Bart Schultz
In a considerable part of these areas there are polders. In the majority of these polders there is subsidence due to shrinkage and oxidation of the topsoil, or extraction of groundwater in the deeper layers. In extreme cases - reclamation of peatlands in the humid tropics, groundwater extraction in urban and industrial areas, or reclamation of soft clay layers - there can be subsidence of 10 to 15 centimetres per year, or even more than twenty centimetres per year. In such cases this subsidence is therefore considerably greater than the sea level rise, which currently averages about 0.3 centimetres per year and in gloomy predictions could increase to one centimetre per year during this century.
An overview of the data I obtained from various scientific articles is shown in the Table below.
|
City or area |
Subsidence in centimetres per year |
|
Tokyo, Japan Semarang and Surabaya, Indonesia Southwest of Taiwan Jakarta, Indonesia Bangkok, Thailand and Holme Fen, England San Francisco Bay area, United States of America and Bolivar Coast Polders, Venezuela Houston-Galveston, United States of America and Ho Chi Minh City, Vietnam Manila, Philippines and New Orleans, United States of America; Shanghai, China and Ganges Brahmaputra Delta, Bangladesh Mekong Delta, Vietnam and Venetia, Italy Mississippi Delta, United States of America |
1 - 24 6 – 20 3 - 17 0,5 - 17 4 - 12 0,2 - 10
4 – 5
2 – 4
1 – 4 0 – 3,5 |
With regard to the data in the table, it should be noted that different publications present different data. This can have various causes. The publications sometimes describe different periods or different parts of the cities or areas concerned, or different methods were used to determine the subsidence.
In the polders, subsidence and to a certain extent sea level rise can have major consequences for drainage and flood protection. It can mean that discharge through discharge sluices will eventually have to be replaced by discharge through pumping stations, as is currently the case for the vast majority of the polders in the Netherlands. With regard to the discharge of excess water in polders, there are basically three situations with regard to internal and external water levels:
- the outside water level is always lower than the inside water level, which normally allows excess water to be discharged through an open connection or discharge sluices;
- the outside water level is always higher than the water level in the polder. In this case, the water must always be pumped out of the polder by means of one or more pumping stations;
- the outside water level varies between the two situations described above, in which discharge sluices are necessary, or a combination of discharge sluices with a pumping station. In fact, this is a transitional situation, while in the long term, in these types of cases, only pumping is possible.
The far-reaching consequences of subsidence for the polders in the floodprone areas have been pointed out many times by various experts and in the scientific press. I have also made my contribution to this. In 2012, I sent the only letter in my life to the editor of a newspaper, the Volkskrant. This was published in the Volkskrant as an opinion article on 12 December 2012 under the headline Climate change is not the problem at all. The headline was made by the newspaper, but I agreed with it. We are now almost ten years further, but in the publicity and certainly also in the political decision-making, soil subsidence unfortunately still plays a subordinate role in the vast majority of countries. Time will tell what the consequences of this will be in the long term.
