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1.4.03 Flood events – the forgotten groundwater

What are the consequences of a groundwater level raised by a flood, particularly for built-up areas near rivers? The MULTISURE project has been seeking answers to this long-neglected question. The scientists involved developed models to enable potential risks and damage to be more accurately pinpointed in future, using Dresden — which is located on the Elbe – as an example.

The specialist community most often perceives a flood as an event involving extreme effluent over the surface of the ground. In addition to flooded surface areas, a rise in groundwater as a result of flooding can also be observed – especially after long periods of flooding in wide valley flood plains. Before now, science rarely factored in the risk to underground structures and infrastructures posed by fast-rising groundwater as a consequence of extreme flooding.

So, how can the risk and damage potential from fast-rising groundwater be estimated? What is the required make-up of models that can predict underground damage due to extreme flooding? These questions were the focus of the project entitled “Development of Multisequential Mitigation Strategies for Urban Areas with Risk of Groundwater Flood” (MULTISURE), led by the Dresden Groundwater Research Center (DGFZ), involving several institutes and running from 2006 to 2009. The aim of the project was to develop tools used to map and assess the hazards, potential damage and risks arising from the interactions between flooding, groundwater and underground infrastructures. The site chosen for investigation was Dresden along with the Elbe valley aquifer and both existing and planned underground building developments.

Two damage models developed

Threat from below: effects on the building structure

Threat from below: effects on the building structure
Threat from below: effects on the building structure

The project partners began by investigating how approaches for estimating damage through flooded rivers can be applied to groundwater-related flooding, and how the two events interact and overlap. The German Research Centre for Geosciences (GFZ) modified the meso-scale damage model developed for river flooding called FLEMOps (top-down approach) in order to estimate the damage caused by rising groundwater. This involved conducting telephone surveys with those affected — specifically regarding damage that occurred outside the flooded surface areas or were caused solely by groundwater. As such, findings could be obtained on handling the groundwaterrelated flooding individually and on the material and financial damage.

A bottom-up approach involved describing damage for types of building and infrastructure in relation to age — and depending on the groundwater dynamic. This enables the remediation measures required to rectify the damage to be determined, and also their costs. Expanding on this, the Leibniz Institute of Ecological Urban and Regional Development (IöR) is developing the new GRUWAD model (damage simulation model for building damage related to groundwater).

Multiple scenarios created

The groundwater risk assessment and depiction used both the above modelling approaches (top-down and bottom-up). FLEMOps and GRUWAD were used for a GIS-based determination of damage caused by groundwater at different spatial resolutions. The basis for this were the scenarios created by the Dresden Groundwater Research Center for the highest groundwater levels under various flood conditions in the Dresden Elbe valley and the implementation of various protective measures (database previously was flood events plus current planning within Dresden).

Threat from below: groundwater escaping to the surface

Threat from below: groundwater escaping to the surface
Threat from below: groundwater escaping to the surface

Interviews conducted

Intensive communication and co-operation among everyone involved is a key in ensuring efficient flood risk management: this includes city and state authorities, associations, scientists and residents. MULTISURE has analysed and evaluated these processes.

The basis for co-ordinated public action in flood prevention is meaningful information – and this also increases risk awareness and personal prevention measures among citizens. The interviews conducted during the course of the project and the analysis of existing means of information and communication were co-ordinated by the Institute for Environmental Communication at the Leuphana University of Lüneburg. The resulting brochure – primarily aimed at the general public – deals particularly with the personal responsibility of those affected and is therefore intended to intensify risk prevention.

The project results were fed by Görlitz/Zittau University into the city of Dresden’s information system. This enables internal access within authorities to key project results. As such, the results can be used to improve authorities’ internal analyses and decisions and to provide the public with information on the risks posed by groundwater-related flooding.

Dresdner Grundwasserforschungszentrum e.V.

Dr. Thomas Sommer
Meraner Straße 10
01217 Dresden, Germany
Tel.: +49(0)3 51/4 05 06-65
Fax: +49(0)3 51/4 05 06-79
E-mail: tsommer@dgfz.de
Internet: www.dgfz.de
Funding reference: 0330755
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