To restore the hydrology of a low-lying area, drains within the project area should be eliminated and other means to restore the natural hydrology should be implemented.  

Why

Disconnecting functioning drainpipes is a measure to purify water by retaining nutrients and organic material 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 closure of drainage ditches, raising the riverbed level and remeandering the river course to rewet an area. These NbS can contributed to climate change adaptation and mitigation and disaster risk preparedness. 

Disconnecting functional drainpipes can stimulate denitrification and help reduce the transport of nitrate to streams and downstream coastal areas. Overall, denitrification acts as a natural filter, removing excess nitrate 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 (NO_3-) and nitrites (NO_2-) 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. 

Disconnecting functional drainpipes can be achieved by either excavating the entire drainpipe or by cutting/crushing the pipes at suitable intervals to prevent water being transported through the pipe system.  

Potential outcomes 

Reduced nitrogen pollution of aquatic ecosystems: Nitrate polluted water from disconnected drainage pipes can, provided that anaerobic conditions are generated, 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.  

ATTENTION

To ensure biodiversity net gains within the project area it is important to be aware that drainage water can be detrimental for biodiversity because drainage water often contains high levels of nitrate. Elevated nitrate levels can be critical for many plant species and therefore biodiversity may not respond positively to the disconnection of functioning drainpipes if nitrate-rich drainage water percolates on the surface of the terrain and/or into the root zone of the plants. The reason behind is that interspecific competition will be affected and species that compete effectively at high levels of nutrients will be favoured. These species are not in general species that are associated with biodiversity net gains. Instead, drainage water should percolate below the root zone to ensure that the outflowing nutrient-rich water does not come into contact with the roots of the plants, but instead into the layers below where denitrification can occur. 

Costs

Implementation (manpower, technology, costs of buying land etc.), operational costs, maintenance and monitoring costs. This text should be qualitative rather than quantitative.