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

The study begins by contextualizing the incidence of urban flooding within rapidly expanding metropolitan areas. Urban landscapes are characterized by an extensive network of roads, buildings, and other paved surfaces, significantly altering the natural water cycle by accelerating runoff and reducing infiltration. The authors argue that understanding these changes requires precise hydrological models capable of simulating surface runoff, infiltration, and subsurface flow processes under varying climatic and land use scenarios. Both SCS-TR20 and SBUH models boast comprehensive frameworks that may be tailored to urban catchments, enabling the dissection of hydrological responses to precipitation events with high spatial and temporal resolution.

Fundamental to their research is the exploration of how different physical and environmental factors affect urban flooding. These include soil properties, topography, storm characteristics, land use patterns, vegetation cover, and urban infrastructure design. The SCS-TR20 model, originally developed to estimate direct runoff from small watersheds, is applied here to quantify the runoff generation process by integrating rainfall data with land surface conditions. It incorporates parameters like curve number (CN), which reflects soil infiltration capacity and land cover, to predict peak discharge during storm events. This allows the researchers to examine how changes in impervious surface area or soil moisture status may intensify or mitigate runoff production in urban catchments.

Simultaneously, the study leverages the SBUH hydrological model, a unit hydrograph-based method that simulates watershed response to rainfall by accounting for soil moisture dynamics. The SBUH model provides a mechanistic representation of subsurface flow and its contribution to streamflow, which is often overlooked in simpler runoff generation models. By embedding soil moisture accounting within the urban context, the authors can capture delayed flow components and baseflow behavior, enriching the flood prediction process. This is particularly important in urban areas where drainage infrastructure can alter natural groundwater recharge pathways.

A critical component of their methodology involves calibrating and validating these models against observed hydrological data from urban watersheds experiencing recurrent flooding. The researchers emphasize rigorous parameter optimization to enhance predictive accuracy. They adopt techniques such as sensitivity analysis to identify which parameters most significantly influence model outputs, thereby guiding priorities for data collection and urban planning interventions. The models’ outputs are then cross-verified through statistical measures, including Nash-Sutcliffe efficiency and root mean square error, confirming their robustness in simulating runoff processes in real-world urban systems.

Their findings highlight that urban flooding is not solely dependent on rainfall intensity and volume but is profoundly affected by intricate interactions between soil characteristics, land cover changes, and urban drainage infrastructure. For instance, increasing impervious surfaces significantly raises the curve number, leading to more immediate and higher runoff peaks. Moreover, poor maintenance or design flaws in stormwater systems exacerbate flooding by reducing conveyance capacity. The study also reveals that antecedent soil moisture conditions before rainfall events play a pivotal role; saturated soils contribute to quicker runoff generation and potential flash floods, emphasizing the need for integrated urban water management that considers soil moisture dynamics.

The implications of this research extend to urban planners, environmental managers, and policymakers. By elucidating how specific landscape and infrastructure features influence flood risks, the models can inform the design of resilient urban drainage networks. Incorporating green infrastructure, such as permeable pavements, rain gardens, and bio-retention basins, can modulate runoff by enhancing infiltration and delaying peak flows, a strategy validated by simulation scenarios using their models. Additionally, the work supports the advancement of early warning systems that integrate hydrological forecasts with real-time rainfall data, offering communities proactive flood risk mitigation.

Significantly, the study advances the hydrological modeling field by adapting traditional models—initially designed for rural or agricultural catchments—to the complex hydrodynamics of urban settings. This adaptation includes customizing parameter sets to account for heterogeneity in urban land use, addressing issues such as localized flooding resulting from blocked drains or topographical depressions. The dual-model approach combining SCS-TR20 and SBUH affords a more comprehensive understanding than reliance on a single modeling framework, as it captures both surface and subsurface hydrological processes with greater fidelity.

Furthermore, the research contributes to global climate change discussions by considering how altered precipitation patterns may interact with urban factors to exacerbate flooding. The authors simulate future storm scenarios based on climate projections, revealing potential increases in flood frequency and severity if urban areas do not enhance their stormwater resilience. This evidence reinforces the urgency of integrating climate adaptation measures into urban development policies, underscoring the indispensable role of hydrological models in forecasting and managing emerging flood risks.

