The beginning of an inverted siphon on the Aqueduc du Gier in Chaponost, France. Photo by Ann Soule

The beginning of an inverted siphon on the Aqueduc du Gier in Chaponost, France. Photo by Ann Soule

Water Matters: Aqueduc du Gier

We recently returned from a place where sophisticated water engineering was commonplace over 2,000 years ago. Lyon, France, was the capital of Gaul during the Roman Empire, and has many fabulous relics of the ancient metropolis from massive amphitheaters to aqueducts. We were hosted by a lovely Lyonnaise family for two nights who took us to see some of the city’s treasures, including sections of the Gier aqueduct near their home (at my request, since Roman aqueducts are about as fascinating as anything can be!).

Called Lugdunum at the time, the city was established after the Romans took over a fortress on La Fourviere hill west of the confluence of the Soane and Rhone Rivers in the first century BC. Being that they had chosen a hill surrounded by valleys, the water supply for their rapidly growing population was limited to wells apparently more than 100 feet deep. And being that they were accustomed to bountiful water, fountains, and the oft-bathing lifestyle, wells requiring manual labor weren’t going to suffice. The solution? Aqueducts.

At this point, my pragmatic brain asks why a city would keep developing on a hill rather than take advantage of the large rivers flowing just below. I also had to wonder wouldn’t it be imprudent in that day and age for a city’s water supply to be dependent on vulnerable bridges? Indeed, we had a walking-tour guide in Old Lyon who wrote it off to “those crazy Romans who used slaves to accomplish all kinds of eccentric projects.”

Now that I’ve done more reading it seems that the Romans had such confidence in the strength and expanse of their Empire that vulnerability of the aqueducts wasn’t a big consideration at the time. It became an issue eventually, but for several centuries the city grew and thrived as a prestigious trade center—albeit profiting from cheap and slave labor.

Roman-era water managers and learned quickly that rivers near urban areas were quickly tainted with waste and runoff, so they ventured into nearby hills to find high quality sources for their aqueducts. They even appraised the apparent health of the peasants utilizing a stream or lake before deciding which one to tap.

Supplying a growing city

Between about 20 BC and 125 AD four aqueducts were built for Lugdunum, each longer than the last.

In addition to water quality, ease and expense of the project would have been factors in selecting an aqueduct’s route. The route had to start higher in elevation and, for Lugdunum at least, required some fancy engineering at the end to get water up to their city on top of a hill.

Normally, tunnels and bridges were used when following the contour of the landscape wasn’t feasible. For deeper valleys, siphons were cheaper than bridges but involve a pressurized pipe made of expensive lead or bronze. The “header tank” is a reservoir at the entry to the siphon, which consists of a water-tight pipe with a relatively steep drop, a flat section (short bridge), and rising section that empties into another tank slightly lower in elevation than the header tank.

The Romans didn’t invent aqueducts but greatly advanced the technology during their day. They used remarkable engineering tools to maintain a constant, gradual drop in elevation, such as a table with a level, called a chorobate. The second aqueduct serving Lugdunum required a double siphon to cross one valley, the only one known in Europe.

We visited impressive sections of the Gier aqueduct in Chaponost, a little west of Lyon. The engineers and laborers had clearly gained confidence for this fourth major project in 100 years: it was the most complex, with four siphons and 25 bridges, and half its 86-kilometer length was in tunnels; the tunnel sections that have been found have manholes every 240 feet for maintenance. The channel was consistent, apparently from tunnel to bridge, about 20 inches wide by 50 inches deep.

The Gier was the longest aqueduct with a net drop in elevation of 100 meters but still delivered water to the highest point yet on Fourviere hill.

This final aqueduct was also the largest, carrying 15,000 cubic meters per day—about 6 cubic feet per second, the volume in Bell Creek during a moderate rainstorm. Combined, the four aqueducts serving Lugdunum delivered 30,000-50,000 cubic meters per day (13 million gallons per day) to serve a population that maxed at 200,000 in the second century AD. Rivers, springs, and wells were used by this time for newer, lower-elevation parts of the city.

Water commissioners called “curator aquarum” were responsible for enforcing rules against illegal diversions and ensuring that aqueducts were maintained.

Speaking of water, it was excessively hot when we were visiting in July of 2019 AD and we learned quickly that Roman-style water bounty and their tradition of public water fountains had apparently died along with the ancient aqueducts. It was not easy to find a place to fill up our terra cotta water jars.…

And yes, another attraction in Lyon was the best women’s soccer in the world—and it was fabulous.

Geek(ier) Moment

Despite predictions of a dry summer, it was fairly cool and wet here in the past month, hopefully evading wildfires and low streamflows for a while. The current flow in the Dungeness River is hovering at 200 cubic feet per second, about half the long-term average for this time of the summer. Irrigators who are able will likely be asked eventually by state water managers to lease their water rights (forego River diversions) to the state trust, to keep water in the river during salmon spawning season.

For the 2019 Water Year (started Oct. 1, 2018):

• Rain in Sequim through July 22 at the Sequim 2E weather station (sea level): Total rainfall= 15.7 inches; New highest temp= 82 deg F on June 12-13; lowest temp still= 15 deg F in Oct.

• River flow at the USGS gage on the Dungeness (Mile 11.2): Max and min flows haven’t changed= 1,870 cubic feet per second (cfs) on Nov. 27 and 77 cfs on Oct. 25. Currently= 208 cfs. Range for the past month= 200-325 cfs.

• Flow at Bell Creek entering Carrie Blake Park: none; Bell Creek at Washington Harbor= summer flow is 1-2 cfs.

Ann Soule is a hydrogeologist immersed in the Dungeness watershed since 1990, now Resource Manager for City of Sequim. The opinions expressed are those of the author and do not necessarily represent policies of her employer. Reach Ann at columnists@sequimgazette.com or via her blog at watercolumnsite.wordpress.com, where photos of the Aqueduc du Gier are posted.

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