HEC-RAS Question 1

[chaa]

I'm doing a waterway analysis using HEC-RAS.
An increase of the channel crosssections in a certain reach of the stream leads to lower velocities but an increase of the water surface elevations. I can't really explain why the water surface is higher. Can anybody help?
Thanks.

 

[Rookie2]

One possible reason, is that if you are analyzing using subcritical flow regime, the program will default to critical depth in when it can't find a subcritical solution. This would make sense for the situation you are describing. The flow is critical in the upstream sections and then goes to subcritical in the wider section downstream. I have seen this on channels with low slopes. Just my thoughts.

 

[chaa]

Thanks Rookie2 for the replay. The flow regime is subcritical in both situations.

 

[Rookie2]

For the replay?

 

[chaa]

Sorry. I mean, reply.

 

[Rookie2]

Sure; If the flow is subcritical in both cases then that doesn't make sense to me either. I am assuming when you say an "increase in the channel crossections" you mean that the downstream cross-section is larger than the upstream cross-section. The calculated flow regime is the most obvious answer, but if that's not the case then you would probably need to provide a little more information. I'm sure there is someone here that could answer your question.

Good Luck

 

[chaa]

By increase of cross sections I mean a new sream alignment; excavating the channel at one of the banks. This is because the structural engineer would like to eliminate/reduce a flow impact area.

 

[cvg]

does the slope decrease or roughness increase in the downstream reach?

 

[Rookie2]

Oh, I totally misunderstood your question. As CVG says it is probably due to a change in slope and/or roughness coefficients.

 

[francesca]

Decrease in velocity means a decrease in velocity head and an increase in potential head (i.e. depth).

 

[chaa]

Thank you all. Francesca, I understand what you said, but...Does it mean that in most cases when you increase a waterway cross section you reduce velocity and increase water depth? For uniform channels doesn't happen.
As a test, I used an old natural waterway (brook) HEC-RAS model and at two locations in separate runs I increased the cross section. The same thing happened; lower velocity, higher depth, associated of course with higher energy grade elev. and slope, larger flow area and smaller Froude #. My problem is that I still can't picture the situation. I apologize for my ignorance.

Again, my flow is subcritical, the slope doesn't really change and roughness is the same.
Thanks.

 

[francesca]

The principal of conservation of energy requires depth to increase when velocity decreases. I don't know what you mean by for uniform channels it doesn't happen. If the depth stays the same when velocity decreases there is something else which has reduced the total energy - e.g. a meander, increased friction, entry loss/exit loss, etc.

 

[Rookie2]

I'm curious, at the cross section immediately downstream of this area where you are improving the stream, does the elevation change between the two models (before improvements, and after). And what is the flow regime at the downstream cross section?

 

[Rookie2]

Just to clarify, when I say elevation I mean water surface elevation.

 

[chaa]

I appreciate a lot your effort and the time you guys spend in trying to help. The flow regime at the immediately downstream section is also subcritical and the water surface elevation does not change between the two models (before and after improvements)- the water surface elevations are calculated from downstream to upstream.
Using Manning for a uniform (say trapezoidal or rectangular channel) for the same discharge increasing the channel width leads to smaller velocity (larger area) and water depth, isn't it? Where is my confusion Francesca?

 

[francesca]

You are basing that entirely on the continuity equation, and supposing that flow area alone determines velocity. In fact, the energy equation trumps the continuity equation in establishing water surface elevation, which is set by the hydraulic grade line.