The research, conducted by Tiku T. Tanyimboh of the University of the Witwatersrand and North Lanarkshire in the United Kingdom, together with Nokuthula R. Nyawo of the University of the Witwatersrand and the Johannesburg Roads Agency, tackles a deceptively simple question: when a developer must choose between conventional piped drainage, rainwater harvesting systems and detention basins, how should that choice be made? The authors argue that previous studies have typically addressed fragments of the problem. Some evaluated hydrologic performance without cost. Others considered water supply reliability for rainwater tanks in isolation, without examining how those tanks alter the behavior of the entire drainage network. Very few combined probabilistic rainfall modelling, full hydrologic-hydraulic simulation of the complete drainage infrastructure, and whole-life cost analysis within a single decision framework.

To close that gap, the team developed a generic, parsimonious and computationally efficient method for sizing rainwater harvesting tanks based on the modal storm, the representative storm event that captures the dominant rainfall characteristics of a site. This SuDS-oriented tank-sizing approach deliberately sidesteps the elaborate continuous simulations and extensive historical data requirements that have traditionally made probabilistic tank design burdensome. By anchoring the sizing calculation to the modal storm, the method delivers tank capacities consistent with the statistical structure of local rainfall while remaining fast enough to embed in iterative design and optimization workflows. The tank sizing also moves beyond the conventional focus on water supply reliability alone, instead treating the tank as a component of the drainage system whose storage volume attenuates runoff and reshapes the hydrographs that downstream pipes and channels must convey.

With tank sizing resolved, the researchers built a multicriteria decision-making framework around the analytical hierarchy process, a structured technique for weighting and combining multiple, often conflicting, objectives. The framework compares alternative SuDS-based integrated drainage designs across infrastructure performance, whole-life costs and a broader set of sustainability factors. The analytical hierarchy process, originally formulated by Thomas Saaty, asks decision makers to perform pairwise comparisons among criteria and alternatives, translating qualitative judgments into quantitative priorities. Here those priorities were complemented by entropy-based criteria weighting, a technique that derives objective weights from the spread and discriminating power of the performance data itself, reducing the subjectivity that can undermine purely judgment-based weighting schemes.

The test bed for the framework was a large, real-world urban residential development on a greenfield site in Johannesburg, South Africa, a city whose climate and regulatory context make it a compelling laboratory for drainage research. The authors modelled the hydrologic and hydraulic response of the complete drainage infrastructure under each design alternative, routing runoff generated by probabilistically derived design storms through the network to assess whether pipes and channels retained adequate freeboard and conveyance capacity. Greenfield sites are particularly instructive because they allow designers to compare conventional and sustainable approaches from first principles, without the legacy constraints that complicate retrofit studies.

One of the study’s most striking results concerns hidden redundancy in existing design standards. The analysis revealed excess flow capacity in the drainage network, which translated directly into high marginal construction costs. The authors attribute this oversizing to implicit constraint duplication within the drainage design standards, meaning that multiple clauses in the governing guidelines effectively impose the same hydraulic requirement, each adding its own margin of safety. When these overlapping constraints stack, the resulting infrastructure is significantly larger, and therefore significantly more expensive, than a technically defensible design requires. In an era when municipal budgets are stretched thin, the finding suggests that simply harmonizing design standards could free substantial resources without sacrificing flood protection.

The cost analysis carried a second sobering message. Operation and maintenance expenditure accounted for approximately 35 to 47 percent of the net present value of the drainage schemes examined. Whole-life costing, which discounts construction, operation, maintenance, rehabilitation and end-of-life costs over the asset’s service horizon, is frequently championed in principle but neglected in practice, with decisions dominated by lowest capital cost. The study’s numbers make clear that ignoring the operational tail of a drainage project’s life distorts the comparison between options. Detention basins, for example, may appear inexpensive at construction yet accrue maintenance liabilities associated with sediment removal, vegetation management and outlet structure upkeep, while rainwater harvesting systems require pump maintenance, tank cleaning and eventual component replacement.

Rainwater harvesting carries an additional dividend that conventional drainage cannot match: it converts a nuisance into a resource. In a water-stressed country such as South Africa, where national planning documents have repeatedly warned of supply deficits, harvested rainwater can displace potable demand for non-potable uses such as toilet flushing, irrigation and laundry. The study’s framework captures this dual function, crediting harvesting designs both for runoff attenuation and for water conservation, while detention basins earn their place through flood peak reduction and conventional systems through their simplicity and familiarity. By scoring every alternative on the same weighted criteria, the framework exposes trade-offs that single-metric evaluations conceal, such as how much additional capital a rainwater harvesting scheme must absorb to deliver a given improvement in network hydraulic performance.

The broader significance of the work lies in its methodological generality. The authors describe their approach as generic, meaning it can be transplanted to other catchments, climates and regulatory regimes by substituting local rainfall statistics, unit costs and design standards. The parsimonious tank-sizing method lowers the computational barrier that has kept smaller municipalities and design consultancies out of probabilistic SuDS analysis, and the multicriteria structure accommodates additional criteria, from water quality improvement to amenity and biodiversity value, as local priorities demand. As climate change allowances force designers to plan for intensified rainfall, frameworks of this kind provide a defensible, transparent and reproducible basis for spending scarce infrastructure funds where they deliver the most value.

For engineers, planners and municipal officials, the practical takeaways are concrete. Audit drainage design standards for duplicated constraints before committing to oversized pipes. Bring operation and maintenance costs to the table at project inception, where they can shape the choice of technology rather than merely haunt the asset register. Treat rainwater tanks not just as water supply devices but as distributed storage that reshapes catchment response. And above all, evaluate drainage alternatives as integrated systems, using structured multicriteria methods that make every assumption explicit. The study suggests that the path to affordable, resilient urban drainage runs not through any single technology, but through smarter, more honest comparisons among them.

Subject of Research: Multicriteria evaluation of sustainable urban drainage design options combining hydrologic-hydraulic performance, rainwater harvesting tank sizing and whole-life cost analysis.

Article Title: Multicriteria Evaluation of Sustainable Drainage Options: Comparison of Conventional Drainage, Rainwater Harvesting and Detention Basins

Article References: Tanyimboh, T. T., & Nyawo, N. R. (2026). Multicriteria Evaluation of Sustainable Drainage Options: Comparison of Conventional Drainage, Rainwater Harvesting and Detention Basins. Water Resources Management, 40(12), Article 527. https://doi.org/10.1007/s11269-026-04878-8

Image Credits: AI Generated

DOI: 10.1007/s11269-026-04878-8

Keywords: sustainable drainage systems, stormwater management, rainwater harvesting, detention basins, multicriteria decision making, analytical hierarchy process, whole-life costing, urban flooding, runoff attenuation, water resources management, Multicriteria, Evaluation