By: Jeffrey Carlson
Like many other ancient cities of its time, the city of Rome was founded on a floodplains. Sitting on the Tiber River, Rome has over the course of several centuries been subjected to the threats of floods. These floods played a role in shaping Roman society into what it is today, and challenged engineers to keep the city protected from floods. This paper aims to cover the major flood control measures implemented throughout the city’s history, and how the modern Tiber embankment walls in particular changed flood responses in the city, leading the city into the flood-resistant place it is today.
Characteristics of Floods in Rome
In order to understand how engineers tackled the challenges that floods floods, one must first understand how and why floods form in Rome. Flooding in Rome is most often caused by rain, which is most common during the winter months in the city and least common during the summer months (Aldrete, 2007). Floods in Rome do not occur suddenly. Rather, the conditions that start floods are built up through gradual rainfall that saturates the soil. The soil in and around Rome can become saturated very fast, as it is composed of fine particles of volcanic ash that make the ground relatively impermeable (Borio et al, 2010). When the soil is saturated enough that it can no longer hold more water, any rain that falls in Rome will become surface runoff. This surface runoff will flow downhill and eventually settle at a low point. If enough runoff water builds up at a low point, a flood will occur. In Rome, floods will usually occur after about 90 days of consistent rainfall that saturate the soil, followed by a few days of more intense rain that produce a large quantity of surface runoff (Aldrete, 2007). High water levels would then stay for 2-3 days before being fully drained, returning the city to normal.
Geographically, the city of Rome is situated on the floodplains of the Tiber River, which has traditionally been the area where surface runoff builds up to produce a flood in the city. Most of the damage caused by flooding over the course of Rome’s history has been due to the Tiber overflowing, but water can build up and cause floods in other low-lying areas as well.
History of Flood Control
Floods in Ancient Rome
The history of flooding in Rome can be traced all the way back to the mythical start of the city, with Romulus deciding to build his city on top of Palatine Hill. The city of Rome eventually expanded to encompass settlements on seven hills, all of which overlooked the floodplains of the Tiber River. These seven hills were all protected from any surges in water coming from the marshes below. With the Tiber River being the easiest way in and out of Rome, the city was pretty well-defended from its enemies while staying well-connected with the outside world. The floodplains are thought to have been unoccupied by residential buildings during the ancient times, outside of a few establishments built for agriculture or trading (Di Baldassarre et al, 2017).
The Tiber River began to be altered by engineering projects as far back as the 6th century BC, with open air canals diverting the flow of nearby streams into the Tiber River (Hopkins, 2007). These canals helped drain the marshes below the seven hills, allowing for construction of buildings in the low-lying areas of the city. One such canal that went through the Roman Forum was eventually covered up and became the Cloaca Maxima. The Cloaca Maxima became the main drainage and sewage system for the Romans, and played a critical role in draining floodwater from the low-lying Roman Forum (Figures 1 and 2). The Cloaca Maxima was hooked up to the drainage systems of bathhouses, toilets, and street drains all across Rome, with all of the wastewater entering the Tiber River in a giant tube that can still be seen today (Figure 3). The Cloaca Maxima was not a perfect drainage system, as floodwater sometimes went back into the city through it. This phenomenon usually occurred during floods that had a high water mark above 15.7 meters above sea level (masl), as at this height the cloaca would be completely submerged underwater, preventing water from flowing out of it (Hopkins, 2007). Because of this, the Cloaca Maxima began to differentiate the severity of floods, with most major floods occurring with high water marks above 16 masl, and minor floods usually occurring with high water marks below 16 masl (Aldrete, 2007). The drainage systems of Ancient Rome had their fair share of flaws, but nonetheless, they marked the first major attempt at controlling floods in Rome.
