Changes caused by climate change in conjunction with increasing population density are leading to a significant increase in flooding caused by heavy rainfall events. The resulting damage to buildings could be significantly reduced by means of preventive measures, which is why the issue of surface runoff must be taken into account at an early stage in construction planning.

Text Hans Starl, Mathias Laudacher (in former function for BVS – Brandverhütungsstelle für Oö. registrierte Genossenschaft m.b.H.)

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In a world increasingly affected by extreme weather events, the construction industry faces the challenge of making resilience to nature a central element of its planning. Surface runoff after heavy rainfall events has become a particular focus of attention in Austria. This is because the enormous amounts of precipitation during thunderstorm events cannot always be drained away in a controlled manner via the existing drainage facilities. The resulting damage to buildings could be significantly reduced by taking preventative measures.

Surface runoff should therefore not only be seen as a task for architects, planners, municipalities and water management experts, but above all as an opportunity to minimize damage and secure the value of buildings in the long term through forward-looking planning and preventive protective measures. The federal state of Upper Austria serves as an example of adapted planning, which since 2021 has required corresponding detailed elaborations and protective measures for building land dedications and building permits depending on the hazard situation. OIB Guideline 3 has also taken this issue into account for years [1].

Upper Austrian foresight

In Upper Austria, the management of slope water is dealt with in a multi-stage and integrated manner, from the zoning of building land to the building permit, which enables effective prevention.

Regional planning procedure

Once the plot of land has been selected, the first step is zoning or an amendment to the development plan as part of the regional planning procedure. In Austria, the conversion of grassland into building land is not carried out lightly, but is carefully examined by the competent planning authority (local council) either through official channels or on the basis of a suggestion. The slope water hazard is already analyzed in this process. The assessment can indicate either a low or high slope water hazard.

Result high slope water hazard

This requires a conscientious examination by the regional planning authority and the department responsible for water management. In this case, a positive zoning decision will only be made if effective slope water protection measures have been planned and approved beforehand or a comprehensive surface water drainage concept has been positively assessed. The regional planning authority is responsible for ensuring that these protective measures are in place, which is guaranteed by a development plan or building land protection contract. In the spatial planning process, great importance is also attached to securing the necessary drainage corridors. These are recorded and preserved through zoning or development plans in order to enable natural and permanently functional, safe drainage.

Result low slope water hazard

Even in the case of a low slope water hazard, a reference to the water management relevance must be included in the dedication procedure. This ensures that it is taken into account in the subsequent construction process.

Suitability of building site

Without a fixed development plan, it is the responsibility of the building authority, represented by the mayor, to check the suitability of a building site. Every applicant – whether natural or legal – must submit all the necessary documents. A key role is played by the assessment of the slope water hazard, which must always be taken into account in the assessment process.

Construction process

If there is a risk of slope water, the slope and surface water situation must be taken into account in the submission documents, i.e. in the construction plan, in addition to the existing requirements. For example, hydraulic engineering solutions for self-protection and – if the planned measures/installations (e.g. building structures, access roads, enclosures, terrain design) change the slope water runoff – for the protection of third parties must be shown in the construction plan. If slope water is discharged into a body of water or a facility is planned in the HQ30 discharge area of a body of water, a water law approval procedure is required. This ensures that construction projects are realized in harmony with the natural waterways and protects both the environment and the infrastructure.

OIB Guideline 3 – Environmental protection as a central component

The topic was also addressed in the OIB guidelines. In OIB Guideline 3 “Hygiene, Health and Environmental Protection”, 2019 edition, point 6.2 stipulates that buildings must be designed in such a way that they are protected against the ingress of rainwater, including surface runoff. Consequently, this requires a detailed examination of the matter and the implementation of measures that go beyond mere waterproofing. This requires a comprehensive approach that ensures both self-protection and the protection of third parties from surface runoff.

