Bart Schultz
In lectures on land reclamation, Professor Adriaan Volker explaned to us that there are essentially two alternatives for developing such polders:
- construct a ring dike and pump out the water. He argued that building deep polders directly on the coast, prone to storm surges and high waves, was inadvisable;
- construct a dam and create a freshwater reservoir and polders. In this way, safety and saline seepage problems can be reduced or even prevented.
The safety and future functioning of deep polders depend on a proper enclosing dam, strong polder dikes, water management systems in the new polders, and the drainage of excess water. The enclosing dam - in the Netherlands called Afsluitdijk, by the way - must be able to withstand storm surges from the sea. How safely such an area would have to be developed is generally based on an economic, social and political decision.
As far as I know, the following projects have been realized based on the latter principle:
- South Korea: Ansan, Daeho, Geumgang, Geumho, Hwaeung, Namyang, Saemangeum, Sapgyo, Seokmun, Seosan, Sihwa, Yeongam, Yeongsangang;
- Japan: Hachirogata, Isahaya, Kahokugata, Kasaoka, Kojima, Nakaumi;
- Australia: Murray Darling;
- Bangladesh: Muhuri Basin;
- India: Kuttanad;
- Netherlands: Zuiderzee;
- North Korea: Taegye Do;
- Romania: Razim.
This involves a total of 25 land reclamation projects covering an area of 738,000 hectares, of which 337,000 hectares have been reclaimed and 401,000 hectares have been created as freshwater reservoirs. The polder surface can be as low as 5-6 metres below mean sea level.
In fact, these are costly land reclamation projects, because in addition to the enclosing dam, the polder dikes also need to be constructed, and discharge from the polders must be achieved entirely or partially through pumping. In some cases, an irrigation system is also present in the polders. Although these areas are generally reclaimed for agriculture, after initial development, cities or industrial estates can be built in parts of these polders. In the Netherlands and South Korea, several newly reclaimed areas, such as Flevoland and Saemangeum have been developed directly for multiple land use.
The decision regarding the location of the polder dikes and the freshwater reservoirs is crucial. Important aspects are:
- depth gradient in the inlet: deeper areas are best suited for the reservoirs;
- soil conditions: fertile soils would preferably have to be reclaimed;
- seepage conditions: permeable topsoils would have to be avoided or reinforced;
- soil load-bearing capacity: thick, soft layers of clay or peat would have to be avoided where possible for the polder dikes.
After the closure, the salty water in the reservoir will gradually be replaced by fresh water. This is due to inflow from the river(s) that flow into it, excess water discharged from the new polders, and precipitation. Water in the reservoir evaporates. Water is often withdrawn from the reservoir for irrigation and drinking or industrial water supply. Excess water is also discharged into the sea at low tide. Therefore, the preferred water level in the reservoir is generally around mean sea level.
In the event of high river discharges into the reservoirs, the capacity of the discharge sluices may be insufficient. In such cases, a pumping station may have been built next to the discharge sluices. Due to rising sea levels and changes in the river basin area of the river(s) flowing into the reservoir, this may also become necessary in the future. For example, a winter water level of 0.40 metres and a summer water level of 0.20 metres below mean sea level is being preferred in the IJsselmeer, and now – more than 90 years after the closure – a pumping station is being built on the Enclosing Dam.
The proper discharge of excess water through pumping stations is essential for water management in deep polders. To reduce the risk of malfunctioning, two or more pumping stations, each powered by different energy sources (electricity and diesel), have often been constructed.
Closing off and reclaiming such areas can have various side effects. These include a reduction in the tidal basin's surface area, or seepage from adjacent areas into the deep polders, potentially causing a drop in the groundwater level in those areas. This can cause agricultural damage or damage to buildings and infrastructure. In such cases, the construction of a peripheral lake, such as that implemented in Flevoland, may be necessary.
Especially in densely populated Asian countries, there is significant pressure to reclaim and develop low-lying coastal areas. The Saemangeum project in South Korea and the proposed Kalpasar project in India are the best-known examples. Careful selection of the areas that could still be developed will be crucial to reap the benefits of significant investments in water management and flood protection.
Environmental problems and complications resulting from land subsidence and, to some extent, climate change are making these developments increasingly vulnerable. The challenge for the future will be to keep the negative effects at an acceptable level and to support the positive effects as much as possible.
