In ancient times, the storm sewers diverted rainwater flowing down from Mount Rahmat away from the platform on which Persepolis is built, thus saving it from being undermined.
Apart from the sewer system, archaeologists have identified three water and irrigation systems at Persepolis, which circulated water over the platform. The Achaemenid engineers constructed the sewer system under the platform before construction of the citadel. The oldest sewage system at Persepolis is dated to the reign of Darius the Great (reigned 550-486 BCE).
Archaeologists believe that by digging out and re-opening the ancient storm sewer system, they will be able to resolve the flood issue that Persepolis has been suffering from in recent years.
The sewer system constructed beneath the platform had a very systematic structure. About two kilometers of the sewer line were discovered during previous excavations. The ducts vary from 60 to 160 centimeters (24 to 31 inches) in width.
Last year, some archaeologists warned the authorities that if no measures were taken immediately to resolve the flood issue, the edifice might be undermined within 10 years.
The penetration of rainwater inside the platform, which has formed a large pool, as well as severe dampness have caused the stonework to crack and crumble.
This is mainly due to the fact that the sewer system constructed by the Achaemenid engineers has been blocked. Critics also contend that subterranean waters generated from the lake formed behind the Sivand Dam, which opened a few years ago, have caused a rise in dampness.
An anonymous staffer with the state Cultural Heritage Organization told the Mehr news agency last year:
“Rainfall is quite heavy on the platform and the rainwater from Mount Rahmat also flows into Persepolis. As a result, year after year, the water gathers and has turned the platform into a massive water reservoir. In addition, the subterranean waters have increased and, since the waters inside the platform have nowhere to go, they have begun to penetrate the foundation of the structures built over the platform and then evaporate through the stonework.”
The Circle of Ancient Iranian Studies (CAIS) in Britain has said the dampness has caused chemical weathering that has brought corrosion and decay, caused the stone masonry walls to crack and gradually crumble. It says lichen are growing and salt residues have appeared on the stones throughout Persepolis, particularly in the Treasury Hall and southwestern section of the structure called the Harem. In addition, the mud-brick structures act as a sponge and are commonly wet.
CAIS says all this has affected the detailed features of the site, with the surfaces of bas-reliefs gradually being worn away layer by layer.
The expert quoted by Mehr said, “Another reason for dampness in the foundation is that pebbles have been spread over the platform so visitors do not walk in the mud during the winter. As a result, less water evaporates from the surface.”
He said: “There are a number of ways to tackle the problem. First, clean and reopen the blocked ancient sewer system, then create small water-channels throughout the platform to direct the water toward the sewers. Also, separating the stonework from the mud bricks, though a massive task, would be effective to prevent passing the water from one to the other.”
Many critics of the government assert that the regime does not care about Iran’s pre-Islamic past and would just as soon see Persepolis fall apart, and therefore will not invest in preservation.
There are undoubtedly many within the regime who feel that way, but there are many others who do not. The Cultural Heritage Organization devotes effort to pre-Islamic sites, and tourism officials see sites like Persepolis as a huge profit center for the government.
A major problem is inertia within the government. A computer search by the Iran Times found officials talking about the need to re-open the Persepolis storm sewer system at least as far back as July 2005. But little has been done despite the passage of seven years.