19 September 2009
Special session on Typhoon Morakot in Taiwan
Posted by Dave Petley
This afternoon at the Chi-Chi earthquake conference in Taiwan the organiser laid on a special session on the impact of Typhoon Morakot in August. This is of great interest to me, given the impact of the landslides, so I thought I’d give summary of the key points. Apologies for the note form – I have done this Twitter-style!
1. The magnitude of the typhoon
For Taiwan this was an extraordinary event. It appears that in terms of river discharge the floods were the largest since records began – over 200 years ago. For example, Prof. Tsai of the National Cheng Kung University showed that for the Gaoping River the peak discharge was 29,100 cubic metres per second. That is the equivalent of the daily water needs of 150,000 people – each second in a single river! These huge floods were driven by extraordinary rainfall. At Chiayi the statistics are as follows:
Total storm rainfall: 3005 mm
Maximum hourly rainfall: 136 mm
Max 3 hour: 325 mm
Max 6 hour: 548 mm
Max daily: 1623 mm
This is close to but not actually quite, the world record rainfall.
2. Why was it so intense?
Prof. Jou of the Taiwan Meteorological Agency suggested that there were two key reasons why the typhoon was so intense. First, the storm slowed down as it crossed the island. Before it made landform it was moving at 20 km per hour. When it came ashore it moved only about 50 km in 24 hours. This led to very high rainfall accumulations. Second it appears that as it formed the typhoon interacted with a substantive monsoon trough, which drew in moisture from the inter-tropical convergence zone to the southwest. This meant that the typhoon generated huge rainfalls on the south edge of the storm which is where the maximum damage occurred. Interestingly, he admitted that one of their dynamic models run the day before the typhoon struck forecast 1900 mm of rainfall. However they didn’t trust the model.
3. Landslide impacts
Very little was presented on the impact of the landslide at Siaolin, but Meei-Lin Ling stated that the death toll was 491 people. She stated that at the moment they have records of 1349 landslides, 46 debris flows and 298 road slope failures. I suspect that that many of the landslides may be classified as debris flows? These landslides cover an area of over 50,000 hectares. Prof. Jenn-Chuan Chern the Deputy CEO of Morakot Post-Disaster Reconstruction Council, suggested that the volume of sediment is 56 million tonnes (I wonder if this is rather low though?). Tainan County had the largest number of failures (515) the Kaohsiung (288) and Chiayi (216). The landslide distribution closely reflects the rainfall distribution. In Sinkai village 32 people were killed by a debris flow. There was also very extensive damage to the highway network – there is some doubt as to whether the road to Alishan (a major tourist area) can be repaired.
4. Other damage and impacts
Coastal flooding was a major impact. The peak of the storm coincided with a high Spring tide, causing major inundation. These flooded areas and those affected by river floods, have had major problems from silt accumulation. In some cases over a metre of slit has had to be removed from houses. This is quite straightforward, but clearing the water and sewage pipe network has been a major headache. Over 500,000 tonnes of wood has been deposited and is having to be removed. This is a substantive task.
5. Human costs
Human impacts are 701 fatalities with another 58 missing. 120,000 houses were flooded, 310,000 houses damaged. 4489 people are being housed in army barracks even now; reconstruction is going to be complex. At present 55 aboriginal tribal villages have been displaced. Government evaluations suggest that 31 of these are permanently unsafe. Therefore resettlement is not simple – the aboriginal communities want to reconstruct their villages in order to maintain their culture (e.g. through targeted schooling), but given the dangers of the sites this is not simple. Government land is being earmarked and NGOs are helping to liaise to find an appropriate solution.
6. The elephant in the room – climate change
None of the speakers wanted to ascribe this event to climate change – there was a real sense of caution about saying anything unwise. This is sensible. The point was made that in recent years the number of landfalling typhoons has been high compared with historic record and also that a dramatic increase in precipitation intensity has been noted. The floods met and even the calculated probable maximum flood of the rivers. Thus, it appears that the rainfall that is now happening is different from the historic record. Is this climate change? They key factor seems to be the interaction of the typhoon and the monsoon. A discussant implied that this could be because the monsoon front is moving. Whether it is climate change or not it does seem sensible to plan for more intense rainfall events. This is going to be very challenging in a place like Taiwan.
The final interesting comment was a note that classifying typhoon intensity by wind, which is the convention, is meaningless when most of the damage is done by rainfall. They suggested that a new classification is needed that combines both wind and rainfall, allowing better forecasting of landslide and flood impacts. Frustratingly I submitted a grant application to DFID ten years ago to develop a scheme to do exactly this, tied to a terrain classification scheme. They didn’t fund it.
All-in-all a fascinating set of presentations that really helped in the understanding of this extraordinary event.
Your questions and thoughts are welcome!
Nice blog! Good to meet you today.
I'd like to make some corrections coz I prepared part of Prof. Lin's slide. 1. The presentation is a quick report to let people realize overall concept about the landslides. That's why we were not focusing on Siaolin village although we had done some preliminary research and data collection.2. The debris flow and landslide classifications came from SWCB and DGH. They do have their own point of views and I just put them in the same figure coz those are official data. The numbers of debris flow and landslide will be re-counted by ourselves ASAP. However, the overall field investigations are very closing to finish stage. More results will be shown from NCDR in less than a month. See you Tuesday.
Thank you for this posting, it was very informative.