Introduction

Design and Mainte.,!.ance of Road Drainal:e System to ·p.revent Floodinl:

SUl:l:ested Solutions to Problems

S.C. WONG Senior EngineerlResearch & Development

Drainage Services Department

April 1998

In the other papers of this Seminar, various cause~ of flooding d';le to problems at the inlets of

the storrnwater drainage system have been presented and discussed. These are summarized as

follows :-

i) blockage of gullies and gully connection pipes by rubbish;

ii) blockage of storrnwater drains by earth, debris and waste from construction sites;

iii) blockage of catchpits and storrnwater intakes by rubbish, vegetation, etc.;

iv) inadequate number of gullies;

v) inadequacies in the design of gullies and storrnwater intakes.

As mentioned previously, flooding in the urban areas will cause much inconvenience to the

public, may incur financial loss and even pose risks to life. Repeated and serious flooding is

therefore unacceptable to the public. As a service to the public, actions should be taken to solve

or mitigate the above problems. The following are some proposals on improvement measures

which will be detailed in the following paragraphs :-

• upgrading of substandard road drainage system

• improvements in maintenance practices

• improvements to the design of gullies and storrnwater intakes

• control oflittering and interference to drainage system due to construction activities

• education of the public

• emergency organization to combat flooding

• improved co-ordination between departments.

Uperadine of Substandard Road Drainaee System

The revised Road Note 6 issued in 1994 sets a new standard for design of gullies for road

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drainage. In particular, this Road Note requires gullies to be at much doser spacing on flat and

nearly flat roads, requires the use of overflow weirs (with vertical or horizontal bars) at gullies

along flat roads, steep roads and blockage blackspots, and the use of mUltiple gullies at sag

points. Since the issue of the Road Note, a proportion of existing roads have been upgraded to

the new standard when they are reconstructed. However there are still a lot of roads which have

not been reconstructed and upgraded. It is suggested that the upgrading of the drainage system

of these roads should be given higher priority. As DSD will soon start to carry out drainage

improvement works recommended by the Drainage Master Plan (DMP) Studies, it is desirable

for the upgrading works to be carried out not later than the DMP improvement works.

Improvements in Maintenance Practices

As one of the main causes of flooding is blockage of gullies, trash grilles and stormwater drains

by rubbish, mud and construction.waste, a good system of inspection and cleansing is therefore

of paramount importance to the proper functioning of these assets.

Frequency oflnspection and Cleansing

The frequency of inspection and cleansing should based on the importance of the road in

question, catchment characteristics and past experience of blockage and flooding. Gullies near

markets, hawker areas, in older districts, near construction sites and at other blockage blackspots

should be cleansed much more frequently than those at other locations. DSD can supply

information on flooding blackspots and drainage complaint blackspots to USDIRSD and HyD.

In addition, gullies and stormwater intakes on trunk roads, primary distributors and in front of

large basements where flooding will have a significant impact should be given more attention.

As suggested in the paper on flooding at West Kowloon, gullies at blackspots should be

inspected and cleansed frequently at an interval of, say, twice weekly. The frequency should be

reviewed regularly and further increased if frequent chokage is still observed. Catchpits and

stormwater intakes liable to chokage by rubbish or vegetation should be inspected and cleansed

before the rainy season and after every heavy rainstorm.

Method of Cleansing

As the number of gullies is very large, especially at new roads where gully spacings are much

closer in accordance with Road Note 6, an efficient method of cleansing is necessary. Cleansing

by mechanical means using gully emptiers together with water-jetting unit is a recommended

method. The gully emptier sucks out the muck; the water-jetting unit flushes the gully

connection pipes and refills the gully sump with clean water. It is essential that all floating

refuse, which is the main cause of blockage, is removed, perhaps by manual means. Flushing

of the gully connection pipe is required as rubbish and oil and grease accumulated in the

connection pipe can also cause chokage.

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Improvements to the Desi&n of Gulljes and Stormwater Intakes

The new Road Note 6 has been used for nearly 4 years since its issue in 1994. It is perhaps

appropriate to review the design standards based on actual experience. As blockage of gullies

and gully connection pipes by rubbish is a serious problem, improvements to the design may be

necessary. Some suggestions include :-

• the use of a sort of trap to exclude both floating rubbish and silt

• use of gully connection pipes of larger size

• use of special large-sized gullies at sag points instead of multiple gullies separated from

each other.

In addition, gullies along steep roads appear to have lower efficiency in trapping the flow.

Further study on an improved gully design for steep roads may be justified.

As concerns catchpits and stormwater intakes, particular attention should be paid to catchpits and

intakes with trash grilles or gratings. These grilles or gratings should be designed to minimize

blockage as far as possible, with due allowance to prevent overflow should blockage occurs. In

addition, catchpits and stormwater intakes should be designed to be large enough to catch the

flow, especially flow of high velocity and running down from steep slopes.

