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Chiang Mai Floods
(for other information about waterways and management, click here)
Chiang Mai urban and peri-urban area is subject to floods every year during the rainy season. While only major floods affecting the city centre make it into national and international news, numerous flooding events affect the city in different ways.
Types of flooding
Three types of floods occur in Chiang Mai province, all caused originally by rain but expressing floods differently in terms of volume of water involved, water flow, associated kinetic energy, source of water, etc.
Surface pluvial floods are quite common and widespread in the basin and valleys of the province. It results from heavy rainfall in a localised area with insufficient drainage. As such, these floods have no warning other than weather forecasts, direct evidence of heavy rainfall and a knowledge of the current drainage capacity in the area. Most floods are however limited to low intensity street flooding, rarely reaching one meter except in cuvette shaped zones where higher depths are possible. It produced an accumulation of rainwater, rarely above one meter. These floods are rarely reported in the news as they are mostly low intensity street flooding or affect poor suburbs of Chiang Mai or occur in farmland. They are however very frequent and some areas of Chiang Mai can experience them several times per year. These floods are caused by insufficient drainage, either due to the lack of infrastructure or a lack of maintenance of the drainage pathways (canals, underground). As a result, these surface pluvial floods are very common in Thai cities, however, Chiang Mai has generally the advantage that these floods are not extensively mixed with sewage waters.
Flash floods are a type of pluvial flood that result from heavy rainfall on a topographic high leading to extreme accumulation of runoff drainage in a localised area over a short time. It typically occurs along the foothills of mountain ranges around the Chiang Mai – Lamphun basin and in subordinate creeks from tributaries. Like surface floods, flash floods have no warning other than heavy rainfall with possibly a few tens of minutes of warning for rainstorm occurring upstream of catchment but within sight. Intense rain events can happen up to 10 or 15 times per year, causing water levels to suddenly rise well above 1 meter with very high velocities, sometimes above 5 m/s (>20 km/h). These floods are short lived, lasting only a few hours but can be quite severe and destructive. Flash flooding is very common and can cause damage to roads, farmland, infrastructure and other constructions but only get reported when the damage is extensive or affect an economically important area. In recent years, it is the only type of flooding that is a direct cause of fatalities.
Figure 1. Examples of floods in the Mae Wang sub-catchment. a. Loss of protective concrete embankment in Octoberr 2024. b. Loss of embankment and significant erosion in October 2024. c. Flash flood sedimentation in September 2024. Rocks are football size, sometimes larger. The water level reached is seen in the flattened grass. d. Debris flow in a small valley arriving directly to the Mae Wang river. Rocks transported by the water are larger than 1 m3.
Fluvial floods in the Ping catchment are restricted to river floods since Doi Tao Lake water level is limited by Bhumibol dam. It occurs in the direct vicinity of waterways and excludes overflowing canals designed as stormwater channels which are more associated with pluvial flooding. Ping floods are the most commonly reported but Mae Taeng, Mae Kuang, Mae Chaem rivers are also commonly affected by floods. Compared to pluvial floods that have limited warning period, river floods can be forecasted around 48 hours in advance for Chiang Mai as intense regional rainfall events are confirmed in the upstream catchment. The peak flow and intensity of the flood in Chiang Mai city can be forecasted with an accuracy below ±10 cm in the 6-7 hours before it hits.
In Chiang Mai, Ping river flooding mostly affect poor low-lying and rural areas that can experience 2 to 5 floods per year with water height below 1 meter and lasting 3 to 4 days. These floods are part of the traditional way of life and not particularly reported in the media. Intermediate and major floods however, affects larger areas and occasionally overwhelm the protected section of the river, leading to flooding of economical areas and cutting major roads. Such floods happen every few years and receive important media coverage.
Figure 2. Arrival time of floods based on river level measuring stations in the Upper Ping
Description of recent major floods
Ping river flooding occurs since there are historical records in the area. A separate section on Mon and Tai experiences with floods can be found here. This section only covers recent floods of the last century.
In 1933, 1937, 1942 and 1945, there are records of significant flooding in Chiang Mai with no specific description. However, the 1942 floods is described in Bangkok has a very significant and lasting 3 months.
1952 is considered as the largest flood of the past century. Two typhoons, Louise and Nona passed over Northern Thailand in August, saturating soils with water. In September, the monsoon through was stationary over Northern Thailand, producing extensive rainfall and causing successive floods peaking with a flow volume estimated between 490 and 830 m3/s. Description of the floods mention that large parts of Chiang Mai were inundated, which, considering the size of the city at the time, would probably include most lands between the river and the walled inner city.
1973 is another year with a significant flood. Cylone Anita in mid-July produced abundant rain but no flooding as it was absorbed by the catchment but a flood eventually occurred at the end of the month. Cyclone Jones at the end of August however, produced the peak flow at 720 m3/s.
