Deep Storm Drain: A Two-Minute Entry and a Tripod Without a Winch
A 6.5 m deep storm drain manhole on a road in Kwun Tong has a 900 mm opening and a fixed ladder. Two workers are to remove silt and a plastic sheet blocking the drain. After overnight rain, water at the bottom is about 0.8 m deep and rising. A tripod is set over the manhole but has no winch line; a rope is tied to a nearby lamp post. The standby worker is also directing traffic 25 m away behind cones, and the radio is in the van. The entry worker says 'two minutes, no harness, the ladder is fine'. A permit signed for 'inspection only' is in the van. Readings taken with a 3 m sampling line at 1 m depth are O2 20.9%, LEL 0%, H2S 0 ppm. No bottom reading was taken. The downstream flow is controlled by a flap valve that a colleague says is 'probably closed'.
1. List the main hazards and explain why the 'two minutes, no harness' plan is unacceptable.
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Hazards:
- water and engulfment: 0.8 m and rising after rain, with a flap valve only 'probably closed'; a surge can trap or drown a worker;
- fall from 6.5 m and ladder slip;
- retrieval: no winch line, and a rope tied to a lamp post is not a rated anchor or retrieval system;
- traffic: the standby is 25 m away directing traffic, so no one is at the entry;
- atmospheric: no bottom reading, and drain silt can release H2S or methane; a 3 m line cannot sample the bottom;
- communication: the radio is in the van.
The 'two-minute' plan is unacceptable because the permit is for inspection only, the hazard controls are absent, and a quick entry gives no time to react to a fall, gas alarm or water rise. A confined space entry must be planned for rescue, not for speed.
2. What sampling and monitoring must be completed before entry, and what equipment is needed?
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Sample at the top, middle and bottom of the 6.5 m drain, including just above the water and near the silt, using a pump and extension tubing long enough to reach the bottom. Test for O2, LEL, H2S and CO at each depth; repeat after ventilation and during entry. Because this is a storm drain, methane and H2S are possible; if readings change with water flow, stop.
Equipment: calibrated multi-gas monitor with a pump, spare batteries, tripod with winch and fall-arrest harness for each entrant, retrieval line, rated anchor, portable blower and duct, radios, barriers and traffic control, and a rescue plan. No entry until the bottom reading is known and stable.
3. What changes are required to the permit, standby and rescue arrangements before any entry?
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The permit must be reissued for the actual task, silt removal, not inspection. It must record isolation of the flap valve, positively closed and locked or controlled by a dam or plug, water-level monitoring, ventilation, sampling results, entry duration and evacuation triggers.
The standby must be dedicated at the manhole, trained, with a monitor and radio, and must not direct traffic; a separate traffic controller is needed. Rescue: tripod or winch with a harness on the entrant, a trained rescue team, a retrieval line, first aid and a clear 999 briefing. If the flap valve cannot be positively controlled or water is rising, do not enter; use remote or pumping methods.