Nearshore bar dynamics at Egmond aan Zee, the Netherlands. -An analysis of bar stages derived by Argus video imagery. Marie-Louise Søndberg Master student Institute of Geography University 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 H m : 1.2 m. From Nov-Jan: 1.7 m and from Apr-Aug: 1.0 m. T m : 5 s. 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 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 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.

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

UNDULATIONINDEX 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: m 1 st bar: m 2 nd bar: Undulation-index: Intertidal bar: 18.7 m 1 st bar: 31.3 m 2 nd bar: 19.9 m

CLASSIFICATION 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 rips Undulating 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 the year.

RESULTS Undulation-index for the intertidal, 1 st and 2 nd bar. Variation and flexibility is greatest for the 1 st 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 – 34.1Variation (2nd bar): 13.2 – 24.1 Variation (1st bar): 14.5 – 51.2

RESULTS Red dots: undulation-index Black line: wave height (h s ) Horizontal Bar: classification. Linear bars: blue. Undulating bars: yellow Strongly undulating bars: red 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 Wave power Same pattern as the wave heights. R 2 =0.21 Average of wave power for the previous 16 days. Linear bars: blue. Undulating bars: yellow Strongly undulating bars: red 16 days gives the best relation between R 2 and the undulation- index.

RESULTS Linear bars: LR; Undulating bars: UR; Strongly undulating bars: SU Dots: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.

RESULTS Systematic model for the bar system at Egmond aan Zee. Showing the three different intermediate stages observed during year 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.

CONCLUSIONS Three bar stages were observed: Linear bars: Undulation-index < 22 Undulating bars: Undulation-index Strongly 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.

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