Abstract
Low impact development (LID) devices or green infrastructures have been advocated for urban stormwater management worldwide. Currently, the design and evaluation of LID devices adopt the Intensity-Duration-Frequency (IDF) method, which employs the average rainfall intensity. However, due to variations of rainfall intensity during a storm event, using average rainfall intensity may generate certain errors when designing a LID device. This paper presents an analytical study to calculate the magnitude of such errors with respect to LID device design and associated device performance evaluation. The normal distribution rainfall (NDR) with different standard deviations was employed to represent realistic rainfall processes. Compared with NDR method, the error in sizing the LID device was determined using the IDF method. Moreover, the overflow difference calculated using the IDF method was evaluated. We employed a programmed hydrological model to simulate different design scenarios. Using storm data from 31 regions with different climatic conditions in continental China, the results showed that different rainfall distributions (as represented by standard deviations (σ) of 5, 3, and 2) have little influence on the design depth of LID devices in most regions. The relative difference in design depth using IDF method was less than 1.00% in humid areas, −0.61% to 3.97% in semi-humid areas, and the significant error was 46.13% in arid areas. The maximum absolute difference in design depth resulting from the IDF method was 2.8 cm. For a LID device designed for storms with a 2-year recurrence interval, when meeting for the 5-year storm, the relative differences in calculated overflow volume using IDF method ranged from 19.8% to 95.3%, while those for the 20-year storm ranged from 7.4% to 40.5%. The average relative difference of the estimated overflow volume was 29.9% under a 5-year storm, and 12.0% under a 20-year storm. The relative difference in calculated overflow volumes using IDF method showed a decreasing tendency from northwest to southeast. Findings from this study suggest that the existing IDF method is adequate for use in sizing LID devices when the design storm is not usually very intense. However, accurate rainfall process data are required to estimate the overflow volume under large storms.
Highlights
To mitigate the negative impact of urbanization on stormwater management, green development concept, such as the Low Impact Development (LID) in the US or the Water Sensitive Urban Design in Australia etc., has been widely advocated to mitigate stormwater runoff, these employ onsite infiltration-storage devices to reduce peak flow rates and total runoff volume [1,2,3,4]
Device designed for storms with a 2-year recurrence interval, when meeting for the 5-year storm, the relative differences in calculated overflow volume using IDF method ranged from 19.8% to 95.3%, while those for the 20-year storm ranged from 7.4% to 40.5%
Findings from this study suggest that the existing IDF method is adequate for use in sizing LID devices when the design storm is not usually very intense
Summary
To mitigate the negative impact of urbanization on stormwater management, green development concept, such as the Low Impact Development (LID) in the US or the Water Sensitive Urban Design in Australia etc., has been widely advocated to mitigate stormwater runoff, these employ onsite infiltration-storage devices to reduce peak flow rates and total runoff volume [1,2,3,4]. The retention effect of a LID device is affected by different storm processes, it is important to use an appropriate design storm when considering LID device design [11,12]. As the front input parameter, storm process distribution is of vital important for designing LID devices and urban drainage systems [19]. Several models can simulate rainfall-runoff processes considering LID devices. SWMM and SUSTAIN can be employed to imitate the hydrological process of a single LID device, but complex model parameters such as pipe networks and GIS need to be taken into account. LID devices reduce runoff mainly through an onsite infiltration process In this respect, collected storm runoff is temporarily ponded in the surface storage layer of the LID device and is overflowed to the city drainage system or via deep percolation to groundwater.
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