Engineering Rome

Tiber River – Destroying and Invigorating the City for Millennia

Ann Albright
brutuslovescaesar brutuslovescaesar Dec 3, 2015


The Tiber River…
Legend tells us that Rome was founded by a brother who, along with his twin, was sentenced to death as an infant. They were put into a basket in the river to be drowned. However, since it was flooded at the time, they were not washed downstream, but instead gently deposited to the entrance of a she-wolf’s cave, who raised them until they were men. Even though this is probably a mythological beginning for the city, there are grains of truth that speak to the strong ties between the city of Rome and the Tiber River. For more information go here.

It is now a forgotten piece of the city, running down to 30 m below the ground height of the heart of the city. Despite its neglect today, the river used to be a source of prosperity, and more often than they would like, despair in regards to the violent floods it could create. One might ask, “How can a city become so successful, even though it is victim to floods every few years?” This report will explain the hydraulic flood plane that Rome is nestled within, the results of the floods, and how they tried to mitigate the issues due to flooding, then and now. Regardless of how, it is irrefutable that the river has shaped and influenced the lives of citizens of Rome from 700 BC to modern times.

Geography and History of the River

The Tiber’s ancient Path

Rome was founded on one of the “Seven Hills of Rome” which are more or less large plateaus. The plateaus were created by erosion of tributaries of the Tiber River. The materials of the hill are generally a soft tuff rock, easily broken down. The result is high plateaus to build homesteads and barriers, but an easily accessible river with means of communication and transportation. The historian Livy writes, “…the river is advantageous since along it are brought food stuffs from inland… but is not exposed to the dangers of enemy fleets.” (Livy Today, some of the main streets of Rome are actually where these tributaries used to run. See Figure 1.

The tributaries ran between a few of the more prominent hills and eventually have been paved over to be some of the larger roadways in the city. The Aquare Sallustinaea ran between the Pincian and Quirinal hills. This was later paved over to become the Via del Tritone, Barberini, and Vittorio Veneto (Heiken, 2005). Another tributary from the Tiber River flowed over the marshes in the floodplains, mainly between the Capitoline and Palentine Hills. This ancient tributary and a few others are still the paths Romans travel today. Figure 2 shows the basin of the Tiber River.

Ancient Rome Features.JPG Drainage Basin.JPG
Figure 1: Ancient Tributaries of the Tiber Figure 2: Drainage Basin of the Tiber (Aldrete,2007)

Flooding and Commerce
The Tiber River is prone to flooding. The records of these floods are not as accurate as we would like, but there are about 33 noted floods between the years 414 B.C. and 398 A.D. (Aldrete, 2007). The river is 250 miles long. It passes through the Tuscan, Umbrian, and Lazio regions, covering an area of 6,623 miles square. It has 42 tributaries feeding it. The Tiber Basin has a backbone of mountains and is surrounded on two sides by seas, which puts it in a very vulnerable flooding position (Heiken, 2005).

The Tiber River floods because the river swells with excess rainwater and it overflows its banks. The soils under-laying the drainage basin are baked by the hot Mediterranean climate during the dry season, which makes them less receptive to water. They will absorb as much rainwater as they are able, but at a certain point they become saturated. When this saturation level occurs any other water on top of the soil will run off of the soil and into the nearest stream or river. The Anio and Nera Rivers drain into the Tiber, as well the 42 other tributaries. The rainy season comes and fills these extremities, which drain directly into the Tiber river. The river fills and swells with the excess water and it eventually overflows, submerging surrounding and low-lying areas. The surface runoff has also been increased in the basin due to the fact that there is a city there. Cities are full of impervious surfaces that used to absorb rainwater, instead of directing it straight into the river. The runoff from impervious surfaces, such as roofs, roads, or sidewalks, can flow very quickly down narrow streets. Since the force of water is proportional to the square of its velocity (see Bernoulli’s equation), this increased velocity is very damaging to the city.

Another way to consider the extent of the flooding is to look at the rainfall data versus the outflow measurements. See Table 1. A map of the drainage basin where rainfall data was collected is in Figure 2. The Ripetta Station is located in Rome.

Table 1. Rainfall and Drainage Data for the Tiber River (Aldrete, 2007)

Month Rainfall(mm)
from 1921-1990
Drainage(m^3/s) as measured
at the Ripetta Station
January 83.02 316.2
February 90.52 356.7
March 81.25 312.1
April 83.62 259.5
May 80.27 228.1
June 63.42 174.0
July 35.52 139.7
August 49.22 128.4
September 85.9 148.8
October 110.9 178.7
November 129.27 254.5
December 105.13 306.8
Figure 3: Tiber River Delta, with Ostia

It is easy to see that the peak rainfall occurs in November, while the minimum is in July. The river, however, isn’t flowing at its maximum capability until Feburary, while its slowest drainage occurs in August. The drainage for the light rainfall corresponds well, but the heavy rainfall clogs the river for a few months before it can flow out at a fast enough rate. Floods usually grow rapidly, yet drain slowly.

