Abstract: The Tai Hang Tung Storage Scheme (THTSS) is an essential component of the West Kowloon Drainage Improvement-StageProject that will be implemented to solve the flooding problem in Mongkok-a congested and densely populated district of the Hong Kong Special Administration Region Over the years rapid development and changes in land use have resulted in significant increases in surface stormwater runoff,and the existing drainage system failed to cope with severe rainstorms.Rather than digging up roads and widening/adding existing drains,an economic drainage improvement plan calls for the construction of a 100,000 m³ underground storage tank upstream of the flooding black spots,below an existing football field in the vicinity of the Tai Hang Tung/Tat Chee Road junction.The upstream storage can serve to attenuate peak floods and protect downstream areas from flooding.Due to the complicated flows in the stormwater drainage system (including subcritical and su percritical open channel flows,pressurized flow in closed conduit,side weir overflows,and transitional flows at junctions),a 1D numerical model of the unsteady flow is developed to study water stages and flow characteristics for different side weir ar rangements,based on the observations provided by a 1:22 undistorted Froude scale physical model under the steady inflow.The key findings of the physical and numerical model studies are reported herein.The studies have resulted in an optimal design of the flood storage scheme; the physical model tests and the numerical model results indicate that THTSS can enhance the flood control capacity to cope with a one in 50 year rainstorm.