Placepoint

Stormwater management

Stormwater management is the way rain and meltwater are led away from and around a property without causing damage. As more of the ground is covered with sealed surfaces such as roofs, asphalt and concrete, the water cannot soak into the ground and has to be handled on the surface instead. Poor management leads to flooded basements, overloaded drains and erosion. Good management keeps the water safe and can at the same time become a green feature on the site.

The professional norm in Norway is local stormwater disposal (LOD): the water should as far as possible be dealt with where it falls, not piped straight into the public network. The three-step strategy from Norsk Vann is the most common framework. Step 1 captures and infiltrates small, frequent rainfall volumes locally (often set at the first 20 mm). Step 2 delays and detains larger volumes in rain gardens, detention basins or green roofs (typically 20-40 mm). Step 3 secures safe flood routes for the rare, heavy events that exceed capacity, so the water runs towards a safe low point and not into buildings. The figures are set by each municipality in local VA-norm standards (municipal water and sewage standards) and can vary.

The requirement is anchored in several places. The Planning and Building Act, section 27-2 regulates the removal of waste water, and TEK17 section 15-8 (External drainage installations with pipe networks. Stormwater and drainage water) requires that stormwater is preferably handled on the site itself through infiltration and detention. In a zoning plan, stormwater often appears as a sequencing requirement: the measures must be in place before the building is taken into use. Many municipalities require a separate stormwater plan as part of the building application.

For a developer, stormwater has become both a cost item and an opportunity that must be considered early in the project. Detention requirements take up land and affect what can be built, especially on dense urban sites. At the same time, stormwater installations have become a means of reducing climate risk: buildings in or near a flood zone see a lower damage frequency with good management, which counts positively in due diligence and can affect the premium for natural perils insurance. Blue-green solutions such as rain gardens and open streams also provide outdoor space that the market values.

In Placepoint you can see flood zones, water bodies and terrain in the map when you assess the stormwater risk on a site. Report: Risk and climate risk says it plainly: stormwater is the hazard without a national dataset, and the topic is covered with the closest proxies instead.

How it looks in Placepoint

In Placepoint you find this in Report: Risk and climate risk:

Stormwater management in Placepoint

From Placepoint's dictionary: Stormwater management

More information: Norsk Vann: stormwater, Lovdata: the Planning and Building Act, section 27-2, Miljødirektoratet

English: Stormwater management (LOD, local stormwater disposal).

Common questions we come across

What is the difference between stormwater and sewage?

Stormwater is rain and meltwater from sealed surfaces such as roofs and asphalt. Sewage in the narrow sense is waste water from toilets and sinks. Modern systems keep them separate: stormwater should preferably be handled locally on the site (LOD), not led into the waste water network, because it puts a load on treatment plants and increases the risk of flooding.

What is the three-step strategy?

It is Norsk Vann's principle for local stormwater disposal. Step 1 captures and infiltrates small rainfall volumes locally, step 2 delays and detains larger volumes in rain gardens or basins, and step 3 secures safe flood routes for the heaviest events. The thresholds are set by the municipality in the local VA-norm standard.

Why has stormwater become a requirement in building applications?

More sealed land and heavier rainfall put more water on the surface. TEK17 section 15-8 requires that stormwater is preferably handled on the site itself through infiltration and detention, and municipalities often add a sequencing requirement in the zoning plan stating that the measures must be complete before the building is taken into use.

Does stormwater management affect the property value?

Yes. Good management reduces damage risk in and near flood zones, can give lower natural perils insurance costs and counts positively in due diligence. At the same time, detention requirements take up land and can limit the utilisation of dense sites.