The two provinces have received immediate flood warnings again despite they are still suffering from heavy rainfall left by Supertyphoon Yagi early last month and Tropical Storm Soulik in the middle of the month or almost every ten days
The Office of National Water Resources (ONWR) have immediately issued two warnings against flooding today for the two provinces especially their low-lying areas, which have suffered from the previous floodings brought by Super Typhoon Yagi and Tropical Strom Soulik. These include Mae Sai’s downtown, Chiang Rai’s downtown, and the city of Chiang Mai down to Saraphi district, where the Ping River passes through.
The water levels in their major rivers are expected to rise up to 1-1.5 metres due to heavy rainfall in watersheds, and the situation could continue up to a week, according to the ONWR.
The ONWR explained that the heavy rainfall this round is a result of the clash between the cold air mass moving down from the land mass of China and the south-westerly winds from the Indian Ocean. Where the Monsoon Trough is passing, there will be disruptive weather of thunderstorms and heavy rainfall. As the land in the areas is already saturated from the previous rounds of rainfall, it’s easy to have run-offs and overflows in the rivers.
The ONWR has also projected that there could be another storm coming from the South China Sea and the organisation is keeping a close watch on it.

The previous storm was Soulik, which had occurred around the middle of September. Fortunately, it did not spin itself into a typhoon and reached Thailand as a tropical depression and quickly dissipated. Earlier, tropical storm Yagi had emerged in the South China Sea and spun quickly when it was close to the shore. It upgraded into a Super Typhoon before it made landfall in Vietnam, prompting a great extent of damage to the country.
Here in Thailand, although the typhoon did not directly hit the country, its influence as a low-pressure area that later dissipated in Myanmar has left great damage to the two provinces as it caused flash floods and muddy waters that flooded the downtowns heavily.
In Chiang Rai especially Mae Sai, the muds have covered properties up to two metres or over, and the clean-up process has not completely finished yet. The Department of Disaster Prevention and Mitigation (DDPM) reported during the first few days after the Yagi incident that over 54,000 households in Chiang Rai were affected, and the damage for Chiang Rai alone is estimated to be up to Bt 30 billion, according to Chiang Rai Chamber of Commerce.


Deforestation
Geo-Informatics and Space Technology Development Agency (GISTDA) has revealed one of the prime causes of this year’s heavy damage from flooding in the areas.
The agency has found that rainfall in the Upper North was above the average this year, and rainwaters have accumulated in watersheds since July, especially those of the Ruak or Mae Sai River, which runs past the border towns of Tachileik in Myanmar and Mae Sai in Chiang Rai, and those of the Kok River in Chiang Mai.
According to the agency, Mae Sai alone had been flooded six times already before being hit by flash floods in early September following the dissipation of Super Typhoon Yagi inland.
The agency pointed out that the downtown of Mae Sai is situated on the landscape called an alluvial fan, which is a triangle-shaped sediment deposit area created when water flows through mountains or steep landscapes and brings sediments down to deposit over time in low-lying areas at the mouth of the mountains. Normally, this landscape is considered one of the most fertile areas on earth as it’s full of nutrients brought and accumulated there and suitable for agricultural purposes.
The area, however, is changed and serves the urbanisation without proper city planning. Located in such an area, the downtown is therefore prone to flash floods, which can be aggravating following improper city planning.
More critically, the watersheds upstream are misused by large-scale monocropping and mining, resulting in a reduction of their capacity to hold water. As the agencies’ satellites show, which were recorded on Sep 13, there were patches of mudslides in contrast to tree covers on the river’s watersheds along the border. Some patches are 20 to 30 metres wide plus tens to hundreds of metres long, the agency noted.

“These satellite photos tell us about one of the prime causes of the mudslides running down and submerging the downtown below,” noted the agency.
Fah Foon FB Page, which is an educational platform specialising in monitoring climate disruptions and land use changes in the country, cited the satellite data from FAO Earthmap that shows tree cover loss and forest burnings over the past 25 years in the Ruak’s watersheds.
The data shows that over 10% of the tree cover loss in the areas occurred over the period while burnt scars occurred every two or three years. These activities suggest the massive use of the areas for mono-cropping although at this point it cannot be proved what crop people grow there. The page has suggested further studies to be conducted for more precise information to facilitate better planning and disaster mitigation in the future.

Where’s next?
As the cold air mass pushes the south-westerly winds downward, rainfall will likely go down south until the season changes into the winter.
The noted climate expert Assoc. Prof. Dr. Seree Supratid and the IPCC’s author, has also made caution and warned people in the Upper North to not to be complacent about the situation as there will still be heavy rainfall despite the absence of tropical storms here.
The latest storm Krathon has shifted its way towards Taiwan and upgraded into a Super Typhoon before making landfall this afternoon. Reuters reported that the storm was weaker but still powerful and at the same time “weird” in the sense that it directly hit the west coast and also hovered off the coast before reaching land, rather than hitting the east coast as usual.
Assoc. Prof. Dr. Seree has warned that despite no storms here, the rainwaters may have accumulated in Myanmar and prompted the watersheds to be saturated. Based on his calculation, the accumulated rainfall in the Upper North this year is 14% over the average.
There is around one month left to keep an eye on the situation as there may be an isolated storm hitting the region. At least seven more storms are projected to occur before the season changes, and their impacts here could not be assessed yet.
However, he has downplayed the impact downstream, in the Central Plains, citing that the floodwaters passing down are still far less than that measured in the major flooding incident in 2011 and the 2022 flooding by over 230% and 80% respectively.
Flooding in the Central Plains could occur if the flood level is similar to that of the 2022 situation, complicated water discharges due to excessive rainfall in particular areas, and tidal surges around the end of the month.
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