[♪ INTRO] When you think of ancient engineering, you might think of Roman roads, or Roman aqueducts, or the Roman Colosseum… Rome gets a lot of credit.

And, yes, the bath houses are cool, but Romans were far from the only people building incredible infrastructure during their time. Civilizations around the world developed clever solutions to their unique challenges and environments.

So here are four ways we can learn from ancient infrastructure. In difficult terrain, like the dramatic cliffs of the central Andes mountains in Peru, you need more than just roads to get from here to there. That’s exactly where the Inca empire thrived until Spanish colonizers arrived in the 16th century.

The Inca empire had a massive network of roads, and where those roads encountered cliffs, the Inca built suspension bridges. The ends of the bridge are secured to the abutments, and tension in the ropes holds you up while you cross. That means that as a person crosses the bridge, the bridge dips and the rope gets pulled taut, creating an upward force to support the weight of the person crossing.

Now, that is the opposite of arch-based Roman bridges, the most common bridges that the Europeans would have known about in the 16th century. Roman arch bridges use compression for stability. The weight of the stones in the arch and the people crossing over top pushes all of the force outward and toward the bridge’s foundations, creating a self-stabilizing, tension-free system.

A few communities in Peru still maintain Inca-style bridges today, which means engineers have been able to study how the bridges are built and why they work. Every year, the Q’eswachaka bridge in Huinchiri, Peru, is rebuilt in a 3-day festival. People twist grasses into ropes, and those into strong braided cables, to replace the previous year’s bridge.

But this bridge is a relatively small, local affair. During the time of the Inca empire, larger bridges with cables as thick as a person’s torso were made of vines, branches and leather, and could support up to 22,600 kilograms. The largest Inca suspension bridges could freely span over 50 meters, held up only by the tension in the rope and the heavy abutments on either side.

A Spanish army once tried to replace a major Inca bridge with a Roman-style one. Between 1588 and 1595, they attempted to build an arch bridge over the Apurímac River to replace a suspension bridge. The problem is that to build an arch bridge, you need a temporary, semicircular wooden structure called a centering in order to shape the arch.

And that is tough to maintain in an Andean gorge. So the Spanish attempt was abandoned. Suspension bridges were the tools for the job.

And at least two hundred Inca suspension bridges spanned canyons by the 1500s, supporting the empire’s vast road and messenger network. Suspension bridges like the Inca bridges were the best way to cross deep gorges until the 19th century brought steel to bridge-building. And these still have unique benefits.

For one thing, Inca suspension bridges are totally biodegradable and sustainable to produce. And today, bridges called stressed ribbon bridges use cables in the deck of the bridge strung between two points to cross long distances, holding pedestrians up with tension much like Inca suspension bridges. The city of Petra is best known for the architectural marvels carved into its cliffs.

It was the capital of the Nabataean kingdom, going back to before 312 BCE. As a stopping point along trade routes between Asia and Rome, the city was economically prosperous, and it flaunted that wealth by building large fountains and decorative pools. These were extra impressive because Petra is five miles from the nearest major water source, a spring called Ain Mousa.

So how do you move water five miles across the desert? With five miles of pipes! The Nabataeans crafted thousands of terracotta pipes that were each about 35 centimeters long and about 14 centimeters wide and strung them together from the spring to the city.

This required an incredible balancing act of fluid dynamics. See, if the incline of the pipes was too steep, then the water would fill them completely, increasing the pressure and eventually causing a leak. But at just the right angle, water can flow smoothly without filling the pipes.

And that angle is four degrees. Archaeologists have found imprints from Petra’s ancient pipelines in the nearby cliffs, and they are at that ideal four degree incline. Some water from the pipelines was also directed into reservoirs to store extra water close at hand.

The reservoirs were covered to reduce evaporation, and had a settling area for sediment, to store the water as long and as cleanly as possible. Petra also got about fifteen centimeters of rain per year, mostly in winter. That’s a little more than notoriously dry Las Vegas.

Now, climate change is causing historic droughts in regions that aren’t built with this kind of water-saving infrastructure. Take, for example, the Canadian prairies. Farms in this western region of Canada rely on snowmelt for water to fill reservoirs called dugouts.

