NbS Long list

Source: NbS catalogue (World Bank, 2021)

The aim is to improve or restore the natural connectivity between the river channels and the floodplains. Reactivating wetland areas would improve its environmental performance and overall resilience of the river system.

Source: NbS catalogue (World Bank, 2021)

Natural inland wetland areas have been used for agriculture in many regions around the world. To make the land suitable for crops, drainage systems are installed to control the water table and provide irrigation.

To reverse the destruction of the wetland, natural water fluctuation must be restored together with the composition of the anaerobic hydric soil formed over a long period. The first step is to remove a section of the underground agricultural tile that is draining the wetland basin or create a ditch plug by building an earthen wall to impound water.

Inspiration maybe be found in the cased of traditional Khmer agriculture system that enhanced both water and sediment natural cycles to irrigate and create soil texture suitable for different crops (Delvert 1961; L’Hamarttan1994).

Source: NbS catalogue (World Bank, 2021)

Levee setback is the process of relocating a levee further back in the floodplain to provide extra space for the river to flood. Levee setback provides the river with more floodplain area to interact with and can result in lower flood elevation.

The new space for the river allows new ecological and recreational activities and provides a greater diversity of floodplain habitats. In addition, this new space could also be used as natural resource production, such as agriculture.

Source: NbS catalogue (World Bank, 2021)

An oxbow is a historical river meander that is cut off from the main channel during the natural process of channel migration, or through man-made channelization. Water levels are maintained through larger flooding events overflowing into the oxbow and groundwater seepage.

Based on the habitat proposed, inlet and outlet structures may need to be constructed to regulate the inflow and outflow of water for the oxbow.

Source: NbS catalogue (World Bank, 2021)

In incised floodplains, a new meandering stream channel is excavated on the original floodplain by raising the stream bed elevation. The former incised channel is then filled, converting it to a floodplain feature. This approach is used in areas where there are few lateral constraints and where flooding on the adjacent land can be increased.

The existing levee/embankment system can be utilised to create flood control basins for water retention (during discharge peaks), or to create additional flow-paths during predetermined discharge conditions. Flood-control basins are facilities that temporarily store river water in adjacent reservoirs to mitigate flood peaks and gradually drain the water back to the main channels after a flood (Ishiyama et al., 2019).

 In some of the embankments (in Vietnam) are already sluices present. In others, they can be created, or an overflowing embankment can be implemented to increase connectivity between the floodplain and the main river.

Source: NbS catalogue (World Bank, 2021)

Bioswales and rain gardens are shallow, densely vegetated ground depressions, with a variety of trees, shrubs, and grasses to collect stormwater from adjacent impervious surfaces

During storms, they become flooded and facilitate ground infiltration and cleaning of stormwater simultaneously. During dry seasons, swales and rain gardens contribute to the quality of public areas. Bioswales are common in streets and other linear infrastructure; rain gardens are common in parks, squares, and private gardens.

Source: NbS catalogue (World Bank, 2021)

Detention ponds are deeper and less biologically diverse bioretention areas than bioswales and rain gardens. Bioretention systems capture and temporally store stormwater during periods of heavy rain (Eisenberg and Polcher 2020).

Detention ponds can be completely filled with water during storms; they infiltrate much of it into the ground; and discharge the overflow into the sewer system. The remainder of the time they remain dry.

Source: NbS catalogue (World Bank, 2021)

Retention ponds are bioretention areas characterized by a permanent body of water and vegetated edges. Unlike detention ponds, they are permanently filled with water. Retention ponds collect stormwater from the surrounding areas; add storage capacity and ease the pressure on the surface water treatment and sewerage systems.

Retention ponds offer the added benefit of storing water for further reuse during drought conditions, while simultaneously providing habitat and enriching the diversity of public green spaces (Iwaszuk et al. 2019).

Source: NbS catalogue (World Bank, 2021)

Flood-based agriculture is a traditional farming practice that harnesses seasonal floods in specific regions. It relies on the natural flooding and recession of rivers, lakes, or floodplains to cultivate crops. This approach reduces flood risks and helps replenish the essential fertile sediments crucial for agricultural productivity.

