Bart Schultz
A significant portion of these areas are polders. Consequently, we regularly encounter flooding in such areas. In publicity, the cause is usually attributed to climate change.
While I take the predictions of the effects of climate change on more extreme rainfall, higher peak river discharges, and sea level rise seriously, I believe that human activities in flood-prone areas play a much greater role in the causes and consequences of flooding than the effects of climate change. This primarily concerns the widespread urbanization that has occurred and is occurring worldwide in flood-prone areas, the insufficient flood protection, and land subsidence.
For example, when we look at the location of cities with more than 5 million inhabitants, a figure published in 2006 by the United Nations Population Office for the situation in 1950 (8 cities) and in 2015 (44 cities) is very illustrative. Almost all cities are located along rivers or in depressions, and primarily in coastal and delta areas. Our ancestors were not stupid and generally lived on the higher grounds. Therefore, urban expansions are generally located in the lower, flood-prone areas.
Flood protection is therefore crucial. Some examples are included in the table below. For comparison, our primary river dikes are based on a risk of failure of 0.08 % per year, while sea dikes and dunes are based on a risk of failure of 0.025 and 0.01 % per year for sparsely populated and densely populated areas, respectively. For regional dikes, values ranging from 10 to 0.1 % per year are used, depending on the value of buildings and infrastructure and the number of inhabitants in the protected areas.
|
City/country |
Risk of failure in % per year |
|
Dhaka, Bangladesh Australia, England and USA (including New Orleans after Katrina) India: cities and industry rural area China: large cities cities rural area British Columbia, Canada |
1 1
1 4 0.5 1 5 0.5 |
The worst-protected city with millions of inhabitants I know is Dhaka, Bangladesh. At least 15 million people live there behind a ring dike that offers protection with a risk of failure of 1 % per year. The table also shows that for many cities elsewhere in the world, flood protection is typically not higher than 1 % per year.
Another important aspect is land subsidence. Sea level rise is currently typically 3 millimetres per year. The highest prediction from the Intergovernmental Panel on Climate Change is 1 centimetre per year. For comparison, the table below shows land subsidence in various cities and regions in flood-prone areas. This shows that land subsidence in these areas is, in by far most cases, much faster than sea level rise.
|
City or region |
Subsidence in centimetres per year |
|
Tokyo, Japan Semarang en Surabaya, Indonesia Southwest Taiwan Jakarta, Indonesia Bangkok, Thailand an Holme Fen, England San Francisco Bay area, USA and Bolivar Coast Polders, Venezuela Houston-Galveston, USA and Ho Chi Minh City, Vietnam Manila, Philippines and New Orleans, USA; Shanghai, China and Ganges Brahmaputra Delta, Bangladesh Mekong Delta, Vietnam and Venetia, Italy Mississippi Delta, USA |
1 - 24 6 – 20 3 - 17 0.5 - 17 4 - 12 0.2 - 10
4 – 5
2 – 4
1 – 4 0 – 3.5 |
As indicated above, massive urbanization is occurring in flood-prone areas worldwide. Due to inadequate flood protection standards and land subsidence, floods are causing increasing casualties and damage. We must hope that in the coming years, those responsible for urbanization in flood-prone areas will better consider local conditions and potential changes due to the interventions and the effects of climate change.
