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