An additional innovative aspect of this study is the use of high-resolution spatial data and advanced computational techniques to implement the hydrological models. Remote sensing technologies and geographic information systems (GIS) are applied to delineate watershed boundaries, map land cover, and analyze slope and soil properties with great precision. The incorporation of these technologies allows for dynamic model updating and supports scenario testing for various urban growth and land use change possibilities. By utilizing such data-driven methods, the authors ensure that their flood impact analyses are both spatially relevant and temporally sensitive.

The multi-faceted modeling framework also evaluates the effectiveness of different flood mitigation strategies using scenario-based simulations. For example, the researchers model the impact of increasing urban green spaces or enhancing stormwater storage capacity on reducing flood peak discharge. These simulations provide actionable insights for urban engineers and policymakers by demonstrating which interventions yield the greatest reduction in flood risks under diverse weather conditions. Consequently, their work serves as a decision-support tool to prioritize resource allocation and infrastructure investments in flood-prone urban regions.

In summary, the comprehensive evaluation and analysis conducted by Dehdehi, Soleimani-Motlagh, and Haghizadeh signify a breakthrough in understanding urban flood dynamics through integrated hydrological modeling. By leveraging the strengths of SCS-TR20 and SBUH models, their study provides a nuanced depiction of how natural, built, and climatic factors coalesce to influence flooding in cities. These insights pave the way for smarter urban water management, resilience building against extreme weather events, and sustainable urban growth.

Looking forward, the authors advocate for further refinement of hydrological models through enhanced data assimilation and coupling with hydraulic simulations to address flooding at the street and neighborhood scales. They also suggest expanding their approach to encompass socio-economic dimensions, thereby assessing the human and economic impacts of flood events alongside hydrological predictions. Such interdisciplinary integration will be vital as urban flood challenges intensify in the face of urban expansion and climate variability.

Ultimately, this pioneering research not only enriches the theoretical hydrology landscape but also offers practical tools for urban flood mitigation worldwide. As cities grapple with the escalating frequency of intense storm events, such scientifically rigorous and technologically sophisticated studies provide a beacon for safeguarding populations, infrastructure, and ecosystems from the devastating consequences of urban flooding.

Subject of Research: Evaluation and analysis of factors affecting urban flooding through hydrological modeling.

Article Title: Evaluation and analysis of the impact of factors affecting urban flooding using SCS-TR20 and SBUH hydrological models.

Article References:
Dehdehi, N., Soleimani‑Motlagh, M. & Haghizadeh, A. Evaluation and analysis of the impact of factors affecting urban flooding using SCS-TR20 and SBUH hydrological models. Sci Rep (2026). https://doi.org/10.1038/s41598-026-58279-3

Image Credits: AI Generated

Nature-based solutions refer to strategies that harness natural processes and ecosystems to address societal challenges. In the realm of stormwater management, this can manifest through green roofs, permeable pavements, rain gardens, and urban forests, all of which help to manage runoff while also providing ecological benefits. Finland has become a fertile ground for implementing these solutions, as its unique climatic conditions and urban structures provide an excellent backdrop for such innovations. The insights derived from this research underline the necessity of integrating NbS within the existing urban planning frameworks to mitigate the adverse effects of stormwater runoff.

One of the pivotal aspects discussed in the research is the need for supportive policies that facilitate the implementation of nature-based solutions. The authors emphasize that merely recognizing the benefits of NbS is not enough; comprehensive regulatory measures must be established to incentivize property owners to adopt these sustainable practices. This often involves initial investments, but with the right policies, the long-term ecological and economic benefits can outweigh these upfront costs. The research highlights successful examples from Finland where policies were tailored to promote NbS, allowing stakeholders to better align their practices with environmental sustainability.

Moreover, the study delves into the importance of stakeholders’ engagement in shifting urban stormwater management practices. Engaging communities, property owners, and urban planners is crucial for the successful implementation of NbS. The authors articulate the necessity of fostering a collaborative environment whereby stakeholders can share insights, resources, and strategies. Public awareness campaigns, workshops, and incentive programs can significantly enhance community buy-in and participation, thus creating a supportive atmosphere for implementing nature-based solutions.

