Groundwater nuisance occurs when user functions are affected by high groundwater levels. It is estimated that in the Netherlands approximately 150,000 homes are susceptible to groundwater nuisance ( KPMG-Grontmij ).
What types of groundwater nuisance are common?
- High indoor humidity and fungoid growth as a result of wet crawl spaces or rising dampness in walls;
- Sweating basement walls;
- Swampy gardens and long-lasting wet green areas in a neighbourhood;
- Damage to urban greenery and trees being blown down as a result of drowning roots;
- Damage to buildings caused by changes in hydraulic uplift pressure underneath the foundation;
- Rutting and uneven subsidence in roads and street pavement.
In the Netherlands, some 150,000 houses are estimated to be prone to excessive groundwater levels ( KPMG-Grontmij ).
Is groundwater flooding increasing due to climate change?
According to the KNMI’14 climate scenarios, winter precipitation is increasing, whilst evaporation remains more or less constant. As a result, groundwater recharge will increase, groundwater levels will rise, and seepage water (groundwater outflow) will increase. All this adds to the probability of problems caused by excessive groundwater levels. Furthermore, in large parts of the low-lying areas of the Netherlands, continuing soil subsidence can also add to the probability of excessive groundwater levels.
What does the map show?
The map shows the degree to which the probability of excessive groundwater levels causing problems for urban functions (buildings, infrastructure, gardens, and greenery) will increase between now and 2050. The map only reflects changes up to 2050; measures to prevent problems in the current situation have not been taken into account.
The Mean Highest Groundwater Level (MHGL) usually occurs by the end of the winter season. The circumstances prevailing at that time are usually taken into consideration in the construction of drainage systems, buildings, and infrastructure.
At locations where the MHGL will rise to 2050, facilities geared to the current situation may no longer suffice and problems may arise due to the higher groundwater levels.
In the current situation, the MHGL reaches particularly close to the surface in low-lying parts of the Netherlands. In order to combat excessive groundwater levels in such areas, large-scale drainage systems have been constructed in the form of ditches and subsoil drainage.
Between now and 2050, the MHGL will rise most sharply at the elevated sandy soils and in the elevated parts of the province of Limburg. However, the probability of problems arising is quite small, because the groundwater lies well below the surface level here. However, on the outskirts of elevated sandy soils and dunes, and in brook valleys, a rising MHGL could cause problems: the groundwater is shallow here and the areas are prone to seepage.
The probability of problems rises sharply at locations affected by soil subsidence and where nearby surface water levels remain relatively high. The map factors in the model set-up in which excessive groundwater levels in urban areas are underestimated vis-à-vis rural areas. The table below shows the limiting values used in the classification.
The maps are based on outcomes of the National Water Model (link in Dutch). This model has been used to determine the effects of the so-called Delta Scenarios. These Scenarios combine the climate changes set out in the KNMI’14 scenarios with potential economic changes. In the model, economic changes are reflected in differences in land use and water consumption ( Deltares research institute, 2016 ). For example, steep economic growth will entail a higher demand for industrial water, increased groundwater withdrawal, and thus (locally) lower groundwater levels.
The “Warm” Delta Scenario was used to determine changes in the probability of excessive groundwater levels. This scenario combines the WH KNMI’14 scenario with low economic growth and generates the relatively highest increase in MHGL, predominantly as a result of a wet winter season, high precipitation intensity in summer, and moderate groundwater withdrawal. Soil subsidence is also taken into account in this scenario.
A map of the current excessive groundwater levels situation – based on a detailed survey – is not available for the Netherlands. National models can predict excessive groundwater levels to a limited extent. In many cases, whether high groundwater levels will cause problems at a particular location will depend on highly local conditions and processes, for which the model resolution is too limited. Nationwide information on the presence and performance of drainage systems is not available either. Consequently, various locations are prone to underestimate or overestimated groundwater levels. However, the changes in groundwater levels up to 2050 can be modelled fairly accurately. That is why this map has been selected.
How can this information be used?
At the municipal scale level, the map gives a rough indication of the zones in which the probability of excessive groundwater levels is increasing. In such zones, additional groundwater level control measures must be considered, or additional construction requirements must be enforced in order to reduce the vulnerability of buildings and infrastructure.
With respect to the interpretation of the map, a factor to be considered is that, especially in urban areas, the probability of high groundwater levels causing problems is significantly influenced by such local conditions as excavations disrupting the soil, building construction features, or local impermeable layers. Such local conditions are not reflected on the map.
The map has a signalling function. At the local scale level, further research can be conducted into factors that are determinative for excessive groundwater levels, e.g.:
- Analysing the current groundwater level dynamics (how high is the groundwater currently rising vis-à-vis the surface level?);
- Surveying the presence and performance of drainage systems that will curb the rise in groundwater levels;
- Analysing the expected volume of soil subsidence and regional policy to control subsidence via water level management;
- The conditions at locations that are already affected by high groundwater levels. These provide a key clue for the identification of locations at which problems caused by high groundwater levels will increase or occur in the future.
