The prediction of bridge-pier scour has been mostly aimed for a single pier of various sizes, shapes, and alignments with flow. For the case of multiple piers, widely used manuals still recommend using single-pier scour formulas with an equivalent diameter, regardless of pier separation and angle of attack. In this paper, we present the results of a new set of laboratory experiments conducted to address the local scour around a group of three piers with different sizes, spacing, and attacking angles. Based on the phenomenological theory of turbulence, we also formulate a semitheoretical framework for calculating the maximum scour depth in the multipier condition, including the single-pier counterpart as a special case. The few coefficients in the final equation were determined by experimental data from this and other studies. Our results provide an understanding of the eddies responsible for the scour process, and show that pier diameter, pier spacing, actual pier width, flow depth, Froude number, and sediment size are all important variables that need to be considered in order to obtain an accurate prediction of the maximum scour depth. Therefore, these results shed new light on current engineering procedures based on using single-pier expressions in the case of multiple piers.

Bioretention systems are widely applied in green stormwater infrastructure, with increasing interest in incorporating recycled materials to improve sustainability. This study develops a novel granular bioretention medium by valorizing water treatment residuals (WTR) through bio-enzyme granulation. A synthetic bio-enzyme formulation transforms fine-grained WTR into durable, water-stable granules (2–5 mm). Laboratory tests and a full-scale field demonstration evaluated hydrological, physical, and pollutant removal performance. The WTR-derived granules exhibited high porosity (>50%), sustained high infiltration rates (>2000 mm/h in laboratory tests and >760 mm/h under field conditions), and effective removal of stormwater pollutants including TSS (98.4 ± 0.2%), TP (69.5 ± 7.8%), NH₃-N (92.4 ± 3.5%), TN (63.0 ± 4.9%), and COD (88.2 ± 5.6%). Scanning electron microscopy confirmed excellent structural stability after five years of operation. By converting a widely generated urban waste stream into high-performance stormwater treatment media, this approach demonstrates a practical circular economy pathway that enhances urban flood resilience while reducing reliance on virgin materials and energy-intensive disposal.

Urban combined sewer systems are increasingly challenged by climate-intensified rainfall, combined sewer overflows, and receiving-water degradation. This study presents a retrospective evaluation of a hybrid green-gray retrofit program implemented in the Zhenjiang Sponge City pilot watershed, China, where green stormwater infrastructure, drainage network upgrades, and a centralized deep tunnel system were integrated within a densely developed watershed constrained by limited space, low native-soil permeability, shallow groundwater, and aging infrastructure. System performance was evaluated using long-term operational observations, representative hydraulic and water-quality monitoring, municipal operational records, and supporting engineering analyses at both facility and watershed scales. The results demonstrated sustained hydraulic functionality after 7–10 years of operation, with approximately 90% of the monitored bioretention systems maintaining effective infiltration rates greater than 80 mm h⁻¹. Event-based monitoring indicated substantial reductions in runoff volume and pollutant loads, including TSS, COD, NH₃–N, and TP. Following implementation, annual combined sewer overflow occurrence at major outfalls decreased from 318 to 24 events, representing a 92.5% reduction. Supporting engineering analyses indicated that green stormwater infrastructure retrofits alone reduced overflow frequency by approximately 41.8% and overflow volume by approximately 61.1%, while integration with deep tunnels increased reductions to approximately 58.8% and 85.3%, respectively. Official receiving-water monitoring records further indicated that Class III or better water-quality conditions were maintained during approximately 74.7% of the monitored days between 2021 and 2026. These findings provide long-term watershed-scale evidence that hybrid green-gray retrofit strategies can integrate green stormwater infrastructure with centralized overflow regulation to achieve sustained overflow reduction and receiving-water improvement in highly constrained urban watersheds.