1.4.01 The consequences of a hundred-year flood – pollution following the flooding of the Elbe

1.4.01 The consequences of a hundred-year flood – pollution following the flooding of the Elbe

When extreme flood events mobilise pollutants as in the Elbe basin, large amounts of harmful sludge and wastewater are distributed over the flood plains – and also residential areas and farmland. A team of scientists investigated the pollution left behind and evaluated the ensuing risks. The result: pollution levels within the soil were generally no greater than they were prior to the flood. Nevertheless, the experts recommend a comprehensive water and risk management system for future use.

In August 2002, heavy rainfall caused extreme flooding of the Elbe and its tributaries, leading to significant contamination of the flooded area. The floods released pollutants from old sources, expanded contaminated river sediments and carried along polluted soil and excavated material from industrial areas and mine dumps. Oil flowed from tanks on private property and both communal and industrial wastewater escaped from overcome sewage plants into the rivers. The water infiltrated residential areas, gardens and farmland, where the solids were deposited and formed a layer of sludge contaminated with heavy metals, organic pollutants and harmful germs. As such, the risks posed to health needed to be clarified as soon as possible.

The first stage saw numerous research institutes and authorities independently examining the effects of the flood on the pollution levels in the groundwater and flooded areas. So that these measurements could be combined and an analysis gained of the overall situation, the BMBF initiated a research project called “Schadstoffuntersuchungen nach dem Hochwasser 2002 – Ermittlung der Gefährdungspotenziale an Elbe und Mulde” (Pollution investigations after the flood of 2002 – determining the potential risk at the Elbe and Mulde). 28 partners worked under the guidance of the centre for environmental research at Leipzig-Halle to investigate the river basins of the Mulde and Elbe from the Czech Republic to Hamburg.

Chronic pollution of river sediment

The numerous pollutants in the Elbe come from many sources. Elements such as arsenic and heavy metals occur naturally across the entire basinPart of the earth’s surface that is drained by a river and its tributaries. We distinguish between above-ground and underground basins. The drainage divide marks the boundaries of the basin. and have always been carried down from the bordering low-mountain regions. Depending on the river dynamics, they are deposited or carried further and cause the “geogenic background contaminationNatural pollution of the ground, water or air resulting from chemical compounds in the rocks of the earth’s crust.. The consequences of a hundred-year flood – pollution following the flooding of the Elbe ” in the waterways. Other sources of pollution include mining and other industrial activities within the region.

Virtually no changes as a result of the flood

The substantial (in part) concentration of heavy metals and organic pollutants exhibited by the rivers during the flood quickly dropped again as the waters receded according to tests. With just a few exceptions, the flood did not significantly increase the amount of pollution in flood plain soils and sediments. However, this should not take away from the fact that the regularly flooded land beyond the dyke beneath the confluence of the Mulde and Saale is heavily contaminated. As the recommended values for use as pasture were way exceeded in terms of dioxin and mercury in many samples, experts recommend the implementation of consistent use management. This would mean that the heavily contaminated hollows and watering holes should not be used and that grazing should only commence after cleaning through precipitation.

The great flood of 2002 also affected locations normally protected by dykes. In the researchers’ view, no acute risk is posed to the health of the population: the analyses indicated only disparate increases in the concentration of pollution. Nevertheless, the measurements indicated that the basic pollution load of the researched area was already pretty high before the flood occurred. (See UFZ final report: Schadstoffbelastung nach dem Elbe-Hochwasser 2002 (pollution load after the Elbe flood of 2002) at www.ufz.de/data/HWBroschuere2637.pdf).

Introducing risk management

Floods and the associated hazards are not going to go away. However, the intensity of the events and the extent of the damage can be reduced. As such, the researchers propose employing preventive water management and land usage in a far more consistent manner than before and consolidating fragmented areas of responsibility for flood-related issues. The aim is to ensure integrative and interdisciplinary water management in the river basin, which is also to include flood risk management. The researchers view integrated pollution management – particularly for the Mulde and Saale – as a potential basis for long-term remediation within central Germany.

A recently developed pollution distribution model is the first step in this direction. The coupling of a hydraulic model with a land and pollution distribution model for the first time enabled scenarios for moderate to extreme flood events to be determined and then fed into a decision support systemSee ►DSS. – a system already in use within Anhalt Bitterfeld. It is also used for current flood exercises and for resettlement operations in order to assess the risk of pollution ingress during flood events.

In order to reduce the risk posed by toxins and harmful germs during future floods, the experts also advocate the creation of handling instructions for dealing with flood sediments, installing pollution sources such as private oil tanks and heaters and commercial chemical stores in locations secured against flooding and devising measures to enhance protection of industrial facilities, sewage plants and other similar installations. The scientists also recommend compiling research results through a database-supported decision support systemSee ►DSS. (DSS) so that the necessary information is available when making decisions in future.

Greater efforts should also be made to keep flood zones free from buildings and inappropriate use as a preventive measure (preventive land use). Abandoning farmland and planting greenery is an effective way to counteract erosion of contaminated soil on river flood plains. Undeveloped land also provides natural retentionIn the field of water management, this term refers to the balancing effect of reservoirs on the outflow of flowing waters; see also ->Retention basins. areas; this method all but eliminates damage to buildings.

In a second project funded by the BMBF called “Flood risk mitigation based on non-structural land use schemes in runoff generation and flood plains” (MinHorLam), scientists investigated the influence of non-structural land use measures on flood risks. One of the topics covered was the risk potential for producers and consumers when pollution contaminates the plant stocks and soils on land used for agriculture and forestry. The researchers devised damage- limitation measures such as changing land use, introducing special types of plant, accumulating pollution in only small amounts and compensation for leaving land uncultivated. The findings gained are being made available to the general public through an Internet platform.