The changes in land use associated with urbanization affect flooding in many ways. Urban development alters the natural landscapes by modifying the surface topography, replacing the vegetated and pervious areas with impervious surfaces, and compacting the soil, which ultimately leads to the increase in direct conversion of rainfall to runoff [1
]. Despite the use of a dense stormwater drainage system, major flooding is often associated with urbanized areas [6
]. The warming climate further intensifies this problem by increasing the frequency and intensity of extreme precipitation [6
] and severe floods [8
]. With the continuous growth in urbanization and rising global temperatures, effective management of the stormwater has become increasingly challenging [12
]. The challenge is further exacerbated by the use of conventional and aging stormwater infrastructure, making communities across the globe vulnerable to increasing flood risk [15
]. The existing stormwater systems are already beeing overwhelmed by the present-day storm conditions in many areas [17
]. Appropriate modifications of existing stormwater systems, including revision and updating the design practices and standards, retrofit of the existing infrastructure, and construction of additional ones, are required to mitigate the increasingly frequent occurrence of urban flooding [18
Traditionally, stormwater is managed by collecting storm runoff through a network of conveyance systems made of gutters, concrete channels, and pipes that help drain the excess water into nearby rivers and lakes [19
]. This system of stormwater management is also known as end-of-pipe practices involving centralized management [20
]. In most older cities, this traditional system has shown limited ability to handle the increased flow volume, quicker time of concentration, and high peak flow [21
]. Consequently, the system is susceptible to causing frequent flooding in the future [22
]. In the early 1990s, environmental managers, architects, and civil engineers proposed an alternative design approach, also known as Low Impact Development (LID), to improve the efficiency of traditional stormwater management. The LID practices are considered to provide better functioning of the hydrological and environmental systems in urban areas [23
]. The approach was proven to significantly control the discharge and peak flow compared to the traditional counterparts [3
]. The LID practices are environmentally sustainable as they shift the focus of urban stormwater management systems from using drainage pipes to designing runoff storage, increasing infiltration, reducing runoff quantity, and improving runoff quality [25
]. The most common LID practices include preamble pavements, bio-retention ponds, rain gardens, infiltration trenches, vegetated bio-swales, and rain barrels that foster retention, infiltration, evaporation, transpiration, groundwater recharge and discharge, and storage [26
]. However, despite their significant environmental and hydrological benefits, implementation and maintenance of LIDs are costly [27
]. Furthermore, little is known about their effectiveness under climate-driven extreme flood conditions [27
Several studies have compared LIDs with traditional methods of stormwater management. Gilroy and McCuen [29
] evaluated two LIDs (cisterns and bioretention pits) versus the traditional stormwater system in Maryland, USA. They pointed out that the conventional off-site stormwater management system is not effective for small watersheds. They also found that the size and spatial location of the LIDs affect their effectiveness in reducing peak flows and total runoff volume. Dietz and Clausen [3
] found the LIDs outperformed the traditional practices for sustainable stormwater management at the watershed scale. A similar study by Hood et al. [23
] in Connecticut, USA, found that LIDs effectively decrease runoff volume and peak flow rate by more than 1000% compared to the traditional stormwater system. The time of concentrations of the hydrographs from the LIDs are also significantly longer for intense storms with short durations and low soil moisture conditions [23
Although several studies have been conducted to evaluate the hydrological and environmental benefits of LIDs, very few have assessed the LIDs under future precipitation [30
]). Zahmatkesh et al. [31
] evaluated LIDs under climate change scenarios in New York City using SWMM and storm data from Coupled Model Intercomparison Project Phase 5 (CMIP5). Their results indicated an overall increase of 48% in annual runoff volume due to climate change, and the LIDs provide nearly 41% and 8–13% reduction of the volume and peak flow, respectively. Zhou et al. [32
] assessed the impact of climate change on urban flood volumes under Representative Concentration Pathway (RCP 2.6 and RCP 8.5) scenarios and the effectiveness of LIDs to reduce the flood volume in the city of Hohhot in northern China. Their findings revealed that flood volume is due to increase by as much as 52% in the future and that the LIDs were more effective in tackling the flood than the climate change mitigation measures through reduction of carbon emission.
