Dr Jonathan Pearson – University of Warwick – Mixing in the Nearshore Zone
Contents
Coastal Research Facility
HR Wallingford
Mixing in the nearshore zone
‘Scheldgoot’ wave - current
facility, Delft Hydraulics
Jonathan Pearson
Dye Tracing Workshop
17th April 2008
Shallow Water Basin
DHI
Mixing in the Near-Shore Zone
Mixing in the Near-Shore Zone
Background
What are the mixing processes?
Sources of pollution:
- Turbulent diffusion (ey)
Offshore boundary
- Outfalls (but becoming cleaner, but to increased regulation)
- Shear dispersion (ky)
Shoreline boundary
- Storm water overflows
e.g. Pattaya Beach
e.g. Nadaoka & Kondoh (1982)
H/L
= 0.026-0.029
oo
H/L
= 0.070-0.080
oo
200
Cross-shore variation
in turbulence
v
- Polluted rivers flowing into
nearshore region
e.g. San Diego
Major source of
pollution :
storm water overflows
b
1 /2
(gd )
Turbulent Intensity
300
100
0
0
1
2
d/d
Water Depth / Depth
@ Breaking, h/hb
b
Mixing in the Near-Shore Zone
Mixing in the Near-Shore Zone
Coastal Research Facility
Coastal Research Facility
Recirculating Flow in the CRF
Flow straighteners
UKCRF
(HR Wallingford)
Aim: On-offshore (transverse)
mixing under waves & current
Beach section
Gates for controlling
longshore currents
Location: seawards of the breaker point
Tests: 4 regular wave conditions
(constant period) superimposed on
longshore current
LONGSHORE CURRENT
D
SUMPS
7.5m
D
10m
B
C
C
B
A
A
SHORELINE
36m
Fluorometers
Access bridge &
instrument carriage
Mutli-paddle
wavemakers
SUMPS
Office
Current generating sumps
1
Dr Jonathan Pearson – University of Warwick – Mixing in the Nearshore Zone
Mixing in the Near-Shore Zone
Coastal Research Facility
Coastal Research Facility
0
19.0
0
500
0
2
4
6
8
Distance from shore, y (m)
Measured waves across
nearshore region
10
12
Long-shore velocity under
current only conditions
Mean Offs hore Wave Ht .=0.057m ( y=15m from S hore)
y(m) H (mm)
1.5
57
2.0
60
3.0
62
4.0
63
30
0
-30
60
Mean Offs hore Wave Ht .=0.086m ( y=15m from S hore)
y(m) H (mm)
1.5
48
2.0
76
2.5
85
3.0
88
4.0
91
30
0
-30
90
0
x=5.35m
0
1.05
x=8.35m
0.70
0.35
0
x=9.85m
0.50
0.25
0
-30
2
4
5
10
15
20
25
Distance from bed injection point, x (m)
0
0.75
3.5
4.0
4.5
5.0
5.5
Distance from shore, y (m)
6
Dista nce fr om shore, y (m)
0
0.53
0
0
H = 86mm
H = 120mm
1.06
y (m) H (mm)
3. 0 1 04
3. 5 1 21
4. 0 1 25
4. 5 1 29
5. 0 1 32
30
H = 57mm
10
0.61
Mean O ffsh ore Wav e H t.=0. 120m (y=15 m from S hore)
60
x=3.85m
1.21
1.59
No Waves
20
0.64
0
250
29.0
40
Normalised Transverse
2
2
-2
Variance, σ (m ) {x10 }
y
80
x=2.35m
1.28
1.82
Transverse variance v’s
distance from injection
30
dispersion
sion c oeffic ient, D y
ansv ers e d isp er
T r Transverse
coefficient,
D2 /(mm
s) 2/s)
(m m
Distance from inlet
gates , x (m)
1.92
Concentration (ml/l) {x10-5}
120
(m s-1) {x10 -3}
Measured
Rough bed conditions
6000
Estimated
Mixing processes due to:
Longshore current
W ave induced turbulence
Stoke s Drift
4000
Mixing processes
seawards
of the
breaker point
Assumed Conditions
T = 1.2s
y = 5.0m
M = 0.01
m = 1:20
γ = 0.69
2000
0
Tracer conc. profiles under waves (H = 0.086mm)
0
40
80
120
Wave Height,
height, H (mm)
Wave
H (mm)
Summary: Increase in Transverse Mixing due to Waves
Mixing in the Near-Shore Zone
Mixing in the Near-Shore Zone
‘Scheldgoot’ wave – current facility
‘Scheldgoot’ wave – current facility
Mean longitudinal velocity
profiles under waves
1000
Aim: Vertical mixing under waves & combined
currents using Laser induced fluorescence
Distance from bed, z (mm)
100
LDA system
Direct Measurement of Tracer
Concentration - 3 dye injection
points (bed, +100mm, +200mm)
Detailed Measurement of
hydrodynamics - Detailed LDA
velocity profiles ( +20 points over
vertical)
overall length: 56 m
width: 1 m
height: 1.2 m
Facility set-up
10
1
Current + Waves (H=120m)
Current + Waves (H=150m)
Current + Waves (H=180m)
0.1
0
Experimental water
Depth: 0.5m
Mixing in the Near-Shore Zone
‘Scheldgoot’ wave – current facility
Current + Waves (H=0m)
Current + Waves (H=60m)
Test conditions - 5 Regular
waves (H=0, 60, 120,150
&180mm), T=1.2s tested
superimposed on current.
