Let’s get a basic idea of what inlet control is from a culvert hydraulics standpoint and compare it to outlet control. When you have a flow through a culvert, there is a limit on the capacity of fluid conveyed through the culvert. Inlet control applies when the culvert barrel has more capacity than the inlet will accept. Outlet control is the opposite and happens when the inlet can accommodate more flow than the culvert barrel has the capability to convey.

Think of it as cash flow. Most of us probably experience inlet control. We dream of the big house and fancy cars. Outside the dream, we realize that we can’t afford that lifestyle. Our income or “inlet capacity” is not at that level. If we tried to live that life, our cash flow would “supercritically” flow out of our life.

Now, imagine you just want a small ranch house and you ride a bike to work. Perhaps, you have the same income or “inlet” as before, but you would rather take more time off over a pay raise. You might say that your outlet controls your income.

Alright, we have a general understanding on the two options of control here. Which one is simpler to change, meaning which one would have the lesser number of variables to adjust to change and make more efficient while still maintaining the same control? The answer is inlet control.

Simply put, inlet control means that the culvert inlet is restricting the flow. You could change the barrel and outlet, yet there wouldn’t be a meaningful effect on the headwater depth. If you want to make any improvements, it must involve the inlet’s ability to channel flow into the culvert. The table below provides some values of entrance loss coefficients for common inlet end conditions. A comparison of the entrance loss coefficients shows that a culvert with a common metal end section can accept nearly twice as much flow at the inlet as a pipe with no inlet improvements. 

Think back to our dream. Without selling everything, how do we improve cash flow? Simple, we increase the inlet of cash in our dream and we are back to our comfort zone.

If you look at the outlet control variables, several variables could change the water head. There are the properties of the culvert like length, slope, and friction factors. Then you also must consider anything that follows the outlet and the tailwater. For example, we are trying to improve our energy usage in our ranch by switching to LED lightbulbs. Then we realize there are many variables that affect the energy usage of a house.

Those analogies might feel like a stretch, but hopefully they added a little clarification and understanding to the ideas of changing inlet and outlet flow control.