Source: NbS catalogue (World Bank, 2021)

The riverbank is an interface of aquatic and terrestrial ecosystems, an area protecting cities from riverine floods and often an important social place with recreational and cultural value. Its renaturation design should also safeguard ecological functions and flood control.

Riverbank and bed renaturation aim to restore the natural dynamic of the river, which may mean restoring its shape, creating natural/physical structures to direct the flow of water, and provide habitat for aquatic species.

Source: NbS catalogue (World Bank, 2021)

Small streams provide a wide array of benefits to communities, such as nutrient and pollution removal, groundwater recharge, and flood mitigation. In some urban areas, streams were previously enclosed by concrete pipes or simply filled in. This could lead to floods, riverbed incision and consequently to severe damages such as building collapses

Daylighting is a technique to remove layers of concrete and recreate the natural shape and dynamic of streams, resulting in increased wildlife and aquatic habitat, and better regulated stormwater runoff treatment and intake (Eisenbert and Polcher, 2020).

Source: NbS catalogue (World Bank, 2021)

Renaturation relies on several bioengineering techniques to recreate the natural course of a river and connect it to its landscape for floodplain and riparian corridor revegetation, riverbank stabilization, and restoration of the riverbed.

The natural river dynamic rests on the use of plants, rocks, and other natural elements, as well as geotextiles and membranes to create ecologically rich and structurally stable environment mimicking natural conditions, while providing space for recreation (Eisenbert and Polcher, 2020).

Creating natural drainage corridors normally involves converting a ditch or storm drain into a natural creek flowing within a multipurpose corridor. They can also be created by rehabilitating and enhancing natural creeks and streams. They greatly reduce the number of drainpipes and other costly technologies required to manage stormwater runoff, and they reduce flood management costs overall.

Stormwater is collected in a traditional pit and pipe network within the road reserves. The pit and pipe network conveys flows to the drainage corridor where wetland conditions treat runoff before discharging to the natural watercourse.

Log terracing is a way to intercept water running down a slope and comprises bedding logs or coir logs in shallow trenches along the corridor. Log terracing is used on burned slopes that have less than 30% of the original ground cover remaining and are at risk of increased erosion, and where there are enough logs of adequate size to construct a semicontinuous line at the desired horizontal spacing. It is better to use coir logs made locally, than to fell trees

Log terracing shortens the slope length and the gradient between each structure, providing stable planting areas throughout most of the slope face. Logs must be installed in trenches and staked into place.

Live staking and joint planting involve the insertion of woody shrub cuttings into the ground in a manner that allows the cutting (stake) to take root and grow. Live stake cuttings can be used to repair small earth slips and slumps. The stakes can help buttress the soil and arching

Silviculture treatments aim to restore riparian areas by replanting trees and vegetation, often with protection from further harvest or removal. These treatments, including thinning, girding, pruning, and coppicing, are used to enhance riparian conditions. The ultimate goal is to establish a riparian buffer zone that can effectively mitigate flooding and erosion, stabilize riverbanks, and preserve the essential connection between the forest and the water system (Gashaw, 2015).

Active restoration involves planting native species to ensure their successful growth. This revival is achieved through sowing seeds or planting seedlings. Sometimes, protection from herbivores, like fencing, may be necessary. It is crucial to remove invasive species beforehand to prevent their return and disrupt the restoration efforts. Starting restoration upstream and progressing downstream can help prevent invasive species from hindering the restoration process. This measure is particularly suitable in areas where the river's physical processes have been significantly altered by human activities, such as dams, levees, bank stabilization, and water diversions (Griggs, 2009).

Passive restoration relies on natural generation and requires minimal human disturbance for successful outcomes. This method encourages natural processes like flooding and seed dispersal. Over time, native plants recolonize the area, and the ecosystem regains its functionality. Monitoring the progress of natural regeneration is vital to ensure success. Passive restoration is particularly effective in climates and soil conditions with inherent resilience, such as relatively healthy soil and wetter climates. In arid climates, recovery may take longer (Eubanks, 2004).