0.04
0.08
0.12
Fluoresced 6G dye passing thro’
lightsheet (~1m up stream)
‘Scheldgoot’ wave – current facility
Vertical concentration
distribution from bed
injection
Vertical variance v’s
distance from injection
Dye particle
Excited dye particle
Fluoresced dye particle
Emitted laser light
250
Distance from injection
5.0m
200
4.0m
3.0m
150
2.0m
1.0m
100
50
0
0.0
Vertical Variance, σz2
{x10-3} (m 2)
Wave
35 Condition No Waves ( ) H=0.06m, T =1.2s ( )
Key
0.2
Mixing in the Near-Shore Zone
Laser / Light sheet generator
Direction of
flo w
0.16
Velocity (m s-1)
Distance from bed (mm)
Longshore velocity, u
160
-3
Water depth, -z (m) {x10 }
-3
Water Elevation, z (m) {x10 }
Mixing in the Near-Shore Zone
∂σ2z {x10-4 }
∂x
30
1.0
1.5
2.0
Concentration (g/l) {x10-4}
2.5
H=0.09m, T=1.2s ( )
50
70
25
20
15
10
5
0
0.5
20
0
1
2
3
4
Distance from source (m)
5
Summary: Increase in Vertical Mixing due to Waves
Facility bed
Fibre optic Laser
lig ht sheet
CCD Camera
Absorbence
filter
Schematic of LIF
2
Dr Jonathan Pearson – University of Warwick – Mixing in the Nearshore Zone
Mixing in the Near-Shore Zone
Mixing in the Near-Shore Zone
Wave Current facility at DHI
Wave Current facility at DHI
Crest-trough
layer
Mean wave
induced velocity
Still water
level
Aim of study: Investigate the near
shore (mainly transverse / crossshore) mixing processes by
fluorescent tracer experiments and
by detailed velocity measurements
Undertow
layer
vv+
Bed, z=-d
Dy = ky + ey =
1:20 plain smooth beach, all wave conditions superimposed on
longshore current (north -south) which approached shorenormal (i.e. no wave driven current)
… we can get a basic estimate of the
overall transverse mixing coefficient in
the surfzone:
Dy =
+ -
Tracer near injection (offshore region)
assuming the idealized case, that mean
velocities can be estimated from the
mass flux of the breaking wave
(v + − v − )2 d 2
+ ey
48e z
… standard solution
gH 4
+ ey
768dez
The turbulent diffusion (ez, ey) can be
estimated from Svendsen’s work [e.g.
Svendsen (1987), Svendsen & Putrevu
(1994)]
νt = Md gd
Inside surfzone
ν t = (0.8(d /db )−4 + 0.2)ν tb
Outside surfzone
how does the simple model compare to
measured data?
