Climate adjusted design values for extreme precipitation and flooding

The extreme weather event Hans in August 2023 hit Eastern Norway hard and led to record-breaking floods in small streams to the large catchments. Similar high impact floods in late summer floods occurred in 1938 and 1789. At the same time, Hans is an example of a type of flood we can expect more of in the future, with both heavy rain showers and large amounts of rainfall over large areas that is already wet after a wet summer. Society can benefit from being sufficiently prepared for the extreme events we have today and can expect in the future. To help us be prepared for future floods, researchers on extreme rainfall, floods and statistical modeling have worked together with owners of important infrastructure and representatives from municipalities in the ClimDesign project. In the project, the researchers used historical measurements of water flow and precipitation to develop new statistical models to calculate the risk of extreme flood and precipitation events from very short-term events (a few minutes) to long events over several days. The calculations are made for places both with and without measurements. Furthermore, extreme rainfall has been calculated for catchments with everything from small to large areas. The researchers also analyzed the connection between extreme events at different locations and explored how extreme events change when the climate changes. Important results from ClimDesign: • How much water comes in one minute, one hour, or one day depends on where in Norway you are. • The new models for extreme precipitation developed in ClimDesign provide higher precipitation intensities than previous generation models. • Completely new and flexible models have been developed to calculate design floods for several durations. • New and more flexible models for calculating design flood values ??where measurements are not available show better results than the existing models. • How much extreme precipitation is reduced when the size of the catchment area increases depends on where you are in Norway (greatest reduction in Eastern Norway and in Finnmark) and season (greatest reduction in summer). • We have analyzed how often we can experience floods and extreme rainfall within an area. This depends on which return period we are interested in. Extreme events with a high return period (e.g. 100-year events) often have smaller spatial extents than events with a shorter recurrence interval (e.g. 10-year events). • Future extreme events of short duration may become even more extreme than previously thought. This is shown by climate models that better describe intense rain showers (convective precipitation) Results from ClimDesign have been published in international and national scientific journals. In addition, several of the results have been implemented in tools and routines that can be used by end users (See, for example, https://klimaservicesenter.no/ivf?locale=nb).

Viktige virkninger fra ClimDesign er økt nasjonalt og internasjonalt samarbeid mellom forskere som jobber med ekstremnedbør og flom samt mellom forskere og sluttbrukere som må takle ekstremnedbør og flom. Nasjonalt kan vi fremheve at samarbeid mellom statistikere, meteorologer og hydrologer er forsterket og inkluderer flere yngre forskere og en PhD-kandidat. Internasjonalt har ClimDesign knyttet faglige kontakter til forskningsmiljøer som jobber med tilsvarende tema. ClimDesign har bidratt til å forsterke kommunikasjonen mellom forskere og sluttbrukere ved å la sluttbrukere fra kommuner og eiere av infrastruktur bidra i forskingen gjennom seminar og befaringer. Slikt samarbeid gjør at man får identifisert kunnskapshull som forskinga må fylle for å bidra til forsvarlig forvaltning på tvers av sektorer. Flere av resultatene er allerede eller vil bli implementert i verktøy som er offentlig tilgjengelige ved Meteorologisk Institutt og ved NVE og vil bli en del av kunnskapsgrunnlaget når anbefalinger for beregninger av ekstremnedbør, flom og klimapåslag skal oppdateres. Effektene av ClimDesign er et samfunn som er mindre sårbart for ekstremnedbør og flom. Disse effektene sikres gjennom å forbedre metoder og verktøy for beregninger av dimensjonerende nedbør, oppsummere nye anbefalinger i rapporter og veiledere samt kommunisere resultater i nasjonale fagseminar og tidsskrift..

The main objective of ClimDesign is to generate user-relevant design values based on intensity-duration-frequency (IDF) and flood-duration-frequency (QDF) curves that account for a changing climate at individual locations and across regions. A project team of experts on floods, extreme precipitation and statistical methods together with major infrastructure owners, will collaborate to handle the challenges emanating from this objective. ClimDesign will move the frontier of knowledge by (i) improved understanding of temporal changes in extreme events and handling non-stationarity in estimating design values, (ii) quantifying the risk of extreme events within a region and (iii) incorporating projected changes in extremes into design values. A particular challenge related to climate changes effects is the expected changes in seasonality of extremes. Spatial analysis of extremes is important for the estimation of an area reduction factor of extreme precipitation, i.e. what is the ratio of a 200-year precipitation event for an area of 1000 km2 compared to a rain gauge? Spatial analysis of extremes is also important for assessing the spatial extension of a 200-year flood and for assessing how often one of the culverts along a road might experience a 200-year rainfall event. The high-dimensional extreme value modelling needed for analysing spatial extremes is a methodological challenge. Selected project findings will be implemented in operational tools, and the project team will suggest updates of existing guidelines for flood estimation. Infrastructure owners, areal planners, municipalities and consultancies’, will use the project results for climate change adapted estimates of design values. This will reduce flood risks and thereby reduce the vulnerability to floods and increase infrastructure resilience. The research tasks will be carried out on case-studies suggested by the stakeholders, ensuring that there is a close link between research questions and user needs.