Renewal agenda requires a clearer understanding of infiltration sewers
As 2050 approaches, much of the Dutch sewer system is reaching the end of its life, with investments of billions of euros around the corner. More and more municipal authorities are opting for DIT sewers (drainage infiltration pipes) but an adequate systematic basis is still lacking with respect to the costs, benefits and operation. A national survey by Deltares found considerable variation in the use of DIT sewers.
The Deltares analysis shows that at least 1,800 kilometres of DIT sewers have already been installed. Because there is almost no structural monitoring, design and management decisions are mostly based on assumptions.
Targeted investments require more knowledge about operation and effectiveness
Installing DIT sewers usually costs 10-20% more than their closed counterparts. Municipal authorities are opting for DIT systems with a view to climate adaptation, drought prevention and the reduction of problems with excess water but the quantitative evidence for these benefits is still limited. The authorities say they would like a clearer picture of crucial factors such as actual infiltration capacity, long-term operation, maintenance requirements, the risks of subsurface pollution spread and the actual impact on groundwater and the urban water balance.
Tackling these knowledge gaps would put the authorities in a better position to assess the efficacy of DIT sewers in their specific circumstances. A clearer picture contributes to informed investment choices and informed management & maintenance.
Practical follow-up study
Deltares recommends that a practical follow-up study should include targeted measurements, in representative locations in the Netherlands, of:
- the infiltration capacity, drainage capacity and/or emptying time by comparison with the expected values
- the long‑term performance of different systems and the influence of cleaning
- the impact on groundwater levels, soil moisture and the spatial extent of these effects
- the impact on the urban water balance
Would you like to participate in the follow-up study or learn more about the findings? Please contact the Urban Water team at Deltares.