Earlier studies were mainly focused on the modeling of LIDs in small areas within the watersheds [33
], providing little understanding about their effects on the overall watershed hydrology [36
]. Large scale hydrologic and hydraulic modelling and optimization of LIDs with conventional stormwater systems remain challenging. Most studies generally consider single types of LIDs, for example, permeable pavements [37
], bioretention ponds/rain gardens [38
], rain barrels [40
], rainwater harvesting/permeable pavement [41
], and bio-swales [42
]. This is mainly due to the modeling difficulties and lack of data to simultaneously simulate and optimize combinations of several LIDs. In some cases, LIDs failed to achieve desired expectations of reducing stormwater runoff and flooding problems since most of the models did not accurately incorporate the physical characteristics of LIDs, such as the size and location [43
]. To gain the full benefit from the combined LID and conventional stormwater systems, optimized implementation, in terms of size, location, and type, plays an important role [19
Therefore, to fill these research gaps, comprehensive hydrologic and hydraulic modeling is required that simulates and optimizes several LIDs under current and future extreme storm events [27
]. There are several hydrologic-hydraulic models, including Storm Water Management Model (SWMM), Western Washington Hydrology Model (WWHM2012), System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN), Urban Drainage and Sewer Model (MOUSE), Distributed Routing Rainfall-Runoff Model (DR3M), Storage, Treatment, Overflow, Runoff Model (STORM), and Long-Term Hydrologic Impact Assessment-Low Impact Development (L-THEA-LID), which are commonly used to simulate the effects of LID on hydrology and water quality [49
]. Among them, the SWMM model has been extensively used in the U.S. and many other countries worldwide [27
]. For the present study, SWMM was selected due to its ability to model dynamic rainfall-runoff properties in urban environment, easy to use graphic user interface (GUI) and its wide applicability for planning, analysis and design related to drainage systems in urban areas. SWMM can simulate the peak discharge, runoff volume, and water quality for short- and long-term rainfall events, making it suitable for designing and implementing several LIDs [46
Different LIDs (e.g., bioretention cells, bioswales, infiltration trenches, rain gardens, and permeable pavements) are incorporated into the model to evaluate their effectiveness in reducing the runoff characteristics (e.g., total flow and peak flow). The efficacy of each LID was evaluated by the ability to turn-on and turn-off specific LIDs and assess the changes in flood level. The study also incorporates the change in storms into the stormwater system design based on different climate change scenarios and models. While design storms derived from historical observations are often used in stormwater management design and planning, they may not represent the potential change in future precipitation. Given that most stormwater systems are designed to last for multiple decades into the future, consideration of future extreme storms is essential. Non-stationary IDF curves are developed using both the observed precipitation and downscaled future precipitation data obtained from the CMIP6 climate models under Sharted Socioeconomic Pathways (SSP585) scenarios. Furthermore, the LIDs implementation requires optimization of their types, sizes, and spatial distributions. The study determines suitable sizes and locations of the LIDs by comparing them with the existing stormwater system for both the historical and future 50 year and 100 year storm events.
3. Results and Discussions
3.1. Rainfall-Runoff Modeling
Comparison of the observed and downscaled daily precipitation for the MIROC6 and CMCC-ESM2 GCMs are shown in Figure 7
a,b, respectively. Results shows that the downscaled precipitation match the observed daily precipitation reasonably well. Overall, the MIROC6 model performed better (R2
= 0.82 and NSE = 0.81) than the CMCC-ESM2 model (R2
= 0.69 and NSE = 0.64). Based on this performance the MIROC6 model predictions under SSP 585 scenario were used to evaluate the LIDs performance under the future design storms.
shows the historical and future IDF curves for the study area. The precipitation intensities are expected to increase in the future for almost all durations and return periods. For example, the historical 100 year precipitation intensity event will be a 25 year event in the future under the SSP585 climate change scenario. On average, the observed 100 year precipitation intensity will increase by 2.5% to 30% by the mid-century, with the increase in short-duration precipitation being more prominent. The stormwater system is highly vulnerable to short-duration extreme precipitation, indicating that climate change will significantly impact the stormwater systems in the future.
3.2. Sensitivity Analysis
The SWMM sensitivity analysis results are presented in Table 4
. The 5% to 15% increase in the sub-catchment’s width, CN, percent Zero impervious (impervious area with no depression storage), and Manning’s n value for impervious surfaces results in the peak flow reduction of 10.75% to 22.44%, 17.29% to 24.74%; 13.89% to 22.44%, and 15.25% to 23.51%, respectively. This indicates the model is equally sensitive to the four parameters we have considered.
