Flood Mitigation at Tan Kah Kee
As Singapore experiences prolonged torrential rainfall during the monsoon season, the water levels in drains and canals along Tan Kah Kee area may exceed 90 per cent of their capacity causing flash floods. These causes cars to be submerged in wheel-high waters affecting the traffic in the area.
Figure 1: Flash Flood at Tan Kah Kee bus stop area
To prevent further occurrence of flash floods along Tan Kah Kee area, Hocklim Engineering designed a Box drain that connects to an existing open drain leading to the main Bukit Timah Canal. This is to ensure smoother flow of surface runoff into the drainage system and to act as a detention tank during excessive rain.
To carry out the construction of the Box drain, Hocklim Engineering had to liaise with NParks for the preservation and clearance of vegetation before excavation work can commenced. Hocklim engaged a certified arborist to carry out a Visual Tree Assessment on all tree species within the proposed work area to identify which tree species should be preserved and transplanted and which can be cleared.
Box drain - Detention Drain along Drainage reserve
Figure 2: Trees to be transplanted were tagged with bright-colored tapes for easy identification.
Figure 3: Ensuring that the tree roots were not damaged during the transplantation
After the site was cleared of vegetation, excavation can take place. Trial trenches were first dug manually to detect any cables under the cable detection plan. After which hydraulic excavators were used to dig up to a depth of around 0.5m and the remaining excavation depth of 1m was carried out using manual tools to prevent any damage to the cables. Any cables discovered had its’ condition assessed by a cable detection specialist and immediately protected to prevent unnecessary damage and as part of additional precaution. Subsequent excavation works were done in the same format to ensure that no cables were damaged. For safety, shoring measures were implemented along the excavated area to prevent any collapse as the site is near to the road.
Figure 4: Manual excavation to ensure cables were not damaged
During the construction as the Box drain site area is less than 100mm away from the 800mm diameter water main, Hocklim drafted a Risk Assessment Plan to mitigate the risk of damaging the water main. In addition, Hocklim also drafted an Instrumentation and Monitoring Plan to monitor excessive vibration in the water main. An Instrumentation Engineer was assigned to site to periodically check and record the vibration data to ensure the vibration levels do not exceed the maximum allowable threshold level.
Figure 5: Vibration meter to measure vibration levels
Due to the urgency of this project, the pre-casting of the U-drain was done in a factory concurrently to the excavation works, thereby ensuring the installation of the U-drain to be smooth and efficient.
After the U-drains are lifted into place, the drain walls are topped up to the required height and a top slab is casted to close off the drain resulting as a Box drain. Once the Box drain has been installed, the overall capacity of the drain has increased by 425.12m³ providing abundant capacity to deal with excessive rain.
Once the casting of the Box drain had been done, the installation of a series of stoplog at the various discharge and entry points of the Box drain was done to enable the Box drain to act as a detention tank during excessive rain.
Figure 6: Installation of U-drains before a top slab is casted to close off the drain
Stoplog System - Remote activated
The Stoplog system was designed and installed by Hocklim Engineering to monitor and alleviate the flash floods by managing the water flow in the Box drain. The primary component of the system is a panel constructed of stainless steel.
The panel is attached to a thick steel wire cable that is powered by a motorized winch that can be operated remotely via a website in real time. In addition, there is a manual hand crank installed as a fail safe option.
The stoplog system uses CCTVs to monitor the flood situation on-site, if the water level in the main canal becomes too high, the stoplog system is easily activated via a website using a remote I/O controller. Once activated, the stoplog at the discharge point prevents any backflow of water from the canal into the Box drain. When all the stoplogs are closed, water is kept within the Box drain, making it function as a detention tank.
Figure 7: Motorized winch and manual hand crank installed
Figure 8: Users are able to control the stoplog via a website that streams the CCTV images in real-time
How it works
When it rains, rainwater will drain into Drainage A and flow into Bukit Timah Canal
Once the water reaches the activation level, Stoplog A will be activated and lowered, preventing water from flowing into Bukit Timah Canal. Rainwater will continue to fill Drainage A and once it is is full, the water will overflow into Drainage B and C. Water is then contained within Drainage B and C, making it function as a detention tank.
When the water level in Bukit Timah Canal has subsided, Stoplog A will be activated and hoisted up. Due to difference in pressure, the flow velocity of water increases causing the water to gush out as it exits into Bukit Timah Canal. This will empty Drainage A allowing rainwater to continue to fill it up. This process will be repeated, as the water flow is being managed by the Stoplog. This will greatly reduce the possibility of flash flooding around the area.
Once the rain has subsided, Stoplog A, B and C will be hoisted to allow water contained in Drainage A, B and C to flow into Bukit Timah Canal. After the drains have been emptied, Stoplog B and C will be lowered while Stoplog A will remain open resuming their default positions.
Figure 9: Rainwater flowing into Drainage A and into Bukit Timah Canal
Figure 10: Once Drainage A is full, water starts overflowing into Drainage B and C.
Figure 11: Stoplog A is raised, allowing water to flow back into Bukit Timah Canal
Figure 12: Stoplog A, B and C is raised to allow water to flow into Bukit Timah Canal.
Lastly, this entire design solution will not be possible without the collaboration with Mr. Tan Hooi Soon of PUB. With his contribution, the design solution of the Box drain had proven effective in mitigating flash floods along Tan Kah Kee area as the overall capacity of the drain had increased by 425.12m³ to adequately deal with the excessive rain. In addition, the installation of Stoplogs is essential in managing the water flow in the Box drain.
This entire system also requires minimum manpower to operate as it can be controlled remotely via a website that streams the CCTV image in real-time ensuring quick response time to activate, making it a reliable, efficient and effective flood mitigation system.