Welcome to Schipol (Amsterdam) – 11 feet below sea level
Schipol – Amsterdam Airport – is about eleven feet below sea level. The name translates as “ship grave.” The origin goes back two centuries when it was a shallow sea, prone to occasional violent storms, sinking many ships.
Schipol today is one of the world’s busiest airports, about twenty minutes by train from the heart of Amsterdam. When I landed here on the weekend, it occurred to me that this terrific engineering work can be both an example of what is possible with good large scale planning, and should also be an alert that not all areas have the same possibilities.
Schipol airport is built on a “polder” the Dutch term for an area below sea level that has been reclaimed from the sea. An estimated 30% of the Netherlands is below sea level and has been created by the dikes or levees that form the perimeter of the polders. Polders have been an essential innovation here going back a thousand years. Community based organizations came together to work together at local levels to construct the dikes and pump the water out with windmills, sharing the expense among the residents and farms.
While Schipol reminds us all that we can engineer some amazing adaptations, it also should point out that all places are not the same. The Netherlands is largely clay based geology. Clay is impermeable. Soils that contain layers of clay are water barriers. Levees built with high clay content can retain water and make it possible to have large polders that are below sea level, pumping out small amounts of seepage.
South Florida, The Bahamas and other areas built on porous limestone are a very different situation. That geologic structure is like a rigid sponge with small channels allowing water to flow through the rock. In these locations, a levee or dike would not keep out the water. Even if a wall were created with a watertight vertical layer, a rising sea level would simply percolate up through the deep substrate.
Schipol reminds us that large scale engineering can do amazing things but that specific geologic structure needs to be considered. In areas with porous limestone a polder below sea level is not a solution. In such areas, a retaining wall may allow the ground level to be built up to a higher elevation to get the area above the highest tide and rising sea level. The question becomes how large an elevated area is economical. The community, the infrastructure for utilities, ports, airports, and supply chains all need to be considered.