Nearshore bar dynamics at Egmond aan Zee, the Netherlands.

-An analysis of bar stages derived by Argus video imagery.

Marie-Louise SøndbergMaster student

Institute of GeographyUniversity of Copenhagen

•What morphodynamic stages appear at Egmond aan Zee, the Netherlands, 2002?

•What is the relation between variations in incident wave climate and qualitative/quantitative changes in the shape and position of the bars?

•And can there be established a rigorous morphodynamic model for the three-bar system?

Objectives:

STUDY AREA

• Hm: 1.2 m. From Nov-Jan: 1.7 m and from Apr-Aug: 1.0 m.

•Tm : 5 s.

Den Helder

Egmond aan Zee

Ijmuiden

MPN

Den Haag

Hoek van Holland

Argus tower40

45

50

WESP

Noordwijk

North S ea

0 100km

E gmondaan Zee

The Netherlands

N

0 30km

•Erosion rate: From Den Helder – Egmond: 0.95 m/year and from Egmond – Den Haag: 0.2 m/year.

• Profile nourishment happens every second or third year.

•The mean grain size is 0.25 – 0.30 mm•The mean tidal range is 1.65 m. Spring: 2.0 m and nip: 1.4 m.

•Winds from SW (23%), W (16%) and NW (12%) dominate. In high energy situations winds from NW dominate.

•Dominated by storm waves.

PROFILE

0 200 400 600 800 1000 1200 1400 1600D istance (m )

-8

-4

0

4

Dep

th (

m)

In te rtida l B ar

1 . B ar

2 . B ar

•Two or three bars.•The intertidal bar is visible at low to moderate wave energy. •The 1st bar is at 1.5 – 2.5 m depth. Wave length: 350 – 900 m. •The 2nd bar is at 3.5 – 4.0 m depth and is often linear. Wave length: 1 km•The beach gradient is between 1:10 and 1:50. From 1 m to 8 m depth it is 1:35 – 1:60.•The net sediment transport is shoreward.•The offshore bar migration cycle is 15 years.

DATA

•Daily low tide images from the Argus tower. Already rectified and mosaiced to single images.

•A total of 220 images from year 2002. Data from March, August and first half of September are missing.

•Wave data from the MPN wave station.

•And a few short profiles from the WESP area. Example of a rectified

and mosaiced Image.

Cam era

0 300 0m

Rectified image cropped

800m

METHODS

0 500 1000 1500 2000 2500 3000m

Intensity and bar crest analysis.•Each image is imported as black/white in the WinChips software.•All pixels in the images has a digital number between 0 (black) and 256 (white). •The lighter areas shows where the waves shoals, which means that the bars and the shoreline becomes visible.•31 profile lines are superimposed and the intensity along each line extracted.

0 100 200 300 400 500 600D istance (m )

120

140

160

180

200

Dig

ital n

umbe

r

- 4

- 2

0

2

4D

epht

(m

)

0 100 200 300 400 500 600D istance (m )

80

120

160

200

Dig

ital n

umbe

r

- 4

- 2

0

2

4

Dep

ht (

m)

Examples of single intensity lines (red) and short profiles (black) Deviation: 30 m (Source: van Enckevort & Ruessink, 2001)

UNDULATIONINDEX

0 1000 2000 3000Shore para lle l d istance (m )

800

600

400

200

0

Sho

rew

ard

dist

ance

(m

)

In tertida l bar

1. Bar

2. bar

Bar crests are plotted in a diagram and a trend line is added. The standard deviation of bar crest position relative to this line is calculated. The standard deviation is an expression of how much the bar undulates. And is here called the undulation-index.

Mean distance: Intertidal bar: 216.1 m 1st bar: 394.9 m2nd bar: 692.1

Undulation-index:Intertidal bar: 18.7 m1st bar: 31.3 m2nd bar: 19.9 m

CLASSIFICATION

Linear bars (LR)

Strongly undulating bars (SU)

Undulating bars (UR)D. 28/1 D. 28/10

D. 2/1 D. 26/4

D. 11/7 D. 22/9Shore parallel direction (3000m)Cross shore direction (800m)

L inear bars (LR )

Strongly undulating bars (SU)

Undulating bars (UR)D. 28/1 D. 28/10

D. 2/1 D. 26/4

D. 11/7 D. 22/9Shore parallel direction (3000m)Cross shore dir

ection (800m)

L in e a r b a r s ( L R )

S t r o n g ly u n d u la t i n g b a r s ( S U )

U n d u la t in g b a r s ( U R )

D . 2 8 / 1 D . 2 8 / 1 0

D . 2 / 1 D . 2 6 / 4

D . 1 1 / 7 D . 2 2 / 9

S h o r e p a r a l l e l d i r e c t io n ( 3 0 0 0 m )

Cro

ss

sh

ore

dir

ec

tio

n (

80

0m

)

Each image is classified on the basis of:•How much each single bar undulates.•Rhythmic forms•Transverse bars•Number of rips

Six images have been chosen to illustrate the three different types of stages observed.Linear bars•2nd bar is visible•No clear rhythmic•Even the intertidal bar is linear•Few ripsUndulating bars•Clear rhythmic forms•Rips Strongly undulating bars•Very clear rhythmic forms•Transverse bar forms•A lot of rips

RESULTS

The nearshore bar crests mean distance to the Argus tower.

Upper figure:The bars have more or less the same position during the year.

 The lower figure is a magnification of the two inner bars in the upper figure.

Lower figure:The bars migrates offshore from January to the end of July and onshore from September to December.

A lot of minor irregularities during theyear.

