Introduction

The Tai Hang Tung Storage Tank Scheme (THTSS) is an essential component of the West Kowloon Drainage Improvement -- Stage 2 Project that will be implemented to solve the flooding problem in Mongkok. Rather than digging up roads and widening/adding existing drains, an innovative and economic upstream storage scheme is devised. The plan calls for the construction of a 100,000m3 underground storage tank below an existing football field in the vicinity of the Tai Hang Tung/Tat Chee Road junction. Under design conditions (maximum flow of 110m3, part of the flood flow along the Tai Hang Tung Road culvert (approx. 8m wide by 2.5m high) and Tat Chee Road culvert will be intercepted by a system of side weirs. The stormwater that overflows into the underground tank will be temporarily stored; the flow diversion/storage serves to attenuate the flood peaks and prevent downstream flooding.

Schematic diagram of the stormwater sewer system with Tai Hang Tung Storage scheme (not to scale)

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Methodology

A 1:22 undistorted Froude scale physical model was designed and constructed to study the feasibility and performance of the THTSS scheme. As a complement, a 1D numerical model of the unsteady flow was developed to study water stages and flow characteristics in the whole system (including the study area and all downstream drains), and to investigate optimal design of the side weir arrangements and channel junctions.
　

Using the experimentally derived side weir discharge coefficients, the performance of the scheme for an unsteady hydrograph has been examined for:

A 75m long trapezoidal side weir, linearly tapering from 14m width at entrance to 1m width at the dead end; and
A system of three rectangular side weirs (all 30m long and 8m wide) shown in the accompanying diagram. The level of the side weir crest zw is 5.5 mPD.

Observed flow in Tai Hang Tung and side weir channels (1:22 undistorted Froude scale physical model)
　

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Results

Both the physical model tests and the numerical model results suggest that the THTSS can improve the flood control capacity; the experiments confirm the key features of the flood attenuation, including a predicted back flow from the Boundary Street Nullh into the Tai Hang Tung sewer and the side weir channel overflows. The maximum flood level appears to be about the same for both schemes. The water level at the flooding back spot decreases by about 0.9 m, the required height of storage tank is approx. 6.6 m implying a storage tank volume of about 117,000 m3.


Computed time history of water level at location A	Computed time variation of water level in storage tank (Scheme 2)

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