Raising the riverbed level is an NbS that can address different types of societal challenges. Raising the riverbed level can improve the natural hydrology for better water management of the river system and with that the many ecosystem services associated with a natural hydrology. Furthermore, raising the riverbed level can also provide biodiversity benefits, although depending on how it is performed and the environmental context. 

When the riverbed is raised, the volume of the channel is reduced and the capacity of the channel to transport water is reduced, leading to a higher water level in the river when water discharge is normal and more frequent floodings of the surrounding land when water discharge increases in periods with high precipitation.  

Raising the riverbed level 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 by reducing CO₂ emissions from the area. Raising the riverbed level can also protect downstream areas from flooding in periods with high levels of precipitation. This is because the higher elevation of the riverbed in the terrain increases the likelihood of flooding, thereby improving water retention within the project area in periods with high levels of precipitation thereby protecting flood-prone areas downstream e.g. critical infra-structure, urban or other land uses.

When raising the riverbed, considerations must be made regarding the relationship between the natural width and depth of the river to ensure that this NbS does not result in overly wide rivers with shallow depths. The material used to raise the riverbed level should consist of sand, gravel and stone in mixture to resemble the natural substrate composition of the river where the intervention is planned. 

Outcomes

Potential outcomes

Reduced climate gas emissions: The reduction in CO₂ emissions will be highest in areas where organic soil contents are high (>6%) and where the water level in the river is as close as possible to the surface of the terrain over a large part of the project area. This creates oxygen-free conditions that slow down the decomposition of the organic matter in the soil thereby mediating the largest reduction in CO₂ emissions.  

Protect downstream areas from flooding: The discharge through the river is reduced, when the surrounding land is flooded. Consequently, water is retained that would otherwise be transported to downstream areas. This can be highly beneficial if downstream areas should be protected from flooding like urban or cultivated areas. The efficiency of raising the riverbed level for flood protection depends on the length of the river reach where the riverbed is raised, the discharge of the river and the characteristics of the surrounding land, since these parameters will all affect the amount of water that can be retained. The efficiency will be highest in low-laying project areas that are sufficiently large to retain large quantities of water.  

Reduced nitrogen pollution of aquatic ecosystems: Flooding of areas with nitrate polluted water can stimulate nitrogen removal by denitrification thereby lowering the transport of nitrogen to downstream river reaches, lakes and coastal areas.   

Potential side-effects 

Methane emission: There is a high risk of methane emissions in areas with standing water. Anaerobic conditions creates favourable conditions for the formation of methane gas through anaerobic decomposition and, as methane is a greenhouse gas, just like CO₂, methane emission may counteract the positive effect of less CO₂ emission. Therefore, it is very important to maintain a water level just below the surface to minimize this risk. 

Phosphorus mobilization: When former agricultural land with high contents of phosphorus is flooded there is a high risk of phosphorus mobilization from the soil that can enter the river and cause eutrophication of downstream river reaches, lakes and coastal areas. Therefore, mitigation measures to reduce this risk should be considering before the intervention. This could be harvesting to remove nutrients in the biomass, top soil removal or other measures. 

Altered hydrology outside the project area: When the groundwater level is raised in a river reach there can be a risk of affecting water level in upstream reaches, drainage pipes and ditches that discharge into the river within the project area. Therefore, the project boundary should be defined so only low-lying areas are included in the project, while higher-lying areas are excluded. This will diminish the risk of negatively affecting drainage conditions outside the project area. 

Raising the riverbed level is a NbS that can help restore the natural hydrology either alone or in combination with other measures like remeandering, closing of drainage pipes and ditches and is therefore an NbS with a high potential for restoring natural characteristics of freshwater ecosystems including many different ecosystem service benefits characterising rewetted areas. To ensure biodiversity net gains it is however important to be aware that high inputs of nutrients can be critical for many plant species and therefore that biodiversity net gain may not respond positively if this NBS is implemented in combination with closure of drainage pipes and/or ditches at the edge of the project area, thereby increasing the amount of nitrate polluted water entering the root zone of the plants.  

Costs

The costs will vary depending on the local socio-economic-environmental settings including manpower, technology, costs of buying land etc. In addition there will be operational costs to monitor the effectiveness of the NbS and maintenance costs if adaptive management is needed to maintain the effectiveness over time.