Hydrologic performance of bioretention system and permeable pavement for potential applications in Hong Kong

Abstract

Stormwater management is always a problem in Hong Kong since its development from a fishing village. Contributed by abundant precipitation, hilly topography, and dense urban development, flooding has been causing enormous economic losses to Hong Kong and is a main focus of local stormwater management. With the construction of many conventional hardcore engineering stormwater management structures in recent decades, such as underground detention tanks and stormwater tunnels, the flooding problem in Hong Kong has been well alleviated. It is now the time to move forward and incorporate more sustainable stormwater management principles and techniques, namely the strategy of low-impact development (LID), into the local practices in Hong Kong. Stormwater should be viewed not only as a problem, but also as a valuable resource.

This research aims at a feasibility study on the possible applications and hydrologic benefits of bioretention and permeable pavements under the local conditions of Hong Kong.

The buildability of infiltration devices in Hong Kong is examined by constructing pilot-scale physical models of both bioretention and permeable pavements in this study. Hydrologic monitoring of these physical models under Hong Kong rainfall events is carried out for at least one wet season. The monitoring data are analyzed to evaluate the hydrologic performance of bioretention and permeable pavements, as indicated by peak flow reduction and volume retention of stormwater runoff. The long-term hydrologic performance is also evaluated by the numerical model SWMM (Stormwater Management Model). After model calibration and validation using field data on the physical model, SWMM isused to simulate bioretention performance for the past ten year precipitation records of Hong Kong under systematic variations of two relevant parameters, namely the exfiltration rate and the area ratio of bioretention to catchment.

Results show that both bioretention and permeable pavements are feasible to be applied in Hong Kong. The hydrologic performance of bioretention is influenced by the precipitation patterns, the size of bioretention, the stormwater storage, and the properties of soil. As in common practice, the available storage of bioretention is much smaller than design rainfalls in Hong Kong. Therefore, peak flow reduction shall not be the target of incorporating bioretention in local storm drain designs. The influence on long-term water balance in the urban area may be considered as the main benefits from bioretention, using the annual retention ratio as a performance indicator. The hydrologic performance of permeable pavements is influenced by the storage depth provided by the gravel layer and the properties of in-situ soil. Considerably good peak flow reduction and volume retention are obtained in the experimental permeable pavements subjected to the local extreme precipitation events. In actually applications, the storage of permeable pavement may be designed to capture the total depth of design storms in Hong Kong, after which peak flow reduction may be obtained.

It is anticipated that this research can provide reference information on both the design and hydrologic benefit estimation of bioretention and permeable pavements practices for applications in Hong Kong.