Aided by:This project (FKZ: 02 WH 0557) is aided by the Federal Ministry of Education and Research (BMBF) within the research focus „Risk Management of Extreme Flood Events (RIMAX)“.

Project partners:Coordination:

Problems and Aims

Steps to the Coupled Model

Perspective

The subsurface water distribution was the reason for many damages in the underground during the August 2002 Flood at the Elbe River.

So around 16% of the flood damages on the state areas of Saxony were caused by groundwater. Around 29 % of the demages were caused by water from the sewer system.

The reasons for the risen groundwater level can be:• infiltration of overland-flow,• flow through sewer systems,• groundwater flow from the recharge areas.

Development of a coupled software system that is capable of simulating the interaction between these three components in case of flooding events is the main focus of the project.

Detecting of the relevant flows between the overland flow, the flow in the sewerage and the groundwater flow by the coupled model is a second aim of the project.

The coupled system will make a contribution to holistic flood risk management and will support the planning of measures for preventive protection of the subsurface infrastructure.

TrimR2DRisoSurf

HYSTEM-EXTRANHAMOKA

PCGEOFIM

• The coupled model system will be applied to the quaternary aquifer of Dresden. The codes PCGEOFIM (groundwater), HYSTEM-EXTRAN (sewerage) and Trim R2D (overland flow) will be coupled for this scale.

• Special problems of feedback between overland flow and sewerage require fine mashed coupled modelling in local areas. Therefore in these local areas the code PCGEOFIM will be coupled with the simulation software RisoSurf (overland flow) and HAMOKA (sewerage).

• Finally, the modelling with the coupled system will allow an integrated flood risk management in urban areas, the planning of protection measures for the sewerage system and the strategy of protection of subsurface building components against the hazard of increasing groundwater or incoming sewerage water.

StrategyThe connection of existing modelling tools that have been approved for their particular area of application is the strategy of the coupling.

The used tools are:• TrimR2D and RisoSurf for the overland flow,• HYSTEM-EXTRAN and HAMOKA for the flow in sewerage,• PCGEOFIM for the groundwater flow.

The coupling software MpCCImanages communication between the single models. The communication comprises synchronisation of the tools, mapping between the different model geometries, and the interpolation between the different time scales of the processes involved.

Test ModelThe coupling will be tested in a test model. The test model must include all relevant processes of the interaction between the three flow paths overland flow, flow in sewerage and groundwater flow.

The part of the overland flow comprise the main river, a tributary stream and a depressed area without outflow.

The sewerage lies under and outside of the flood area.

The upper layers of the groundwater model must be subdivide finely because the processes of interaction between groundwater and sewerage and accordingly the infiltration of surface water are focused in these layers.

Process AnalysisThe process analysis has two steps.

The definition of the relevant water definition of the relevant water flowsflows between the individual components of the flow system is the first step. It includes the diverse states depending on the expansion of the flood. Each of the individual models works with his own boundary condition (INPUT / OUTPUT).

The definition of the timesdefinition of the times for the coupled modelling is the second step. In Dresden the river Elbe overflows at a high of 5 m gauge Dresden. This date is the beginning of the coupled modelling.

The finish of the coupled modelling is different. The interaction between the groundwater model and the flood model closes with the sinking of the flood under the high of 5 m gauge Dresden. The time of the communication between sewerage model and the groundwater model depends on the level of the groundwater table.

Spatial and Time CouplingThe exchange of the grid based coupling data is the main task of the coupling software MpCCI. Therefore, MpCCI must perform the neighbourhood searchneighbourhood search and must interpolate between the geometric components. These are volume and flat elements for the groundwater model, flat elements for the overland flow model and points for the sewerage model. The coupling data are the high of the water surface h [m] and the water flux rate q [m/s].

The coupling algorithmcoupling algorithm must consider the difference of the dimension of time steps. The time steps of the overland flow and the sewerage flow processes range to ∆t ≤ 1s. The groundwater flow processes go on with time steps ∆t ≥ 1h.

Therefore the overland flow and the sewerage flow processes will be coupled bilaterally in a subsystem with a little time step.

Development of a Coupled Model forFlood, Insfrastructure and Groundwater in Urban Areas

Th. Sommer, Dresden Groundwater Research CenterD-01217 Dresden, Meraner Straße 10, Fon: ++49-351-4050665; Fax: ++49-351-4050679; Email: tsommer@dgfz.de

Developmentcoupled modeling

system

Strategies forintegrated flood risk management

Detecting relevant flows

Alluvial clayFine sandCoase sand, with gravelAquiclude

Overland flow / Sewerage

Groundwater

Neighbourhoodrelationships

Detail area“Mickten”

Detail area“City”

Detail area“Laubegast”

Model area

Quaternary aquifer in Dresden

Flood area August 2002