The Tai Hang Tung Storage Scheme (THTSS) aims to solve the flooding problem in Mongkok – a highly densely populated commercial and industrial district of West Kowloon, Hong Kong. For a 50-year design storm, a maximum upstream inflow of Q = 110 m3/s must be conveyed within acceptable water levels. The heart of the flood detention scheme is a 100,000 m3 underground storage tank, a triple side weir system, and transitions to the upstream inlets and downstream channels. The THTSS design was developed from a combination of heuristic reasoning, physical and numerical model studies. The performance of the THTSS during the heavy rainstorm of June 2008 is reviewed. The complex hydraulics in the triple side-weir system is first studied via a 2D numerical solution of the shallow water equations using a shock-capturing finite volume method; the flow behavior and weir overflow distributions are compared with experiments. Measured flood levels in the surcharged system are studied using a 1D unsteady model for key storm events and different weir level settings. The audit offers insights into the hydraulic control and optimal operation of the system.