Estimating Rainfall Design Values for the City of Oslo, Norway—Comparison of Methods and Quantification of Uncertainty

Julia Lutz,Lars Grinde,Anita Verpe Dyrrdal

doi:10.3390/w12061735

Julia Lutz, Lars Grinde + Show 1 more

Open Access

https://doi.org/10.3390/w12061735

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Journal: Water	Publication Date: Jun 17, 2020
Citations: 13	License type: CC BY 4.0

Affiliation: Norwegian Meteorological Institute

Abstract

Due to its location, its old sewage system, and the channelling of rivers, Oslo is highly exposed to urban flooding. Thus, it is crucial to provide relevant and reliable information on extreme precipitation in the planning and design of infrastructure. Intensity-Duration-Frequency (IDF) curves are a frequently used tool for that purpose. However, the computational method for IDF curves in Norway was established over 45 years ago, and has not been further developed since. In our study, we show that the current method of fitting a Gumbel distribution to the highest precipitation events is not able to reflect the return values for the long return periods. Instead, we introduce the fitting of a Generalised Extreme Value (GEV) distribution for annual maximum precipitation in two different ways, using (a) a modified Maximum Likelihood estimation and (b) Bayesian inference. The comparison of the two methods for 14 stations in and around Oslo reveals that the estimated median return values are very similar, but the Bayesian method provides upper credible interval boundaries that are considerably higher. Two different goodness-of-fit tests favour the Bayesian method; thus, we suggest using the Bayesian inference for estimating IDF curves for the Oslo area.

Highlights

Around midnight on 4 August 2019, an intense thunderstorm surprised the forecasters at the Norwegian MET office (MET Norway) and the population of Oslo city
Both the Generalised Extreme Value (GEV)-mML and GEV-Bay estimates are lower than Gumbel-RV for short return periods, similar to return periods from about
20 to 50 years, and slightly higher for the longest return periods. This means that the 10- and 20-year return values that are amongst the most-used return periods do not deviate much between the methods, while the 200-year return values increase significantly, especially with GEV-Bay

Summary

Introduction

Around midnight on 4 August 2019, an intense thunderstorm surprised the forecasters at the Norwegian MET office (MET Norway) and the population of Oslo city. The thunderstorm divided the city into two, with dry conditions in the west and intense rainfall in the east. The return values for the shortest durations were estimated to exceed the 200-year level, resulting in urban flooding and a large amount of damage to property and infrastructure. Such events have become quite common in the southern part of Norway, partly due to an increase in the frequency and intensity of short-duration rainfall [1], and partly due to urbanisation and vulnerable infrastructure.

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