Photo: BMLFUW/Rita Newman
Flood Control
The disaster events of the floods of August 2002, August 2005 and spring 2006 caused considerable damage. Floods can destroy existences and endanger lives. Which measures can protective water management take to offer protection against floods? How can people protect themselves personally?
Austria is a water-rich and mountainous country. It has a share in the East Alps, which cover almost two thirds of the national territory. But also extra-Alpine regions such as the Mühlviertel (mill quarter) or the Waldviertel (forest quarter) in the north of Austria are located at a higher altitude and mountainous. Consequently, only 38 percent of the federal territory are suited as permanent settlement areas.River valleys and basins have therefore always been the essential areas where settlement developed. Very early hydraulic engineering was used to reduce the risks for humans. Protective water management has a long tradition in Austria. However, every flood control system can provide protection only up to a certain extent. Certain risks will always remain.
It is therefore one of the key tasks of the Federal Water Engineering Administration not to respond to flood events on a short-term basis, but to develop a future-oriented strategy and planning. Apart from active flood control the measures of passive flood control, sufficient flood discharge and retention, are again gaining importance. The principle has to be to lay down and implement the protection of the water and the protection against the water in the framework of modern protective water management.
However, apart from the responsibility of public protective water management the population concerned also bears responsibility itself.
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Organisation of flood control
The organisation of flood control is divided into three major tasks on federal level, which are assigned to -the following administrative units:
1. Protective hydraulic engineering: Federal Water Engineering Administration: The care of all
waters except for the torrents and waterways is the responsibility of the Federal Water Engineering
Administration (Bundeswasserverbauung, BWV).
2. Torrent control: Torrent and avalanche control: Torrents and their catchment areas are the
responsibility of the Forest Engineering Service in Torrent and Avalanche Control (WLV) at the
Ministry of Life.
3. Maintenance and development of the waterways: Federal Waterways Administration: As
waterways, the rivers Danube, March and Thaya are the responsibility of the Federal Ministry of
Transport, Innovation and Technology (BMVIT).
The legal bases are provided in the following instruments:
- Water Rights Act (WRG 1959)
- Hydraulic Engineering Assistance Act (WBFG 1985)
- Technical Guidelines for the Federal Water Engineering Administration (RIWA-T 2006)
- Forest Law
- Regional planning legislation and spatial planning of the Federal Provinces
The European Water Framework Directive, which was transposed into Austrian legislation in 2003 by means of the amendment to the Water Rights Act, provides a comprehensive regulatory framework for measures in the field of water management.
Basics of water management
How do floods develop? Precipitation falls on earth in the form of snow or rain. It depends on the quantity of precipitation, the plant cover, the terrain and the type of soil and underground material to which extent precipitation sweeps away and thereby feeds the groundwater, evaporates, or is surface run-off in drains, brooks and rivers.
As precipitation is distributed over time, usually water flows in domestic rivers all year round. This “baseflow” of running waters is subject only to the natural, seasonal fluctuations.
Floods develop where a precipitation event exceeds a normal event in terms of duration and/or intensity, snow-melting occurs, or even both.
In the case of flood the extent of flow exceeding the baseflow is called flood wave. It is characterised by its maximum height and the time required to reach the flood peak and to return to the baseflow.
The discharge of a flood is like that of the precipitation event and depends on the size, location and characteristics of the catchment area of the water concerned. The smaller the catchment area and the stronger the precipitation event, the shorter is the alert time. The time until the arrival of a flood peak is thus in many regions not sufficient to take spontaneous protection measures. To be truly appropriate, flood protection therefore requires preventive activities.
In most cases the groundwater is closely connected with the surface run-off in a drain. Cushioned and with a time delay, the fluctuations of the discharge of the surface water body continue in the groundwater.
Therefore a flood event causes in most cases also high groundwater levels which, however, usually last much longer than the flood event itself. Moreover, in specific situations high groundwater levels can occur also without any visible connection to a flood event. High groundwater levels are of special importance where parts of buildings can be reached or surmounted.
