The government of Upper Austria prepares a tender for a state-wide hazard map of flash flood endangered areas away from permanent water bodies. Areas prone to inundation shall be delineated using computer simulations. The boundary conditions for these simulations (design rain under climate change, initial soil moisture conditions) must be derived.
Change in design rains and soil moisture data in combination with daily rain-sums for the reference period and future period give the bandwidth for the selection of initial conditions and allow future situations to be accounted for.
Flash floods are a major threat to settlements away from permanent waterbodies. Using climate indicators, changes in the initial conditions for the simulation of inundation areas could be investigated considering the future meteorological conditions.
Variation of input conditions and impacts on flash flood inundations and water depth
Case Study Description
The case study deals with flash floods occurring away from permanent water bodies. High intensity storm events introduce an overland flow and flood situation before the water reaches a river or creek. Today, the federal government seeks to prepare state wide maps, indicating zones prone to flash flooding. Instead of deriving classical flood hazard maps indicating recurrence intervals and flood inundations for certain recurrence intervals, 'safe zones' from flash flooding will be mapped. Classical inundation mapping is planned in a second step, considering a more detailed simulation approach.
For the flash flood simulations, standardized boundary conditions, initial conditions and inputs under current and future climate situation will be derived. Whereas current design rainfalls are available in Austria, altered design rainfalls (or relative changes) are not. Further, the preconditions in the rainfall-runoff situation remain to be defined.
For the client, basically two aspects are of interest, because the government of Upper Austria plans to contract out a project for a statewide hazard-map:
- What assumption should be made for the initial loss and continuous loss in a design process?
- How do design rainfalls alter under climate change conditions?
The investigation of flash-floods and the triggering preconditions showed, that rain exceeding a certain amount can trigger a flash flood independent of reconditions, although the necessary rain must be higher if soil is less saturated following less antecedent rain. As the first results show a big influence of preconditions on the discharge of a flash-flood, we recommend to use a highly saturated soil.
The SWICCA IDF curves showed lower intensities that the corresponding Austrian design rains. SWICCA indicators show an increase in rain intensities for all emission scenarios. Therefore, it can be assumed that design-rains should increase accounting for future climate conditions. The client will set the rain intensities for his project of a hazard-map considering these insights.
The use of design rainfalls and their altering for different future scenarios introduces a new and important part in the design of mitigation structures and evaluation of future risk mapping. It is expected, that allowing the consideration of increased design values in the design process, leads to more sustainable mitigation structures. Still, from a funding perspective, users (engineering and political level) are required to set this design strategy to be standard.
The time scale at which climate change impacts are considered is estimated to be more than 20 years. Depending on the availabilty, altered design discharges for 50 years from now are also useful.
Flash flooding is - by nature - a process linked to small temporal and spatial scales. For spatial scale, catchment of several km² up to max 100 km² are considered, where permanent river reaches build the lower boundary. The temporal scale is typical linked to rainfall events of 15min to 2-3 h duration. The spatial extent of high intensity rainfall is as well limited to several kilometers.
The stakeholders in Upper Austria are aware that the design values from the SWICCA portal are subject of considerable uncertainties and do not correspond to national design values. Nevertheless, they are very interested in relative changes of design rainfall values under climate change conditions. The largest problem is currently the eligibility of protective structures. In this case, guidelines for the application of 100-year design rainfall must first be defined. Taking climate change into account would be the next step. The statistic classification of flood events in the last two years has been transmitted by department of water management and flood protection of Upper Austria and was taken into account for the case studies. The processing of the preconditions as runoff-control component was demanded by the decision makers in Upper Austria and classified as very important.
For part (a), altered design rainfall under climate change conditions, the following indicators have been used and compared:
- SWICCA portal: “Precipitation Intensity Duration” (formerly: “Intensity Duration Frequency IDF”)
- Portal of National Hydrological Service http://ehyd.gv.at: “Bemessungsregen” (design precipitation)
For part (b), we used wetness1 and wetness2 and soil water content to get an idea about the former and future situation. For the detailed investigation other indicators available at the SWICCA portal could not be used as we had to compare rainfall sums of past flash flood events with their initial conditions. To filter out such events, we need precipitation data from rain gauges in high temporal resolution (at least in 15 min intervals, otherwise convective precipitation events cannot be identified as such) and a data source of occurred damages (possible minor events with no damage are not subject of investigation). The SWICCA-indicators wetness 1+2 or precipitation could not be used, as they only were representative for the period 1970-2000 and included no information about the situation in 2016. On the other hand we unfortunately got no data from former flash flood events in that period from 1970-2000.
