Reliability Analysis of the Nezahualcoyotl Dike in the Aztec City of Tenochtitlan
G.A. Torres Alves (TU Delft - Civil Engineering & Geosciences)
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Abstract
Before the Spanish conquest of Mexico, around the year 1519, the Valley of Mexico was a closed basin. As a result, at the bottom of the valley, an extensive system of shallow lakes, lagoons, and swamps was formed due to precipitation and permanent river’s discharge from the Sierra Nevada mountains. This lacustrine system occupied around 1000km2 of the total surface of the valley. Lakes Zumpango, Xaltocan, Chalco, Xochimilco, Texcoco, and Mexico were distinguished. The capital of the Aztec empire, Tenochtitlan, was founded and built on an island in the middle of Lake Mexico.
The Aztecs were known for their impressive constructions and hydraulic structures. At the time of the Spanish conquest, they had a complex system of approximately 95 hydraulic structures (Palerm, 1973), of which the most impressive one was the Nezahualcoyotl dike. This structure was roughly sixteen kilometers long, eight meters’ height and three and a half meters’ width. Its principal function was to protect the city of Tenochtitlan from high water levels in Lake Texcoco.
Nowadays, there are no remains of the dike and most of the lakes were drained. The purpose of this thesis is to characterize the lacustrine system and the Nezahualcoyotl at the time of the Spanish conquest of Mexico City dike by using historical documentation and present-day climate and terrain data. This in order to assess the reliability of the dike as a flood defense mechanism and to compare it to modern safety levels. The dike was tested for one failure mechanism: Overflow. A Markov chain and Copula models are proposed in order to create a synthetic time series of precipitation and evaporation. Through a hydrological balance, the water elevation at Lake Texcoco was obtained. In this way, it was possible to provide an estimation of the water level fluctuation in the lake each year during the wet season. In total, a thousand years of synthetic data were generated.
To the author’s knowledge, this is the first time that an attempt is made to compare the Aztec design criteria with present time standards. This research illustrates, from an engineering point of view, the possible design criteria of the Nezahualcoyotl dike and the uncertainties surrounding it. This work can be used as a guideline to assess the reliability of other ancient structures or present-day constructions all over the world whose design is largely based on informal criteria where information for the reliability assessment is scarce.