Design and Maintenance of Road Drainage System to Prevent Flooding

Flooding in West

c.o. TAM Senior Engineer, Mainland South Division Drainage Services Department

Experience Sharing

I. 1997 was a "special" year. It was special in many aspects, among which were the abnormal rainfall - heavy and concentrated in short durations of time - which might be due to the El Nino Effect, and the severe and frequent flooding thus caused in Mongkok area, particularly along between Prince Edward MTR station and Mongkok MTR station. As a result of the coincidence in timing with the Reclamation and Hinterland Drainage Improvement works (the portion of the latter in Mongkok catchment could only be completed in late June 1997), the public and the media always connected the flooding in Mongkok with the West Kowloon Reclamation.

2. In fact, except for the most severe flooding near Prince Edward MTR StationIPlaying Field Road, which is a local low spot and the capacity of the stormwater drainage system there was exceeded in 1997, and some of the Yaumatei areas where there were drainage improvement works being carried out that required flow diversion and affected the capacity of the existing storm water drainage system, most of the other small scale flooding cases were due to blockage of the inlet of the storm water drainage system, i.e. roadside gullies, by various rubbish and debris including newspaper, commercial promotion leaflet, cardboard, plastic bags, plastic bottles, foam plastic lunch box, mud or even hardened cement paste. Coupled with the abnormal rainfall pattern - heavy and concentrated in short durations of time (sometimes during early morning before the scavengers started working and v.ith surfaces covered with a lot of rubbish left by pedestrian the night before), these small scale flooding were also very significant. As a matter of fact, in the mind of the citizens, all kinds of flooding cases, no matter large scale or small scale, are not desirable because a flooding case with a flood water level only up to the ankles of the pedestrian causes similar degree of inconvenience to pedestrian as that with a flood water level up to 0.5 m high. As a related

• 18 - point of information, one may need to disti nguish between flood water that rises above pedestrian pavement surfaces and those only stays on the surface of the road and is below the top of the kerb. Although the latter will also cause inconvenience to pedestrians cross ing the roads and pedestrians when passing-by vehicles cause splashing of the flood water onto them, it cannot be totally avoided as the total area of the openings in the roadside gully gratings are incomparable to the total surface area of the nearby road and pedestrian pavement that receive the rainfall, and, furthermore, certain hydraulic head is required to force the stormwater into the underground storm water drainage system through the roadside gullies. In fact, in the design of the spacing of the roadside gullies, ponding up to part of the slow lane has been assumed. Of course, if there are lots of rubbish/debris in the roadside gullies and/or the gully connection pipes, the effective cross-sectional area will be reduced and the hydraulic headloss will be much greater than the design values. The flow velocity through the roadside gullies and gully connection pipes will be affected and a greater hydraulic head will be required, in other words, the depth and extent of ponding around the roadside gullies will have to be greater than expected and more inconvenience will be caused to the citizens.

3. From the Government's point of view, every feasible piece of work has to be done to meet the needs of the citizens and all Government servants have to act in unison to achieve the goal of minimizing both the possibility and extent of flooding so as to avoid the disruption of the normal economic activities and inconvenience to people. As indicated in the last paragraph, whilst we are working to eliminate deficiencies in the existing stormwater drainage system to prevent major flooding cases, we still have to work diligently to maintain the existing storwmwater drainage system in such a way to keep the highest efficiency of it in draining away the stormwater within the shortest time. To achieve this, much work has to be done, not just to regular cleanse the main drains, but also to avoid blockage of the inlet, i.e. roadside gullies, by rubbish and debris. As mentioned in the last paragraph, the situation will be more critical if rainstorms affect in the early morning before the working hours of the scavengers. I did walk along Nathan Road, from Yaumatei to Prince Edward MTR Station several times in the recent months after 8 p.m. and before 8 a.m. and I found that the amount of rubbish on the pedestrian pavement, both after 8 p.m. and before 8 a.m., was terribly tremendous, particularly near , Pioneer Centre, junction with Mongkok Road in front of (IFO) Chase Manhattan Bank, junction with Argyle Street (IFO Shui Hing Department Store), Hui Chiao Commercial Building, Bank of East Asia Building, Bank Centre (IFO Chartered Bank), Hollywood Plaza, Sino Centre, Nathan Centre and Chow Tai Fook Centre, so that these sections of Nathan Road appear just like big "Refuse Collection Points". This is unacceptable, not only from the point of view of giving negative publicity to tourists, thus adversely affecting the

- 19- resurrection of the to urist indllstry. bllt also becallse if there comes heavy rainfall in the early morning, all the rllbbish, in particlllar hllge amount of commercial promotion leaflets and plastic bags, will be brought down to the nearby roadside gullies and block the inlet, i.e. gullies or the gully connection pipes, and render the stonnwater drainage system ineffective.

