5TH International Congress on Technology - Engineering & Science - Kuala Lumpur - Malaysia (2018-02-01)

A Sensitivity Analysis For Improvement Of Stormwater Interception Ratio Estimating Of Vegetative Swale Lid Facility

Progress of urbanization means the increase of impervious area ratio and change of terrain of urban area. These lead to changes in hydrological phenomena, such as increase in peak stormwater(runoff) according to rainfall. To prevent this problem, LID facilities have been attracting attention recently and their application is expanding. Accurate evaluation is required for effective design of LID facilities. Currently, in Korea, the evaluation method of the non-point pollution reduction facilities is used as it is to evaluate LID facilities [1]. This evaluation method was devised considering the large-scale reservoir facilities. However, direct application to LID facilities, generally designed for small-scale, may result in over-estimation or under-estimation of the facilities. Also, the empirical equation for calculating the rainfall interception rate (RIR) in the existing evaluation method was derived using rainfall data at only four stations. Since this empirical equation does not take into account various of rainfall properties across the country, this equation can lead to inaccurate facility assessment. In this study, we improved the empirical equation. The existing RIR equation is based on the design rainfall. However, the ratio of LID facility area and Contributing Area (ratioL/C) is more important to LID facilities than the design rainfall [2,3], so we present the empirical equation based on the ratio of LID facility area and Contributing Area. the LID facilities do not deal directly with rainfall, bur rather processes stormwater from rainfall. So first, we calculated the stormwater interception ratio (SIR) through long-term runoff simuation using EPA SWMM. For this purpose, the region, where long-term rainfall and runoff data were observed, was built in SWMM. The hydrological phenomena of the region were reproduced by using the observed data to correct the parameters related to the runoff. After that, SWMM-LID module was used to organize the vegetative swale composed of the selected parameters through literature review[4,5,6,7]. Table 1 is shown used parameters. After that, by analyzing the effect of design specifications of the LID facility on the SIR, the validity of the SIR equation based on ratioL/C was verified. Also, the SIRs were calculated by using the rainfall data of 61 stations across Korea in order to examine effect of rainfall properties on the SIR. By averaging the SIRs for rainfall properties and by regression analysis the SIR empirial equation of vegetative swale based on ratioL/C was devised. As a result of the sensitivity analysis of the SIR according to the design specifications, change amount of 0.01 was shown when the facility depth was increased by 200 mm (Fig. 1). It is considered that the influence of depth of LID is very small in the standard ratioL/C, and it means the validity of the estimation of the SIR equation based on the ratioL/C. Sensitivity analysis of the SIR according to rainfall properties showed a maximum variation of 0.2 (Fig. 2). It is considered that the influence of rainfall properties is large, so rainfall properties should be considered when formulating the SIR empirial equation. Based on these results, we devied the SIR empirial equation which can minimize regional error by averaging SIRs for rainfall properties. The devied empirical equation of vegetative swale is as follows: SIR = 0.2351 ln(ratioL/C) + 1.0255 The application of the exisiting RIR equation is likely to underestimate or overestimate the performance of the LID facilities(vegetative swale). By using the results of this study, it is possible to eliminate errors due to the application of the RIR and reduce errors due to insufficient rainfall properties. This make it possible to evaluate the performance of LID facilities relatively accurately, and it is expected to be helpful for facility design and evauluation.
Jeonghyeon Choi, Okjeong Lee, Sangdan Kim