Extreme rainfall, ageing pipe networks, and densification pressure are pushing stormwater management to a breaking point across the Follo region. But solutions exist — and the toolbox is larger than many realise.
On 25 September, around 35 politicians, municipal administrators, and business representatives gathered at NMBU in Ås for a working session on flooding and stormwater. The message from the experts was both sobering and action-oriented: the problems are real and growing, but we already know what works.
The challenge is converging from multiple directions
More frequent extreme rainfall events, the kind that made Storm Hans a reference point for worst-case scenarios, are hitting pipe networks that are old and undersized, in towns with ever more closed concrete surfaces. Drinking water quality has declined from good to moderate in some areas, and the investments needed are pushing up fees. "We cannot engineer our way out of this with pipes alone," said NMBU researcher Ulf Rydningen.
Rydningen: Think in three steps, not one big pipe
The answer is a three-step strategy: first, collect and infiltrate everyday rainfall locally through rain gardens, permeable surfaces, and green roofs; then delay and retain heavier rain in beds and lowered public spaces that serve as temporary basins; finally, secure safe flood pathways through the terrain for the most extreme events. This approach is cheaper over its lifetime than building ever-larger pipes — and it produces greener, more pleasant places to live as a bonus.
Di Marino: Nature-based solutions are not an add-on — they are the standard
NMBU researcher Mina di Marino presented fresh research from four Norwegian municipalities showing that many have strong ambitions for blue-green infrastructure, but that implementation is uneven. Particularly concerning: the requirement in national planning guidelines to justify in writing when nature-based solutions are not chosen is rarely followed at the zoning level. "Grey pipe-and-concrete solutions are no longer the default," di Marino emphasised. Stavanger stands out as a leading municipality by embedding concrete requirements throughout both its strategic plan and zoning regulations, a model the Follo municipalities can replicate. She emphasised that nature-based solutions have multiple functions beyond storm water management, such as increasing well being as well as adapting to heat waves and freeze/thaw winter conditions.
From the room and the experts: Five things that are urgent
The discussion produced a clear picture of what municipalities need:
- 1. Stronger cross-disciplinary collaboration across planning, water and wastewater, and parks departments (Nordre Follo's stormwater network was highlighted as a model others want to copy)
- 2. Better maps of safe flood pathways before new zoning plans are adopted
- 3. Competence-building through NMBU's continuing education courses
- 4. Stormwater charges as a steering mechanism
5. Coordinated applications to the Norwegian Environment Agency's funding stream for streams flowing to the Oslofjord.
A concrete example: in Ås, the question of whether Hogstvedtbekken can be opened as a green waterway has been answered with a yes — a hybrid solution with an open stream at the surface and existing pipes below ground is entirely feasible and recommended.
The key recommendations to municipalities are clear
- Adopt nature-based solutions as the default in all new plans and require written justification for deviations;
- Introduce minimum blue-green factor requirements in town centres and densification areas;
- Commission flood-pathway maps for priority locations before new zoning plans are approved;
- And allocate funding for pilot projects, rain gardens, stream openings, and lowered public spaces that can serve as learning arenas for the whole region.
Storm water events are increasing. The question is whether the water runs through a plan, or through someone's basement.