1987 has seen a flood reaching +4.53m in Chiang Mai
1989 has seen a intermediate flood reaching +3.82m in Chiang Mai.
1994 saw a first flood occurring in mid-August after heavy monsoonal rainfall and reach _+4.12m. In mid-September, typhoon Harry crossed over Northern Thailand and produced another flood at +4.43m. Between 1994 and 1996, the city centre of Chiang Mai has been flooded 5 times, notably +4.27m in 1995.
2001 is characterised by extensive flooding in Mae Chaem caused by typhoon Usagi. Levels reached +7.98m above the river bed, equivalent to 794 m3/s causing significant local damage but no flood was recorded in Chiang Mai city.
2005 is likely the largest flood of the past 100 years and occurred as a series of five floods from mid-August to October. It started in the second week of August with a low pressure bringin heavy monsoon conditions over Northern Thailand with 100 to 200 mm of rains precipitated regionally. It caused numerous local floods, flash floods and mudslides in the region. The Ping rose at 12-14 cm per hour and reached +4.90m (747 m3/s) for 8 hours totalling 51 hours of flooding.
More rainstorms in the following weeks created additional flash flooding, notably in the foothills of Doi Suthep, followed by tropical storm Vicene producing a second major floods at +3.80 m (485 m3/s). A week later, Typhoon Damrey followed, dropping in excess of 300 mm of rain over three days in Chiang Dao (while there was little rain in Chiang Mai). A day later, it produced a +4.93 m flood (750 to 912 m3/s) equivalent to 1.68 m of water in the floodplain that was maintained for 82 hours. While the floods of 2005 lasted 3 to 7 days individually in most areas, some rural and poor districts of Chiang Mai have been continuously flooded for a month.
The 2005 floods have caused around 1 billion baht of damage, affected 250000 people and the initial August flood lead to 5 deaths due to the lack of preparation.
2006 had a medium flood in Chiang Mai at 526 m3/s at the end of July, caused by a strong monsoon and another three minor floods in September and October.. The year is however noticeable for floods in Uttaradit and Sukhothai in May, caused by the early heavy rainfall of typhoon Chanchu which show that floods are also possible outside the August-October period.
Figure 3. Flood profile (in volume per second) for several major floods. The blue line is the hourly record of river height translated into water volume. the red line is the base flow, fed by groundwater rather than rainfall runoff.
2011 is well known as the great flood that lasted almost half a year in some parts of Thailand and made immense damage in the Central Plains and Bangkok. It started at the end of July with the tropical depression Nock Ten dropping 100 to 200 mm/day and was the largest rainfall event of the year. However, since the soil was not water-saturated at that time, only 430 m3/s of flow were recorded in Chiang Mai, which was below the flood threshold of 3.7 m.
By October however, Northern Thailand had experienced 5 tropical storms and a heavy monsoon giving a total wet season rainfall 20 to 60% above normal. In early October, a 50 to 70 mm/day rainfall, combined with necessary dam releases produced a flood reaching +4.95 m in Chiang Mai (700 m3/s). This flood made the Bhumibol dam to reach full capacity, leading to the extensive floods of the Central Plains.
The flood waters covered 335 km2 of land in Chiang Mai and caused 7 to 8 billion baht of damage that are included in the estimated 45 billion baht of flood damage that Thailand experienced in 2011.
2022 saw an important flood on the 5th of October, reaching +4.75 m (630 m3/s) in Chiang Mai. It was caused by heavy rainfall due to a stationary monsoon through over Northern Thailand producing a high runoff and a necessary release from the Mae Ngat reservoir.
2024 started with the landfall of Typhoon Yagi, one out the four ever recorded category 5 cyclone in the South China Sea, that passed over Thailand on the 8th of September, causing marginal floods in the Ping river. On the 22nd of September, tropical depression nº8, also from the South China Sea, caused abundant rainfall over Northern Thailand for several days, producing landslides, slope failures and the closure of the Chiang Mai – Lamphun railway for a few days. This rain event caused the first major flood on the 26th of September, stabilizing at +4.45 m then followed a few hours later by a second runoff pulse bringing the flood to +4.93m (600 m3/s).
On the 3rd of October, the monsoon through moved and stalled over Northern Thailand bringing abundant moisture and extensive rainfall on an already oversaturated catchment. A very significant proportion of this rain was brought to rivers and the Ping reached its peak flow on the 5th of October at +5.30 m (656 m3/s) affecting very large areas of the city but also rural area downstream and narrower floodplains in tributaries.
The flood lasted 102 hours in Chiang Mai but some farmland were constantly flooded from mid-September to the end of October. The flooded surface in the basin represented 245 km2 and an unknown surface outside the basin and a preliminary estimation of the cost of these floods is around 10 billion baht.
