Nature-based Solutions for a Climate-Resilient Geul River Valley

Water security
BelgiumGermanyNetherlands

Nature-based solutions (NbS) can reduce the likelihood of droughts and floods. For the river basin of the Geul in the Netherlands, Belgium, and Germany, the effects of 10 landscape interventions were modeled using the SWAT+ model (Soil and Water Assessment Tool Plus). Examples include greening urban areas and road infrastructure, reforestation, promoting runoff delays in stream reaches, and implementing changes in the management of agricultural landscapes. The analysis shows where, and to what extent, nature-based measures can contribute to slowing down the journey of water through the landscape.  

Key Takeaways

  • The development of new forests is the number one nature-based measure to mitigate against flooding and waterlogging.
  • Natural solutions not only combat waterlogging, but also can address water shortages.
  • By taking an integrated approach to the landscape, nature-based solutions have the power to slow down water runoff.
  • For the first time, a cross-border, detailed hydrological model has been built for the entire Geul River Basin.
  • The model incorporates local differences in soil, relief, infrastructure, land use, and vegetation cover.

Project Details

Started: 2022 – Completed: 2023
Ref: ​221350  | Status: Completed
Title: Retaining and slowing down water in the Geul River Valley — Using nature to combat flooding and drought 

Background

On July 14 in 2021 the Geul River valley in the south of the Netherlands was hit by a flood wave caused by extreme rainfall. The vast majority of the water came from neighboring Belgium headwater catchments. The development of a detailed and cross-border model was therefore important to calculate the effectiveness of risk-reducing measures. In addition, better knowledge of spatial variation of landscape hydrological factors can be used to promote planning of mitigating measures.

By using nature to capture and slow down water, the landscape can absorb, store, and release more rainwater through adjustments in design and use. This ‘natural sponge effect’ is an approach that Natuurkracht, a coalition of nature organizations, wants to bring to the attention of the public. They have commissioned a detailed, cross-border model to map the entire Geul River Basin for climate- and future-proof water management.

Water expert locating the hight of the water during the time of the flooding in 2021
Samenstelling van de bodem weergegeven naast een handboor: 30 cm bodem kan niet veel water bergen en bij veel regen loopt het water door snel door het landschap

Activities

Like any model, the SWAT+ model provides a simplified representation of the actual processes that occur during precipitation and runoff. This model is used worldwide to interpret the hydrological effects of measures.

Changes on the scale of the Geul River Basin (339 km²) can be calculated by defining unique combinations of slope percentage, soil, and land use. For this model, the Geul River Basin has been divided into a detailed mosaic of more than 39,000 HRUs (Hydrologic Response Units). This allows the model to quantify the effects of measures for both an entire river basin and small sub-areas.

The aim of the study is to determine the extent to which proposed natural measures contribute to reducing peak discharges. Using the model developed for the entire Geul River Basin, most promising nature-based measures have been calculated for each of the sub-basins. The measures are always based on the nature of the sub-basin, so not all measures are applied everywhere, but interventions are used selectively in the most promising areas.

Findings

The study presents six possible solutions relating to:

  • Increased rainwater collection by vegetation;
  • Increased rainwater absorption by the soil;
  • Longer retention of groundwater in the soil and increased water absorption by the deep subsoil;
  • Slowing down runoff in rough terrain;
  • Slowing down and retaining water collected in tributary streams and dry headwater valleys;
  • Slowing down the stream runoff and providing space for natural floodplains.

It is also possible to provide an initial assessment of the effect of a single measure at a specific location on the entire water management of the river basin. The model shows which natural measures can reduce the risk of flooding of the Geul River and Gulp tributary.

A particularly illuminating aspect of this study is the journey of the raindrop on its way to the Geul Catchment outlet, which was closely tracked. This journey provides starting points for measures to slow down and retain water with the help of nature.

The 10 modeled interventions were:

  • Development of new forests
  • Conversion of fields to natural grassland
  • Development of infiltration strips
  • Removal of underground drainage
  • Filling in excavated stream extensions to promote drainage
  • Shallow, widen, and roughen upstream stream courses
  • Utilize storage capacity in the valley plain
  • Utilize road verges
  • Reprofile sunken roads
  • Convert urban source areas into green sponges

These measures fit in seamlessly with other important tasks for the Geul River Valley management, such as improving landscape quality, biodiversity, drinking water security, combating drought, eco-tourism/green recreation, carbon dioxide storage, and preventing the washout of nitrogen-rich erosion material.

Measures can contribute to reducing the discharge wave during high water and improving water availability in dry periods by promoting water storage in the landscape.

Project Areas

The Geul River Basin covers more than 33,400 hectares, of which 52% is located in the Netherlands, 42% in Belgium, and 5% in Germany. This landscape is characterized by high plateaus, steep slopes, and numerous valleys with and without perennial streams. Another characteristic feature is the alternation of (small) forests and open areas with grasslands, fields, and orchards. The whole area is crisscrossed with copses and hedges. It is a bocage landscape, in which the variation in soil and (limestone) rock has resulted in characteristic flora and fauna. The soil in the Geul River valley is loamy in many places. This makes it vulnerable to compaction due to rainfall followed by intense sunshine. The hard top layer that forms (silt) hardly absorbs any water. Nevertheless, the Geul catchment is characterized by the fact that the deeper (geological) subsoil has a large water storage capacity. The thickness of the soil is low in Belgium and Germany, which means that water storage during heavy rainfall is lower than in the Dutch part where soils are much deeper.

In many places, the hilly landscape is no longer able to properly process large amounts of rainwater. Villages and towns are often so paved and asphalted that water has no choice but to rush through the streets instead of seeping into the ground. Increased scale has further exacerbated the problem: on large, sloping plots without trees, shrubs, and scrub, the natural delays  on runoff have disappeared. In addition, climate change will cause extreme wet and extreme dry weather to occur more frequently, as shown by the recently published KNMI climate scenarios. This calls for urgent measures that will have a lasting effect.

Geographic scope: Regional

Downloads and Links

Rapport Stroomgebied Geuldal Water vasthouden en vertragen in het Geuldal (2023) Dutch only

Samenvatting Stroomgebied Geuldal Water vasthouden en vertragen in het Geuldal (2023) Dutch only

Presentatie Natuur-gebaseerde interventies voor demping van piekafvoeren – SWAT plus model stroomgebied de Geul (Acacia Water 2023) Dutch only

Rapport Stroomgebied De Geul – Effectiviteit van natuur-gerelateerde maatregelen ter vermindering van overstromingsrisico’s (Acacia Water 2023) Dutch only

In Cooperation With

Commissioned by:

Natuurkracht

Funded by:

Natuurkracht

Project partners:

Acacia Water Bureau Stroming H+N+S Landschapsarchitecten
Water expert locating the hight of the water during the time of the flooding in 2021

Contact

Maarten Waterloo

Ecohydrologist

+31 6 83 56 08 09
maarten.waterloo@acaciawater.com
dr-maarten-waterloo-0b35477

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Drought, Flooding, Modelling, NbS, SWAT

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