The ongoing flooding in the East of the Chao Phraya River. Credit: ONWR

Authorities rush to discharge massive water volume from last rains to keep Bangkok safe from great flooding risk

They nod in agreement that the flooding this year is far different from the “great flooding” in 2011, which had devastated a large part of the country and the Central Plains and caused the massive economic loss worth over US $46 billion

Water management authorities from the Office of National Water Resources (ONWR) and the Royal Irrigation Department are racing against time to try to discharge excessive overflows left in the Lower North and the Central Plains by the two tropical storms last month in order to brace for possible more rains brought by Monsoons and next tropical storms forming in the seas.

Two tropical storms of Conson and Dianmu, hitting the country during September 6 to 13 and 23 to 24 respectively, have generated nearly 11 billion cubic meters (cu m) of water. The authorities concerned have managed to discharge over 4.6 billion cu m of the water to the sea, leaving over six billion cu m left to be discharged from the Lower North and the Central Plains, hit the hardest by Dianmu.

They have to rush the work as there is a Monsoon trough passing the Central Plains, while there is a report of another two tropical storms forming in the seas that could run past the country in the next ten days. The authorities cannot say at this point whether or not they will hit the country or the Central Plains, which is experiencing the massive overflows from the last two storms.

Credit: ONWR

The best approach is trying to clear the excess water so that there will be some room to take the new water if any, according to Dr. Somkiat Prajamwong, the outgoing Secretary-General of the ONWR.

“We cannot say at this point whether the two new storms will affect us or not, but we will try to clear the water left in the Central Plains as quickly as possible to ensure that we (Bangkok and the Central Plains) will be safe from the next rains if any. And if they come, I just hope that they cross past the lower part so they will cause less impact,” said Dr. Somkiat.

The overflows

The overflows and floodwaters caused by Conson and Dianmu play a critical factor to determine whether the capital of Bangkok and its peripheries will experience severe flooding this year or not. At this point, they are at risk of being inundated as the water is being discharged downstream. Some locations of Bangkok which have no embankments to protect them will see minor flooding, according to the authorities.

To clear all the six billion cu m of the water, the authorities concerned are now discharging the water with the maximum capacity of the river channels as well as irrigation services and facilities.

The main river of Chao Phraya and over ten vast fields and irrigation networks will be the key to success. Up to 80% of this excessive volume of water will be discharged through the river channel, while around 900 million cu m will be diverted into the vast fields, according to Dr. Somkiat.

The water discharge situation in the Central Plains, as of September 30, 2021. Credit: ONWR

Dr. Somkiat said Dianmu caused rains to fall straightforwardly to the Lower North and the Central Plains, resulting in major tributaries of the Chao Phraya bearing the heavy burden of water discharging.

As the water is running down from Sukhothai to Nakhon Sawan province, where all major tributaries merge, the water discharged through the main water monitoring station there, known as C.2, has become stable this week, being around 2,600 cu m per second or so.

However, water from rainfalls in the lower area, as well as minor tributaries including Sakae Krang in the West, has added into the discharged volume of water, prompting it to become excessive beyond the maximum discharge capacity of the river channel at the Chao Phraya River dam in Chai Nat province, which is around 2,800 cu m per second or so.

The authorities decided to divert the water into the vast fields along the river so that the dam (C.13) can help keep the discharge volume below its maximum discharge capacity.

The vast fields to help take the excessive water. Credit: ONWR

However, further down the dam, the Pasak Jolasid dam, which was overly filled up by the same Dianmu, is discharging its excessive water to ensure dam safety too; around 1,200 cu m per second, thus complicating the situation.

The authorities are trying to regulate the water traffic to lessen the impacts on residents in the East while slowing down the water to be flowing into the Chao Phraya River in Ayutthaya province, where the river channel there can discharge less water volume than the Chao Phraya dam by over half, or around 1,100 cu m per second.

Dr. Somkiat said people in some districts of Saraburi, Lop Buri, and further downstream in the East are at risk of over 1-meter flooding, but the authorities will try to clear all the water on this side within the next 20 days.

They will also try to divert the water in the Chao Phraya River further down to some other vast fields and the Bang Pakong River to expedite the work. The water in the mainstream river could be cleared within the next ten days and Bangkok could be spared from the next rains if any, Dr. Somkiat said.

The 2011 great flooding

Dr. Somkiat has downplayed concerns that residents in the Central Plains especially in the capital of Bangkok could face severe flooding like that in 2011, which had submerged over 30,000 sq km in the Central Plains for months. (97,000 sq km in 65 provinces were affected in total. The 2011 flooding was ranked the fifth in terms of flood magnitude, but was the first if ranked by flooding duration (158 days), according to the Royal Meteorological Society’s research paper, the 2011 Thailand flood: climate causes and return periods published in 2013.)

He cited critical factors that have a role in this, saying they are different. These include the water stored in major dams in the North and the excessive volumes of water generated by tropical storms.

Dr. Somkiat said in 2011 all major dams in the country were almost full of water following a series of storms that hit the country (It was around 95% of the storage capacity of the dams of Bhumibol, Sirikit, Kwae Noi, and Pasak Jolasid, combined or over 23 billion cu m of water. The water stored at these dams now is around 48% of their storage capacity or just nearly 12 billion cu m, according to the ONWR.)

Credit: ONWR

The excessive water of the 2011 incident was over 15 billion cu m, while the volume this year is six billion cu m, being far less than that of the year 2011, according to Dr. Somkiat.

His explanation is in line with other water experts from the Foundation of National Disaster Warning Council, who have cited similar factors.

However, the foundation notes that although the current situation would not be like that in 2011, flooding could locally occur following infrastructure and properties in the areas which become disruptive to water flows. More critically, the local climate is apparently shifting, prompting local flooding and risks to occur to the people, the foundation notes.

“People should brace for possible flooding and not be complacent as we have become sensitive to it due to our untidy city planning and the changing climate.

“There is no need for severe storms or rains to come. We could face local flooding like we are experiencing at the moment,” said Noppadol Makthong, the foundation’s spokesperson.

I Deputy PM Gen Prawit Wongsuwan inspects the situation at the ONWR’s war-room. Dr. Somkiat explains the situation.
Credit: ONWR