The Cloaca Maxima was also by no means the only defense Rome had against floods. Due to the frequent flooding, sediment and debris deposited in the aftermath of floods naturally raised the level of the city so that it was more resistant to future floods. Many buildings in Rome today are built on top of the ruins of other buildings because of the heightened elevation the ruins provided. While most of the time the process of rising the elevation of the city was natural, there were also deliberate attempts to raise the elevation of areas of Rome by humans, most notably in the Roman Forum, which in some sections sits on top of 8 artificial layers of debris (Aldrete, 2007). The other big measure used to prevent floods were embankments around the Tiber River, which were often not successful as they weren’t built high enough. The ancient Romans also had plans to divert and impair the flow of the Tiber using canals and dams. However, none of these plans were ever fully implemented, due to the uncertainty of the results of the projects as well as fear of divine punishment given the sacredness of water in Roman mythology (Aldrete, 2007). The conditions in the Tiber River were also heavily regulated by administrative authorities at the time. Records show that Marcus Vipsanius Agrippa, the curator of all Roman works under Emperor Augustus and one of the most influential architects/engineers of the Roman Empire, had worked to keep the riverbed clear of debris, erected embankment walls, and improved conditions within the Cloaca Maxima in order to control floods (Long, 2008).
In total, there are about 33 recorded floods occurring in ancient Rome, encompassing the time period from 414 BC to 398 AD (Aldrete, 2007). The records were relatively vague and did not mention high water marks or damage caused by floods. The lack of information in the records suggests that the Romans may have rather indifferent towards the effects of floods, as many of them lived in the hills. This assumption that Romans were indifferent to floods is further backed up by archaeological evidence of the distribution of public and private buildings. Contrary to what one might expect, a majority of the commercial and entertainment buildings of Rome existed in the floodplains, while about 85% of private homes existed in the hills above the floodplains and would be left untouched during a flooding event (Aldrete, 2007). The ancient records of floods also maybe suggest that the drainage systems were well maintained, and damage from floods were quickly repaired.
During the Middle Ages, the techniques the Romans used to prevent flooding fell into negligence. Debris from bridges and buildings built up in the Tiber, drains became clogged, and aqueducts fell into disuse. The buildup of debris in the Tiber River had raised the level of the riverbed, making the floods more common and more devastating. During the 16thcentury, the negligence towards the Tiber River could no longer be ignored, as the city was hit by several devastating floods. The four worst recorded floods in Rome’s history occurred during or around this century, with the high water marks for floods reaching over 18 masl in 1530, 1557, 1598, and 1606 (Aldrete, 2007). During the worst of these floods in 1598, water reportedly completely submerged the first story of most buildings in the floodplains, with some buildings in the low-lying Jewish ghetto reportedly having up to three stories submerged at the peak of the flood (Aldrete, 2007). At this time, most of Italy was taking part in the Renaissance movement. During this period of time, there was a renewed fascination in the works of the ancient Romans within the city of Rome. At the same time, the city was under the control of various popes, who had the desire to revitalize Rome as the capital of the Christian world. For the Catholic church, the floods were seen as an embarrassment, as the floodwaters had destroyed many religious artifacts and buildings and gave religious pilgrims a poor first impression of the city (Long, 2008).
In the aftermath of the flood in 1557, the Catholic church decided to take action and hired physician Andrea Bacci and military engineer Antonio Trevisi to come up with a way to control the floods (Long, 2008). Despite their differing academic backgrounds, both men came to similar conclusions in their research. Both men believed that ancient Romans had less floods because they kept the Tiber River and the drains that flow into it clear of debris. Buildup of debris in the river had led to the riverbed rising in elevation at a pace which the city had not kept up with. On top of that, debris was reducing the flow of water out of the city, making it easier for water to buildup in low-lying areas. In their research on the causes of floods, Bacci recommended returning to the methods of the ancient Romans in order to prevent flooding. These methods mainly included dredging the river, providing more water to the city, keeping drains clean, and appointing a government official to care for the river (Long, 2008). Trevisi recommended the construction of trenches between the Vatican and Trastevere to help prevent further flooding. Only one part of his trench was fully implemented, in the form of a moat around Castel Sant’Angelo that can still be seen today as shown in Figure 3. It is unclear which other recommended measures were implemented, but records do show that flood levels in Rome have never reached the levels seen in the 16thcentury, meaning that many of the recommended flood control measures were likely implemented going into the 17thcentury (Long, 2008).
The floods of the Renaissance also brought about a change to the way floods were recorded. Around the early 13th century, markers began to be used to mark the high water points of extreme floods. While this innovation was a welcome change (especially after a nearly 400-year gap in any sort of reliable records of floods during the dark ages), the accuracy of these pre-Renaissance markers is questionable. During the Renaissance, the accuracy of these markers were greatly improved in two ways. Firstly, the number of markers being placed for floods increased drastically, with three or more markers often being used to mark the high water point throughout the city (Aldrete, 2007). Secondly, the measured height of these markers became far more accurate thanks to the installation of hydrometers at the banks of the Tiber. While there were multiple hydrometers installed, the hydrometer installed at the Ripetta Wharf on the Tiber River quickly became the standardized hydrometer used to record all flood high water marks down to the centimeter (Aldrete, 2007).