Finland’s regulatory landscape serves as a fascinating case study for examining the intersection of policy, community engagement, and environmental management. The Finnish approach has seen local governments play a critical role in facilitating these solutions, often through financing and educational initiatives. The authors illustrate how municipalities have taken initiatives to not only promote knowledge about NbS but also to provide fiscal support for property owners willing to adopt such measures. This aligns perfectly with broader goals of enhancing urban resilience against climate change while maintaining ecological balance.

Throughout this research, the authors present compelling arguments for the necessity of monitoring and evaluation frameworks to assess the success of implemented nature-based solutions. Merely installing a green roof or rain garden is insufficient without a systematic approach to tracking its performance over time. The need for robust data collection and analysis is a recurring theme that emphasizes the value of continuous improvement in urban stormwater management strategies. By understanding what works and what does not, cities can refine their approaches, drawing lessons from both successes and setbacks.

In addition to environmental benefits, NbS can have significant social outcomes. The research highlights how enhanced green spaces not only improve aesthetics but also promote community interaction and mental well-being. The potential for nature-based solutions to serve as community hubs cannot be overstated. Urban environments that integrate NbS not only mitigate stormwater issues but also contribute to increased public health and social cohesion. This multifaceted approach underscores the need for policies that explicitly recognize the wide-ranging benefits of NbS beyond mere functionality.

As cities worldwide grapple with the challenges posed by climate change and urbanization, the Finnish experience sheds light on essential strategies for sustainable urban development. The research articulates how other regions can draw inspiration from this model, advocating for tailored policy mechanisms that cater to local contexts while emphasizing the necessity of comprehensive stakeholder engagement. Policymakers are urged to recognize that NbS is not just a trend; it represents a shift toward resilient urban landscapes that prioritize ecological integrity alongside economic feasibility.

To further bolster these efforts, the authors suggest leveraging technology in promoting and managing NbS. Digital tools, including geographic information systems (GIS), can aid in mapping potential sites for intervention and assessing the ecological performance of implemented solutions. Combining traditional urban planning with modern technology widens the scope for innovative approaches in stormwater management, paving the way for more informed and efficient strategies.

The dialogue surrounding nature-based solutions in urban settings is a rich and evolving conversation, particularly as more data becomes available through ongoing implementations. The insights derived from Finland serve as a foundation upon which other cities can build their frameworks, fostering a sense of unity in tackling global challenges like climate change and biodiversity loss. With the proper blend of policy support, community engagement, and technological innovation, the vision of sustainable urban environments managed through nature-based solutions is not merely aspirational but achievable.

The ongoing discourse on NbS also hints at a broader cultural shift in how society views nature. As urban spaces increasingly encroach upon natural habitats, the collective recognition of the essential services that these ecosystems provide could lead to a paradigm shift in urban planning. The outcomes of this Finnish case study could thus resonate far beyond its borders, inspiring global efforts aimed at integrating nature harmoniously into the fabric of urban living.

As we anticipate the publication of this research in “Ambio,” it stands as a testament to the evolving understanding of how nature-based solutions can transform urban spaces for the better. The call to action for integrating these practices into regulatory frameworks sets the stage for a new era in urban development, one that embraces environmental sustainability as an intrinsic component of city planning. By prioritizing nature in every decision, towns and cities can pave the way for resilient future generations, ensuring that nature thrives even in the bustling heart of urban life.

In conclusion, the insights from Saarinen, Leskinen, Reini, and their colleagues are more than just academic findings; they represent a clarion call for action in urban planning and policy. Nature-based solutions are not peripheral additives but central to sustainable urban development. The ongoing implementation and refinement of these strategies will be key to overcoming the challenges posed by climate change, enhancing urban resilience, and fostering healthy communities. The lessons learned from Finland could indeed inspire global movements towards greener, more sustainable urban futures.

Subject of Research: Nature-based solutions in urban stormwater management in Finland

Article Title: Policy and regulatory measures supporting the implementation of nature-based solutions in urban stormwater management of private properties: Insights from Finland

Article References:

Saarinen, A., Leskinen, P., Reini, A. et al. Policy and regulatory measures supporting the implementation of nature-based solutions in urban stormwater management of private properties: Insights from Finland.
Ambio (2025). https://doi.org/10.1007/s13280-025-02253-2

Image Credits: AI Generated

DOI: 10.1007/s13280-025-02253-2

Keywords: nature-based solutions, urban stormwater management, Finland, policy, environmental sustainability, community engagement, green infrastructure