Wetland drainage for agricultural purposes have used surface ditches, subsurface permeable pipes (drains), or both, to lower the groundwater depth. Thereby, excess water from the plant root zone and underlying soil can enter the pipes through perforations and flow away from the field to a ditch or another outlet. Ditch and drain blocking and filling are therefore a measure that can contribute to raise water levels thereby contributing to restore the natural hydrological dynamics/hydraulic connectivity within the area. Ditch and drain blocking and filling resemble disconnection of functioning drainpipes and can rarely stand alone as an NbS. Instead, it should be implemented in conjunction with other nature-based techniques aimed at restoring the natural hydrology of an area.
Rivers, lakes and wetlands before NbS have been implemented
Rivers, lakes and wetlands after NbS have been implemented
Ditches and drains within the project area should be eliminated together with the implementation of other means to restore the natural hydrology of the area.
Ditch and drain blocking and filling is a measure to purify water by retaining nutrients and delaying transport of water to downstream areas, thereby improving water management. This measure is often implemented in conjunction with other types of NbS like raising the riverbed level, remeandering the river course to rewet an area. All of these NbS can contributed to climate change adaptation and mitigation, as well as disaster risk reduction.
Ditch and drain blocking and filling can contribute to a reduction in climate gas emissions if applied in low-lying areas with intermediate to high contents of organic carbon in the soil. Furthermore, this NbS can also enhance carbon dioxide sequestration from the atmosphere. Ditch and drain blocking and filling can also stimulate denitrification in rewetted areas and help reduce the transport of nitrate to streams and downstream coastal areas.
Overall, denitrification acts as a natural filter, removing excess nitrates from drainage water entering the area thereby reducing the risk of nutrient pollution of downstream ecosystems. Nitrate is reduced through denitrification, which is a natural process by which bacteria convert nitrates (NO3-) and nitrites (NO2-) into nitrogen gas (N2), which is released into the atmosphere. This process only occurs under anaerobic conditions, meaning in the absence of oxygen, and water saturation is therefore a prerequisite for this process to occur. Organic matter should also be present in the soil to serve as an energy source for the denitrifying bacteria.
Ditch and drain blocking and filling can be implemented by either excavating the entire drainpipe or by cutting/crushing the pipes at suitable intervals to prevent water flow. When drainage pipes from systems outside the project area are encountered, it's essential to ensure that the drainage water percolates deeper than the root zone and that it doesn't trickle onto the terrain surface, although the latter has been the normal procedure. Existing ditches within the area should be covered along the entire stretch or at appropriate intervals to rewet the area.
Reduced climate gas emissions: The reduction in CO2 emissions will be highest in areas where organic soil contents are high (>6%) and where the groundwater level within the area is sufficiently high to create oxygen-free conditions that slow down the decomposition of the organic matter in the soil thereby mediating the largest reduction in CO2 emissions.
Reduced nitrogen pollution of aquatic ecosystems: Nitrate polluted water from disconnected drainage pipes can, provided that anaerobic conditions are created in the soil, stimulate nitrogen removal by denitrification thereby reducing the transport of nitrogen to downstream river reaches, lakes and coastal areas. Factors such as temperature, pH, soil moisture, and substrate availability influence the rate and efficiency of denitrification in rewetted areas. Warmer temperatures generally promote higher rates of denitrification, while acidic conditions can inhibit the process.
To ensure biodiversity net gains within the project area it is important to be aware that high inputs of nitrate can be critical for many plant species and therefore that biodiversity net gain may not respond positively within the area if nitrate-rich drainage water percolates into the root zone of the plants. This will affect interspecific competition and favour species that compete effectively at high levels of nutrients. These species are not in general species that are associated with biodiversity net gains. Instead, disconnected drainpipes should be placed below the root zone to ensure that the outflowing nutrient-rich water does not come into contact with the root zone, but instead into the layers below where denitrification can occur.
