Controlled weirs in HEC-RAS 3.1.3

[hgb1]

Hello,

I would like to model a dam in HEC-RAS 3.1.3 using unsteady flow.

The upstream boundary has different flows (Q) VS. time. The water level has to be constant, because of a retention water level elevation. Therefore the weir has to be a controlled weir. This means the weir opening has to be larger when more flow comes into the model and smaller when Q is lower. This assures the constant retention water level elevation.

Is it possible to model such a controlled weir in HEC-RAS 3.1.3? Unfortunately I could only read about uncontrolled weirs in the users manuals.

Have a nice day!

 

[blueoak]

I think you have 3 choices:
1. Use a gate for the model (like a radial gate on large dams)
2. Just try setting the weir coefficient so high that your water level change is minimum.
3. Set the width of the weir very wide. You will have some elevation changes but more like a labyrinth weir.

Also, if your weir is the downstream boundary you can enter a rating curve under the unsteady flow editor as a boundary condition.

I am interested in how it is supposed to be done though.

 

[hgb1]

Thank you very much, blueoak.

I must confess that I did not try it out. I found out that a steady flow model for my problem is absolutely adequate, because of the use of the flood design (Q1000).

This task was given within a lecture of my study. The aim is a preliminary draft for a weir size with a water-power plant. Therefore I have to calculate the hydraulics manually. I just wanted to use HEC-RAS to get an idea how it is looking and to become more familiar with the appliance.

The interesting point is that there has to be supercritical flow directly behind the overfall. But the results from HEC-RAS show subcritical flow. Also, the critical depth is not plotted on the longitudinal section behind the weir, only respectively one point on the very left and on the very right.

Of course, I am interested to solve this problem with HEC-RAS, in order to become more familiar. But it is not absolutely necessary, because my manually calculated results are correct.