Earthen mounds helped ancient Dutch settlers thrive in coastal flood zones. Could historical engineering help us fight against rising seas?
By Sarah Derouin
For centuries, fighting rising sea levels has been a regular part of life for people in the Netherlands. Since the early 20th century, archeologists have been digging into how humans adapted and thrived in these flooded environments.
Settlers in the low-lying region of what is now the northern Netherlands built elevated platforms on the land, protecting their homes, crops, and livestock from the rising ocean and frequent flooding. These constructed landforms were highly successful, allowing settlers to thrive in low-lying lands for more than 1,500 years.
Although most research documenting such early engineering practices has been presented to archaeological societies and in archaeological journals, these ancient adaptations to flooding might be of interest to other modern communities, especially those concerned with coastal management and policy.
“I wanted to show the results of our research to a wider audience than just archaeologists,” says Annet Nieuwhof, an archaeologist at Groningen Institute of Archaeology at the University of Groningen in the Netherlands and lead author of a new paper in Ocean and Coastal Management. “I think we can contribute something to the problems of today with rising sea levels, because that is exactly what people in the past were also dealing with.”
Living in salt marshes
Salt marshes on the northern coast of the Netherlands have been home to settlers since around 650 BCE. The fertile soils that attracted settlers to the area were created by the same processes that threatened their settlements: floods. This land experienced storm surges and flooding, as well as moderate sea level rise—about 3 to 4 centimeters of rise per century—which inundated their homes.
To deal with flooding, settlers built raised platforms of sediment called terps. A terp is a dwelling mound specifically built to allow humans to adapt to a regularly flooded area.
“The terp has a long history,” says Nieuwhof. She and her colleagues pieced together the history and evolution of terp settlements in the northern Netherlands by using a large data set of archaeological research collected over the past few decades, including excavations of terps and age dating of buried finds within the platforms.
Early terps were built for a single home and were modest in height—only about 0.4 to 1 meter above what were likely the highest expected flood levels.
The evolution of terps
Terps were not simple piles of dirt, says Nieuwhof, but engineered structures that were resistant to erosion and could support a house without sagging. Early terps were built for a single home and were modest in height—only about 0.4 to 1 meter above what were likely the highest expected flood levels.
Over time, this subtle rise was not enough protection. In addition to the slow sea rise, colonizers living on terps during the pre-Roman Iron Age (about 500 to 1 BCE) began to unintentionally undermine their settlements.
“They dug ditches to reclaim the area, to cultivate the area, and they started plowing,” says Nieuwhof. “That caused oxidation of the peat and subsidence—that made the whole area very vulnerable to the sea.”
Under threat of inundation, colonizers increased both the height and area of the platforms. Nieuwhof explains that eventually, individual terps grew together, creating raised surfaces several meters high and big enough to include room for gardens or even fields. These expanded terps hosted small communities, sustaining populations of 15–20 people per square kilometer.
Historic solutions for modern problems?
Understanding how humans have dealt with past sea level rise could be helpful in present-day coastal management strategies.
“That type of perspective is really valuable and important because it helps us to look at long-term responses of landscape to human manipulation.”
“This [paper] is a really nice synthesis of how people interacted with this coastal landscape over more than a thousand years,” says Elizabeth Chamberlain, a postdoctoral fellow at Tulane University who was not involved with the study. “That type of perspective is really valuable and important because it helps us to look at long-term responses of landscape to human manipulation.”
Chamberlain mentions that directly applying the same engineering solutions to present-day conditions isn’t a one-to-one approach.
One big difference is the number of people at risk: Terps supported up to 20 people per square kilometer, but “in contemporary times, the population density [in that area] is roughly 488 people per square kilometer,” says Chamberlain. In places like Bangladesh, she notes, population density is even higher, averaging more than a thousand people per square kilometer on the delta plain.
Even with these population differences, Chamberlain says it’s really useful to have this kind of historical information, especially for region-specific strategies for at-risk coasts. “You can pull the components that are most valuable for present-day society and try to engineer better and more sustainable systems by building with nature,” she says.
Adjusting to sea level rise will likely mean societies will have to adjust to new protections.
“In the Netherlands, we trust just our dikes, I think, a bit too much,” says Nieuwhof. She says it may be time to revisit the older strategies for new ideas. “We have to try other things and experiments—perhaps lower dikes to allow some flooding, perhaps live on terps again in some areas.”