3.3. Calibration and Validation of SWMM
Four storm events were used to calibrate and validate the model. Figure 8
illustrates the observed and simulated flow rate for the four precipitation events. Overall, the model captures the hydrographs reasonably well, particularly the rising and recession of the hydrograph. The peak flows are slightly underestimated for the calibration period, whereas they are overestimated for the validation period (Figure 9
). The model underestimates the low flows in the validation period, but these low flows are not important in terms of stormwater system design [72
]. The NSE values for the calibration storms are 0.81 and 0.79, while R2
values are 0.83 and 0.81 (Table 5
). The NSE values for validation storms are 0.80 and 0.84, and R2
values are 0.82 and 0.87. Overall, the results indicate that the SWMM model reasonably reproduces the flow hydrograph for the historical period with the existing stormwater system, land use, and hydromorphic conditions of the watershed. The calibrated SWMM model is then used to simulate storm runoff for different past and future storm events and LID scenarios.
3.4. Effects of Existing LIDs on Runoff
The effectiveness of the existing LIDs to reduce flow rate at the drainage outlet was assessed by turning on and off the specific LIDs (i.e., infiltration trench (IT), permeable pavement (PP), bio-retention (BR), rain garden (RG), and rain barrel (RB)). Figure 10
shows the flow rate at the outlet with and without considering the existing LIDs in addition to the conventional stormwater system. The result shows that the existing LIDs play an important role in reducing runoff from the drainage area. Additionally, the effectiveness of the individual LIDs are assessed by replacing all the existing LIDs with a specific LID. Figure 11
shows the percentage reduction of flow rate when a single type of LID replaced all the existing LIDs in the stormwater system. As expected, the flow reduction varies depending on the types of LIDs.
The maximum flow rate reduction achieved compared to the baseline scenario with IT, PP, RG, BR, and RB were 84%, 63%, 36%, 33%, 29%, and 21%, respectively.
The baseline scenario considers the existing stormwater system, which include both the conventional stormwater and LIDs. The results indicated that infiltration facilities namely IT and PP have greater potential to reduce the surface runoff and peak flow than the storage facilities. The outcomes of single type LIDs in reducing surface runoff were consistent with the studies conducted by Xie et al. [76
] and Bai et al. [77
3.5. Effects of Proposed LIDs on Runoff
In addition to the existing stormwater system, we considered additional LIDs to further reduce the runoff. Six LID scenarios, based on the types of property and runoff coefficients of the sub-catchments, were considered to find the optimal combination of LIDs in the study area. Figure 12
shows the percent reduction in total flow by the different LID scenarios for 50 and 100 year storms events in the past and future periods. All the scenarios reduced the flood risk but with different magnitudes.
The average reduction of total flow for the historical (HIST 50 and 100) and future (CC 50 and 100) storm events are 29.8%, 34.3%, 52.5%, 62.0% 15.8% and 65% for LID scenarios S1, S2, S3, S4, S5 and S6, respectively.
The combination of RB+ BR+ IT (S6) offered maximum reduction in total flow (80% for 50 year and 70% for 100 year event), while IT + BR (S2) provides only 45% and 34% flow reduction for 50 and 100 year historical events, respectively. When single type of LIDs is considered, PP only (S3) reduces the flow by 65% and 57% for 50 and 100 year historical events, respectively. In contrast, BR only (S5) has the least effectiveness in reducing the total flow.
The different LID scenarios considered also performed well in minimizing the peak flows, but at lower percentage compared to the reduction in the total flow (Figure 13
). The comparison results among the six scenarios are similar to that of the total flow. For example, the RB+ BR+ IT (S6) provides maximum reduction in peak flows for all conditions (50 and 100 year historical and future storm events), whereas the BR only (S5) has lowest reduction in peak flow. On an average the overall reduction of peak flow for the S6 and S2 scenarios are 51.25% and 7%, respectively. For the single type of LIDs, S3 (PP) showed best performance in reducing peak flow (50% and 43% for the 50 year and 100 year historical storm events). For all the scenarios, the percentage reduction of total and peak flows for the future period are lower compared to the historical period. This indicates a better implementation of the LID mixes using a full optimization approach to improve their effectiveness under increased future storm and flood events.
Among the single LIDs, infiltration-based facilities (IT and PP) were better in reducing the surface runoff while the combined LID model S6 (RB+ BR+ IT) and S4 (PP + BR) performed better compared to any other combination under historical and future climate change scenario. Amalgamation of filtration and storage-based LID facilities provide a greater control over the surface runoff and peak flow. Hence, optimal combination of LIDs is necessary for obtaining full benefits from the green infrastructure development for stormwater management. Overall, the appropriate implementation of LIDs can help reducing the likelihood of flooding from increased extreme precipitation, and thus, assist tackling the urban stormwater flooding issues in the future with additional benefit to the region water quality, ecology, and socioeconomic. Therefore, this study will help the water managers and urban developers to better understand the effectiveness of different LIDs in tackling future storm events and flooding problems in the city of Renton and other identical areas.