares that they have acknowledged such
quotations by providing detailed references in an approved format. The author declares
that they understand that either or both unidentified and unreferenced copying
constitutes plagiarism, which is one of a number of serious offences under the
University's Code of Practice on the Use of Unfair Means. The author declares that the
word count is correct and they have counted the words using the instructions given in
the Departmental Student Handbook. The author declares that ethical approval was
gained for the research the dissertation is based on, and that any ethical issues raised by
the research were given full consideration during the execution of the research.

---

Acknowledgements
I would like to thank Dr Stuart McLelland for providing me with excellent support and
guidance. I would also like to thank Brendan Murphy for his assistance during my
experimental stage. Thanks also go to my friends and family for their support and believe in
me.

---

The Effects of Relative Submergence
on the Hydrodynamics of an Open
Vegetated Channel


200909858
Supervisor: Dr Stuart McLelland











25/04/2012

Department of Geography
University of Hull, UK

---

Abstract

The presence of aquatic vegetation has been shown to have major impacts on the morphology
and ecology of fluvial and coastal environments. This study investigates the interaction
between relative submergence and flow transition at the front of an aquatic canopy. Flume
experiments with simulated vegetation were conducted to collect data. The results are in
accordance with previously published literature. As the flow encounters the front of the
canopy, the velocity adapts at a scale x0. The shear stress is reduced to approximately 10% of
pre-canopy values. Strong coherent turbulences grow and penetrate a certain distance into the
canopy, dividing the in-canopy flow in two regions. The turbulence transition length scale is
termed L

T. Generally, LT 3x0. As submergence decreases the vertical growth of the shear
layer becomes restricted. The critical depth is H/h2.5. It has been found that the previously
used method of evaluating LT is not applicable in depth-limited flow. A modified equations is
therefore proposed here: LT=9eUH( )-1. The fluid deceleration scale xo is relatively
unaffected by changes in submergence. This area of research is particularly actual with
regards to the need for river restoration.

---

Contents


1. Introduction .......................................................................................................................... 1
2. Literature Review ................................................................................................................. 2
2.1 Ecological Impact ............................................................................................................. 2
2.2 General Flow Parameters and Unobstructed Flow ........................................................... 2
2.3 The Mixing-Layer Analogy .............................................................................................. 4
2.4 Canopy Characteristics ..................................................................................................... 6
2.5 Relative Submergence ...................................................................................................... 8
2.6 Transition at the Front of the Canopy ............................................................................... 8
2.7 Context of this Study ........................................................................................................ 9
3. Methodology ....................................................................................................................... 10
3.1 Experimental Setup ......................................................................................................... 10
3.2 Measurement Techniques and Post-processing .............................................................. 13
3.3 Basic Calculations and Flow Parameters ........................................................................ 15
4. Results and Discussion ....................................................................................................... 17
4.1 General Flow Parameters ................................................................................................ 17
4.2 Unobstructed Flow .......................................................................................................... 19
4.2.1 Velocity Profiles ....................................................................................................... 19
4.2.2 Turbulence Profiles ................................................................................................. 22
4.2.3 Shear Velocity and Stress ....................................................................................... 22
4.3 Transition to Canopy Flow ............................................................................................. 24
4.3.1 Velocity Profiles ....................................................................................................... 24
4.3.2 Shear Velocity ......................................................................................................... 27
4.3.3 Turbulence Profiles ................................................................................................. 27
4.3.4 Drag Coefficient ..................................................................................................... 31
4.2.5 Transition Length Scales LT and x0 ......................................................................... 32

---

4.4 Relative Submergence .................................................................................................... 33
4.4.1 Velocity ................................................................................................................... 33
4.4.2 Turbulence .............................................................................................................. 33
4.4.3 Drag Coefficient and Transition Lengths ............................................................... 36
4.5 Effectiveness and Limitations ......................................................................................... 38
4.5.1 Simulated Vegetation and Canopy .......................................................................... 38
4.5.2 Flume Setup and Measurement Techniques ........................................................... 39
5. Conclusion ........................................................................................................................... 40
4. Reference List ..................................................................................................................... 42

---

List of Figures

Figure 2.1 Theoretical (a) velocity and (b) Reynolds stress profiles in open-channel flows .... 3
Figure 2.2 Theoretical (a) velocity and (b) Reynolds stress profiles in vegetated flows .......... 5