The Flood of 1870
A major turning point in responses to floods occurred in 1870, when Rome was set to become the capitol of the unified Kingdom of Italy. What was supposed to be a monumental occasion for the entire nation was ultimately ruined by a flood on December 27th, 1870, when flood waters reached a high water mark of 17.27 masl (Aldrete, 2007). Unlike the floods of the Renaissance, the causes of the flood of 1870 appeared to be almost entirely natural, as the timing of the flood was not out of line with post-Renaissance flood patterns, which consisted of floods reaching above 16 masl roughly every 25 years (Aldrete, 2007). The high water mark for the flood, however, was seen as an extreme anomaly compared to the floods of the time, as the high water mark was the highest seen since the 16th century’s record-breaking floods (Aldrete, 2007). The flood was seen as an embarrassment for the new nation, and triggered debates at a national level on how to bring floods under control once more.
With radical change in Italian politics came a radical change to flood control in the city of Rome, and a commission was formed to prevent severe floods of the Tiber from ever happening again. Many proposals were thrown around as to how to control the river, including implementing some of the plans to divert the flow of the river that the ancient Romans never fully implemented (Aldrete, 2007). Ultimately, the commission settled on the cheapest option for flood control. Taking after other western European cities of the time such as London and Paris, large embankment walls were built along the Tiber River to prevent flooding (Di Baldassarre, 2017). The embankment walls were built to reach a height of 18 masl, a level that was higher than the high water marks for all floods after the 16th century. With their construction commencing in 1875 and their completion in 1910, the embankment walls have proven to be the most successful flood control measure the city had ever taken, with its impact on Roman society still being felt today.
Legacy of the Embankment Walls in Modern Rome
Today, the city of Rome is a far more flood-resistant city than it once was thanks to the embankment walls constructed around the Tiber River. Made out of impermeable travertine stone as shown in figure 5, the embankment walls have helped prevent rainfall from permeating into the soil beneath the city. Instead, rainfall is sent downstream, where it will not pose a flood risk to the city. Since construction began on the embankment walls, only 3 major floods have been able to advance beyond the Tiber River’s banks and into the streets of Rome (Aldrete, 2007). Of these three floods, none of them caused any significant damage to buildings or streets in Rome. The last of these major floods occurred in 1937, meaning that Rome has lived free of the threats of major floods from the Tiber for over 80 years. Having been free of flood threats for such a long time, Roman society has begun to be influenced less by the floods of the Tiber and more by the embankment walls themselves.
Construction of the Embankment Walls
The most immediate change that the embankment walls brought about to Roman society came during the period of their construction. The Tiber River in the 1870’s was not the desolate place that it once was during ancient times. In order to construct the walls, many structures already existing on the riverfront had to be demolished. Bridges in particular had to be heavily changed in order to accommodate for the construction of the embankment walls (Aldrete, 2007). In some cases, the ruins of older bridges can be seen in the river underneath the newer bridges (see Figure 6). Not all bridges were destroyed, and some bridges, such as the Ponte Sisto, have retained their original structure (see Figure 7). Bridges were also by no means the only structures that had to be demolished to make way for the embankment walls. Many residential buildings also had to be removed for the construction process. The low-lying Jewish ghetto in particular saw the destruction of many homes nearby the Tiber River in order to make way for the embankment walls. Even the Ripetta Wharf, which had proven crucial in the measurements of flood heights, was demolished in the construction process, along with every other wharf on the Tiber River (Aldrete, 2007). The construction of the embankment walls and the demolition that accompanied it profoundly changed the area surrounding the Tiber River, making it almost unrecognizable from the river it once was when construction was completed.
