An Application of the ACER Method to Norwegian Water Level Data: A Comparison of Return Level Estimates

Abstract

In this Master's thesis we have used data from eight different locations in Norway to estimate the distributions of extreme water levels. The locations used are Oslo, Heimsjø, Honningsvåg, Narvik, Harstad, Tregde, Andenes and Viker, and four different methods have been used for the estimation. We use two established and widely used methods, the Annual Maxima Method (AMM) and the Peaks-over-Threshold (POT) method. The AMM uses the observed annual maxima from each location, while the POT method uses the exceedances of some high threshold in relation to the data. In addition, we also use two approaches that are less known and less used, the Revised Joint Probability Method (RJPM) and the Average Conditional Exceedance Rate (ACER) method. In the former, a distribution for extreme surges is found and used together with information from numerical tide predictions. We also take into account the interaction between tides and surges where applicable. The ACER method uses only the series of measured sea levels, but tries to account for dependence by a cascade of conditioning probabilities. We find that for return periods up to 20 years the return levels are usually very similar for all methods except RJPM, which seems to underestimate these values. The data intensive methods of POT, RJPM and ACER seem to agree for the 200 year levels, while particularly the Gumbel method seems to overshoot the levels in comparison to the other methods.