The applicability of LID facilities as an adaptation strategy of urban CSOs management for climate change

The magnitude and frequency of extreme rainfall due to climate change is increasing. Increasing rainfall causes serious hydrological problems in cities. Rainfall does not infiltrate the soil, but mostly flows through the sewer pipes into the stream. Most old urban watersheds have combined sewer pipes. When rainfall exceeds the capacity of the combined sewer pipes, sewage mixed with stormwater overflows the sewer pipes and flows directly into the stream. This is called Combined Sewer Overflows (CSOs). CSOs enter the stream with non-point source pollutants accumulated on the surface and pollute the stream. CSOs are one of the major water quality problems in older urban watersheds. This can be solved by replacing the combined sewer pipes with separated sewer pipes, but in reality it requires astronomical costs. As an alternative, the Low Impact Development (LID) technique has recently been introduced. In this study, we analyzed the effects of climate change on CSOs in urban watersheds and applied LID techniques to offset the effects. The LID facility was applied with the most commonly used Bio-Retention cells.

Assessing the effects of climate change on urban watersheds: a review and call for future research

Considerable efforts have been made to control and manage hydrology and water quality of watersheds impacted by urban development through construction of stormwater control measures (SCMs). Climate change (CC) could, however, undermine these efforts through intensifying precipitation and hydrologic extremes. Although the impact of CC on water resources has been well-documented, its impact on urban hydrology remains less studied. CC may complicate sustainable urban hydrology, which can cause reduction in SCM efficiency with changes in precipitation pattern (i.e., change in duration, frequency, depth, and intensity). More intense precipitation may result in reduced runoff reduction and treatment efficiency given that SCMs have the finite surface storage volume and surface infiltration capacity. Determining the functionality of various SCMs under future climate projections is important to better understand the impact of CC on urban stormwater and how well these practices can build resiliency into our urban environment. The purpose of this review is to provide the needs and opportunities for future research on quantifying the effect of CC on urban SCMs and characterizing the performance and effectiveness of these systems under existing and projected climate scenarios. A summary of the modeled constituents as well as the stormwater and climate models applied in these studies is provided. We concluded that there are still limitations in exploring the impact of future change in meteorological variables will influence the operation of SCMs in the long-term. Previous studies mostly focused on the impacts of CC on urban runoff quantity and only handful studies have explored water quality impacts from CC such as potential changes in water temperature, metals and pathogens. Assessing pollutant removal efficiency of SCMs such as bioretention, infiltration trenches, dry and wet swales, rooftop disconnections, wet and dry ponds, which are common practices in urban watersheds, also needs more attention. Analysis on the cost of adapting SCMs to CC to maintain the same performance as current climate conditions is also recommended for future research.

Comprehensive Performance Evaluation of Green Infrastructure Practices for Urban Watersheds Using an Engineering–Environmental–Economic (3E) Model

Green infrastructure practices could provide innovative solutions for on-site stormwater management and runoff pollution control, which could relieve the stress of nonpoint pollution resulting from heavy rainfall events. In this study, the performance and cost-effectiveness of six green infrastructure practices, namely, green roofs, rain gardens, pervious surfaces, swales, detention basins, and constructed wetlands, were investigated. The comprehensive performance evaluation in terms of the engineering performance, environmental impact, and economic cost was determined in the proposed engineering–environmental–economic (3E) triangle model. The results revealed that these green infrastructure practices were effective for stormwater management in terms of runoff attenuation, peak flow reduction and delay, and pollutant attenuation. It was suggested that for pollution control, detention basins can efficiently reduce the total suspended solids, total nitrogen, total phosphorus, and lead. The implementation of detention basins is highly recommended due to their higher engineering performance and lower environmental impact and economic cost. A case study of a preliminary cost–benefit analysis of green infrastructure practice exemplified by the Pearl River Delta in China was addressed. It suggested that green infrastructure was cost-effective in stormwater management in this area, which would be helpful for sustaining healthy urban watersheds.