Societal Shifts Due to the Embankment Walls
Once the construction of the embankment walls was completed, the Tiber riverfront quickly became cut off from the rest of the city. As part of the many cost-saving measures implemented during the construction process, the embankment walls were built as steep as possible in order to use less travertine during their construction (Aldrete, 2007). The steepness of the embankment walls have severely impaired pedestrian access to the Tiber River, which had once been granted by the river’s many wharfs. Today, the Tiber River is almost invisible from the street level above, allowing it to fall into negligence and become the home of many homeless encampments. For many urban planners, the Tiber River is seen as a wasted opportunity for the addition of more public spaces (Rankin, 2015). Lack of any human activity on the riverfront has even allowed the water in the river to decline in quality, as seen in Figure 8. Although the floods may have stopped, reintegrating the river into the city will remain a challenge for years to come.
As the river below the embankment walls have become deserted, daily life on the streets above has thrived for the most part. The embankment walls have helped stimulate large amounts of development in the Tiber’s floodplains as the city experienced a population boom in the 20th century (see figure 9). This population boom has helped shift the public perception of floods in the city back towards that of the ancient Romans who were not pre-occupied with the threats of flooding. One study by Di Baldassarre et al. (2017) showed that public memory of high water level events in Rome had a half-life of 2.5 years, meaning that it only takes 2.5 years after a flood for half of the people in Rome’s floodplains to forget about a flood. This social phenomenon, coined the “levee effect”, is observable in other flood-prone cities as well, although it is not usually as strong as observed in Rome. While the study in question focused on minor floods from recent decades, the results have likely been driven by the lack of major floods from the Tiber in the past 80 years, . This relative lack of worry about floods is consistent with the historical trends in the city, as the floods in the 16th century and 1870 have shown that the people of Rome find no need to address problems related to flooding of the Tiber unless it ruins the perception of the city to outsiders. Overall, the embankment walls have been massively beneficial to the residential and commercial areas surrounding the Tiber River.
Future Outlook on Flood Control in the Tiber River
With all of this information, it is clear that the embankment walls have been effective in preventing the Tiber River from flooding into the streets of Rome. Because of them, the city is arguably in a position where it does not need to implement new flood control technology in the Tiber River and can instead focus on improving existing measures within the city. Going forward, the negligence of the Tiber riverfront is potentially concerning, as the Renaissance has shown what can happen when negligence of the river gets out of hand. Many projects have aimed to combat this negligence by reintegrating the Tiber Riverfront into public life in Rome, bringing sufficient maintenance of the river along with these projects (Rankin, 2015). While a few projects have reached beyond their design phase, none have truly been as successful as one would hope. As of today, the Tiber riverfront remains a vast no man’s land cutting through the center of the city.
Flood Risks in Rome’s Historic City Center
While Rome may have been free of floods from the Tiber for the past 80 years, it has not been free of floods altogether. The saturation of the relatively impermeable soil beneath Rome has still caused some minor floods, particularly in the city’s historic center. As the rest of the city has developed with modernized flood control measures, the low-lying regions of the city center, including the Roman Forum and the Campus Martius, have lagged behind, making them particularly vulnerable to floods. Today, floods in the historic city center have been heavily influenced by the area’s drains and pavement, which are often centuries older than the drains and pavement in the rest of the city.
One of the most notable flood control measures that can be seen in the city today are the stormwater drains. Most of the drains in the city are catch basin drains, including the drain shown in Figure 10. These catch basins allow any solid debris that enters the drain to sink in the catch basin to a level where it wouldn’t pose any immediate harm to drainage pipes. (get diagram). In the event of heavy rainfall, the drains may overflow and allow the sunken debris in the catch basin to return to street level. Most of the drains in the city are still hooked up to the sewage systems used by the ancient Romans, including the Cloaca Maxima. There are some obvious concerns about the structural integrity of drainage systems that are over 2000 years old, but the Cloaca Maxima and other old drainage systems in Rome have generally not caused problems in recent decades due to regular maintenance (Hopkins, 2007). The few drains in the city that aren’t well maintained could create an initial flood risk due to water backup. Despite some water backup in drains creating an initial flood risk, most well-maintained drains will still effectively remove water from low-lying areas in the aftermath of a flood. Whereas floods before 1870 had high waters marks that lasted for 2-3 days, it is rare today to see floodwater more than 12 hours after heavy rainfall thanks to the modern drainage systems.
