Observations and Predictions of Ripple Development on a
Complex ShorefaceArthur C. Trembanis1, L. Donelson Wright1,
Carl T. Friedrichs1, Malcolm O. Green2,
Terry M. Hume2
1Virginia Institute of Marine Science, College of William and Mary, Supported by theNational Science FoundationINT-9987936
2National Institute of Water and Atmospheric Research, New Zealand Supported by theNZ Foundation for Research Science and TechnologyFRST-CO1X0015
Questions Addressed• How do complex substrates differ from uniform
substrates in terms of bedform dynamics?• How do bedforms on contrasting substrates vary in
space and time? • How well do existing models account for those
differences?
A field study of the effects of spatially varying substrates on bedform evolution was conducted on a shoreface on the east coast of New Zealand’s North Island under similar physical forcing conditions that
included two significant storm events.
Main Points• Rough areas composed of large ripples sharply contacted
smooth surfaces covered by fine sand.• The two facies types maintained their positions despite
highly energetic conditions over a seven-month period.• Wave orbital ripples were the dominant roughness feature.• Roughness gradients were enhanced during storms. • The Nielsen model predicted the small ripples observed at
the smooth site. Wikramanayake model had the best fit to the ripples at the rough site.
NEWZEALAND
TASMANSEA
SOUTHPACIFIC
175ºE
37ºS
NEWZEALAND
TASMANSEA
SOUTHPACIFIC
175ºE
37ºS
900KHz 20m range
Study Area
1.701.60.2016Fine
3.88.70.7522Coarse
cr
(dyn/cm)^2s
(cm/s)D50
(mm)H
(m)Facies
Not exact scale
Shoreface Slope New Zealand and Middle Atlantic Bight)
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
00 1 2 3 4 5 6 7 8 9 10
Distance offshore (km)
Dep
th (
m)
Pauanui Tairua Duck Cedar Island
Shoreface Slope New Zealand and Middle Atlantic Bight)
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
00 1 2 3 4 5 6 7 8 9 10
Distance offshore (km)
Dep
th (
m)
Pauanui Tairua Duck Cedar IslandPauanui Tairua Duck Cedar Island
= 0.86º
ADV
EMCM
OBS
ABS
ADCP
BatteriesData logger
Sediment trap
CTD
Surface Buoy
Steel FrameExtension arm
Instrumented Benthic Tripod
2.7 m
Observed HydrodynamicsS
peed
(cm
/s)
Per
iod
(s)
Spe
ed (
cm/s
)
Per
iod
(s)
0
10
20
30
40
50
60
70
80
2/16/01 2/23/01 3/2/01 3/9/01 3/16/01 3/23/01
Date
0
2
4
6
8
10
12
14
16
Uwsig UcTavg 6 per. Mov. Avg. (Tavg)6 per. Mov. Avg. (Uwsig) 6 per. Mov. Avg. (Uc)
Date
Uorb
Uc
Ts
TC Paula
Sp
ee
d (
cm/s
)
Pe
riod
(s)
h = 22 m
z = 0.70 mab
Observed Bedforms
Rough Site
= 15-25 cm
= 75-120 cm
900 kHz
Smooth Site
= 5-8 cm
= 20-30 cm
Bed Altimetry
-5
0
5
10
15
2/15/01 2/22/01 3/1/01 3/8/01 3/15/01 3/22/01
Date
ele
va
tio
n o
f th
e b
ed
(c
m)
0
5
10
15
20
25
30
35
40
Rough Site Fine Site 12 per. Mov. Avg. (Fine Site) 12 per. Mov. Avg. (Rough Site)
Comparison of Bed Activity
=15-25 cm
=75-120 cm
=5-8 cm
=20-30 cm
Diver Observations Rough Smooth
Ele
vatio
n (c
m)
Rough
Smooth
Altimetry records from contrasting sites
2
50
( )
( 1)orbsigU
s gd
Roughness from Ripple Models
Grain Size and wave parameters used in two ripple models to estimate roughness length
Nielsen, P., 1981. J. Geophys. Res. 86
Wikramanayake,P.N., 1993., MIT
28b