And snow packs are becoming increasingly unpredictable. But researchers have actually proposed that something like the Nabataeans’ covered reservoirs, which dramatically reduced evaporation, could help preserve water in dugouts. The Gunditjmara people in southeastern Australia also have hydraulic engineering structures that are thousands of years old.

The difference is, they weren’t really out to manage the water, but something in it: Eels. Using local volcanic rock, the Gunditjmara people built a vast aquaculture system at a site known as the Budj Bim Cultural Landscape to manage short-finned eels, which they caught in woven grass baskets, cured, and traded with neighbors. One eel trap on Lake Condah was carbon dated to about 6,600 years old, with modifications added as recently as 500 years ago, which suggests that the trap was used continuously that whole time.

The Budj Bim aquaculture system is made of hundreds of yards of channels dug into the ground and dams built out of basalt blocks. These direct the flow of water to catch eels in baskets and ponds as they swim downstream. Computer models of the channels and ponds showed that the Budj Bim aquaculture system could hold eels at different stages of growth, and gave the eels a protected area in which to grow.

Today, Budj Bim is still managed by the Gunditjmara Aboriginal Cooperative. Sections of it are still being uncovered, like a 25-meter-long channel revealed by bush fires in 2020. And Budj Bim has a lot to offer to scientists studying Australia’s eels and wetland ecology.

An eel can live up to 50 years old, and will spend most of its life in freshwater until one day, it migrates out into the ocean to reproduce. These trips can be incredibly long, and they can even have overland segments. But what sparks that migration, and how the eels traverse the rivers to the ocean, remains a topic of ongoing research.

Scientists are working with the Gunditjmara to track eels’ paths through Budj Bim and out to sea. Because the aquaculture system was built with an understanding of eel migration and the changing water levels throughout the year, researchers can observe it for clues about eel biology. The more scientists understand about eels, the better we can protect them in the face of climate change, commercial fishing, and other threats to their wetland habitats.

Persian cities in 400 BCE took hydro-engineering to another level: they irrigated fresh water into their cities, and used architecture and wind to cool their homes and even make and store ice. Water in this arid environment came from alluvial aquifers, which are shallow sources of groundwater at the heads of valleys. And using an underground aqueduct called a qanat, which was invented about 3,000 years ago, engineers could direct that water to their cities.

Instead of using pipes, they dug tunnels large enough for a person to walk through, with several vertical shafts to let in fresh air. Rather like the Nabataeans, they had to find the right angle to keep the water flowing, and they also had to avoid these underground tunnels eroding away. Many qanats in Iran are still in use today for tasks like irrigating crops.

But the water also had a couple of other, unexpected uses: air conditioning and ice-making. Both very valuable in a desert, and ancient Persia engineered solutions without refrigerants or electricity. Many buildings had tall, chimney-like structures called bâdgir, or wind-catchers.

They would catch and redirect the prevailing winds to the basement, where the air would be cooled by a pool of water brought in by the qanat. Then, because there was constantly air coming in through the bâdgir, it would fill up the building from the bottom and push hot air out the top. Qanat also brought water to Persia’s ice houses, called yakhchals.

Yakhchals have three main parts: a long, rectangular pool for water; a wall as long as the pool, to provide shade; and a round building with a conical roof, to store ice. To make ice, people began by filling the pool with a shallow layer of water and allowing it to freeze overnight in winter. Then the ice was broken up into pieces and covered with water again, which was allowed to freeze overnight.

This process was repeated for eight days or until the ice was about two meters thick, and at that point it would be cut into blocks and stored in the building. Layers of ice would be separated with layers of straw and wood for insulation and to keep them from sticking together. By one estimate, a yakhchal would only lose about one fifth of its ice to melting over the course of nine months.

The same architects who performed that analysis also found that if they could build a new yakhchal with modern insulation materials like polyurethane foam, their yakhchal would only lose 6% of its ice to melt over nine months. These methods of climate control and ice-making that don’t use electricity could be sustainable alternatives to fossil-fuel intensive air conditioning around the world. A wind-catcher at the visitor center of Utah’s Zion National Park uses this passive cooling strategy to keep the building up to 16 degrees Celsius lower than the outdoor temperature, and bâdgir are still widely used in Iran.

While not all of these strategies directly inspired modern engineers, many of them are still in use today. Humans have been humans for a long time, and humans are pretty smart! It just goes to show that this ancient engineering know-how can directly help us today, to build a greener, smarter world.

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