Pavement in the city has also presented a unique challenge for flood control within the historic city center. Many of the older roads in Rome’s historic center and Trastevere are paved with Sampietrini bricks. The Sampietrini paving technique, first used in St. Peter’s Square during the 16th century, consists of Sampietrini stone blocks hammered into a sand bed below it (see Figure 11). The gaps of sand bedding in between Sampietrini blocks allow for this type of pavement to be more permeable than other types of modern pavement used in the city, even though the Sampietrini blocks themselves aren’t permeable. The gaps in the bedding allow for the absorption of water by the soil beneath this type of pavement, helping reduce the threats of floods even when drains aren’t nearby the pavement (Zoccali et al, 2017). Today, Sampietrini paving covers over 100 kilometers of Rome’s urban road network, but it is by no means the most common form of pavement in the city (see Figure 12).
Despite their permeability being helpful in preventing floods, Sampietrini pavement is not the most structurally sound form of pavement, and will often take heavy damage from water. Heavy rainfall in the city has, over many years, eroded away the sand located between the joints of Sampietrini bricks, subjecting the pavement to many forms of distress that can harm the structural integrity of roads. Examples of eroded joints in Sampietrini pavement can be seen in figures 13 and 14. Furthermore, erosion from both water and vehicles on Sampietrini roads can lead to the formation of depressions in the pavement. During a minor flood event, water can gather in a depression and create a risk of hydroplaning for vehicles that drive over the depression (Zoccali et al, 2017). Examples of depressions in pavement can be seen in figure 15. These distresses, combined with the general fact that Sampietrini bricks are very slippery when wet, have contributed to this type of pavement being very undesirable for vehicles to drive on, especially after heavy rainfall.
Future Outlook on Flood Control in the City
From observation of drains and pavement in Rome’s historic city center, it is clear that there are many downsides to using drains and pavement that are centuries old. Their function has deteriorated over time, and both the drainage systems and the Sampietrini pavement have become far more expensive to maintain than their modern counterparts (Zoccali et al, 2017). Despite their downsides, the drains and pavement in the city center is still more reliable than the soil beneath the city. Both have helped prevent Rome’s streets from becoming impassable messes of mud and sewage in the aftermath of floods, as they did in the flood of 1870. While they may not have as large of an impact as the embankment walls, the maintenance of the drains and pavement in the historic city center is just as important for flood control during minor floods. As both Bacci and Trevisi revealed during the Renaissance, the best flood control measures that the city can take are often just the maintenance of the existing flood control measures.
With a vast history encompassing more than 2500 years, few things have remained as constant in Rome as flooding. Floods have completely altered the landscape of Rome, with complex drainage systems, artificial hills, and embankment walls leaving the area unrecognizable from the marshes it was built above. The construction of flood control measures over the years have not only protected the city effectively, but have also provided fascinating insights into Roman’s attitudes towards floods. In the modern day, most of the old methods of flood control are still working despite centuries of use wearing them down. While floods have been controlled well since 1870, the city still has room to prepare for future floods and bring the city’s flood control infrastructure into the 21st century.
Aldrete, Gregory S. Floods of the Tiber in Ancient Rome. Johns Hopkins University Press, Baltimore, 2007. doi:10.1353/book.3303
Borio, L., and Peila, D. “Study of the Permeability of Foam Conditioned Soil with Laboratory Tests”. American Journal of Environmental Sciences, vol. 6, no. 4, pp. 365-370, 2010.
Di Baldassarre, G., Sacca, S., Aronica, G. T., Grimaldi, S., Ciullo, A., and Crisci, M. “Human-flood interactions in Rome over the past 150 years”. Advances in Geosciences, vol. 44, 2017, pp. 9-13. doi:10.5194/adgeo-44-9-2017
Hopkins, J. N. N. “The Cloaca Maxima and the monumental manipulation of water in archaic Rome. The Waters of Rome, no. 4, pp. 1-15, 2007.
Long, P. O. “Hydraulic Engineering and the Study of Antiquity: Rome, 1557-70”. Renaissance Quarterly, vol. 61, no. 4, pp. 1098-1138, 2008. doi:10.1353/ren.0.0320
Rankin, Tom. Rome Works. Peruzzi Press, 2015.
Zoccoli, P., Loprencipe, G. and Galoni, A. “Sampietrini Stone Pavements: Distress Analysis Using Pavement Condition Index Method”. Applied Science, vol. 7, no. 7, p. 669, 2017. doi:10.3390/app7070669