Flash Flood events have been derived from the following data sources:
- publicly available database of fire brigade operations in the State of Upper Austria (For this study, data was available only for the months May – September 2016): http://intranet.ooelfv.at/
- Regional Newspaper articles
- Precipitation sums (5 min intervals) of 63 rain gauges from Upper Austrian Hydrological Service (data available until 31.12.2015)
- For year 2016: Precipitation sums (5 / 10 / 15 min intervals) of privately owned rain gauges from Weather Underground Portal (no other data sources available): https://www.wunderground.com/wundermap
To be able to deduce on the initial conditions, we used the following indicators:
- Accumulated precipitation sums before the respective flood event (12h / 24h / 48h / 72h / 7d / 14d before the flash flood event) from rain gauge data sources stated above
- ERS-1/2 AMI 50 km Soil Moisture Time Series: http://rs.geo.tuwien.ac.at/
- ERS-1/2 AMI 50 km Soil Water Index Time Series: http://rs.geo.tuwien.ac.at/
- Assessment of relative changes of design rainfall based on pan-European indicator (provided by SWICCA)
- Relative changes to design discharges (varying durations and intensities) from pan-European data are applied to local design rainfalls.
- Simulation of flash flooding considering varying inputs (durations and intensities)
- Assess preconditions given at past flash flood events. Prior rainfall intensity and/or sums may be derived from pan European or local rainfall data set.
- Simulation of flash flooding (hydrology and hydraulic) using different preconditions.
- Derivation of the sensitivity of flood extends / inundations onto initial loss variations.
For part (a) and (b)
4. Derive flood extend maps (local data (e.g. DEM)
5. Derive flood inundation maps based on local data (e.g. DEM)
6. Comparison of flood extend and inundation maps due to original and altered design rainfall input. Present results to the client and describe the effects of preconditions and altered design rains on flash floods.
We found that there are many data regarding precipitation that deal with daily rain sums or seasonal values. But there is little information about precipitation with shorter duration in the SWICCA portal by now. Background is, that climate change data is currently limited to larger scale (time and space) and is as well limited with regard to convective processes included in the climate models. Still, for the purpose of modelling flash-floods and evaluating future changes, indicators for intense rains with short duration would be valuable. In this project, we used a relative changes indicated for short durations and applied these to national data sets as a first approach.
Also hosting real time data for precipitation with short time resolution will be a major value for the upcoming database because it would be of use for many water managers (flashfloods, sewage systems operation etc).
The importance in simulating local flash floods is based on two aspects. First, local flash floods are a hazard to existing property. Thus, the flood mapping is a first step to identify local hot spots in the project area and to develop mitigation strategies. Secondly, erecting new property is limited in flash flood prone areas. Further to the current situation, future flash flood prone areas (due to climate change) can be identified.
Climate services as such can provide new and adapted inputs to models as they are updated. Authorities and consulting engineers can access the latest data sets and apply them to the design process where relevant.
Without the use of data from the climate service the effects of changed inundation areas could not be assessed and this would lead to future damage to settlements, because flash flood prone areas would not be detected in advance.
"The provincial government of Upper Austria expects a substantial benefit from the results in SWICCA ... the project provides a valuable discussion basis for aiming to set new, lasting design values. Thus, the questions raised at the project beginning could be answered complete and served us in our decision making processes". Felix Weingraber, Amt der OÖ Landesregierung, Abt. Oberflächengewässerwirtschaft.
With regard to cost benefit analysis, potentially higher standards will be applied in regulations for permitting residential areas beforehand. The more frequent occurrence of losses due to climate change (if applicable) is considered from the beginning. Although a cost benefit analysis is not carried out at this point, the positive effect in properly considering potential changes is seen.
In Austria, different rules and guidelines exist on how flood risk management is implemented. The stated HQ100 level is as well applicable in the context of funding of mitigation structure. A cost benefit analysis is to be made, comparing cost for the mitigation measure and the avoided flood damages. Still, fluvial floods are generally included in this course whereas pluvial floods (as given in the case study here) are not. A catchment based assessment of the flood damage and mitigations is a possibility to as well consider pluvial flooding in the funding process of mitigations structure. Aside from public funding for mitigation measure, inputs covering future altering of design values are valuable add on in any case, supporting the sustainable and lasting design of retention structures and other measures.
So far, SWICCA values and results are not implemented in the design and assessment of mitigation structures or risk maps. Still, the value is recognized and results and method may be considered.
DI Günter Humer, Consulting and Engineering Company Dipl.- Ing. Günter Humer GmbH
Tel:+43 7732 4146
DI Dr. Stefan Achleitner, Unit of Environmental Engineering, University of Innsbruck
Federal government of Upper Austria, Section of surface water bodies
Purveyor: DI Günter Humer, Consulting and Engineering Company
Dipl.- Ing. Günter Humer GmbH
Tel:+43 7732 4146
DI Dr. Stefan Achleitner
Unit of Environmental Engineering, University of Innsbruck
Value added by Copernicus Climate Change Service:
Client: Federal government of Upper Austria, Section of surface water bodies
Inundation due to flash flood for 100 year return period