1.4.04 The underestimated threat posed by groundwater – damage assessment and prevention after the hundred-year flood

In the wake of the flood of August 2002, Dresden had more to tackle than damage to buildings and infrastructure: the groundwater had also risen by up to six metres in places and was taking a long time to recede. A team of local researchers and engineers therefore set about investigating the consequences of the flood beneath ground. The aim was to enable early detection in future of risks to underground facilities and the groundwater so that protective measures can be implemented. To achieve this, the experts modelled the dynamics of the rise in groundwater and analysed the nature of the groundwater as a supply of drinking water and the potential risks due to infiltrating pollution.

During the flood of 2002, groundwater levels underneath the Elbe valley way exceeded anything observed in decades. The triggers for this were the heavy rainfall over 12 and 13 August, the ensuing overflowing of the Elbe tributaries and the flood of the Elbe itself. The groundwater had an impact on building structures both above and beneath ground – their functional capability and stability were significantly affected by the fast-rising levels.

As the surface floodwater receded, it was then possible to assess the impact of the underground floodwater on the body of groundwater beneath the city of Dresden. This groundwater is a major source of drinking and process water and plays a major role in the stability of buildings and the urban environment. Scientists and engineers from TU Dresden and the Dresden Groundwater Research Center joined forces with local engineering firms and dedicated themselves to achieving this task through the research project entitled “Hochwassernachsorge Grundwasser Dresden” (Dresden groundwater: cleaning up after the flood), which was led by the city’s Environment Office. The experts started with short and medium-term consequences, and investigated them in line with the following focus points:

• Further development of the groundwater model to determine the effects of the groundwater dynamic on buildings and potential damage to buildings,
• Investigation of changes to the nature of the groundwater in the wake of significantly risen levels,
• Analysis and evaluation of potential groundwaterrelated damage ensuing – as a result of flooding – from contaminated areas (abandoned waste), sludge deposits or waste,
• Evaluation of the risks posed by unsealed wastewater channels (contaminant discharge).

The aim was to use Dresden as an example to evaluate flood-related damage to a body of groundwater underneath a city for the first time and to use this to derive action recommendations for administration, affected companies and citizens.

Model recording the groundwater dynamic

The nature and course of the groundwater flooding differed throughout the city, with wide areas – mainly more than a kilometre away from 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. – displaying a rise in the groundwater level after the flood wave that was then extremely slow to recede and others showing a brief significant rise that then dropped back down rapidly. The experts developed a computer groundwater model to record these different dynamics, which also factored in the basic structure of buildings beneath ground – primarily the historical town centre and the infrastructure. This enabled the project team to simulate the effect of different flood protection measures on the groundwater too.

The investigations of the nature of the groundwater took place on three levels: the working group took samples from a wide area in autumn 2002 and then spring and autumn 2003 to see how the quality had progressed. Investigations also found isolated pollution ingress at abandoned waste sites. The third element took the form of sample site-specific investigations of the natural sedimentDeposits in water bodies created through settling (sedimentation) of mineral and/or organic solids. We distinguish between clastic (suspended matter, sand, rock), chemical (substances separated from aqeuous solutions, e.g. carbonate) and biogenic sediments (deposited organisms or their remains, e.g. coquina)., conducted in the lab. This should enable statements to be made on the discharge and conversion of substances where polluted wastewater reaches the groundwater from the sewage system. The researchers simulated scenarios with different water levels and pressures in the sewers and the aquifer.

Drinking water unaffected

The experts were able to dispel fears regarding the nature of the groundwater by comparing the readings of specific water characteristics and pollutants values taken prior to the flood. The changes as a result of the flood were then only detectable for three months afterwards; they posed no threat to the drinking water.

The results at the abandoned waste sites investigated varied depending on the substances present and the flow conditions. Increased groundwater levels and flow speed released contaminants from their respective source. The experts detected slight increases in pollutant concentrations in the upper groundwater, plus vertical displacement within certain groups of substances. No significant lateral spread of pollution as a result of the flood was observed.

To enable them to investigate risks posed to the aquiferIn the field of hydrogeology, an aquifer (from the Latin words aqua = water, and ferre = to bear or carry) is an underground layer carrying groundwater. Aquifers are also referred to as water-bearing strata. Different types of aquifer are distinguished depending on the individual rock type (and thus the rock’s porosity). from unsealed channels, the project team simulated a local unsealed sewer system under pressure and flow conditions akin to a flood. This showed that the ammonium load typical of municipal wastewater only spread a small amount as a result of the flow speed and limited amount of unsealed areas.

Identifying risks, developing protective measures

The experts used the distances between the groundwater table and the surface of the terrain (depth to groundwater table) from August 2002 to December 2003 to develop a method for detecting risks posed to underground building areas. The parameters used included the intensity and duration of the groundwater flooding, the highest water levels, the speed of the rise in level and the minimum depths to the groundwater table. As a result, it was possible to determine the risk potential for 68 measurement sites within the city.

The assessment of the nature of the groundwater and the flow modelling based on various flood scenarios enabled conclusions to be drawn for use in town planning. According to the project team’s findings, town planning should always factor in the threat posed by a rise in groundwater. The investigations also confirmed the effectiveness of the planned protection measures, namely mobile shoring with inner-city Dresden and flood relief wells.

In light of the results of their investigations, the researchers recommend that the processes of groundwater rising be constantly factored into the preparation and execution of measures to tackle flooding, with building precautions forming an essential component. The identification of “danger zones”, where increased groundwater levels can be expected, forms the basis for this. As such, prompt measurements of the groundwater dynamic and the identification of the highest groundwater levels using a current groundwater flow model are required.

Project website www.gwz-dresden.de/en/home-groundwater-center.html

Read more:

1.4.05 Avoiding dyke breaches

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1.4.03 Flooding and groundwater