Abstract: Green infrastructure (GI) is becoming a potential alternative to traditional urban drainage system due to their reputable performance in surface runoff control, non-point source pollution abatement, infiltration enhancement, and groundwater recharge, etc. However, the objectives of surface runoff control and groundwater protection may conflict with each other. With the enhanced infiltration of GI, runoff can be better controlled, but it may result in a higher risk of local groundwater mounding and quality deterioration, particularly in shallow groundwater areas. In addition to optimizing the design of an individual GI, allocating GIs optimally in suitable locations can possibly strike a better balance between surface runoff control and groundwater protection. This study utilized a two-way coupled semi-distributed hydrological model, SWMM-MODFLOW, to evaluate the surface-subsurface hydrologic performance of GI in the watershed scale. Landscape metrics, which is a group of indicators in landscape ecology representing the spatial pattern of land use, were used as indicators to quantify the spatial allocation of GI in this study. The mathematical relationships between landscape metrics and regional surface (i.e., peak reduction and volume reduction of surface runoff) and subsurface (i.e., peak and average groundwater table rise) hydrological performance indicators of GI were investigated. Landscape metrics provide a multi-scale approach to link smaller-scale distributed GI with larger-scale GI systems, and relate urban hydrology to urban planning and landscape ecology. The results of this study provide more quantitative evidence for GI planning at regional scale, particularly in areas with groundwater concerns.

Abstract: Green infrastructure (GI) controls surface runoff and restores pre-development hydrological conditions in an efficient and environmental-friendly way. However, shallow groundwater restricts the implementation of GI due to possible groundwater contamination, flooding of basement and decline of surface runoff control efficiency. Thus, it is challenging to strike a balance between surface runoff control and groundwater protection. Some design methods have been proposed, but knowledge on how to allocate LID practices to prevent or minimize these problems is limited. This study presents a loosely-coupled hydrological model, named SWMM-MODFLOW, that allows simulating the two-way interactive behavior between LID practices and groundwater. Based on a calibrated model, the impact of spatial allocation of GI on surface hydrological performance (i.e., peak and volume reduction of surface runoff and total discharge) and groundwater dynamics (i.e., peak height of groundwater mound) under different environmental conditions (i.e., initial groundwater table depths, rainfall intensities, and in-situ soil types) was examined at an urban catchment at the Kitsap County, Washington State of the U.S. GI showed good performance in surface runoff reduction even in shallow groundwater environment. They enhanced the formation of groundwater mound but the effect was localized. Porous pavements showed better performance than bioretention cells, particularly in the peak reduction and volume reduction of surface runoff. Bioretention cells allocated aggregately showed slightly better performance in peak reduction and volume reduction of surface runoff and total discharge than that allocated distributedly. In addition, initial groundwater table depth and in-situ soil type showed more significant impact on groundwater dynamics, while, rainfall intensity showed greater impact on surface hydrological performance of GI. The results of this study provide some references for allocating GI in the catchment scale with an objective of balancing the surface and subsurface hydrology conservation.

Abstract: Green infrastructures (GI) are effective in controlling surface runoff, reducing non-point source pollution, and restoring pre-development hydrologic cycle. Studies have been performed to evaluate the appropriate designs and planning strategies of GI to maximize their benefits in water quantity and quality management. However, very few studies considered their influence on groundwater and the influence of groundwater on their performance, which are beneficial to GI planning particularly in shallow groundwater areas. In this study, a surface-subsurface coupled hydrological model, SWMM-MODFLOW, was utilized, which was calibrated and validated based on the monitoring data in one urban catchment in Silverdale, Washington, U.S. Based on the calibrated model, the influence of GI spatial allocation on surface runoff and groundwater table dynamics was evaluated. The implementation ratio, aggregation level and location of GI all influenced the surface runoff and groundwater table dynamics. With more GI practices implemented, more runoff can be controlled and the groundwater table would rise more, but the uniformity of regional groundwater condition was not significantly affected. Besides, targeting at high runoff control efficiency and uniform groundwater condition, the near-optimal spatial allocation of GI for different planning objectives and environmental conditions (i.e., rainfall characteristics, groundwater conditions, and soil properties) was determined. More GI practices tended to be implemented in areas of wetter climates and gentler regional groundwater hydraulic gradients. GI practices tended to be allocated spatially distributedly for all different conditions, but more upstream (downstream) areas would be preferable for areas of steeper (gentler) groundwater hydraulic gradients and when groundwater management (surface runoff control) was more focused on.