The mouth of the river is where sediment that has taken a journey along the river will most often be deposited. These deposit will eventually build up and thus extend the land. This area of deposit and growth is known as a delta. The delta of the Tiber River is now so large that you need a map to see it. Today, the delta covers an area of 58 square miles above sea level and 193 square miles below sea level(Aldrete,2007). The area occupied by the Fiumiccini Airport used to be the Port of Claudius, a major commercial port of Rome. This port was created by Emporer Claudius in 54 A.D. and extended by Trajan in 110 A.D. to become the largest man made harbor in the world at the time (Aldrete, 2007). The port of Ostia Antiqua was also an influential port and served to help make Rome into a formidable trader and naval power. After the fall of the Roman Empire, the port cities also lost much of their trade, and citizens were forced to abandon them. Seeing Figure 3 below, one could imagine the extents of this city. It would have been bustling with trade as up to 100,000 citizens lived their daily lives.

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Figure 4: Ostia Antiqua, Ancient Roman Port City

There has been some extensive research done in the 20th century on the flooding of the Tiber, by Aiessandroni and Renedia. They have done a thorough analysis on the Tiber basin in correspondence to rainfall. According to their research the river overflows when there are about 90 days of frequent rains, thus saturating the soil. Then, in the following ten days, there are large daily rains that cause a high surface runoff to occur.

The flood of 1937 was notable because it was one of the first times rainfall data was collected in the basin, and then corresponded to the levels of the flood. It rained eight inches in the mountains and basin, which resulted in a flood of 32 inches above ground level, with a flow of 2800 cubic meters per second.

Summary: Functions of the River
How it Helped

As mentioned above, the river played a critical role in the decision for the location of Rome. It became a fruitful trading route, helped move large quantities of goods, such as stones of tuff or travertine, into the city for the creation of infrastructure. Also, before the aqueducts came into creation, it provided a source of reliable and clean water for the citizens. The periodic flooding of the river also helped invigorate the surrounding marshlands, depositing new soil for crops, which animals could then graze on.

How it Hurt

The floods could also have devastating consequences. They could be violent, destroying bridges, homes, and field, in an instant, not to mention the casualties they caused. A particularly heavy flood could leave the city underwater for days at a time, submerging the most popular gathering grounds including the Pantheon, Piazza Navona, and Castel S’Angelo.


Quantity and Coverage

Table 2. Flooding Data (Aldrete, 2007).

Table 2. Flooding Data
Date Source
414 BC Livy 4.49.2-3
363 BC Livy 7.3.2
241 BC Augustine, De Civitate Dei 3.18; Orosius 4.11.6
215 BC Livy 24.9.6
203 BC Livy 30.26.5
202 BC Livy 30.38.10-12
193 BC Livy 35.9.2-3
192 BC Livy 35.21.5-6
189 BC Livy 38.28.4
181 BC Plutarch, Numa 22.4
156 BC Iulius Obsequens 16
60 BC Cassius Dio 37.58.3-4
54 BC Cassius Dio 39.61.1-2; Cicero, Ad Quintum fraterm 3.7.1
44 BC Horace, Carmina 1.2.1-20
32 BC Cassius Dio 50.8.3
27 BC Cassius Dio 53.20.1
23 BC Cassius Dio 53.33.5
22 BC Cassius Dio 54.1
13 BC Cassius Dio 54.25.2
AD 5 Cassiodorus Chonicon 604: Cassius Dio 55.22.3
AD 12 Cassius Dio 58.26.5
AD 15 Tacitus, Annales 1.76
AD 36 Cassius Dio 58.26.5
AD 69 Plutarch, Otho 4.5
Reign of Nerva Sexus Aurelius Victor, Epitome 13
Reign of Trajan Pliny, Epistuale 8.17
Reign of Hadrian SHA, Hadrian 21.6
AD 147 SHA, Antoninus Pius 9.3
Ad 162 SHA, Marcus Aurelius 8
AD 217 Cassius Dio 79.25.5
AD 253 Sextus Aurelius Victor, De Caesaribus 32
AD 371 Ammianus Marcellinus 29.6.17-18
AD 398 Claudian, De Bello Gildonicio 41-43

Piazza Navona Flooded.JPG
Figure 5: Piazza Navona during 1870 Flood

Aldrete provides an explanatory passage for each of the previous dates, and explains why he thinks each was a flood. He needs to provide an explanation for his claim because records from this time are very vague, and most don’t mention the word, “flood”. Instead, they are described as quickly flowing water, or just give descriptions about certain monuments being underwater. The data from this time period is unreliable. In ancient Rome, the flooding was just apart of life; it came, it did some damage, you moved on and repaired. For example, in 13 BC the General Balbus won a triumph, and to celebrate himself he also opened the Balbus Theater. He hoped to parade around the city with Emporor Augustus in attendance. However, when the day of the inauguration for his theater and his triumph came, the city flooded, turning the theater into a temporary swimming pool (Aldrete, 2007).

The flooding of the river reached large portions of the city, including many popular destinations inlcuding Piazza Navona(See Figure 5), Campo de Fiori, the Ghetto, Church of Santa Maria Sopra Minerva, the Pantheon, St. Peter’s Square, and Castel S’Angelo. The earliest quantifiable data comes from marble flood markers, the oldest dating back to 1277. Today, hundreds are scattered around the banks of the Tiber. These are small plaques, located outside of ancient ports and churches in order to mark the height of water during floods. A good example is from one of the worst floods in Santa Maria Sopra Minerva, Piazza di Minerva(see figure 6). The water reached 18.95 meters above sea level on December 24, 1530, Christmas Eve (Aldrete, 2007). This is the second highest recorded flood. The highest flood occurred in December of 1578, when the water reached 19.56 meters above sea level.

Figure 6: Flood Marker