Designing water management strategies which minimize trade-offs between greenhouse gas emissions and flooding risks

25 oktober 2021

Eline Anvik

The water systems of cities and their surrounding rural areas are often interwoven, which adds another layer of complexity to the challenge of designing adaptive and resilient water management strategies. In particular, cities in peatlands must face the challenge of how to minimize soil subsidence and greenhouse gas (GHG) emissions in the rural areas, without enlarging the impacts of flooding in the urban area. To contribute to our collective understanding how to deal with these conflicting goals, we examine the case of the Waardassackerpolder, a peatland polder of 15.7 km² just South of Amsterdam. The polder is shaped like a bathtub, with urban area with mean elevations ranging from +0.63 m to -0.72 m relative to datum, flanked by rural areas with mean elevations of 2.64–2.28 m below datum.

We assessed the impacts of two water management strategies on soil subsidence, GHG emissions and flooding risks in the timeframe 2021–2051. The first strategy reflects the current policy, with surface water levels in the rural areas that are maintained at 76–103 cm below ground surface. The second strategy reflects a rewetting of the rural peatlands, with surface water levels raised to the ground surface. In both strategies, the surface water levels are periodically lowered to compensate for soil subsidence. First, we assessed the impacts of both strategies on soil subsidence and GHG emissions. Second, we used the resulting surface elevation models as input for an assessment of the time it takes before the first roads in the urban area are flooded, after the surrounding embankment of the rural areas is breeched. All assessments were made with the Tygron Geodesign Platform, with a resolution of 1 by 1 m.

The cumulative soil subsidence of the current policy was 26.8 cm, which resulted in an emission of 24.9 t CO₂-eq ha^-1 y^-1 and mean elevations of the rural areas of 2.91–2.50 m below datum in 2051. After the breech of the embankment, it took 35 hours before the first roads in the urban areas were flooded. The scenario with raised water levels resulted in 1.2 cm subsidence in 30 years (95% decrease) and a GHG emission of 8.9 t CO₂-eq ha^-1 y^-1 (64% decrease). The mean elevations of the rural areas in 2051 were 2.66–2.29 m below datum. As a consequence, less water could be stored in the rural areas after the embankment was breeched. This resulted in a 21 hour window before the first urban roads were flooded.

In this case, trade-offs between reducing GHG emissions and flooding risks were limited, i.e., 64% reduction of GHG emissions could be obtained with 40% less, but still enough time to safely evacuate the residents of the urban areas. In cases where trade-offs are more pronounced, we suggest designing water management strategies that result in slightly more soil subsidence in key rural areas, to allow for adequate evacuation times in urban areas. The Tygron Geodesign Platform can be used for integrated impact assessments that can support such design challenges.

Source: van Hardeveld., H. (Waternet). 2021. Designing water management strategies which minimize trade-offs between greenhouse gas emissions and flooding risks. Managing ecological and climate risk, Risk & Resilience. AIWW 2021.