Abstract: Porous pavement is one of the most widely-used and efficient low impact development (LID) practices. Given its high surface infiltration capacity and storage volume, it can recover natural hydrologic regimes in an urban area through reducing, delaying and retaining surface runoff. However, its performance is highly uncertain in shallow groundwater environment (i.e., some areas in Hong Kong) since the high subsurface moisture content may lower the infiltration and exfiltration of porous pavement, resulting in greater outflow (i.e., surface runoff and underdrain flow). This study utilized a laboratory experiment to investigate the hydrologic performance of porous pavement under different rainfall patterns and groundwater table depths. The surface runoff and underdrain flow of the porous pavement were collected, and their flow rates were measured. Then, the peak and volume runoff reduction rates were calculated to quantify its runoff control performance. To deduce the influence of porous pavement on the quantity and quality of groundwater, the variations of groundwater table depth and the water depth inside the pavement reservoir were also monitored. The groundwater mounding height, time for wetting front to reach the groundwater table, as well as the time for groundwater mound to dissipate were then calculated. The results demonstrated the hydrologic performance of porous pavement in a relatively controlled condition, which can complement findings obtained from in-situ monitoring and support the design of porous pavement in shallow groundwater areas.

Abstract: The hydrologic cycle, as well as the interactive behaviors between hydrologic regime and ecosystem have been greatly altered mainly due to overwhelming urbanization and remarkable transform of land cover. Low impact development (LID) practices are efficient systems to control surface runoff, remediate non-point source pollution, mimic natural hydrologic conditions and particularly recover water – ecosystem interactive behaviors through natural-mimicked processes of interception, infiltration, detention/retention, etc. Storm Water Management Model (SWMM) is one hydrology – hydraulic model that has been widely used to evaluate the hydrologic and environmental benefits of LID practices in urban catchments. However, SWMM highly simplifies the simulation of groundwater system, and it even separates LID practices away from groundwater system, making it infeasible to evaluate the performance of LID practices in shallow groundwater conditions. This work modified the current SWMM by considering the interactions between LID practices and shallow groundwater (called SWMM-LID-GW). The model was tested in an urban catchment with porous pavements and bioretention cells implemented in shallow groundwater. Long-term dynamics of LID underdrain flow, surface runoff and total hydrologic load of the catchment retrieved from the model were compared with field monitoring data. SWMM-LID-GW showed better performance in simulating the hydrologic performance of LID practices (i.e., underdrain flow) and the urban catchment (i.e., total hydrologic load) in shallow groundwater than the current SWMM. In addition, SWMM-LID-GW was further tested through a group of hypothetical case studies on the same urban catchment with various native soil types, rainfall patterns, and groundwater levels. SWMM-LID-GW showed improved performance than current SWMM for a wide range of conditions, proving it to be a versatile modeling tool to evaluate hydrologic performance of LID practices in shallow groundwater.

Abstract: Bioretention cells, as one representative low impact development practices, have been proved to be effective in controlling surface runoff, removing pollutants and recharging groundwater. However, they are often not recommended in shallow groundwater areas due to potential groundwater pollution, reduction in runoff control performance and groundwater drainage through the underdrain. Most design guidelines only require a minimum distance between bioretention cell bottom and seasonal high groundwater table without guiding the design of bioretention cells to mitigate the problem of shallow groundwater. This study therefore proposed some design recommendations of bioretention cells for different rainfall runoff loads, native soil types and initial water table depths. A variably saturated flow model was employed to conduct event-based simulations on one single hypothetical bioretention cell in shallow groundwater, which was calibrated using experimental and simulation data of an on-site bioretention cell. A wide range of climatic and geophysical factors (i.e. initial groundwater depths, native soils, rainfall runoff loads) and bioretention designs (i.e. media soil types and underdrain sizes) were considered. Surface runoff reduction, time before groundwater mound formation, as well as maximum height of groundwater mound were evaluated. Less-permeable media types (i.e. sandy loam) are recommended in areas with many extreme rainfall events (i.e. 40 – 70 mm/h or larger) and of shallower groundwater, which can better protect groundwater from mounding and possibly contamination although may slightly compromise the runoff control performance. For areas having seasonal high groundwater table of 0 – 1 m below bioretention bottom, underdrain is recommended to maintain good infiltration capacity without draining groundwater. However, underdrain is not recommended for areas of groundwater table always near or above the bioretention bottom, only if an impermeable sheet is added. Generally, groundwater interference is a concern only when groundwater table is above 1 – 2.5 m below bioretention bottom and runoff loads are very high. The results of this study overall could benefit the implementation of bioretention cells in shallow groundwater areas, and the establishment of relevant design guidelines.