Figure 3. River Ping during the second major flood of 2024. Top figure is river level at P.1. (Nawarat Bridge) where the horizontal line at +3.7 m is the flood threshold. Bottom figure is the rate of change in meters per hour showing 3 pulses of rainfall runoff leading to peak flow during the night between the 5th and 6th of October.
Causes of flooding
Heavy rain is the obvious cause of flooding but there are a few conditions required for this to happen. For minor floods, the rainy season itself will cause floods every couple of years but these do not affect areas with flood protection such as Chiang Mai city. For medium and major floods, the downpour can be caused by tropical storms and depressions (i.e. 1973, 1994, 2005, 2024) or a heavy monsoon (i.e. 1952, 1989, 2005, 2011, 2022, 2024). At a seasonal scale, the El Nino Southern Oscillation (ENSO), guided by the surface temperatures in the Pacific Ocean, as a significant influence. La Nina, which cause higher rainfall average (and as a result, milder burning season), is more likely to produce significant floodings as it was the case in 1973, 2011 and 2022 but not always (1998-2000 and 2008 were La Nina without significant floods). El Nino, on the other hand, provide less wet conditions but it doesn’t stop intense storms to reach Northern Thailand and local flooding remains a possibility such as Cyclone Usagi in 2001 that cause important flash flooding damage. Significant major floods also occurs in neutral years such as 1952, 2005 and 2024.
It also requires the soil in the catchment to be water-saturated beforehand. Storms that caused extreme rainfall early in the season (May, June, July) can produce flash and surface floods and local damage but rarely leads to river floods as rainfall is mostly absorbed. The effect of ENSO on how early the catchment is saturated is significant. Independently from meteorological factors, before the 1980s, the human influence on the catchment was relatively minor and all historical floods prior to that time are strictly natural in their intensity, particularly the water level reached.
Figure 5. Long term variations over the past 100 years of peak flow during floods and minimum flow in the dry season and a compaison with the El Nino - La Nina status.
Anthropogenic factors applies at different level, with different impacts with positive and negative outcomes.
-Deforestation is a common suggestion as the cause of increased flooding in the last two decades, particularly from activists environmentalist that wish to have additional protection on forested areas. However, factually, the effect of deforestation on river floods is weak. While water levels in Chiang Mai city have reached records height in recent years (2005, 2022, 2024), the peak flow (in liters per second) is not abnormally higher in recent years, indicating that the runoff from the catchment hasn’t increase compare to historical averages. While deforestation has been significant during the end Rattanakosin period to the 1980s, reducing forest cover in Ping catchment from 83% in 1973 to 67% in 1989. It hasn’t significantly changed since 1989 when the logging ban was introduced and forest cover has in fact increased by 3.5% between 1990 and 2009 in the Upper Ping. There are also numerous studies that show that the role of deforestation in river floods is unlikely to be an isolated factor. The impact on flash floods can however be significant. Interestingly, from a strictly water management point of view, there is some evidence that deforestation with land converted into rice paddies can have a positive effect as it would increase water infiltration and provide higher water availability during droughts.
Figure 5. Evolution of the forest cover in the Upper Ping catchment since 1960s show that deforestation has basically halted since the late 1980s (Ekkawatpanit et al., 2013).
- River encroachment occurs when land is acquired from river and flood corridors and other water bodies through landfill, rock dumps but also planting aquatic species inducing siltation. The issue was known for quite a while but become a major problem in the 2005, 2011, 2022 and 2024 floods. It has been described as restricting the width of the Ping as much as 1/5th of its original size. Encroached land is occupied by housing, restaurants and government offices and creates a significant bottleneck effect in the inner city.
- Urbanization and industrialization of the Chiang Mai – Lamphun Basin has developed Chiang Mai into a primate city in Northern Thailand. It resulted in a lot of infrastructure and development along the Ping river and in the floodplain. The phenomenon is not new as the naming of areas in Chiang Mai, even in older parts of the city, show that some lands were reclaimed in historical times from the floodplain and toponyms such as หนอง (‘nong’) in their suffixes (i.e. Nong Hoi, Nong Pratheep, Nong Phueng, Nong Prakang, …) are interpreted as ‘swamp’. Such areas would have been prone to be flooded by any rise in the base flow of the river in a natural system.
In the last few decades, Chiang Mai has considerably grown with a population 20x larger and an abundance of elevated roads and extensive landfills for large housing estates have been constructed in the last 4 decades. These urban structures significantly disrupt the flow of floodwaters and reduce the capacity of the floodplain to absorb floods.
