BUILDING THE ULTIMATE DAM

From CHAPTER TWO (pp. 13-41):

The world's earliest dams depended on massive bulk to hold back water, and their builders did not utilize mathematical formulas to calculate the best theoretical shapes to serve this purpose. Instead, basing their structures on known precedents, or perhaps relying entirely on intuition, the builders would pile up earth, rock, timber, and so on until the structure's weight (or mass) could resist the force exerted by a reservoir without sliding or tipping over. Gradually, a massive tradition of dam design developed around this process, fostering construction of structures that resist hydrostatic pressures merely because of their size.


Although designs exemplifying the massive tradition can be based on sophisticated engineering analysis, the basic principle underlying the tradition is simple: accumulate as much material as possible, thus increasing the likelihood that the structure will achieve long-term stability. Largely due to this conceptual simplicity, massive dam technology was employed for the earliest known dams; at the turn of the twenty-first century, most of the world's water storage structures remain gravity dams in the massive tradition.


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In contrast, the structural tradition [of dam design] relies less on bulk than on a dam's shape to resist hydrostatic pressure. Consequently, adding bulk to a structural dam does not necessarily increase its strength. While dams built in the structural tradition (namely, thin arch dams and multiple arch buttress dams) require less material than comparable massive structures, the principles underlying their operation are not immediately obvious and they have not been built in large numbers. Nonetheless, the structural tradition can be traced back to the days of the Roman Empire, when Roman engineers built the first known arch dam and several buttress dams.


After the fall of Rome, the structureal tradition (as well as large-scale dam building in general) waned, but it did not disappear completely. For example, during the thirteenth and fourteenth centuries, Ilkhanid Mongols in Persia built some of the most noteworthy arch dams of the premodern era, including the 190-foot-high Kurit Dam, which stood as the world's tallest dam for over 500 years. By the beginning of the twentieth century, a resurgence of interest in the structural tradition was under way; John Eastwood's efforts to promote the multiple arch dam consitute a part of this movement.

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