THE B2BN TUNNEL JOB
Tackling Chaeng Watthana’s floods
The challenge is that the huge drains must be built underground, close to the route of the Pink Line electric railway.
Chaeng Watthana Road, or Highway No. 304, is notorious for being struck by flash floods. Within a matter of minutes heavy rain can turn the road into something verging on a small canal, which usually takes at least three hours to drain.
The road runs through Pak Kret district of Nonthaburi and the Bang Khen and Laksi districts of Bangkok.
After years of complaints from residents, the Ministry of Transport has finally decided to try and solve the problem once and for all.
The Department of Highways, the state agency that oversees public roads, has received a 2-billion-baht budget to lay a subterranean water-drainage system under the flood-prone road to increase the speed at which it can be drained.
The project, however, will take a couple of years to finish. That means residents and motorists in the area must deal with the effects of flash floods for a while longer.
The case of Chaeng Watthana Road highlights one of the problems with Thailand’s rapid urbanisation as more people migrate to the capital. In the old days, the area was green, populated with fields and ponds that helped absorb and retain water.
But land development projects over the last three decades has stripped away this natural drainage system. One section of the road that connects from Vibhavadi Rangsit Road to Pak Kret is crowded with several growing communities. The Government Complex is also located on this section.
As new residential projects spring up, the road is also “lower” due to all the newly filled ground. At the same time, the natural watercourses that once served as natural drains have all but disappeared.
TEAM Consulting Engineering and Management Co was hired to conduct a technical study into the feasibility of the new underground drainage system, and conduct public hearings with local residents.
The company began its study on Sept 30, 2017 and submitted it to the government at the end of July.
Meanwhile, the Highways Department will offer bids for construction by the end of this year, according to Praphatphao Awakul, director of the department’s maintenance work development and management section.
Construction work will proceed on a 12.2km section of Chaeng Watthana Road from the area around Tiwanon Road to the Lak Si Monument, according to Awirat Suksomart, a water resources engineer at TEAM Consulting.
The section will be broken up into two parts. The first part will stretch for 7.4km from the Tiwanon Road area to Khlong Prapa, in Nonthaburi province. The other section will run for 4.8km from Khlong Prapa to the Lak Si Monument in Bangkok.
The study showed that the first section only has small underground drains on either side of the road. Its authors recommended enlarging these to between 1.2m and 2.3m in diameter.
The challenge is that the huge drains must be built underground, close to the route of the Pink Line electric railway. Construction of the Pink Line is ongoing in the middle of both Tiwanon and Chaeng Watthana roads.
As such, the contractor must use extra labour and a more complicated engineering method to lay the underground drains.
In areas with sufficient pavement space, the contractor will excavate the road and lay the pipes. But at any part of the route where it is impossible to excavate the road, the contractor needs to use another technique called “pipe jacking”.
This requires the use of a prefabricated pipe, which must be threaded through the ground. During the execution of this process a hydraulic jack will be required to force the pipe forward into the ground.
However this will affect more traffic lanes than if the authorities simply close one channel.
It was therefore decided that any work requiring the pipe-jacking technique will be carried out at night to mitigate its impact on traffic.
Additionally, a new water pump station will be built near the only existing one next to Bang Talat canal. This is the closest point to where the canal drains into the Chao Phraya River.
“We expect this will greatly improve the speed at which floodwater can be drained,” said Mr Awirut.
The second section of the project, from Khlong Prapa to the Lak Si Monument, will undergo a similar process of laying more drains underground.
Those that use the pipe-jacking technique in this section of the road are designed to receive water that can then be drained into the Prem Prachakon canal, which in turn empties into the Chao Phraya.
Some of the new drains are designed to receive water that will later be emptied into the Chao Phraya via Khlong Lat Phrao canal, which is connected to the Rama IX giant water tunnel.
“When this project is completed, we are confident the excess water that follows a downpour will be completely drained within 30 minutes — compared to three hours at present,” said Mr Awirut.
The closed channel and pipe-jacking techniques have been chosen for this project because they are cost-effective and have the least impact on traffic lanes, he said.
The Chaeng Watthana project has already undergone three public hearings.
Most people who attended them said they agreed with the project. Some called for more studies to be conducted to determine the exact drainage capacity of the Bang Talat canal, said Phattharawadi Nguatchai, an expert on public participation and relations who is working on the project.
Some observers pointed out the canal’s drainage capacity should be improved before it is included in the new system, she said.
Other important views summarised from the hearings argue that drains of larger sizes should be considered for use in the long run, as the volume of rainwater is expected to rise in the future.
Attendees were also informed that any contractor who wins a contract to implement the Chaeng Watthana project should not already be engaged in several other large-scale projects, to ensure timely implementation.
More studies were recommended to prevent certain scenarios, such as incidents where water flows back into the sewerage system and stops wastewater flowing out when the Chao Phraya is experiencing high water levels, said Ms Phattharawadi.
Meanwhile, street vendors were repeatedly blamed at the hearings for polluting water sources and stopping some of the water from being drained into the river.
Other questioned whether the Department of Highways has in place a proper water-treatment programme for the water that will be drained into the Chao Phraya, she said.