Implementation (manpower, technology, costs of buying land etc.), operational costs, maintenance and monitoring costs. This text should be qualitative rather than quantitative.
Specific location: Kylldal catchment of the Kyll River near Steinebruck
Which ecosystem type: Wetlands
Title/name of the NbS: Drain blocking, ditch filling and wetland restoration
Summary: This project restored a wetland by e.g blocking drainage pipes and ditches and therefore using NbS to enhance the water quality by lowering the nitrogen and phosphorus concentrations, mitigate flood risks because of lower peak flows and lower risks of drought because of longer water retention and therefore lowering the risk of biodiversity loss.
Contact: Silke M. Nauta, silke_nauta@hotmail.com
Relevant links to documentation: https://www.researchgate.net/publication/378588891_Micro-Catchments_Macro_Effects_Natural_Water_Retention_Measures_in_the_Kylldal_Catchment_Germany
Specific location: Kvorning, River vally to Nørreåen in Middle Jutland
Which ecosystem types: Peatlands and Wetlands
Title/name of the NbS: Rewetting of peatlands, blocking of ditches and drainage pipes, remeandering of small streams and grazing from cattle.
Summary: The project near Kvorning is blocking ditches and drainage pipes to rewet the lowland area and therefore using NbS to reduce climate gas emission, lowering nitrogen content to Randers Fjord and recreate/improve nature and biodiversity. The project also aims to improve the recreational value of the area.
Contact: Martin Nissen Nørgård, mail: marno@nst.dk and Mogens Wiedemann Daabeck, mail: mogdaa@lbst.dk
Relevant links to documentation: https://project-merlin.eu/cs-portal/case-study-01.html and https://naturstyrelsen.dk/ny-natur/klimalavbundsprojekter/klima-lavbundsprojekt-ved-kvorning-i-noerreaadalen
Specific location: Vosborg Enge near Nissum Fjord in western Jutland
Which ecosystem types: Agricultural drained lowlands
Title/name of the NbS: Rewetting by blocking of ditches and drainage pipes. Flood control because of water retention in the project area.
Summary: Blocking ditches and drainage pipes will rewet the area and therefore use a NbS to reduce climate gas emission and lowering nitrogen content to Nissum Fjord. The project also aims to create a dynamic mosaic of shallow lakes, marshes and meadows.
Contact: Henning Fjord Aaser, mail: henfa@nst.dk
Relevant links to documentation: https://naturstyrelsen.dk/ny-natur/lavbundsprojekter/vosborg-enge-lavbundsprojekt and https://naturstyrelsen.dk/media/q1xbhmr0/2021_vosborg-enge_detailprojekt-rapport-fra-cowi.pdf
F. Tanneberger, L. Appulo, S. Ewert, S. Lakner, N. Ó Brolcháin, J. Peters, W. Wichtmann, The Power of Nature-Based Solutions: How Peatlands Can Help Us to Achieve Key EU Sustainability Objectives. Adv. Sustainable Syst. 2021, 5, 2000146. https://doi.org/10.1002/adsu.202000146 https://onlinelibrary.wiley.com/doi/full/10.1002/adsu.202000146
Hoffmann, C.C., Nygaard, B., Jensen, J.P., Kronvang, B., Madsen, J., Madsen, A.B., Larsen, S.E., Pedersen, M.L., Jels, T., Baattrup-Pedersen, A., Riis, T., Blicher-Mathiesen, G., Iversen, T.M., Svendsen, L.M., Skriver, J., Laubel, A.R., 2000a. Overvagning af effekten af retablerede vadomrader (surveying ˚ the effect of re-establishment of wetlands). Teknisk Anvisning (technical instruction) fra DMU nr. 19. National Environmental Research Institute, 112 pp. (in Danish). https://www2.dmu.dk/1_viden/2_publikationer/3_tekanvisning/rapporter/ta19_4udg.pdf