4. Last summer, USD, HyD & DSD had a meeting on the cleansing the stonnwater drainage system, in Mongkok areas. It was reported that the three departments carried out work on a regular basis:

(a) USD swept the streets four times a day, cleared the gullies once every two week (once a week for flooding black spots),

(b) HyD cleared all gully connection pipes before rainy season each year and once or several times more during rainy season as the need arises, and

(c) DSD cleansed the main drain on the flooding "red route" 3-4 times a year and additional cleansing will be done during rainy season to deal with flooding situations. For example, the main drains along Nathan Road at Mongkok were cleansed once a month from June to August last year.

5. The meeting also came to a consensus of coordinated action to keep the stonnwater drainage system clean and telephone numbers of USD action officers for various districts were passed to HyD and DSD so that whenever HyD or DSD staff found any blocked gullies, they could immediately refer to USD for action.

6. The arrangement is very useful and DSD has made use of this communication system to refer many cases to USD for action. However, from the observation so far, the problem of gully blockage still persists and there are many cases when we inspect the same series of gullies every week (USD staff may have cleaned them after each of our referrals to them), blockages continued to be found. It may suggest that the accumulation of rubbish/debris inside gullies is much quicker than what weekly cleaning can take care of. Therefore, in the Yau Tsim Mong District Mlmagement Committee meeting held in February, DSD raised a suggestion to the meeting that USD might need to increase the frequency of cleansing of gullies twice a week and the USD representative agreed to the suggestion and promised to start the arrangement for flooding black spots in Mongkok starting from April. To facilitate co-ordinated action by USD, HyD and DSD in the maintenance of the road drainage system, it will be useful if USD can report regularly to a coordinator, may be District Management Committee or DSD, the record of cleansing of gullies giving dates and locations of the work with copies sent to HyD, DSD and District Office as well so that the cleansing work by HyD and DSD can be programmed in a cost effective and co-ordinated manner. It will also help meeting the media reporting actions done by the SAR Govemment as a whole.

- 20- Legal aspects

7. In the aforesaid Yau Tsim Mong Di stri ct Management Committee meeting, the chairlady of the meeting, i.e. DO(YTM), also enquired about the enforcement by USD against litteri ng on street and it was reported that some actions had been done in November last year and there were some prosecutions done. However, as illustrated in the previous paragraph, the littering problem seems to be unsolved yet and perhaps more resources are required to specifically point out the problem to the public and take prosecution action, particularly at areas with high pedestrian flow, such as near large commercial centre, banks, MTR exits, bus stops, especially those in series, and traffic signal junctions. All actions against littering should not be short-lived and should continue until the undesirable situation of litter all along the street disappears. Recently it is known that there will be a new "Clean Hong Kong Campaign" in June. It is hoped that more prosecution actions would be done so as to give a better deterrent effect. As a related point of information, as observed on site, the scavengers may have difficulties to remove some fine particles/rubbish from the street surface and these fine parties/rubbish eventually appear inside the gullies somehow. USD may need to find some ways to help solve the problem.

8. Another source of problem that adds to the flooding problem is the illegal discharge of muddy/cementitious materials into public drains from construction sites. A meeting to coordinate actions of various Government departments, including EPD, HKPF, USD, BD, HyD, HAD and DSD, against illegal discharge of muddy/cementitious materials into public drains from construction sites at Yau Tsim Mong areas has been set up by DSD last year and three meeting has been held. The conclusion is that among the three available Ordinances to deal with the problem, namely CAP. 228 - Summary Offences Ordinance, CAP 132 - Public Health and Municipal Services Ordinance, and CAP. 358 - Water Pollution Control Ordinance, CAP.358 is considered the most effective one to prevent people from the malpractice as it entails imprisonment and the heaviest fine. Therefore, it has come to a consensus in the meeting that all departments will help EPD to spot out construction sites having the malpractice and refer to EPD for enforcement action. From the record of DSD it is found that there are some positive responses from some of the contractors/Authorized Persons for the construction sites after the standard letters are issued to the contractorl Authorized Person for the construction sites with copies to EPD for enforcement actions and other departments (including HKPF) for information, as these contractors/ Authorized Persons did carry out some rectifications promptly. This is the first step forward and further work has to be done to further improve the situation.

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