General Directorate of Water Management

Natural Water Retention Measures

Natural Water Retention Measures (NWRM) are multi-functional measures that aim to protect and manage water resources and address water-related challenges by restoring or maintaining ecosystems as well as natural features and characteristics of water bodies using natural means and processes. Their main focus is to enhance, as well as preserve, the water retention capacity of aquifers, soil and ecosystems with a view to improving their status. NWRM have the potential to provide multiple benefits, including the reduction of risk of floods and droughts, water quality improvement, groundwater recharge and habitat improvement. The application of NWRM supports green infrastructure, improves or preserves the quantitative status of surface water and groundwater bodies and can positively affect the chemical and ecological status of water bodies by restoring or enhancing natural functioning of ecosystems and the services they provide. The preserved or restored ecosystems can contribute both to climate change adaptation and mitigation. One of the most detailed analysis of NWRM was completed at EU level. A guideline document was also produced as part of the analysis, including a list of measures by sector (https://www.nwrm.eu/).

The measures cover the agricultural, forestry, hydromorphology and urban related proposals, some of which are selected and briefly shown here (https://www.nwrm.eu/).

Examples of measures supporting water retention in the agriculture:  

1. Establishing meadows and pasture - – Due to their rooted soils and their permanent cover, meadows and pastures provide good conditions for the uptake and storage of water during temporary floods. They also protect water quality by trapping sediments and assimilating nutrients. The measure offers the potential for temporary flood storage, increased water retention in the landscape and runoff attenuation. Soil cover is maintained at all times with rooted vegetation, this reduces the surface flow of water and allows greater infiltration to the soil. Rates of soil erosion are considerably lower than arable land with potential benefits for water quality.
2. Buffer strips and hedges - Buffer strips are areas of natural vegetation cover (grass, bushes or trees) at the margin of fields, arable land, transport infrastructures and water courses. Due to their permanent vegetation, buffer strips offer good conditions for effective water infiltration and slowing surface flow; they therefore promote the natural retention of water. They can also significantly reduce the amount of suspended solids, nitrates and phosphates originating from agricultural run-off. Buffer strips can be sited in riparian zones, or away from water bodies as field margins, headlands or within fields (e.g. beetle banks). Hedges across long, steep slopes may reduce soil erosion as they intercept and slow surface run-off water before it builds into damaging flow, particularly where there is a margin or buffer strip alongside.
3. No and Low Till agriculture - Intensive tillage can disturb the soil structure, thus increasing erosion, decreasing water retention capacity, reducing soil organic matter through the compaction and transformation of pores. No-till farming (also called zero tillage or direct drilling) is a way of growing crops or pasture from year to year without disturbing the soil through tillage. No-till is an agricultural technique which increases the amount of water that infiltrates into the soil and increases organic matter retention and cycling of nutrients in the soil. In many agricultural regions it can eliminate soil erosion. The most powerful benefit of no-tillage is improvement in soil biological fertility, making soils more resilient. Low till agriculture, also known as conservation or reduced till applies to arable land. It consists of a combination of a crop harvest which leaves at least 30% of crop residue on the soil surface, during the critical soil erosion period and some surface work (low till). This slows water movement, which reduces the amount of soil erosion and potentially leads to greater infiltration.

Examples of measures supporting water retention in forestry:

1. Maintenance of forest cover in headwater areas - Headwaters are the source areas for rivers and streams, crucial for sustaining the structure, function, productivity and complexity of downstream ecosystems. They are vital to hydrologic cycling as they are one of the main areas where precipitation contributes to surface and groundwater. Headwaters are typically less intensively used than downstream areas. In many headwater areas, extensive agriculture, forests or other semi-natural land cover types predominate. Forests in headwater areas have a beneficial role for water quantity and quality. Forest soils generally have better infiltration capacity than other land cover types and may act as a “sponge”, slowly releasing rainfall. In areas of high relief, afforestation of headwater catchments can contribute to slope stabilization and may reduce the risks associated with landslides. On the other hand, afforestation of headwaters in dry areas may lead to reduction of discharge.
2. Creation of sediment trapping ponds in woodland (but it can also be transplanted to non-forestry area too) - constructing small ponds in forestry minor stream brooks, slows down the water velocity and helps to unload fine sediment.
3. Land use conversion: Afforestation is one such land conversion in which trees are planted on previously non forested areas. Depending on the tree species planted and the intensity of forest management, afforestation may have more or less environmental benefits. The NWRM related benefits include potentially enhanced evapotranspiration associated with growing forests and better water holding capacity associated with forest soils. The greatest environmental benefits are probably associated with planting of indigenous broadleaves and low intensity forestry. It should be mentioned that afforestation in dry areas can cause or intensify water shortage. Even though afforestation may reduce available water supply at local scale, forest cover increases water supply regionally and globally, in particular through the intensification of the water cycle.

Examples of natural water retention measures related to hydromorphology:

1. Floodplain restoration and management: a floodplain is the area bordering a river that naturally provides space for the retention of flood and rainwater. Major floodplains’ roles have been lost due to land drainage, intensive urbanization and river channelization. The objective is to restore them, their retention capacity and ecosystem functions, by reconnecting them to the river.
2. Re-meandering, reconnection of oxbow lakes and similar features: the measures play a significant role in slowing down the water velocity. The new form of the river channel creates new flow conditions and very often also has a positive impact on sedimentation and biodiversity
3. Natural bank stabilization Riverbank represents both natural and artificial terrain following the river flow. In the past, lots of artificial banks were built with concrete or other types of retention walls, therefore limiting rivers’ natural movements, leading to degradation of the river, increased water flow, increased erosion and decreased biodiversity. River bank re-naturalisation consists in recovering its ecological components, thus reversing such damages and especially allowing the bank to be stabilized, as well as rivers to move more freely.
4. Lake restoration: A lake is a water retention facility. It can store water (for flood control) and provide water for many purposes such as water supply, irrigation, fisheries, tourism, etc. In addition, it provides important habitats for numerous species of plants and animals. In the past, lakes have sometimes been drained to free the land for agriculture purposes, or have simply not been maintained and have silted up. Restoring lakes consists in enhancing their structure and functioning where they have been drained in former times.

Examples of natural water retention measures related to the urban sector:

• Green roofs: green roofs are multi-layered systems that cover the roof of a building with vegetation and/or green landscaping over a drainage layer.Green roofs are designed to intercept rainfall, which is slowed as it flows through the vegetation and a drainage layer, mimicking the predevelopment state of the building footprint.
• Rain gardens: rain gardens are small-scale vegetated gardens used for storage and infiltration. Rain gardens are typically applied at a property level and close to buildings, for example to capture and infiltrate roof drainage. They use a range of components, typically incorporated into the garden landscape design as appropriate.
• Retention ponds: Retention ponds are ponds or pools designed with additional storage capacity to moderate surface runoff during rainfall events.  They consist of a permanent pond area with landscaped banks and surroundings to provide additional storage capacity during rainfall events. Retention ponds have good capacity to remove urban pollutants and improve the quality of surface runoff.
• Permeable surfaces: Permeable paving is designed to allow rainwater to infiltrate through the surface, either into underlying layers (soils and aquifers), or be stored below ground and released at a controlled rate to surface water.

Source: https://www.nwrm.eu/measures-catalogue

More examples and useful techniques can be found at: https://water.jrc.ec.europa.eu/nbs.html