Stage 3: Identify and plan measures

Stage 3 is where planning becomes concrete. Having identified where pollution comes from and set targets for what to achieve, you now need to decide what you will do and who will do it.

The directive is explicit that IUWMPs should prioritise green and blue solutions wherever hydraulically and environmentally appropriate, such as swales, infiltration systems, retention ponds, constructed wetlands, green roofs and porous surfaces (WOODS BALLARD et al. 2015; EUROPEAN PARLIAMENT & COUNCIL OF THE EU 2024). These measures are more cost-effective over a whole-life horizon that delivers co-benefits. Your plan also needs to name responsible actors for each measure and distinguish between what is already in place and what is new. Document how you evaluated green and blue alternatives so that your plan demonstrates compliance.

Urban runoff pollution is generated at the surface, travels through the drainage system, and eventually reaches a receiving water body. This source–pathway–receptor logic is already established in some countries’ water regulation as the conceptual basis for identifying where pollution risks arise and where they can be managed (ENVIRONMENT AGENCY 2017). Measures can intervene at any of these points, and the most effective plans usually combine all three layers.

At the source, the goal is to prevent pollution from accumulating or being washed off in the first place. This includes road sweeping programmes, replacing zinc and copper roofing materials, managing construction runoff, and limiting impermeable surfaces. Source measures are often the cheapest and most durable, but they require action by many different actors, building owners, highways departments, developers etc., so coordination is the main challenge. Check whether your city has building codes or planning conditions that could mandate less-polluting materials in new developments.

Along the pathway, between the surface and the drain, the primary goal in combined sewer contexts is to reduce the volume of runoff entering the network, particularly from cleaner surfaces, so that hydraulic loads on the treatment plant and the frequency of overflows are reduced. Nature-based solutions can contribute in several ways:

• Swales, constructed wetlands and bioretention systems intercept runoff and provide treatment before it reaches gullies
• Infiltration trenches allow water to percolate into the ground where subsoil and groundwater conditions permit.
• Green roofs and porous pavements reduce the volume and speed of runoff at the surfaces that generate it.

National regulation determines which surfaces and groundwater setting allow infiltration. Planning of measures including nature-based solutions must be coordinated with these regulations, since where a surface is too polluted to infiltrate safely, the runoff may be better directed to the combined sewer for treatment at the WWTP than released to ground.

At the level of the sewer network and its overflow points, the goal is to manage the volumes and loads that enter the system, reducing the frequency and severity of overflows. This involves optimising storage and treatment capacity, managing combined sewer overflows, and where necessary building new infrastructure. Reducing the volume of clean stormwater entering combined networks through source and pathway measures is consistently more cost-effective than expanding downstream treatment capacity (JIA et al. 2017). 

WATERUN Tool 3 MUST-B conducts mapping of NbS potential at source.

WATERUN Tool 4 Risk-based Decision Support System provides a library of treatment options that can help you select adequate measures.

Not every surface needs the same treatment.

• Clean surfaces (roofs, vegetated areas) can be disconnected from the combined sewer through retention, infiltration or rainwater harvesting, reducing the hydraulic load on the network; harvested rainwater can also substitute for potable water in non-potable uses (BELMEZITI et al. 2013).
• Low-moderate polluted runoff can be intercepted and treated before entering the system through swales and infiltration trenches that can be applied in residential and low-traffic areas.
• Medium polluted surfaces can be treated locally through constructed wetlands and bioretention basins. Well-designed systems have reported suspended solids removal in the range of 50–80% and meaningful reductions in metals under stormwater conditions, though performance is highly dependent on design, loading and maintenance (WOODS BALLARD et al. 2015).
• Highly polluted runoff (heavily trafficked roads, industrial yards) is often more appropriately directed to the combined sewer for treatment at the WWTP than infiltrated locally to prevent groundwater pollution.

Field results from the WATERUN project’s demonstration sites give a practical sense of what nature-based solutions can realistically deliver. At Santiago de Compostela, sustainable urban drainage systems installed in an industrial area achieved around 50% reduction in organic carbon concentrations and 50–60% reduction in microplastics and PAH levels. At Aarhus, green infrastructure at residential sub-areas demonstrated measurable reductions in metal concentrations (WATERUN CONSORTIUM 2025). These results are encouraging and illustrate that performance depends heavily on matching the measure to the pollution type and concentration.

WATERUN Tool 3 MUST-B helps plan decentralised NbS interventions for urban blocks, estimating the scale of measures needed.

Build at least three scenarios:

• business-as-usual,
• a low-impact retrofit focusing on quick wins,
• an ambitious option combining runoff management with other municipal objectives (mobility, greening, building renovation).

For each scenario, estimate the load reduction per sub-catchment (using the simple estimates from Stage 1 if no model is available), the approximate cost range, the timeline, and the co-benefits. Present the comparison in a format that non-technical decision-makers can work with, for instance a one-page summary table showing what each scenario achieves against the Stage 2 objectives, what it costs, and what the main implementation risks are.

Pay attention to governance. A technically excellent scenario can fail if no department owns maintenance or if critical surfaces are on private property. The biggest constraints are often organisational, not technical (STOPUP CONSORTIUM 2025).

WATERUN Tool 3 MUST-B models how block-level NbS interventions aggregate to reduce catchment-wide overflow volumes. See the dedicated sub-factsheet for the operational workflow

Stage 3 Checkpoint. Before moving on, confirm that:

• each measure is matched to the pollution severity of the surface it serves,
• green and blue options were genuinely evaluated before any grey alternative,
• each measure is sized for the runoff volumes from Stage 1
• at least three scenarios have been compared on cost, effectiveness, and feasibility,
• there is a named responsible actor for implementation and maintenance of each measure,
• the package collectively meets the Stage 2 objectives.

----> You can directly continue to Stage 4 of the IUWMP Journey or go back to Stage 2.

This IUWMP Journey stage is part of seecon's practical interpretation of Annex V of Directive (EU) 2024/3019 within the WATERUN project. It is not official EU guidance. The EU Commission implementing acts on methodologies are due by 2 January 2028 (Art. 5(6)). See the Stage 0 for details on how the four stages were developed.