Low-level storage offers great potential for water storage in Diemen
In many cases, measures such as disconnecting downspouts and rainwater infiltration predominate in reports on climate adaptation. However, in flat areas with poor infiltration capacity and high groundwater levels, low-level storage constitutes an efficient measure to combat waterlogging caused by extreme downpours.
Four teams of Amsterdam UAS students enrolled in the “Climate-proof City” minor have explored the options for measures in four different residential areas of the municipality of Diemen. They all arrived at the same conclusion: low-level storage in public outdoor areas will considerably boost climate-proofability.
Students calculating the effectiveness of measures
Four teams of students have each calculated the storage and discharge capacities of different neighbourhoods in the current situation. The neighbourhoods concerned accommodate houses constructed in the 1980s and 1990s which are due for renovation in the near future. The teams subsequently calculated the storage and discharge capacities after the implementation of several measures. The point of departure was for the measures to be implemented simply and without any high additional costs during a scheduled redesign of a public outdoor area.
Low-level storage offers a great deal of additional capacity!
In the current situation, the neighbourhoods already boast a storage capacity of 40 to 60 mm in public outdoor areas. Additional water storage can be achieved through various (combinations of) measures. Discharging water to a depression (to be created or existing) in an outdoor area will increase this capacity particularly substantially. The students have calculated the storage and discharge capacities of the following adjustments:
- Hollow roads: Roads featuring a central gutter will increase the storage capacity by 10 mm;
- Lowered pavement: Lowering the pavement by 10 cm will increase the storage capacity by at least another 10 mm;
- Expanding rainwater sewer system: Optimising the rainwater sewer system to a 09 standard downpour system will discharge 30 mm of water in an hour;
- Low-level storage: Draining rainwater directly via the streets to “depressions” such as lower-lying public gardens, playgrounds or football fields, or to (the verges of) drainage canals will add another 30 to 50 mm.
The students showed that the neighbourhoods were thus capable of draining or storing a total downpour of 120 mm or more within an hour.
Video: animated short film of the measures, created by the students. From 05:06 on, it shows how water is being drained and stored during a highly extreme downpour involving 120 mm an hour.
Low-level storage: flat polders and high groundwater levels
The above measures are effective because the neighbourhoods are located in a flat polder landscape featuring distinctive drainage canals, an even surface level, houses with similar ground levels, poorly permeable soil, and at times high groundwater levels. The calculations showed the effect of having rainwater infiltrate into public gardens to be negligible. An added advantage of the measures is that lowering the pavement and constructing hollow roads will reduce sand-filling requirements during renovations.
Lessons to be learned
Implementation of the measures carries several additional advantages:
- Implementation during a renovation will have virtually no impact on the budget;
- Renovations will require less sand-filling;
- Public outdoor areas offer sufficient storage capacity for the water falling on roofs. This will take the sting out of the risk dialogues with residents. Such dialogues will then only address depaving and greening gardens to combat heat stress and promote biodiversity;
- Perforated pipes may be used to optimise rainwater sewer systems to a 09 downpour standard (larger diameters will be quite difficult in the crowded subsoil): so-called Drainage Infiltration Transport (DIT) sewers. During periods of drought, these sewers can infiltrate surface water in the neighbourhood, thus keeping trees vital and wooden piles wet. Their performance was established during the dry period in the summer of 2018 in two Diemen neighbourhoods featuring DIT sewers. However, it is becoming increasingly difficult to accommodate larger diameter rainwater sewers in the crowded subsoil;
- The neighbourhoods in the two Diemen residential districts are comparable to many other such (new) districts;
- Flat polder landscape, even surface level, houses with similar ground levels, poorly permeable peat and clay soil, and occasional high groundwater levels. In such cases, the measures outlined appear promising as well.