For more information on flood risk zoning in Austria, see www.hochwasserrisiko.at.
Private initiative and residual risk of floods
Personal responsibility begins already when purchasing a specific piece of land: Due to the small share of permanent residential space in the federal territory and the high share of area that is already built up or sealed by traffic routes, trade and industry, land is a scarce, as also non-augmentable resource. Every day about 20 hectares of land are consumed in Austria for roads, houses and industrial plants.
As a result of this settlement pressure, plots of land in well suited locations have become expensive and scarce. Thus, in many Austrian regions, we have witnessed the trend to extend building land to areas potentially at risk of being flooded, as, in many cases, they might be the only plane lots still available in such regions.
Already before buying a piece of land situated in an area potentially at risk of being afflicted by natural disasters, information on potential hazards should be obtained from the municipal authorities or the official services responsible for water engineering. That’s the only way to avoid purchasing a permanent flood risk or potential damage for the mere sake of a favourable price or a beautiful location.
Private preventive safeguards in building and rehabilitation
When planning new buildings or rehabilitating existing ones, it is indispensable to take into account information on potential flood risks or high groundwater levels. Construction methods and materials suitable for the situation and respective risk levels contribute to reducing potential damage and costs linked to it, like, e.g. doing without cellars in the case of high groundwater levels, designing building apertures in a way allowing for their quick and secure closing in the event of flooding.
News and information about the usual local measures and appropriate building methods and materials are distributed by the building authorities of the municipalities and Federal Provinces.
Residual risk in flood control
There is no 100% protection against floods. Thus, according to the relevant guidelines, flood protection for residential areas and/or areas benefiting from higher protection levels will be tailored to a flood event statistically occurring every 100 years on average, with local deviations being possible.
This means a safeguard from flood events, but it does not mean that events statistically occurring every 100 years cannot occur more often than that or exceed projected flood discharge levels. Consequently, in the case of floods exceeding calculated flood levels we have to reckon with inundations also in protected areas, “behind the dike”.
Constructional basics
Without appropriate counter-measures floods and high-standing groundwater will cause a risk to buildings, parts of buildings as well as the associated outside structures. The relevant impacts result from the flow of the water, from the pressure exerted by the water as well as directly from the water which enters the buildings.
Over the medium and long term it is more cost-efficient and easier to plan and implement measures for the protection against the water than to restore potential damage.
Constructional measures for damage minimisation
At the building itself and its surroundings various measures can be taken to protect against damage caused by flood or high-standing groundwater:
Protection against groundwater:
- Waterproof basement (bitumen-based waterproof basement, waterproof concrete basement system).
- Avoid water backup from the sewage network.
Protection against floods:
- Prevent access of water to the building by means of water barriers (flood-protection constructions, sand
bag barriers, mobile protection systems, etc.).
- Seal to inhibit water from entering directly at the building (waterproof walls, ceilings, various closings,
stop-log systems, sand bags, waterproof windows/doors etc.).
Prevention inside buildings:
- Use building materials which are waterproof or water-insensitive and which have as little hollow space as
possible.
- Criteria for the choice of materials: Renewability, restorability, good drying conditions etc.
- Transfer heating facilities, current distribution boards and precious furniture to the upper floors: In flood-
prone parts of buildings, use mobile furniture only.
- Do not use oil heating in flood-prone regions (risk that oil escapes), fix oil tank against uplift.
- Preventive planning: Sufficiently dimensioned staircases facilitate the vacation of the building.
Other preventive and protection measures
- Preparation of the households: develop personal alarm plan
- Technical preparation: Store sand bags, mobile closing components, pumps, and wait.
- Keep equipment available for use and store supplies.
- Observe flood warnings of the hydrographical services of the Provinces.
02.07.2008, Lebensministerium Öffentlichkeitsarbeit