In this case, we can notice how the higher section of the gallery has a circular shape: this is not by chance. If we think about our house pipes, their sections are rounded. This is because the tubular section is the one that can transport the highest volume of water in the least space. As sometimes happens in history, humanity thinks to have invented something new, but in fact, we only copied Mother Nature.
In this case, we can see that the upper part of the section is still rounded to testify the first phase, represented by the flow of water at full load. The high pressure, likely to be at about a few thousand cubic meters per second, dug the section. In a secondary phase, the water regime decreased, and the water flow looked similar to what a free river flow looks like. In this phase the corrosive and erosive process continued and the section was eroded along is lower sides, giving it the classing keyhole shape that we see today.