Control of Littering and Interference with Drainage System due to Construction Activities

To reduce blockage of gullies and stormwater drains due to discharge of litter and construction

waste, it is necessary to step up law enforcement and use other administrative control measures.

The laws applicable to littering are the Public Health and Municipal Services Ordinance

(PHMSO) (Cap. 132) and its Regulations, and the authorities for enforcing the laws are Urban

Services Department and Regional Services Department. USDIRSD are therefore requested to

step up policing and law enforcement.

As a related issue, USDIRSD are requested to urge their workers to ensure that no rubbish is

swept into gullies during street sweeping.

The law for controlling discharge of mud, cement paste and construction debris into gullies and

stormwater drains is mainly Section 9 of the Water Pollution Control Ordinance (WPCO) (Cap.

358), for which the authority is the Environmental Protection Department (EPD). Previously,

PHMSO and the Summary Offences Ordinance (Cap. 228) were also applicable; however after

WPCO has taken effect, most ot the relevant sections of PHMSO and Summary Offences

Ordinance have been deleted. Besides law enforcement, other administrative control measures

(e.g. through the Buildings Ordinance) may also be effective (or control of discharge of mud and

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wastes from construction sites.

To prevent illegal discharge of mud and construction debris into the stormwater drainage system,

DSD, HyD and USDIRSD are requested to inspect gullies adjacent to construction sites. If

discharge of muddy water, construction debris or earth into the public drainage system is found,

please immediately report to EPD (the relevant Local Control Office) for action.

For construction works of Government departments and private building lots, control by the

Engineer, the Engineer's Representative and the Authorized Persons are essential in 4 respects:-

• ensuring that there is a proper temporary drainage system to intercept all runoff from the

site and discharge to drainage systems outside the site. This includes the installation and

proper maintenance of silt traps to prevent discharge of mud and construction debris

debris outside the site;

• ensuring that there is no unauthorized interference with the public stormwater drainage

system. Before approving any temporary or permanent works on the public stormwater

system, the engineer/architect should ensure that the capacities of the drains are not

reduced, and any flooding risk is properly managed;

• ensuring that mud and construction waste is not discharged by the contractor into the

public stormwater system;

• ensuring that permanent drainage works newly constructed are regularly inspected and

desilted before hand-over to DSD.

The site staff should exercise more stringent control over contractors, and take strong action if

non-compliance posing a high flooding risk occurs. For construction works 'lf private building

lots controlled under the Buildings Ordinance, DSD has started to remind Authorized Persons

that contractors failing to comply with the law may face prosecution.

Education ofthe Public

Law enforcement and administrative control are just remedial measures after the undesirable

actions (discharge of rubbish, mud and construction waste) are committed. As a preventive

measure, the public should be reminded that littering onto streets and discharge of mud and

wastes into stormwater drains are two of the major causes of flooding. For the benefit of the

society, such actions should be avoided. DSD is already preparing a video to educate the public

in this respect. As a long term measure, DSD staff should bring out this point in interviews with

the media whenever appropriate, and in other formal or informal occasions.

Each year at the arrival of the rainy season, DSD issues letters to Authorized Persons of private

building lots, other Government departments and DSD contractors requesting them to properly

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drain the storm water of their sites, and prevent discharge of mud and construction waste into

public sewers and drains. In this year, we shall continue to do so.

Emereency Oreanization to Combat Floodine

During rainstorms, DSD carries out emergency clearance of choked gullies, stonnwater intakes

and stormwater drains in response to complaints. In addition, DSD sends gangs of workers to

flooding blackspots and chokage blackspots to carry out urgent preventive clearance. In this

year, DSD will deploy more resources to respond to emergencies during the rainy season,

through extending the service hours of its Direct Labour Force, and requiring term contractors

to provide emergency gangs as well.

As USDIRSD also have gangs of staff and plant available for routine cleansing of gullies, it is

suggested that similar emergency, gangs be fonnedto carry out clearance of gullies at blackspot

locations (e.g. near markets and hawker areas) at times ofrainstonns. This will assist DSD in

relieving possible or actual flooding at these locations more efficiently.

Improved Co-ordination between Departments

The inlets of the drainage systems are designed, constructed and maintained by at least 3

departments: DSD, HyD and USDIRSD. Other departments such as TDD and CED are also

involved in the design and construction of the road drainage system. In lIddition, EPD and

USDIRSD are involved in law enforcement against illegal discharge into public drains and

littering. A concerted effort is therefore required in order to tackle the identified problems and

minimize flooding. In order to enhance co-ordination, it is suggested that meetings amongst

departments be held regularly to share experience, to discuss possible improvement measures

such as those suggested above, and to have the measures implemented. A suggested forum is the

District Management Committee (DMC) meeting of the District Boards.

Conclusion

This paper has described various measures to combat flooding due to problems of the inlets of

the drainage system, and the actions DSD has implemented or is planning to implement. As

pointed out earlier, a concerted effort amongst responsible departments is necessary to solve the

problem. It is hoped that this paper will provide material to stimulate co-operation and co­

ordination between departments in this respect.

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