Climatic changes in short duration extreme precipitation and rapid onset flooding - implications for design values

The goals of the project are an increased understanding of extremes in short-term precipitation, how they affect rapid but short-term intense floods (such as flood and urban floods) and how changes in climate can change the intensity and frequency of these types of phenomena. Particular focus was on the development of methodology to include information on climate change and climate uncertainties in the calculation of return values and return periods for these types of extremes. The results of the project are summarized in the report Climatic changes in short duration extreme precipitation and rapid onset flooding - implications for design values available on the website of the Norwegian Climate Services Center. (Https://cms.met.no/site/2/klimaservicesenteret/rapporter-og-publikasjoner/_attachment/13537?_ts=163df95ff7b) Main results of the project: - Quality assurance and analysis of historical observations of extreme precipitation on a short time scale (hourly values) show that a majority of the stations have seen an increase in extreme precipitation values on this time scale over the last 30-50 years. - A reduction in flood size has been observed in a number of areas over the past 50 years. This is particularly the case in catchments where snowmelt makes the main contribution to flooding. - An increase in the frequency of rain-generated floods has been observed over the same period. - A gradual shift from snowmelt-generated to rain-generated floods has been registered in Norway. - Analysis of modelled future changes in extreme precipitation on short time scales (down to hourly values) shows that the changes are considerably larger than for extreme precipitation on a daily time scale previously studied. An analysis of 10 regional climate models indicates that the 200-year event for extreme hourly rainfall will increase by +40 to + 50% in a high emission scenario (rcp8.5). - Future changes in floods in small drainage areas (which are typically vulnerable to flash floods) indicate that the increase in 200-year 3-hour floods is 20-25% higher than the changes in daily 200-year floods. - Hydrological model development has reduced the error in estimated run-off by about 50% compared to the previous version of the model. There has also been great progress in getting good hydrological simulations in drainage areas where there are no hydrological measurements against which the model can be calibrated. - An interactive map tool with Intensity-Duration-Frequency curves has been developed for the whole of Norway. These provide return values of extreme rainfall from 10 to 1440 minutes and return periods up to 200 years at all observational sites as well as for a grid of 1 * 1 km resolution covering the whole of Norway. The tool is available through the Norwegian Climate Service Center (https://klimaservicesenter.no/faces/desktop/idf.xhtml)

Prosjektet har gitt et betydelig bedre nasjonalt kunnskapsgrunnlag på ekstrem nedbør med kort varighet og påfølgende styrtflom. Det er foretatt analyse av historiske observasjoner av ekstremnedbør og flom samt analyse av modellerte framtidige forandringer i ekstremnedbør på korte tidsskalaer. Resultatrapport: Sorteberg m.fl., 2018: Climatic changes in short duration extreme precipitation and rapid onset flooding - implications for design values. Rapport 1/2018, Norsk klimaservicesenter. ISSN: 2387-3027. Lenke: https://cms.met.no/site/2/klimaservicesenteret/rapporter-og-publikasjoner/_attachment/13537?_ts=163df95ff7b Interaktive nasjonale kart av returverdier på nedbør fra 10 minutts til døgnlig varighet er gjort tilgjengelig: Lenke: https://klimaservicesenter.no/faces/desktop/idf.xhtml Resultatene vil danne grunnlag for at NVEs nettbaserte verktøy for beregning av dimensjonerende flommer (NEVINA) også vil kunne gi estimater på fremtidig forandring i momentanflommer.

The project focuses on improving our understanding of short duration (less than 24 hours) precipitation extremes, their relationship to rapid onset flooding (e.g. flash floods and urban floods), and the variability and sensitivity of these processes to climate change. Special emphasis is on incorporating information about climate change and climate change uncertainties into design values suitable for extreme precipitation and rapid onset flooding events. The project explicitly addresses several key foci of the call: It aims to increase understanding of how changes in climate may change the occurrence and intensity of natural hazards and to incorporate this knowledge in a useful way for impact and adaptation purposes. Project summary The project have direct participation and finacial contribution from the two main Norwegian agencies for road and railway work (The Norwegian National Rail Administration(Jernbaneverket) and the Norwegian Public Roads Administration (Statens Vegvesen)). The project specifically adresses the dissemination of the main results (WP4) and will have direct involvement with user communities, through workshops. End-user results and data sets will be disseminated through the Norwegian Climate Service Centre's (NCSC) dissemination system and the project includes funding of an interactive web-based tool which will be accesible through the NCSC webportal. Underlying data will be available through the national e-infrastructure for scientific data (NorStore). The project incoporate work from two PhDs which has already been funded and will fund one additional PostDoc. This will add directly to the national competence building. While longer duration floods and precipitation have been addressed in earlier research projects, this is the first project on short-duration extremes. Thus,we believe the project will be very beneficial by increasing the national scientific knowledge of the topic.