Modelling the impact of water sensitive urban design on pluvial flood management in a tropical climate

Increasing urbanization and population growth in Thailand have altered the natural urban environment and waterways, requiring metropolitan Bangkok and the surrounding areas to develop a more sustainable and effective stormwater management plan. Pluvial flooding has become a particular challenge for these areas, especially due to frequent, high intensity rainfall events associated with a tropical climate that conventional stormwater management is not always able to effectively accommodate. This study therefore aims to introduce a newer stormwater management approach, Water Sensitive Urban Design (WSUD), which is supported by dynamic mathematical modelling, to explore and emphasize its ability in managing pluvial flooding for a mixed residential, commercial, and industrial area of peri-urban Bangkok. Different landscape architectural designs from the Thammasat Nava Nakorn Smart District Project, including bioswales, bioretention cell, raingarden and detention pond were used as the basis of the performance evaluation. A Personal Computer version of the Stormwater Management (PCSWMM) was applied to understand flooding behaviour under a 2-year design storm (72 mm for 24 hours). Design options to improve performance of the individual designs and their connectivity with respect to runoff volume and flood, frequency, duration, and severity were then evaluated. WSUD performance was varied from one feature to another depending on the design size and its ability to capture runoff volume. The WSUD scenarios had less impact on surface flood duration and peak surface flood rate, but arguably volume reduction is more important than peak reduction for pluvial flooding. Flood reduction benefits under each feature were smaller for the entire system (Nava Nakorn) compared to the localized catchment area which is attributed to the fact that WSUDs area shared a small proportion of the entire study area. Simulation result for the localized impact showed that flooding volume was reduced by between 6 – 78 % for all scenarios when bioretention cells were considered and up to 100 % for all scenarios that included bioswale, raingarden and detention pond designs. Volume reduction for the entire Nava Nakorn Estate ranged between (-14 –3.2 %) for the highest impact scenarios under each WSUD design. Furthermore, the findings indicated that flood volume and its severity were greatly decreased when all WSUDs being established together with the maximum reduction of 7.5 % for the entire Nava Nakorn Estate. Increasing percentage of pervious area (surface area occupied by the design) would result in better performance of the designs but would also require higher investment cost.