Applying MUST-B in IUWMP Stage 3: Scenario Development for Decentralised Nature-Based Solutions

MUST-B is the primary WATERUN tool for three Stage 3 tasks of the IUWMP Journey: mapping where decentralised NbS can be deployed, assessing the expected effectiveness of NbS at each location, and building comparable scenarios at different levels of ambition. It works at the block level — the spatial scale at which decentralised measures are actually sited, designed and implemented — and aggregates results to the system scale.

Stage 3 of the IUWMP Journey requires identifying measures, assessing their effectiveness, and comparing deployment scenarios (see Stage 3 factsheet for the full requirements and principles). This sub-factsheet explains the operational workflow for doing that using the MUST-B Planning Toolkit. It assumes that MUST-B has been set up for the city and that the Stage 1 baseline analysis of your IUWMP Journey is complete (see the Tool 4: MUST-B factsheet for setup and data requirements).

1. Review Stage 1 baseline from your IUWMP Journey to set priorities

Before building scenarios, review the evidence from Stage 1 of your IUWMP Journey to identify where NbS deployment would make the biggest difference.

From the outputs of MUST-B’s application Step 1 (Define your study Data), identify which blocks generate the most unmanaged runoff, which are connected to combined sewers (and therefore contribute to CSOs), and where existing management capacity is already in place.

Cross-reference with the other WATERUN tools if available: CleanCityCover’s pollution source inventory shows which surfaces are hotspots; the Risk-based DSS’s baseline risk map shows which discharge points exceed tolerable thresholds. Together, these three sources identify the target zone — the blocks that contribute most to the problem and where intervention would deliver the most benefit.

2. Map NbS deployment potential

Tool 4: MUST-B systematically identifies available space for decentralised measures within each block: pervious areas suitable for infiltration or bioretention, impervious areas that could be converted to permeable paving, rooftops suitable for green roof retrofitting, and existing green spaces that could accommodate additional storage or infiltration capacity. The output is a block-by-block potential map.

Review the potential map critically. Not all identified potential is equally actionable:

• Cross-check against soil and groundwater data. Blocks with high groundwater or low infiltration capacity may not suit infiltration-based NbS. In the Aarhus case study, infiltration was ruled out entirely based on local conditions, and the analysis focused on retention-based measures instead.
• Note ownership and institutional constraints. Potential on private land, in heritage zones, or above critical underground infrastructure may be restricted. MUST-B maps physical space, not institutional feasibility — that judgement is yours.
• Identify blocks where NbS potential aligns with planned urban development, road reconstruction, or building renovation. These represent implementation windows with lower marginal cost and should be flagged as near-term priorities.

Document the gap between theoretical and actionable potential in the IUWMP. This distinction matters for realistic scenario building.

3. Define deployment scenarios

Build at least three scenarios at different levels of ambition:

• Conservative: deploy NbS only on blocks with high potential, no ownership constraints, and alignment with already-planned projects. This represents what can be achieved in the near term with minimal additional investment.
• Moderate: expand deployment to all blocks with high or medium potential where constraints can be resolved through routine coordination — integration into scheduled road maintenance, agreements with public landowners, use of municipal green spaces.
• Ambitious: deploy NbS on all technically feasible blocks, including those requiring proactive investment, land-use negotiation, or dedicated infrastructure projects.

For each scenario, specify: which blocks are included, what NbS type is proposed per block (bioretention, infiltration, permeable paving, green roof, or combination), and what assumptions are made about sizing and performance. Be explicit about the connectivity parameter — the fraction of impervious runoff actually routed to the green infrastructure — because the Santiago case study showed this matters more than the size of the measures themselves.

4. Run scenario simulations and compare against targets of Stage 2 of your IUWMP Journey

For each scenario, MUST-B calculates the change in runoff volume discharged to the sewer system, peak flow during design events, CSO overflow frequency at each overflow point, and estimated pollutant load reduction. Results are produced at block level and aggregated to sub-catchment and system scale.

Compare the results against the objectives of Stage 2 of your IUWMP Journey. The key questions:

• Does the conservative scenario already meet the overflow reduction targets? If so, the IUWMP can adopt achievable near-term objectives with confidence.
• Does the moderate scenario close the gap? If so, the IUWMP should document the coordination and phasing needed to get there.
• Does even the ambitious scenario fall short? If so, decentralised NbS alone are insufficient. The IUWMP must document what additional measures — network storage, real-time control, end-of-pipe treatment — are needed to close the remaining gap. This is important: demonstrating the limits of NbS is as valuable as demonstrating their potential, because it makes the case for complementary investment defensible.

5. Prepare the scenario comparison for the IUWMP

Present the three scenarios in a standardised comparison format suitable for decision-makers:

• Map: spatial extent of NbS deployment per scenario, colour-coded by measure type. This shows stakeholders where intervention is proposed and where it is not.
• Table: for each scenario, show total blocks included, total NbS area deployed, estimated runoff volume reduction (%), estimated CSO frequency reduction (%), and estimated pollutant load reduction (%). Include cost estimates where available from MUST-B’s cost–performance analysis.
• Gap analysis: for each scenario, show the remaining gap between performance and the objectives of Stage 2 of your IUWMP Journey, and what additional measures would be needed to close it.

This comparison is the core deliverable for Stage 3 of your IUWMP Journey. It provides the evidence base for the city’s decision on which level of ambition to adopt, and makes transparent the consequences of each choice.

MUST-B quantifies the hydraulic and load-reduction impact of NbS scenarios. However, a scenario may significantly reduce overflow frequency while still leaving individual discharge points above tolerable environmental or health risk thresholds. Where the Risk-based DSS is available, run the preferred scenario through its risk recalculation workflow to verify that the hydraulic improvements translate into acceptable risk levels (see also the Risk-based DSS factsheet).

At the end of this workflow, you have three deliverables for the documentation of Stage 3 of your IUWMP Journey:

• An NbS potential map with documented constraints, distinguishing theoretical from actionable potential.
• A scenario comparison table showing the system-scale impact of conservative, moderate and ambitious deployment levels against objectives of Stage 2 of your IUWMP Journey.
• A gap analysis identifying where decentralised NbS alone are sufficient and where additional measures are needed.

These outputs feed directly into Stage 4 of your IUWMP Journey (where the selected scenario is assessed at system scale) and into the IUWMP’s phasing and implementation plan, which should align NbS deployment with the urban development windows identified in the potential map.