- Regulated agricultural practices are now widespread requiring significant amount of waters during the dry season, provided by large scale water management supplied by large reservoirs, deeper weirs and dams and various canals. This infrastructure has profoundly changed the seasonal flow of the Ping river and has an impact on floods by limiting the flow and promoting siltation.
- Climate change is occasionally blamed for the observed increase in flooding but no evidence show such visible effect.
Figure 5. Approximate map of urbanization of Chiang Mai show a slow growth from its founding in the late 13th century through the Ratannakosin and Siamese eras. The arrival of railways in Chiang Mai, then roads, and airport increased substantially the urban area in the middle of the 20th century but explosive urbanization started in the 1980s
Consequences
The most evident effect of the Ping river flooding is damage to human habitation and infrastructure. In recent years (post-2005), warnings and awareness of the population is sufficient to avoid human casualties and authorities preparation limit the extend of a humanitarian crisis. However, some damage are unavoidable and in standard houses, threshold in water levels make sanitation unusuable for +0.4 m of water, +0.7 to +0.9 m will make electricity unusable (although likely before) and for levels as high as +1.4 m, 85% of residents in low income areas are generally forced to evacuate. These low income housings also have extensive damage due to swell, warp and decay of surfaces and walls even for level below +0.4 m. In such households, complete loss is also possible as they represent 80% of cases reported. In houses built with more durable materials, the damage is less extensive but humidity penetrating walls, floors and ceiling can require a full refurbishment as well as an inspection of the electrical system and sometimes sanitation.
In rural areas, households are generally better adapted to floods and can sustain minor floodings without too many issues. However, agricultural fields are heavily affected, particularly vegetable farms that are often a complete loss.
A more extensive description of financial loss and various types of damages is available elsewhere.
Aside from direct water damage, the Ping river carry around 1 to 1.5 g/l of sediment in flood waters and it can go up to 8 g/l for the rising flood front. As major floods last several days, a significant quantity of these sediments are deposited as mud in the streets of Chiang Mai. In the 50-100 m from the river, the mud is sandy but it’s mostly fine mud further away. The thickness vary from 15 cm near the river to 5-10 cm as far as a few hundred meters from the main channel and 1 cm or less in areas further than half a kilometer. The average quantity of mud for the flooded area (counting 2 cm of mud) is 33 kg/m2, which means that every few meters of a flooded major road has more than 1 ton of mud to be removed in post-flood clean-up.
Figure 5. a. Aerial picture of Chiang Mai city centre in the 2024 floods (source unknown). b. Post-2024 flood clean up (Thai Royal Army); c. Aerial picture of Chiang Mai train station in the 2024 floods (source unknown); d. Chiang Mai bus station during the 2005 floods (Chaipimonplin, 2010).
The velocity of waters in the main channel of the Ping river can also be responsible for significant erosion, mostly in rural areas where river banks and some structures are unprotected. It can happen in the outer parts of meanders, mismanaged weirs and bridges or when bank protection fails. For flooding in tributaries of the Ping and smaller creeks, erosion, sedimentation and damage can be extreme. Flash floods are known to wash bridges, rase some villages and create substantial damages to roads. In fortunately more remote areas, it is not uncommon for such flash floods to carry boulders of several tens or even hundred of kilos downhill, producing a continuum with debris flow.
Aquifers are also at higher risk of contamination during floods due to microorganisms, sewage, oils, agricultural & industrial wastes, chemicals etc. reaching the water table. However, in Chiang Mai, this risk is not natural as recharge zones are far from the flood plain, on the edge of the basin. The infiltration under the flood waters is a very slow process of percolation and can only be hastened if the saturated soil depth reaches the rising aquifer. This wetting of the sediment separating the surface from the aquifer takes many days and is not a major concern in Chiang Mai. The highest risk of contamination is man-made, through inadequate capping of wells in flooding areas, giving direct access of flood water to the water table with a very hydraulic conductivity.
One historical consequence of Ping river floods is a major change in the river channel called avulsion. These have only occurred 5 or 6 times since the founding of Chiang Mai and none occurred in living memory. The most well-known avulsion is the Ping Hang that made the Ping flow east instead of South at the level of highway [3029] (second ring road) to then flow parallel to highway [106] (Old Chiang Mai-Lamphun road). There is only very low probability that an avulsion could occur in the current Ping, but considering the rate of urbanization in the basin, an avulsion would be catastrophic as it was the case for the Indus river in 2010 or Kosi river in 2008).
Figure 5. a. Loss of bank protection (concrete slabs) and damage to a bridge in Mae Wang river; b. Loss of bank protection (wood piles), erosion and levee failure in Mae Wang river; c. Post-flash flood (~4 hours) in Nam Hu creek with large pebbles accumulation and record of water level in grassland. d. Debris flow in Hill 876 NE thalweg in Mae Wang.