Stage 3: Identify and plan measures

The directive is explicit that IUWMPs should prioritise green and blue solutions wherever possible as the first option considered, such as swales, infiltration systems, retention ponds, constructed wetlands, green roofs and porous surfaces (WOODS BALLARD et al. 2015; STEFANAKIS 2019; Directive (EU) 2024/3019). 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. 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, the connection between the surface and the drain, the goal is to slow, filter and retain runoff before it enters the sewer network. This is where nature-based solutions can intervene:

• Swales, constructed wetlands and other bioretention systems intercept road runoff before it reaches gullies, and have the potential to remove 50-80% of suspended solids and reduce metal pollution significantly (MANGANGKA et al. 2016)
• infiltration trenches allow water to percolate into the ground,
• green roofs and porous pavements reduce the volume and speed of runoff from roofs and car parks.

These measures work best when targeted at the surfaces that generate the most pollution and have the strongest hydraulic connection to the network.

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).  System-level measures are typically the most expensive and should be considered after source and pathway options have been exhausted.

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

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

Not every surface needs the same treatment.

• Clean surfaces are candidates for rainwater harvesting with minimal treatment and can be considered a water resource. (BELMEZITI et al. 2013). 
• For low-moderate pollution standard green infrastructure such as swales, infiltration trenches or porous pavement have well-proven filtering capacity and are cost-effective for residential and low-traffic surfaces.
• Medium-High polluted surfaces need high-performance systems, such as constructed wetlands, bioretention with engineered media, or conventional treatment before any discharge or reuse (ROY-POIRIER et al. 2010). Their design should match the pollutants it aims to remove.
• Heavily contaminated surfaces may need source control as the primary response, potentially combined with pre-treatment. Industrial yards, heavily trafficked roads with high metal loads are unlikely to be fully treated by downstream nature-based measures.

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 PROJECT 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 PROJECT 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:

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

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