=15-25 cm =75-110 cm
Sidescan Sonar/Diver Observations
Model Results- Rough Site100 200 300 400 500 600 700 800
5
10
15
20
25
30
35
40
45
hours
dim
ensi
onle
ss
Psi Mobility Parameter
100 200 300 400 500 600 700 800
20
40
60
80
100
cm
Lambda vortex ripple length
100 200 300 400 500 600 700 800
2
4
6
8
10
12
14
16
cm
Eta vortex ripple height
100 200 300 400 500 600 700 800
5
10
15
20
cm
Kb bed roughness
100 200 300 400 500 600 700 800
5
10
15
20
25
30
35
40
45
hours
dim
ensi
onle
ss
Psi Mobility Parameter
100 200 300 400 500 600 700 800
20
40
60
80
100
cm
Lambda vortex ripple length
100 200 300 400 500 600 700 800
2
4
6
8
10
12
14
16
cm
Eta vortex ripple height
100 200 300 400 500 600 700 800
5
10
15
20
cm
Kb bed roughness
Ripple length
Leng
th (
cm)
Ripple height
Hei
ght (
cm)
Nielsen, 1981
Model Results- Smooth Site
Nielsen, 1981
=5-8 cm =20-30 cm
Sidescan Sonar/Diver Observations
100 200 300 400 500 600 700 800
20
40
60
80
100
120
140
160
180
200
hours
dim
ensi
onle
ss
Psi Mobility Parameter
100 200 300 400 500 600 700 800
10
20
30
40
50
60
70
cm
Lambda vortex ripple length
0 100 200 300 400 500 600 700 8000
2
4
6
8
10
cm
Eta vortex ripple height
100 200 300 400 500 600 700 800
2
4
6
8
10
12
cm
Kb bed roughness
Ripple Height
100 200 300 400 500 600 700 800
20
40
60
80
100
120
140
160
180
200
hours
dim
ensi
onle
ss
Psi Mobility Parameter
100 200 300 400 500 600 700 800
10
20
30
40
50
60
70
cm
Lambda vortex ripple length
0 100 200 300 400 500 600 700 8000
2
4
6
8
10
cm
Eta vortex ripple height
100 200 300 400 500 600 700 800
2
4
6
8
10
12
cm
Kb bed roughness
Ripple Length
Ripple length
Leng
th (
cm)
Ripple height
Hei
ght (
cm)
Smooth Site Model Estimated Ripple Height
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
2/15/01 2/22/01 3/1/01 3/8/01 3/15/01 3/22/01Date
Heig
ht (
m)
Nr (Nielsen) Nr (Wikramanayake)
12 per. Mov. Avg. (Nr (Nielsen)) 12 per. Mov. Avg. (Nr (Wikramanayake))
Hei
ght (
m)
Date
0.0
0.05
0.1
Wikramanayake
Nielsen
Rough Site Model Estimated Ripple Height
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
2/15/01 2/22/01 3/1/01 3/8/01 3/15/01 3/22/01Date
He
igh
t (
m)
Nr (Nielsen) Nr (Wikramanayake)
12 per. Mov. Avg. (Nr (Nielsen)) 12 per. Mov. Avg. (Nr (Wikramanayake))
Comparison of Ripple Model ResultsRough Site Height
Hei
ght (
m)
Smooth Site Height
Wikramanayake
Nielsen
0.0
0.1
0.2
Roughness Comparison Smooth vs Rough site
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
1 51 101 151 201 251 301 351 401 451 501 551 601 651 701 751 801
Burst Hour
Smooth Site Rough Site 6 per. Mov. Avg. (Smooth Site) 6 per. Mov. Avg. (Rough Site)
Ro
ug
hn
ess
len
gth
(m
)Bed Roughness Comparison- Two sites
Rough
Smooth
Rou
ghne
ss L
engt
h (m
)
0.0
0.1
0.2
Conclusions
• Spatial gradient in roughness between facies types is strong and persistent
• Roughness gradient enhanced during storms• Smooth site experiences greater bed mobility• Two widely used ripple models tested against field data
– No single model captured behavior at both sites– Nielsen model fairly well predicted ripple height and length at
smooth site– Wikramanayake had a better fit to ripple height and length at
rough site
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