The Effect of Temporal Length of Current Measurements on the Derived Design Level

The duration of current measurements is often short, ranging from a few weeks up to a year. Application of extreme value statistics to derive design levels requires relatively long time series. To mitigate the lack of long-term measurements, the Norwegian standard NS9415 for fish farm design requires the design level of 50-year return period to be derived by multiplication of the current maximum in month-long current measurements by a prescribed conversion factor of 1.85. Here we use twelve data sets of yearlong coastal current measurements to explore the validity of this factor. For each yearlong time series, a design level of 50-year return period is calculated by extreme value statistics and used to calculate estimates of the conversion factor. The mean value of the resulting conversion factor is close to that of NS9415, 1.85 and 1.80 at 5 and 15 m depth, respectively. However, the spread in values is great, both geographically and between months. A conversion factor ranging from 1 to 4 reflects different relative dominance of the driving forces at different coastal regions and different seasons. The absence of a significant seasonal cycle in the conversion factors calculated here, illustrates the difficulty in adjusting for season. The results illustrate and quantify the uncertainty and — often — the lack of conservatism in design levels derived from month long current observations.