Final answer:
A bulge in the water tank's vertical side is likely due to it being made from flexible material, high pressure, or insufficient support, rather than being overfilled.
Step-by-step explanation:
A bulge in the vertical side of an open water tank could be the result of various factors, including but not limited to, the tank being made of a flexible material, exposure to high pressure, or lack of proper support. If a tank is overfilled, it is more likely to overflow rather than bulge. A bulge might indicate that the sides of the tank cannot exert an upward force on the fluid due to their inability to withstand a shearing force, as demonstrated in Figures 11.8 and 11.13, where the bottom of a container supports the entire weight of the fluid in it. To prevent leaking or bursting due to tightly constricting the expanding gas, rigid containers often have air gaps to allow them to expand and contract without stressing.