Stage 1: Understand Your Situation

Begin with mapping what you already know. If available, assemble available documentation, such as:

• Sewer network maps (combined and separate)
• Locations of combined sewer overflows (CSO) and discharge points
• Storage structures and pumping stations
• Wastewater treatment plant information
• Flood records and incident reports

Check if the data is up to date and completed, based on experience of colleagues working in the field. Then, ask the following questions:

• Where are known problem spots?
• Are storage volumes known?
• How often they spill and how much do they discharge?
• Which areas flood first?
• Where are the sensitive receiving waters?

Figure 2: Example of sub-catchment delineation with surface classification and stormwater network mapping. Source KHADKA et al. 2019

Divide the drainage area map into manageable units, for instance by sewer districts, overflow catchments, pumping station zones as well as natural topography. These sub-catchments will make it easier for you to understand rainfall behaviour, which we look at next. Use hydrological catchment boundaries, not administrative ones. If your city receives wastewater from surrounding municipalities, you will need to account for those flows. Record the data source, date of last update and confidence level in the information shown.

WATERUN Tool 4: Must-B conducts block-level drainage mapping.

Annex V of the EU Directive 2024/3019 requires a dynamic understanding of wet-weather flows. In practice, this means translating rainfall behaviour into concrete implications for your sub-catchments, overflows and receiving waters.

Collect and review:

• Historical rainfall series
• Flow measurements at key points
• Overflow frequency records
• Past design assumptions

The reliability of the information can be checked by asking whether records include wet years and extreme weather events, if measurements were continuous and if monitoring points are representative of critical sub-catchments.

For each sub-catchment, consider how it reacts to rain:

• Estimate imperviousness (% sealed surfaces)
• Identify storage structures and capacities
• Identify overflow points
• Estimate response type (fast / moderate / buffered)
• Identify where discharged water flows

Once you overlayed this information with the drainage map, you can test a few representative rainfall scenarios, such as a wet year or a high-intensity event to see which areas are most sensitive. If a hydraulic model exists, simulate representative rainfall conditions. If climate projections are available, you can apply a stress test, by simulating a stronger peak rainfall (WILLEMS et al., 2012), and seeing which overflows will spill more often and which waters become more vulnerable.

If rainfall and flow data are weak, you can install temporary flow meters at critical overflows, add rainfall gauges in representative areas, and monitor data for 6-12 months to improve the baseline reliability. Focus on the gaps that most affect your risk picture. Online turbidity measurements can serve as a cost-effective surrogate for pollutant loads (METADIER & BERTRAND-KRAJEWSKI 2012).

• WATERUN Tool 4: Must-B simulate runoff under various rainfall scenarios
• WATERUN Tool 3 Risk-Based DSS  simulates urban runoff flows and pollutant transport in the sewer network during rainfall events, calibrated with local field data through the SSWM hydraulic modelling.

Urban runoff is not uniform: a small proportion of surfaces typically generates a disproportionate share of pollution (PATON & HAACKE 2021; REVITT et al. 2022). Your job is to find out which ones.

Figure 3: Ten patterns of diffuse pollution source areas in urban environments, ranging from spatially uniform to concentrated hotspots. Source: PATON & HAACKE 2021

Gather the following information to start with:

• Water quality monitoring results
• Incident logs
• Complaint records
• Industrial discharge permits
• Land-use maps

Next, map which surfaces generate the most polluted runoff, for instance: industrial areas, high-traffic roads, roofs with metal leaching, construction areas, etc. (GALSTER & HELMREICH 2022). If data is sparse, you can conduct targeted rainfall sampling, visual inspections at discharge points, surface-based screening and expert judgement. Do not solely rely on land-use maps since pollution patterns at street level are not always explained by mapped land use alone (LIU et al. 2013).

Remember: not all surfaces need treatment! Identifying clean areas lets you focus resources on genuine hotspots. Clean runoff may be suitable for rainwater harvesting. (BELMEZITI et al. 2013).

WATERUN Tool 2 Clean City Cover addresses this stage. Check the special factsheet on ‘Identifying where pollution comes from and ranking the pollutants’

WATERUN Tool 1 Portable monitoring provides the possibility for screening micropollutants and PAHs directly in the field.

EU Directive 2024/3019 requires an estimation of pollution loads during rainfall, including nutrients, suspended solids, microplastics and relevant pollutants. You can combine existing monitoring data with targeted sampling during rainfall and surface-based screening or even consult the archive of complaints and environmental incidents.

For each sub-catchment, score pollutants based on:

• Observed concentration (if monitored)
• Likely generation intensity (traffic, roofing, industry)
• Toxicity
• Sensitivity of the receiving water

Urban runoff is a significant pathway for exceedances of Environmental Quality Standards, particularly for metals and PAHs (EEA 2024; GNECCO et al. 2005). Consider both regulated substances and pollutants of emerging concern. Bear in mind that first-flush dynamics can dominate annual loads (KAYHANIAN et al. 2012; METADIER & BERTRAND-KRAJEWSKI 2012). The ranking helps guide monitoring refinement, informs Stage 2 target setting and prevents over-focus on low-risk parameters.

WATERUN Tool 2 Clean City Cover addresses this stage. Check the special factsheet on ‘Identifying where pollution comes from and ranking the pollutants’

For each overflow or runoff discharge point, assess its downstream context. Consult existing environmental and regulatory information, such as:

• Ecological and chemical status of the receiving water under the Water Framework Directive 2000/60/EC
• Environmental Quality Standards for priority substances (EUROPEAN PARLIAMENT & COUNCIL OF THE EU 2008)
• Bathing water classifications and temporary restrictions (EUROPEAN PARLIAMENT & COUNCIL OF THE EU 2006)
• Drinking water protection zones
• Natura 2000 or other protected areas
• Documented rainfall-related exceedances or incident reports

Ask: does the discharge reach a sensitive or protected water body? Is there evidence that rainfall events coincide with deterioration? If the answer indicates potential impact, flag the location as a priority zone. Where a water body is already impacted, distinguish between locally caused and upstream-caused pressures so you can target your own measures (MUELLER et al. 2020).

WATERUN Tool 3: Risk-based DSS addresses this stage.

Stage 1 Checkpoint. Before moving on, confirm you have:

• a) a drainage map divided into sub-catchments with documented data quality,
• b) an understanding of how each sub-catchment responds to rainfall,
• c) pollution sources and clean areas identified,
• d) a preliminary pollutant ranking, and
• e) receiving water sensitivities listed.

If any are missing, fill the gaps before continuing. Document any known trends in urban growth, densification, land-use change, and climate projections for rainfall intensity (WILLEMS et al. 2012). These will matter when you set targets in Stage 2 and size measures in Stage 3.

----> You can directly continue to Stage 2 of the IUWMP Journey or go back to the Overview (link).