The big flood
of 13/14 August 2005

- what the statistics show.

Peter K. Wingfield-Digby

  

              The recent wide-scale flooding in and around Chiang Mai caught many of us by surprise. My wife and I, who have recently moved to a new house about 25 km. north of Chiang Mai, experienced the unwelcome novelty of having to leave our house by boat in order to get to higher ground. The inside of our house survived-just, thanks to the help of our neighbours and the liberal use of sandbags-but many other people were probably not so fortunate.

                There have been many photos in the local and national press, illustrating the gravity of the flooding, but what do the statistics show? To find out, I turned to the data collected by the Hydrology and Water Management Centre for the Northern Region, which keeps track of changes in the water level of the River Ping at various points along its course, as well as measuring the volume of water flowing through the system. Using their data, some of which is available on their website (www.hydro-1.net), we can get a good idea of what happened over the weekend of 13/14 August. Rather than give a complex table of numbers, it is most convenient to represent the changes in the form of graphs.


Source of data: Hydrology and Water Management Centre for Northern Region

                Figure 1 : shows the changes in water level, and in the volume of water discharged, at the Nawarat Bridge in the centre of Chiang Mai, for the six-day period from 13 to 18 August. (On the website the symbol P.1 is used to designate the water station at Nawarat Bridge.)

               As indicated by the discharge curve and the scale shown on the right-hand side, Saturday began as a quiet day in the centre of the city. Water was being discharged along the river at less than 50 cubic metres per second (m3/sec). The rate of discharge increased only slightly during the morning but then accelerated in the afternoon and evening, reaching about 400 m3/sec by midnight. The rate of discharge accelerated even faster in the early hours of Sunday, reaching almost 700 m3/sec by 6 a.m. on Sunday. After that, the rate of discharge started to level off, reaching its peak of almost 750 m3/sec by 6 p.m. The rate of discharge finally started to fall early on Monday. The rate of discharge was back down to just over 500 cubic metres per second by the end of Monday, to 300 cubic metres per second by the end of Tuesday, and to 200 m3/sec by the end of Wednesday.

            As one would expect, the graph showing the water level over time mirrors that for water discharge. At Nawarat Bridge the water level was at about 1.5 metres at the beginning of Saturday. The water level rose dramatically during Saturday afternoon and evening, reaching about 3.5 metres by the end of the day. During the early hours of Sunday the water level was rising very fast, and had reached 4.5 metres by 6 a.m. After that there was a levelling out, with the maximum level of 4.9 metres being reached by 6 p.m. This level was maintained until the early hours of Monday morning, and then the water level started to fall, falling to 4 metres by the end of Monday and to 3 metres by the end of Tuesday.


Source of data: Hydrology and Water Management Centre for Northern Region


              Living well upstream from Chiang Mai, I wanted to find out whether the events upstream had unfolded in the same way as they had at Nawarat Bridge. They had, but in an even more dramatic fashion. P.67 (Ban Mae Teng, A. San Sai) is the first point north of the city for which records are kept. Figure 2 compares the water levels at P.67 and at P.1 over the six-day period from 13 to 18 August. The graphs highlight the much faster rate of increase in the water level at P.67 compared to P.1. On Saturday the water level at P.67 had started out at little more than a metre, but by the end of the day it had reached a level of almost 6 metres, and the level climbed even higher on the Sunday. This means that in little more than 24 hours there had been a rise in water level of 5 metres. At Nawarat Bridge the equivalent rise was about 3.5 metres.


               For those who prefer tables to graphs, Table 1 summarizes the changes in water level at the two stations, P.1 and P.67, over the two days. For those of us who live upstream from Chiang Mai, the most telling point is that the water level



Table 1: Changes in water level at Stations P. 67 and P. 1,
13 and 14 August 2005

at P.67 rose by more than two metres (or more than six feet) in just six hours, from 6 p.m. to midnight on the Saturday. The largest hourly increase occurred between 7 and 8 p.m. when the water rose a massive 60 cm (2 feet) in just one hour.

             The use of the expression "flash flood" seems fully justified. We live in hope that the authorities can do something to mitigate the effect of these "flash floods", or at least give residents reasonable advance warning of what they can expect.


      
Peter K. Wingfield-Digby
www.pwdigby.com



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