Q full vs Q dynamic vs Q normal

Mr. Dickinson, I´m sorry but I can understand that theory.

It´s true that when you have dynamic flow exist a force that can increase the capacity of the pipe because the slope of the water surface is diferent to the bed slope.

However that it´s true only if you can be sure that the critical depth is the boundary condition, but can you be sure about that if you are not sure about the capacity of the pipe?

If the critical depth is the real boundary condition for the flow you can find that in this example the pipe start to be in a pressure condition after a flow greater than 31.5 m3/s, it means that the capacity will be increase in 1.6 times the maximium capacity using the Manning equation. It means that we are overdesigning all the pipes when we use the permanent flow equations?

In other hand if you always use a critical depth as a boundary condition you can see that the critical depth for a flow of 100 m3/s is 3.64 m so even with that flow you have 11 cm of free space at the end of the pipe. I can´t believe this situation.

I think that the problem is with the boundary condition because you can´t assume a critical depht for a pressurized flow. In that way. If the boundary condition is not the critical depth, the water surface at the end of the pipe will be increase so the slope of the water surface start to decrease. If this situation happens you are going to obtain after some time a energy slope equal than the bed slope an the capacity will be again the maximum capacity estimated with the Manning equation.

However if I´m wrong and the capacity estimated with the dynamic flow equations is greater than the capacity using the permanent flow equations, i´m interesting to know the answer of these questions.

Why if in this example we have a constant flow the results and the capacity are different if you use the dynamic flow equations and the permanent flow equations?

How can I calculate the capacity with the dynamic flow equations, without the model, to be sure that the flow is not pressure and the boundary condition is the critical depth. Otherwise I think that if you don´t know if the flow produce a pressure condition in the pipe, you can´t assume a critical depht as a boundary condition, so the results of the model are untrustworthy.

Thanks for your answer.

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Comments

Edgar Javier Guevara Garzón March 27, 2009 at 10:14pm

Mr. Dickinson, I´m sorry but I can understand that theory.

It´s true that when you have dynamic flow exist a force that can increase the capacity of the pipe because the slope of the water surface is diferent to the bed slope.

However that it´s true only if you can be sure that the critical depth is the boundary condition, but can you be sure about that if you are not sure about the capacity of the pipe?

If the critical depth is the real boundary condition for the flow you can find that in this example the pipe start to be in a pressure condition after a flow greater than 31.5 m3/s, it means that the capacity will be increase in 1.6 times the maximium capacity using the Manning equation. It means that we are overdesigning all the pipes when we use the permanent flow equations?

In other hand if you always use a critical depth as a boundary condition you can see that the critical depth for a flow of 100 m3/s is 3.64 m so even with that flow you have 11 cm of free space at the end of the pipe. I can´t believe this situation.

I think that the problem is with the boundary condition because you can´t assume a critical depht for a pressurized flow. In that way. If the boundary condition is not the critical depth, the water surface at the end of the pipe will be increase so the slope of the water surface start to decrease. If this situation happens you are going to obtain after some time a energy slope equal than the bed slope an the capacity will be again the maximum capacity estimated with the Manning equation.

However if I´m wrong and the capacity estimated with the dynamic flow equations is greater than the capacity using the permanent flow equations, i´m interesting to know the answer of these questions.

Why if in this example we have a constant flow the results and the capacity are different if you use the dynamic flow equations and the permanent flow equations?

How can I calculate the capacity with the dynamic flow equations, without the model, to be sure that the flow is not pressure and the boundary condition is the critical depth. Otherwise I think that if you don´t know if the flow produce a pressure condition in the pipe, you can´t assume a critical depht as a boundary condition, so the results of the model are untrustworthy.

Thanks for your answer.

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