[Member's News] Earthquakes, Tsunamis and Floods – Un-learning the Past

By David Meigh


 

It is with some sadness that I have watched the news footage from Palu, a city I lived in for a year when working on the Gumbasa irrigation project in the valley. I had seen experienced this before from three years living in Banda Aceh, then returning after the tsunami to rebuild irrigation schemes. During some 30 years working around Indonesia and South East Asia it appears Indonesians have unlearnt the past. Traditional villages are based on highest ground, have timber framed buildings with flexible joints that sway in earthquakes but don’t collapse. Traditional houses on Nias have a unique arrangement of inclined floor beams that flex to absorb earthquake forces. Indigenous houses close to rivers are built on stilts well above flood levels. At the coast, villages were set back behind belts of mangroves and children were taught stories about tsunamis so knew to run to higher ground instead of the beach to pick up fish.

      

 

1. Traditional Nias house raised to avoid flooding  2. Cross-braced at base to absorb earthquake forces

Now status demands houses are made of rigid masonry, based at ground level wherever there is space and often close to a rivers where it often floods. Many organisations think that by building a concrete framed buildings all within will be safe. However, usually the steel, especially at floor slab to column joints, is not only insufficient, it is just hooked around the column steel and pulls out easily during earthquakes. Correct practice is to deeply anchor slab and joist steel into the columns. Modern multi story buildings, such as those in Palu and at Gunung Sitoli on Nias in 2005, were not built to the earthquake design codes and had their floor slabs collapse down on each other, killing hundreds.

    

 

2. Collapsed newbuild house in Bengkulu earthquake.  4. Multistory collapse in Nias, April 2005 

(Source Google and Corbis via Getty images)

Flooding is another disaster area with a lack of planning specially as many cities are built on flood plains (given that name for a reason). Urban areas have 3 to 5 times the run off than the rice fields they are often built on. Do the spatial planners take this into consideration. No, they confine the rivers between walls to maximise land use. The population further exacerbates the situation by encroaching into drains and river banks with illegal houses and treat watercourses as a convenient garbage disposal system. Richer areas have drainage designed to evacuate water quickly in concrete drains so that flood peaks concentrate in the low-lying areas where the poor live. The catchment areas have been stripped of forest and poor agricultural practices, especially for crops such as cassava, ensure erosion and sedimentation in rivers reduces the flow capacities. 

 

5. Catchment absorption (opposite of runoff) by landuse. 6. Common method of garbage disposal

Then there are tsunami which happen infrequently but historically on a regular basis. For cities at risk there still is no effective warning both from national or local systems. In fact, a major earthquake should be enough warning, but many people, especially the most vulnerable, were still caught by the waves, or people were trapped under buildings poorly designed against earthquakes.

Since 2003 the BNPB have been recording the number of different types of disaster. Their graphs show an alarming and progressive increase that can only become worse with climate change. 

In addition, city areas are infilling with buildings and impermeable surfaces and expanding into marginal and disaster risk areas. Most cities have grown organically, well ahead of any spatial plan, and are wall to wall buildings with little green space or community recreation facilities that could absorb rainfall. The city directive to have 30% green space is a planner’s pipe dream. 

 

7. BNPB Chart for disasters occurring in Indonesia over the past 10 years                                                      

I can see a time in the future when all of the government’s income is spent reacting to disasters. 

What is required is a pro-active approach. Several recent studies point to Padang and West Sumatra coastal cities being at high risk to a 10 to 15 m high tsunami as the last one was in 1833 and has a recurrence interval estimated at 200 years. Whilst in 1833 the number of people at risk was in the thousands, now they are in the hundreds of thousands. The 2009 earthquake was a warning, but it did not release the pressures building up in the Mentawai reach of the Sunda Megathrust. A large earthquake (M 8.8) and tsunami are likely sometime in the coming decades opposite this coast. The tsunami would reach the shores of the Mentawai Islands within 5-10 minutes and would reach the mainland, including Padang, within 20-30 minutes of the earthquake. In most areas, there is not enough time to wait for official warnings or to reach high enough land. An earthquake lasting more than 30 seconds should be warning enough.

 

 

8. Conjectured devastating future tsunami (EOS, footnote 2). 9. BNPB Tsunami Risk Map of Padang

To be proactive the government should:

  1. Update the tsunami warning system of sea buoys but also have a local system of siren warnings from mosques and high buildings.

 

  1. Build a line of tsunami shelters with several stories at regular intervals of perhaps 500 m set back some 500 m from the coast in at risk cities. Such structures would have an open ground floor to allow the wave to wash through and be built with the latest earthquake resistant technology. The structures could double as shelters for flood displaced people, car parks, schools, community centres or train stations if a coastal train line was planned.

 

  1. Upgrade the BNPB flood emergency arrangements with each kampong in the risk area forming tsunami and flood associations that would have a database of all vulnerable people (old, disabled and children) in the community. There would be allocated carers with overlapping responsibilities to ensure all are included for assistance to the shelters within 20 minutes.

  2. Once the shelters were built, emergency evacuation exercise should be carried out to ensure all can reach a shelter within the 20 minute time limit.


WRITTEN BY:

David Meigh MSc, CEng, MICE

 

David Meigh MSc, CEng, MICE is an independent international engineering consultant with extensive experience in the planning, design and management of water resources systems in South East Asia. He is currently working on the Enhancement of Water Security Investment Project with the Asian Development Bank, (ADB). David worked  on the Gumbasa Irrigation Project intermittently between 1978 and 1980 and full time for a year in 1983 when he lived in Palu. He is the acting Chairman of the UK Institution of Civil Engineers, (ICE), Jakarta Local Association.