01-01 31-01 02-03 01-04 01-05 31-05 30-06 30-07 29-08 28-09 28-10 27-11 27-12D ate

200

400

600

800

Offs

hor

e di

stan

ce (

m)

01-01 31-01 02-03 01-04 01-05 31-05 30-06 30-07 29-08 28-09 28-10 27-11 27-12D ate

210

220

230

240

250

260

O

ffsho

re d

ista

nce

(m

)

360

380

400

420

440

RESULTS

01-01 31-01 02-03 01-04 01-05 31-05 30-06 30-07 29-08 28-09 28-10 27-11 27-12D ate

10

15

20

25

30

35

40

45

50

55

Un

dula

tioni

nde

x (m

)

01-01 31-01 02-03 01-04 01-05 31-05 30-06 30-07 29-08 28-09 28-10 27-11 27-12D ate

10

15

20

25

30

35

40

45

50

55

Un

dula

tioni

nde

x (m

)

01-01 31-01 02-03 01-04 01-05 31-05 30-06 30-07 29-08 28-09 28-10 27-11 27-12Da te

10

15

20

25

30

35

40

45

50

55

Un

dul

atio

nin

dex

(m)

Undulation-index for the intertidal, 1st and 2nd bar.

Variation and flexibility is greatest for the 1st bar so results from the 1st bar will be used in the classification.

The classification is still a result of a three bar system.

Variation (Inter. bar): 13.5 – 34.1 Variation (2nd bar): 13.2 – 24.1

Variation (1st bar): 14.5 – 51.2

RESULTS

Red dots: undulation-indexBlack line: wave height (hs)Horizontal Bar: classification. Linear bars: blue.Undulating bars: yellowStrongly undulating bars: red

01-01 31-01 02-03 01-04 01-05 31-05 30-06 30-07 29-08 28-09 28-10 27-11 27-12D ate

0

1

2

3

4

5

Hs

(m)

0

10

20

30

40

50

60

Un

dula

tioni

nde

x (m

)

•There is a good qualitative agreement between wave heights and the bar stages. •Linear bars appear under or after periods with very high waves.

•Undulating bars appear after periods with moderate wave heights,

•Strongly undulating bars appear after a long period without high waves.

•Generally the undulation-index decreases quickly when the wave heights increase.

RESULTS

01-01 31-01 02-03 01-04 01-05 31-05 30-06 30-07 29-08 28-09 28-10 27-11 27-12Da te

0

40000

80000

120000

160000

P0 (

Wm

-1)

0 5000 10000 15000 20000 25000 30000 35000P0 (W m -1)

10

20

30

40

50

60

Un

dula

tioni

nde

x (m

)

Wave power

32

22

0

THgP s

•Same pattern as the wave heights.

R2=0.21

Average of wave power for the previous 16 days.Linear bars: blue.Undulating bars: yellowStrongly undulating bars: red

16 days gives the best relation between R2 and the undulation-index.

0 5 10 15 20 25 30P 0 Re la tio n d u rin g n u mb e r o f d a ys.

5

10

15

20

25

Rel

atio

n w

ith u

ndu

latio

nin

dex.

R2

RESULTS

L R U R S U

0

0 . 5

1

1 . 5

2

2 . 5

Hs

(m)

A

L R U R S U

4

5

6

7

8

T (

s)

B

L R U R S U

- 4 0 0 0

0

4 0 0 0

8 0 0 0

1 2 0 0 0

Ene

rgy

dens

ity (

Jm-2

) C

L R U R S U

- 2 0 0 0 0

0

2 0 0 0 0

4 0 0 0 0

6 0 0 0 0

P0

(Wm

-1)

D

Linear bars: LR; Undulating bars: UR; Strongly undulating bars: SUDots:mean value; Error bars: standard deviation

•The values for linear bars are biggest for all the dynamic parametres.

•If there had not been missing images from the summer months, then it is possible that strongly undulating bars would have had the lowest values.

RESULTSL i n e a r b a r s ( L R )

S t r o n g l y u n d u l a t i n g b a r s ( S U )

U n d u l a t i n g b a r s ( U R )

L o n g s h o r e d i r e c t i o n ( 3 0 0 0 m )

Cro

ss

sh

ore

dir

ec

tio

n (

80

0m

)Inc

rea

sin

g e

ne

rgy

lev

el

Systematic model for the bar system at Egmond aan Zee.

Showing the three different intermediate stages observed during year 2002.

The energy level is increasing from strongly undulating bars through undulating bars ending up with quasi-linear bars.

DISCUSSION

•Swells

•One bar

•Responds faster to changes in wave energy because of the smaller sediment volume in the bar.

•Does not have an outer bar to dissipate the incoming wave energy.

•Narrow surfzone and steep gradient

•Storm waves

•Three bars

•Slow system because of the large sediment volume, but during storms the morphology will respond relatively quickly.

•Has an outer bar that will dissipate the incoming wave energy.

•Wide surfzone and a flat gradient

Comparison of the Wright & Short (1984) model and the model for Egmond.

0 200 400 600 800 1000 1200 1400D stance (m )

-10

-5

0

5

Dep

th (

m)

Egm ond aan Zee

Palm Beach

CONCLUSIONS

•Three bar stages were observed:Linear bars: Undulation-index < 22Undulating bars: Undulation-index 22 - 42Strongly undulating bars: Undulation-index > 42

•Wave heights, periods etc. are larger for linear bars than for undulating bars and strongly undulating bars.

• The variation in wave power explains 21 % of the variation in the undulation-index, with a lag of 16 days.

THANK YOU FOR YOUR ATTENTION!