Bart Schultz
By far the majority of the polders in the world were originally created for agricultural land use. The water management systems and flood protection provisions were tailored to this. However, there are exceptions where a polder is developed directly for urban areas. Generally, however, the choice in such cases is not to reclaim the land, but rather to raise the level to a certain level above high water.
In depressions, river valleys, and coastal and delta regions, our ancestors typically settled on higher grounds. Due to the rapid growth of the urban population, particularly in emerging economies, most urban expansions in recent decades have been realized in flood-prone areas.
When urbanization occurs in a polder developed for agricultural use, this can have far-reaching consequences for the water management system and flood protection provisions. Regarding the water management system, this primarily involves a significant increase in paved surface area in the form of roads, squares, and buildings, resulting in reduced infiltration into the subsoil and accelerated discharge of excess rainwater.
With regard to flood protection, there is an enormous increase in the value of public and private assets in the area, as well as of residents at risk of damage, evacuations, and in extreme cases, death due to flooding.
Design standards and management, operation and maintenance would have to be adjusted based on such changes. Unfortunately, in many cases, this appears to be happening insufficiently. When flooding then occurs, reference is usually made to the effects of climate change. These effects can certainly play a role as well, but generally, this role is more limited than that of the effects of urbanization.
In addition to the growth of cities with more than 5 million inhabitants, the impact of urbanization in polders can be illustrated, for example, with two examples of rapidly growing cities: Dhaka in Bangladesh and Jakarta in Indonesia. For these cities, the level of flood protection is not higher than a probability of failure of at most once every hundred years.
In 2006, the United Nations Population Division published maps showing the cities with more than 5 million inhabitants in 1950 and 2015. In 1950 there were eight cities, and in 2015 there would be 45. Illustrative is not only the rapid growth, but also the location, as almost all of these cities, and certainly their expansions, are situated in flood-prone areas.
Dhaka is one of my more extreme examples of a flood-prone city along a river. In a previous contribution to Flevolands Geheugen, I wrote about The dike around Dhaka in Bangladesh. After flooding in 1987 and 1988, it was decided that a western ring dike and an eastern ring dike would be made around Dhaka. The western ring dike was completed in 2001. At that time, about 10 million people lived surrounded by this dike. To my knowledge, more than 23 million people now live in the Dhaka area. It is difficult to determine how many of them live within the endiked area, but the population growth in the endiked area must be considerable.
Jakarta is a rapidly growing coastal city. I recently wrote about The Polders in Jakarta. Here, too, there is rapid population growth, meaning the Jakarta area now has 42 million inhabitants, making it the largest urban area on Earth. A large part of Jakarta consists of polders, which are also facing rapid land subsidence, making it increasingly difficult to maintain water management systems and flood protection provisions, let alone improve them.
As the maps of cities with more than 5 million inhabitants already indicate, there are many other cities in flood-prone areas. For all these cities, it is of great importance that water management systems and flood protection provisions are continuously adapted and improved. Let us hope that this indeed happens, because otherwise we will face increasingly severe catastrophes.
