Physical Model Study for Smithfield Flooding

Team Members: Prof. JHW Lee, Dr. D Yu, Dr. DKW Choi, CKC Wong

Introduction

Experiment

Results

Proposed Improvement

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Introduction

On 24th June 2005, a major flooding occurred at Smithfield Road.  Extensive overshooting from a stepped drainage channel was observed. The stepped channel has a complicated flow structure, including different hydraulic features such as a stepped channel, a 132-degree bend, and two trash grilles. As the rainstorm was not very heavy, it was unknown why the flooding occurred. A study is carried out to investigate the detailed physical model of the channel and develop improvement measures.

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Experimental Setup

Two physical models of the Smithfield drainage channel are used:

• a straight channel plywood model

• an undistorted perspex model.

The straight channel is built for the purpose of studying the characteristics of the stepped channel flow, and to isolate the effect of the bend. The perspex model includes the full length of the steep cascade as well as detailed steps in the channel. Experiments are performed in several configurations: without trash grilles, and four alternative grille arrangements.

Experiments show that with a straight channel, the flow capacity is sufficient to prevent flooding on 24th June 2005. This suggests that the sharp bend as well as the trash grille installation played a part in the flood.

Overview of Smithfield drainage channel model

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Results

The flow down the Smithfield drainage channel is a high velocity supercritical flow. The overshooting flooding flow is caused by:

1. the sharp channel bend between the upper and lower channels; and

2. the trash grilles located just at the entrance to the steep cascade leading to the downstream drainage system.

The sharp bend gives rise to a highly aerated spiral flow, which in turn causes significant lateral overflow. At the trash grille, the flow splits to form an overshooting flow above the covered cascade, onto the street level. The overshooting flow accounts for about 60 percent of the total overflow.

Spiral flow downstream of bend

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Proposed Improvement Work

The proposed modifications are:

1. Reposition of trash grilles;

2. Extension of the cover part of the lower channel; and

3. Inlet modification

After the implementation of the improvement works, no flooding has since been observed in the 2006 wet season.

Extended cover at the lower channel

Left: Model (Q = 12.3 m³/s), Right: Prototype

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