Nourishments and shoreline changes

uninterrupted coast Egmond-Bergen

(The Netherlands)

Ruud Spanhoff (RIKZ) and Stijn de Keijzer (University of Amsterdam)

Kathelijne Wijnberg and Stefan Aarninkhof (WL | delft hydraulics)

Coastview meeting, Bologna, Italy, 4/5 March 2004

Nourishments Bergen-Egmond since 1999

Objective

• Monitor a nourished beach with Argus video remote sensing technology in terms of one or more CSI’s defined in the Coastview project

• Analyse relation between seasonal wave forcing and

beach evolution

Study area

• Egmond (NL), Argus site: Jan van Speijk lighthouse

• Beach nourishment: • June 30 – July 5, 2000 • location: y = 0 – 800 m• volume 0.2x106 m3

Data set

Morphology: 48 monthly intertidal bathymetries (IBMapper)

t range : June 1999 - April 2003

y range: -680 m to +680 m

z range: 0/-0.4 m NAP to + 0.9 m NAP

► +0.2 m NAP contour (Mean Tide Level)

► Beach width (0m to +0.9m contour)

Hydrodyn.: 3-hourly observations of:

► Hrms, Tpeak, angle of incidence (θ0) (YM6, ELD)

► Water level (IJmuiden, Petten)

Methods (1): morphological data analysis

• Temporal evolution of alongshore-averaged MTL position

• Temporal evolution of spatial patterns in MTL position

set of spatial patterns empirically defined by EOF method (empirical eigenfunctions)

• For comparison to seasonal wave forcing: • linear subsampling to 3-hourly interval• Hanning filter, 90 days window width

Methods (2): seasonal wave forcing

• Cross-shore component of nearshore wave energy flux:

evaluated at 6m –NAP depth contour

• Extract seasonal signal:

• Hanning filter

• 90 days window width

218

cos( )sn rms gE gH c

The data (1)M

on

thly

In

tert

idal

Bath

ym

etr

ies

(aft

er:

Caljou

w /

Nip

ius) Egmond station

‘Jan van Speyk’

June 1999-

August 2001

Egmond station‘Jan van Speyk’

October 2001-

April 2003

Mon

thly

In

tert

idal

Bath

ym

etr

ies

The data (2)

The data (3)Mean : 57 m 57 m 0 mSt.dev. : 29 m 16 m 25 mVariance : 838 m2 211 m2 (25%) 627 m2 (75%)

Results EOF analysis

Variance explained by EOF1: 83.1%

= beach nourishment

Example: MTL position (+0.2m contour)

Reconstructed MTL positionbased on EOF1 only

Correlation between seasonal wave forcing and alongshore-averaged contour evolution

Correlation between seasonal wave forcing and amplitude evolution EOF 1

Correlation between seasonal wave forcing and beach width evolution

Conclusions

At Egmond beach :

• Position of the shoreline largely (75%) determined by positioning and amplitude of naturally occurring spatial patterns, rather than cross-shore movement of the beach as a whole

• Beach nourishment incorporated in dominant spatial pattern (explaining 83% of spatial pattern variance) within months

• Seasonality wave forcing of subordinate importance in observed beach evolution

• Regarding prediction of beach evolution at seasonal scale we need to assess the relation between bar evolution and beach evolution (e.g. delayed beach response?).

Longshore position 2942

June 1999

September 1999

March 2000D

istance from R

SP H

eight (m)

9 June 1999

20 December 1999

5 October 1999

15 March 2000

5 September 2001

11 September 2001

77 hours NNW-storm (315-330)

Variance explained by EOF1: 83.1%

= beach nourishment

December 2003

Conclusions

Present case / in general:

• Local problems (1-2 km, 1-5 years) can only be properly understood/tackled by involving larger scales (tens of kilometers/years)

  

This uninterrupted coast (Egmond-Bergen):

• 3D features important, also on a decadal timescale

Local shallows (spacing 2km) on the breaker bar

Interplay between bars and shore

Conclusions (continued)

After the shoreface nourishment(s):• NW-storms straighten the bar/

SW-storms induce crescentic shapes between the shallows

(local shoreline: S-N oriented)

Conclusions (continued)

• Shoreface/beach nourishment quite satisfactory in terms of CSI’s (4-5 years stead of 2 years)

Negative side-effect probably could have been prevented

Argus (with its high resolution) helpful :• At a local scale, and as a complementary

instrument in studies of larger scales

In the design of a new nourishment ?

10 O

ctob

er 2

000

24 N

ovem

ber

200

0124 hours S

W-storm

SW

-storm (30 hours)

NW

-storm (25

hours)

Dec

embe

r 20

00

1214

16

29 December 2001

22 February 2002South 96 hoursNorth 13 hours

24 February 2002North 38 hours

28 February 2002South 25 hoursWest 19 hours