Floodplain reconnection is relevant in various geographic and environmental contexts where rivers and floodplains have been altered or disconnected due to human activities.  

Why  

Floodplain reconnection can address many different societal challenges, but the efficiency will depend on the degree to which the natural hydrological processes are restored, the size of the floodplain area, the spatial location of the floodplain relative to flood prone areas as well as other features of the site including the level of the terrain and soil characteristics.   

Floodplain reconnection can protect downstream areas from flooding by providing additional storage capacity for waters when inundating, thus contributing to climate change mitigation and disaster risk preparedness. The significance will depend on the size of the floodplain and location relative to flood prone areas.  

Increased inundation in reconnected floodplains can also enhance demobilisation of sediments (sediment trapping) and associated nutrients like phosphorus thereby reducing the risk of eutrophication of downstream ecosystems like lakes.  Floodplain reconnection can also lower sediment transport within the river, reflecting that water flow is reduced within the river. Floodplain reconnection can also improve the quality of surface/drainage water before it enters the river, by stimulating denitrification thereby helping to reduce the transport of nitrate to downstream areas. All of this can therefore contribute to better water management. 

In agricultural landscapes, reconnection of floodplains can enhance water retention. This can be highly advantageous in periods of drought and contribute to a more sustainable agricultural practice, and thus also climate change mitigation and adaptation, and disaster risk and preparedness.  

How

Floodplain reconnection is often a key component of river restoration projects aimed at improving the functioning of degraded riverine ecosystems. Reconnection often involves restoration of meander bends (remeandering), removing levees, or breaching embankments to allow river water to access the floodplain. Removing levees that have disconnected the river from its floodplain is one of the most direct methods of floodplain reconnection. This allows floodwaters to inundate the floodplain during high-flow events. Remeandering or restoring the course of a river channel to increase its sinuosity and reconnect it with its historic floodplain can also promote floodplain reconnection. This may involve excavating new channels or modifying existing ones to create meanders and oxbow lakes thereby promoting interactions between the river and its floodplain. Remeandering can involve recreating bends, riffles, and pools to stimulate natural flow dynamics. Planting native riparian vegetation on floodplains can help stabilize soil, reduce erosion, and enhance habitat quality. 

Protect downstream areas from flooding: Floodplains act as natural buffers against flooding by absorbing and attenuating floodwaters. When floodplains are reconnected, they can absorb excess water and slowly release it back into the river over time, helping to delay and reduce the magnitude and duration of downstream flood peaks. This can reduce the risk of flood damage which is highly beneficial if downstream areas should be protected from flooding like urban or cultivated areas. Thereby, floodplain reconnection can also delay the timing of peak flows downstream by temporarily storing floodwaters on the floodplain. This delay can provide additional time for downstream communities to prepare and respond to flooding events, potentially reducing the severity of flood impacts. 

Improve water quality: Under flooding, floodplains act as natural sediment traps, capturing suspended sediment and preventing it from being transported downstream. This process helps to reduce turbidity and sedimentation in the river, improving water clarity and quality. Furthermore, this process enhances immobilisation of sediment-bound phosphorus that may otherwise cause eutrophication of downstream ecosystems. Floodplains can also serve as effective filters for nutrients such as nitrogen and phosphorus, which are often carried by surface runoff from agricultural fields or urban areas. Vegetation and soil microbes on the floodplain can absorb and assimilate these nutrients, reducing their concentrations in the water and mitigating the risk of nutrient pollution of downstream river reaches, lakes and coastal areas. Floodplains also provide ideal conditions for denitrification that may help reduce nitrate levels in the river and, consequently, the transport of nitrate to downstream ecosystems. Factors such as temperature, pH, soil moisture, and substrate availability influence the rate and efficiency of denitrification. Warmer temperatures generally promote higher rates of denitrification, while acidic conditions can inhibit the process.   

Potential side effects 

Methane emission: Floodplain reconnection can potentially increase methane emissions due to the creation of anaerobic conditions in wetland soils, which are conducive to methane production by microbial activity. The risk of methane emissions is particularly high in former agricultural areas with standing water. Anaerobic conditions create favourable conditions for the formation of methane gas through anaerobic decomposition and, as methane is a greenhouse gas this this considered a negative side effect.  

Phosphorus mobilization: When former agricultural land 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, topsoil 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.  

ATTENTION  

Floodplain reconnection is a NbS that can help restore the natural hydrology of an area either alone or in combination with other measures like raising the riverbed level, closure 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 that characterise rewetted areas.  

To ensure biodiversity net gains within the project area, it is 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. Groundwater dependent vegetation like rich fens may also suffer under prolonged floodings, especially during the growth season, and possible conflicts related to the implementation of other legislation like the habitats directive should therefore be considered. Equally important is the immobilization of sediment-bound phosphorus which enriches the site with nutrients, affecting the vegetation that develops in the area.  

Costs

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