Consultancy Modelling and Optimization

Investigation effect halving measurement frequency in trendcalculations of the coastline

By the National Institute for Coastal and Marine Management/RIKZ every year measurements of the coastline of the Netherlands are examined to validate if some norm concerning a basis coastline has been satisfied. The meaning of these yearly tests is to be in time if the coastline is expected to be landwards. With the coastline measurements of the last ten years and with the method of linear regression the prediction of the coastline of the next year and the linear trend are calculated. A comparison of the prediction of the coastline and the basis coastline gives a validation of the norm.

In the figure above the lineair trend is positive and the predicted coastline (TKL) is below the basis coastline (BKL).

The question of this research is the influence of the halving of the frequency of the measurement of the coastline. To analyse the halving of the frequency the coastline measurements are divided in the classes 'all', 'even' and 'odd' years. 'All' means the yearly measurements, 'even' and 'odd' the two-yearly. In the figure below linear regression has been applied to these three classes.

It appears that the differences between the calculations of the trends and the TKL's are enormous. The influence of the halving of the frequency of the measurements of this lane are enormous.

In the next figure an areal image is given of the coast area Delfland. The different colors and the difference between the values of the TKL-BKL's are a rate of the influence of the halving of the frequency and the difference berween ‘even' en ‘odd' years.

The colors in the figure mean :

• Blue, both the linear trend and the difference TKL-BKL are positive (++)

• Green, the linear trend is positive and the difference TKL-BKL is negative (+-)

• Red, both the linear trend and the difference TKL-BKL are negative (--)

• Light blue, the linear trend is negative and the difference TKL-BKL is positive (-+)

Red grafics means no fullfillment of the norm. Red lanes are considered to be of special interest with respect to sandsupplements.

The results of this investigation are given in the dutch report 'Effect halvering meetfrequentie op berekende trend kustlijnligging' (june 1998)

More information about the subject is given on Waterland - 'Dynamisch handhaven van de kust'.