1.4.02 3ZM-GRIMEX project – coupled models simulate flood scenarios

When a flood occurs, the danger is not only posed by submerged land but often by rising groundwater as well. This spreads out under the earth and can cause a great deal of damage. Up to now, most water flow simulation systems have treated these processes separately or as two components, and it has not been possible to map surface water, the sewage system and groundwater together. A research project led by the Dresden Groundwater Research Center interlinked computer-aided models to provide a better assessment of how these components interact during flood conditions.

Floods in recent years have caused a terrible amount of damage. The amount of damage in Dresden alone amounted to around a billion euro in August 2002 – roughly 10% of the total damage nationwide. Groundwater counts for 16% of the damage to property in Saxony: it has therefore become apparent that floods can affect groundwater even in urban areas. The floodwater generally takes two courses as it spreads beneath the earth:

• Surface water getting into the groundwater and spreading. The groundwater flowing into the receiving watersThis term is used in the field of hydrology to describe the channels, such as watercourses and soil drainage, via which water can flow into a water body in the form of wastewater, rainwater or drainage water. Examples of natural receiving waters are open streams that take in and discharge water from other watercourses, groundwater bodies and discharge systems. from the hinterland backs up.
• Surface water spreading via infrastructures such as the wastewater sewage system (technogenous regionsUnderground water supply and disposal networks (sewage systems) are also described as technogenous regions.) to areas outside the region flooded directly.

Researchers at the Dresden Groundwater Research Center used model support to begin mapping the interaction between surface water and groundwater in flood conditions immediately after flooding occurred. The aim was to assist the clean-up in the wake of flooding and to improve prevention measures. At that time however, only individual technical model solutions were available for interactions between surface water and groundwater and between groundwater and the sewage system: they were based on simulation programs relating to a single component – surface water, the sewage system or groundwater.

Coupled modelling: three zones – one system

This is where the BMBF project “Development of a 3-Zone model for groundwater and infrastructure management after extreme flood events in urban areas” (3ZM-GRIMEX) was able to help. The scientists making up the project team developed an innovative modelling tool for the state capital Dresden that maps the 3ZM-GRIMEX project – coupled models simulate flood scenarios interactions between the following hydraulic components in extreme flooding on the basis of existing models: surface water effluent, effluent in the technogenous zone and groundwater. This coupled modelling system enables the development of solution strategies for designing and safeguarding the underground infrastructure networks, for managing groundwater-related flooding and for supporting town-planning decisions.

The experts used coupling software from the Fraunhofer Institute to link together simulation programs with a proven record in mapping significant flows of water during a flood. In doing so, they had to take into account both the time and spatial differences between the individual model components. To ensure that the coupling was successful, full awareness of the fundamental connections within the system – comprising sewage network, surface water and groundwater – and the time and spatial scales of the flow processes was required. A scale determined how a certain feature of a process was recorded and rendered measurable.

Computer-aided coupling process

Computer-aided coupling is based on the strategy that the individual modules – models for surface water, the sewage network and groundwater – calculate their respective water levels and throughflows as separate instances and then exchange these calculations. Each module then supplies these “coupling variables” to the others. The coupling software ultimately combines the information from the individual modules (blending). If, for example, sewage elements, groundwater levels and flooded surface areas are blended together, it is possible to determine which residents are affected by adverse weather and warn them in good time.

The programs used provided the project team with different challenges depending on the field of application. Take the sewage network for example: the hydrodynamic sewage network calculation did not require an especially high level of performance from the computers and the level of data in most areas was also very good. However, the only way to integrate the effect of the sewage system on the dynamic of the groundwater was to use greatly simplified approaches.

Practical application in Dresden

The focus of the first processing phaseA phase is a spatial area in which the physical parameters and chemical composition of the relevant matter are homogeneous. was on the individual models. This depended on harmonising their spatial relationships and recording all relevant water flows that have an effect during flooding and need to be mapped in the modelling system. The researchers created a general water flow schematic for this purpose, which formed the basis for the coupling activities. The main thing was to ensure adequate mapping of temporary components such as flood relief wells, flooded surface areas and overly backed-up sections of sewer. These algorithms were tested in a synthetic test model, which involved the team of experts trialling first the couplings and then all three instances.

The entire system has since been implemented in Dresden. The coupled modelling has enabled the exchange of water between surface water effluent, effluent in the sewage system and groundwater to be calculated for a variety of flood scenarios. Experts also used the new system to identify hotspots with latent flood risks due to emergent sewer water. The transfers from groundwater into the sewage system were able to be localised and quantified. The influence of emergent sewer water on groundwater only has a local effect during flooding, but depending on the intensity can lead to a significant rise in groundwater in the hotspots.

Project website (only in German) http://www.gwz-dresden.de/dgfz-ev/forschungsbereich/3zm-grimex.html

Read more:

1.4.03 Flooding and groundwater

Read more:

1.4.01 Consequences of a hundred-year flood