Cooperation between authorities and planners Hazards caused by surface runoff cannot usually be solved on just one construction site and therefore represent a cross-sectional issue in many areas. It usually requires the interaction of all parties affected by water runoff in order to realize functional and sustainable solutions. These aspects also result in the resulting role of the responsible authorities, which demand this overarching consideration in relation to an adapted and specific detailed elaboration to minimize risk, even if the risk is often not perceived in this form by potentially affected parties in this phase of the construction process. However, it is precisely this cooperative approach of all those involved in the construction process that ensures in the long term that construction works not only meet current standards, but can also withstand future challenges.

Risk awareness creates a basis for prevention

However, successful implementation of preventive measures requires not only appropriate legislation, but also risk awareness on the part of property owners. A study carried out in Upper Austria by experts from the EPZ Elementary Damage Prevention Center shows that this still requires a great deal of awareness-raising: they carried out analyses based on surface water maps at over 150 locations in the hazard areas following real events. The risk assessment of the properties examined showed that 60% of these locations had a moderate to increased risk from surface runoff, but 90% of these property owners were unaware of the risk potential, e see diagrams 1 and 2. This currently rather underestimated risk perception in relation to the damaging effect of surface runoff is also the same in other parts of Austria or Germany, as confirmed by similar study results [2, 3].

Consideration of surface water in existing and new buildings

The entry points of surface runoff into buildings are varied, but every building can be protected – whether existing or new. The cost-benefit comparison between protective measures and potential damage is always positive due to the usually low water levels (< 30 cm). The EPZ’s surface water map uses simulation models to calculate where there is a potential danger from surface runoff and generally distinguishes between three hazard classes:

• Low hazard: no further protective measures required, apparently no or only a low hazard from surface runoff recognizable
• Moderate hazard: protective measures required for property protection (self-protection)
• Increased risk: Protective measures for property protection (self-protection) are necessary and their effects on third parties (third-party protection) must be evaluated.

The EPZ surface water map is used for risk assessment and, where necessary, for the development of preventive protection measures for existing buildings and new construction projects.

For identified “problem areas”, there is a wide range of cost-effective, mostly structural, protective measures that can be implemented on the property or object. This means that the “increased risk” hazard class is not an unavoidable problem, but rather the project-specific implementation of preventive protective measures. If these are already taken into account during construction, there are usually no additional costs. The surface water map will be published in 2024 in the HORA platform (Natural

Hazard Overview & Risk Assessment Austria) and is available for all of Austria at www.hora.gv.at available.

Summary

Safety through planning: The forward-looking and detail-oriented approach in the Austrian spatial planning and construction process reflects the need to understand and respect water as a “shaping” element of construction. Incorporating the issue of surface runoff into construction planning lays the foundation for sustainable, safe and nature-integrated structures that are better equipped to meet the challenges of climate change.

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Bibliography

[1] Land Oberösterreich: Hangwasser, available at https://www.land-oberoesterreich.gv.at/260919.htm, checked on May 15, 2024.

[2] Zahnt, N. et.al.: Herausforderungen durch pluviale Ãœberflutungen – Grundlagen, Schäden und Lösungsansätze, 2017, available at https://www.researchgate.net/publication/321317505_Herausforderungen_durch_pluviale_Uberflutungen_-_Grundlagen_Schaden_und_Losungsansatze, checked on May 15, 2024.

[3] GDVGesamtverband der Deutschen Versicherungswirtschaft e.V., GfKUmfrage im Auftrag des GDV, 2016, https://www.gdv.de/resource/blob/32404/11 f81def69f956e3654fc1df0eb2ae0e/download-keineangstvor-ueberschwemmung-data.pdf, retrieved on April 2, 2021.

[4] Starl, H.: Gebäudeschäden in Österreich als Folge von Hagelschlag und pluvialen Überflutungen: Preventive building protection measures under consideration of climate change-induced changes, dissertation, TU Graz, 2023.