Methodology technical feasibility NbS 3

The objective of this map is to showcase which wetlands are suitable for reconnection with the main river. Our methodology for analysing suitable wetlands is based on two guiding principles:

• Suitable wetlands should show signs of water stress/deterioration and reconnecting the wetland to the river may reverse (part) of the deterioration process. 

• The degree of difficulty of reconnection further determines the suitability. For example, if the distance between the wetland and the river is large, the required investment for reconnection may become prohibitive.

Approach

The spatial potential of the selected NbS types is mapped by overlaying different geospatial datasets, like hydrologic, soil, climatic data. This provides a first high-level indication to the effective geographic scalability of NbS. The suitability for a specific NbS is, where possible, classified in 4 classes: highly suitable, moderately suitable, marginally suitable and not suitable.

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Each suitability factor dataset was scored using the following classification: 

• Score of 0:  Not a suitable area given the specific suitability factor

• Score of 1:  Moderately suitable given the specific suitability factor

• Score of 2:  Highly suitable given the specific suitability factor

After classifying each individual suitability factor, an aggregated suitability score was calculated by summing up all the scores to yield a single aggregated total suitability score. The suitability class - highly, moderately or marginally suitable, thus depends on the aggregate suitability score of all factors. 

Furthermore, should any of the factors render an area unsuitable (score 0), regardless of the scores from other factors, the area is automatically labelled as unsuitable.

Suitability components for reconnecting wetlands to the river

To assess the potential suitability for reconnecting riverine wetlands four suitability factors were combined. 

Suitability factor 1: Relative height of a wetland

The median elevation was calculated for each wetland area. Additionally, the median elevation of the river was determined at the buffered endpoint of the shortest line connecting the wetland to the river. The relative height of a wetland was then computed by subtracting the median elevation of the wetland and the median elevation of the river point closest to the wetland. 

The following classification system was used:

• Highly suitable: The wetland is elevated less than 10 meters above the nearest river.¹

• Moderately suitable : The wetland is elevated more than 10 meters but no more than 25 meters above the nearest river

• Not suitable: The wetland is elevated more than 25 meter above the nearest river

• 1 The global digital elevation model used as input has a vertical accuracy bias, especially in wetland areas with vegetation. Consequently, these wetlands may tend to have a higher elevation in the digital elevation model as compared to the actual elevation. 10 meters was therefore selected as a threshold, although it is on the upper end. 

Suitability factor 2: Water stress

Disconnected wetlands are, for the purpose of this study, defined as wetlands that show signs of water stress, i.e., a shortage or deficit of water, either due to disconnection with the main river or other factors such as drought or human-induced causes like overuse of water resources. 

To compute which wetlands show signs of water stress we utilised the Water Occurrence Change Intensity product from the Global Surface Water Explorer. This product shows where surface water occurrence increased, decreased or remained invariant between 1984 and 2021. For each wetland we calculated the percentage of area that shows a decrease of surface water. The following classification system was used:

• Highly suitable: The wetland shows signs of severe water stress, with more than 20% of the wetland area showing a decreasing trend in surface water occurrence. 

• Moderately suitable: The wetland shows signs of water stress, with 5 to 20% of the wetland area showing a decreasing trend in surface water occurrence. 

• Not suitable: The wetland has no water stress, with less than 5% of the wetland area showing a decreasing trend in surface water occurrence. 

Suitability factor 3: Distance to the river

For each wetland area, the shortest distance to the river was calculated. The following classification system was used:

• Highly suitable: The wetland is located less than 2.5 kilometers from the river and the investment to reconnect this wetland to the river is limited.

• Moderately suitable: The wetland is located between 2.5 and 5 kilometers from the river and the investment needed to reconnect this wetland to the river is moderate.

• Not suitable: The wetland is located more than 5 kilometres for a river branch and the investment needed to reconnect this wetland may be high. 

Suitability factor 4: Size of wetland

The size of each wetland was calculated. The following classification system was used:

• Highly suitable: The wetland is larger than 3 square kilometers and reconnecting this wetland has a significant impact. 

• Moderately suitable: The wetland is between 0.5 and 3 square kilometers and reconnecting this wetland has an impact. 

• Not suitable: The wetland is smaller than 0.5 square kilometers and reconnecting this wetland has no significant impact.