Shape of concrete water tanks

Which are the optimum dimensions of a water tank built with concrete? This is a very common question posed during the design of water structures. A generally accepted thumb rule (Ref.1) is the following one:

  • Volume less than 500 m3: Rectangular+Reinforced concrete. Height=2-3m
  • Volume between 500 m3 and 10.000m3: Circular + Reinforced concrete. Height=3-5m
  • Volume more than 10.000 m3: Circular + presstresed concrete. Height=5-8m
Additionally, it can be said that one tank is cheaper than two tanks with half of the capacity as it is shown in the figure.

Cost of the tank with cover per cubic meter of water (Ref.2)

Optimum Diameter/Height ratio (Ref.2)

However, there are some objections to the given thumb rule. Hydraulic constraints must be taken into account for the final design because water machinery and gadgets (I mean pumps, pipes, valves, expert and remote control systems and such) may be more expensive than the structure. What is more, cylinders are more difficult to be built than straight walls and, in fact, my latest design was an enormous reinforced concrete retangular structure to store 30.000 m3 of clean water.
Floor plan
Cross section