Mixing in the Near-Shore Zone
Mixing in the Near-Shore Zone
Shore
Paddle
LDA velocity measurements
1:20 beach
Detailed Measurement of
hydrodynamics - Detailed
velocity profiles at
y=1,2,3,4,5,6m from shoreline
Cross-shore variation in wave height
200
180
Wave Wave
Height,
H (mm)
Height, Hs (mm)
Direct Measurement of Tracer
Concentration - 3 dye injection
points (offshore, around breaker
point & in surfzone)
Test conditions - 3 Regular waves
(sop 2, 3.5, 5%) & 1 Random
Ho ~120mm
160
140
120
100
80
60
Sop ~ 3.5%
Sop ~ 5.0%
Sop ~ 2.0%
Sop ~ 3.5%- RANDOM
40
20
0
0
1
2
3
4
5
6
7
SWL(m) y (m)
DistanceDistance
fromfromshore,
Mixing in the Near-Shore Zone
(a)
LDA velocity observations
Tracer measurements
Z Y velocity
a) near surface,
b) near bed
0.4
0.3
0.2
Velocity (m/s)
Mixing in the Near-Shore Zone
Constant head of
fluorescent dye injected
into facility
0.1
0
0
1
2
3
4
(b)
5
Velocity (mm/s)
-0.1
-0.2
0.3
-0.3
Blue - vertical direction
(s)direction
GreenTime
- on/off
250
Measured
cross-shore
velocity
0.1
0
0
1
2
3
-0.1
-0.2
-0.3
Time (s)
Time series
a) near surface,
b) near bed
4
5
Distance
from Bed (mm)
Distance from bed (mm)
Velocity (m/s)
0.2
Distance from shore (m)
Plan view
1m
2m
3m
4m
5m
6m
200
150
100
Direction of flow
50
resultant dye
concentration measured
at a number of locations
down stream to
determine the mixing
coefficient / processes
0
-100
-50
0
50
100
150
Cross-shore velocity (mm/s)
200
250
300
Cross-shore velocity (mm/s)
3
Dr Jonathan Pearson – University of Warwick – Mixing in the Nearshore Zone
Mixing in the Near-Shore Zone
Mixing in the Near-Shore Zone
Tracer measurements
Tracer results
Plan view
Inshore, y=2m
Current + waves H=0.12 T=1.2
Dye collected by pumping
samples from flow (for 180
seconds)
Approximate
breaker point
60
Off shore
Concentration (ppb)
40
x = 0.55 m
x = 0.95 m
x = 0.35 m
x = 0.75 m
… into suitable containers and
analysed by fluorometer to
determine dye concentration
Concentration (ppb)
50
In shore
30
Surfzone
20
10
Direction of flow
0
2500
2000
1500
1000
500
0
-500
-1000
-1500
-2000
-2500
Offshore, y=5m
Current + waves H=0.12 T=1.2
-10
Location from y=2m
Approximate
breaker point
location from y=2m
70
Concentration (ppb)
60
Off Shore
In shore
y=
2m
3m
5m
Dy~
80x
60x
2x
40
Concentration (ppb)
50
Offshore
x = 2.35m
x = 3.15m
x = 1.35m
x = 3.75m
x = 1.85m
30
20
10
0
2000
1500
1000
500
0
-500
-1000
-1500
-2000
-10
location from y=5m
Location
from y=5m
Mixing in the Near-Shore Zone
OutSide –
SurfZone
Estim ated
Mixing process es due to:
Lon gsh ore curren t
Wave in d uced tu rbulence
S tok es D rift
4000
0.01
0
T = 1 .2s
y = 5.0 m
M = 0 .01
m = 1 :20
γ = 0.6 9
0
40
Wave
35 Condition No Waves ( )
∂σ2 z {x10-4}
∂x
30
80
120
0.1
1
1.5
(m)
10
Inside
Surfzone
H=0.09m, T=1.2s ( )
50
70
Increase in Vertical Mixing due to Waves
20
15
10
5
0
10
Wave height at breaking, Hb
H=0.06m, T=1.2s ( )
20
25
0
0.01
Increase in Transverse Mixing due to Waves
Assum ed Con dition s
2000
W Wave
a ve height,
H e ighH
t,(mm)
H (m m )
Vertical
Mixing
(NonBreaking)
0.1
0.001
0.001
Measu red
Rough bed con diti on s
6000
Vertical Variance, σz 2
{x10-3} (m 2)
1
Summary
This EPSRC study
Pearson et al. (2002)
Harris et al. (1963)
Harris et al. (1963)
Inman
{Fi ld} et al. (1971)
Tanaka
{Fi ld} et al. (1980)
Theoretical Mixing -this
d
Measured on-offshore dis pers ion
coefficie nt, Dy (m 2/s)
Measured on-offshore dispersion
coefficient, Dy (m2 /s)
10
Tra nsve rse dispersio n
…comparisons in the surf zone for all measured conditions
Transve rse dispersion coefficient, Dy
2
(mm
coefficient,
D /s)
(mm 2/s)
Mixing in the Near-Shore Zone
1
1
2
3
4
Distance from source (m)
5
This EPSRC study
P earson et al. (200 2)
Harris et al. (1963)
Harris et al. (1963)
{Fi ld} et al. (19 71)
Inman
Tanaka et al. (1 980)
Theoretical Mixing -this
Transverse mixing increases in surfzone &
coefficient varies by orders of magnitude
0.1
0.01
Summary: Transverse mixing increases in surfzone &
coefficient varies by orders of magnitude
0.001
0.001
0.01
0. 1
Wave height at breaking, Hb
1
1. 5
10
(m)
4