Figure 3.1 Side view of flume setup ........................................................................................ 11
Figure 3.2 Vegetation patch - (a) diagram and (b) picture ..................................................... 11
Figure 3.3 Picture of simulated canopy in flow conditions ..................................................... 12
Figure 3.4 Overhead view of canopy with ADV positions ..................................................... 12
Figure 3.5 ADV head geometry - (a) diagram and (b) picture ............................................... 14

Figure 4.1 (a) Bed and water surface heights. (b) Relative flow depths ................................. 18
Figure 4.2 Pre-canopy velocity profiles on a regular and logarithmic scale ........................... 20
Figure 4.3 Pre-canopy (a) one-dimensional turbulence intensities and (b) Reynolds stress uw
and turbulent kinetic energy k ................................................................................................. 21
Figure 4.4 Relationship between shear velocity and maximum Reynolds stress .................... 23
Figure 4.5 Overhead In-canopy velocity profiles for Run 6 - (a) Transition at the front of the
canopy and (b) logarithmic profile at M8 ................................................................................ 25
Figure 4.6 Adaption of velocity shear at the top of the canopy .............................................. 26
Figure 4.7 In-canopy bed shear stress ..................................................................................... 28
Figure 4.8 In-canopy turbulence profiles - (a) one-dimensional turbulence intensities at M8
for Run 6 (b) Reynolds stress and turbulent kinetic energy adaption for Run 6 ...................... 29
Figure 4.9 Normalized turbulent kinetic energy for Run 6 at M8 ........................................... 30
Figure 4.10 Overhead Growth of turbulent shear layer (a) relative penetration depth (b)
relative maximum Reynolds stress .......................................................................................... 30
Figure 4.11 Fully developed in-canopy velocity profiles for all Runs .................................... 34

---

Figure 4.12 Velocity adaption compared for different flow depths - (a) relative in-canopy
deceleration (b) velocity shear gradient .................................................................................. 34
Figure 4.13 Normalized Reynolds stress and turbulent kinetic energy profiles for all runs .. 35
Figure 4.14 Turbulence behaviour compared for all flow depths - (a) ratio of wake turbulence
to total turbulence budget (b) turbulence penetration height .................................................. 35
Figure 4.15 Overhead Transition length scale evaluated with two different shear velocities 37

List of Tables

Table 3.1 Flume slope settings ................................................................................................ 13

Table 4.1 General slow parameters ......................................................................................... 17
Table 4.2 Shear velocities and bed shear stresses evaluated with different methods .............. 23
Table 4.3 Canopy parameters .................................................................................................. 24

---

List of Symbols



Symbol
Unit
Description
a
[m-1]
canopy packing density
A
[m]
frontal plant area
CD

single patch drag coefficient
CDa
[m-1]
total canopy drag
CSL

canopy shear layer parameter
D
[m]
plant patch diameter
d50
[m]
mean grain diameter
ec
[ms-2]
canopy dissipation
Fr

Froude number
g
[ms-2]
gravity
H
[m]
flow depth
h
[m]
vegetation height
hp
[m]
shear layer penetration height
k
[ms-2]
turbulent kinetic energy
LS
[m]
total mixing layer extent
LT
[m]
turbulent transition length scale
ps
[ms-2]
shear production
R
[m]
hydraulic radius
Re

Reynolds number
SE

energy slope
SV

flow depth gradient over vegetation
tdiff
[s]
diffusive time scale
u
[ms-1]
horizontal velocity

[ms-1]
vertically averaged flow velocity

[ms-1]
shear velocity
f
[ms-1]
friction velocity at the top of the canopy
u'
[ms-1]
horizontal turbulence intensity
UH
[ms-1]
horizontal velocity at the top of the canopy
uw
[ms-2]
Reynolds stress
v
[ms-1]
lateral velocity
V
[m]
volume influenced by plant patch

---

v'
[ms-1]
lateral turbulence intensity
w
[ms-1]
vertical velocity
w
[m]
channel width
w'
[ms-1]
vertical turbulence intensity
x
[m]
horizontal displacement
x0
[m]
fluid deceleration length scale
xc
[m]
horizontal displacement relative to front of canopy
y
[m]
lateral displacement
z
[m]
vertical displacement
z
[m]
laterally averaged height
z0
[m]
zero displacement height
zi
[m]
interpolated height
e
[m]
shear layer penetration depth
U

velocity shear
x
[m]
horizontal patch spacing
y
[m]
lateral patch spacing


von Karman constant

[Pa s]
dynamic viscosity of water

[kgm-3]
water density

[Nm-2]
bed shear stress

---

Document Outline

• Title Page 1
• Acknowledgements
• Title Page 2
• Abstract
• Contents
• List of Figures and Tables
• Symbols
• Dissertation Final

---