What characterizes a free outlet in hydraulic engineering?

• A. An outlet where the tailwater is lower than the critical depth
• B. An outlet that allows maximum flow at all times
• C. An outlet that restricts backflow during heavy rainfall
• D. An outlet only used for drainage purposes

Answer: (A)

A free outlet in hydraulic engineering is characterized by the condition where the tailwater is lower than the critical depth. This condition ensures that the water flows out of the outlet freely and unobstructed, as the back pressure exerted by the tailwater is minimal. When the tailwater level is lower than the critical depth, the flow can be considered as free flow, enabling the outlet to operate efficiently and at its design capacity.

In this scenario, critical depth refers to the specific depth of flow at which the energy in the water is minimized, creating optimal conditions for water to exit the outlet without being impeded by downstream conditions. The feasibility of maintaining this state is crucial for the effective design and management of hydraulic systems, particularly in managing stormwater and ensuring that flooding risks are minimized.

The other options do not accurately capture the essence of what defines a free outlet. For instance, maximum flow at all times is not necessarily linked to the specific condition of the tailwater being lower than critical depth. Similarly, the restriction of backflow during heavy rainfall does not define a free outlet and instead ties into different drainage system considerations. Lastly, limiting the use of an outlet solely for drainage purposes does not encompass the broader context of free flow principles in hydraulic systems.