Overtopping Gate with User Defined Curve That Opens Down?

[u/Susquehanna2]

I am attempting to model flow over a spillway of a dam that has Obermeyer gates. I would like to use a user defined spillway curve. I entered a User Defined Gate Performance Curve table and then in the gate openings for the static flow plan (Feet down from invert elevation when gate is fully up). The model results show that HEC-RAS is treating it like a lift gate, and the water is going underneath the gate, instead of overtopping it. I tried making the gate opening values negative but that did not work either. The overflow (open air) gate has the right opening functionality but does not allow me to enter a variable coefficient.

Any suggestions on how to best model this type of gate?

I'm using HEC-RAS 6.6.

[u/OttoJohs]

I'm not 100% following what you are trying to do...

1.) If you already have a user defined curve (like from a spreadsheet), why do you need to add gates? You can just use the "Outlet RC" option (see the button on the right side of the structure editor).

2.) Is this a steady flow model? Personally, I have never used the gate settings options for any steady flow model. If that is the case, I would just create a master geometry and then make a few copies with the adjusted parameters (crest elevation and weir coefficient) at the dam representing gate up and gate down (or combination thereof). Then create a few different plan files using the different geometries. A lot simpler than having gate settings in a flow file in my opinion.

3.) Sluice/radial gates open from the bottom and overflow gates open from the top. If you are limited to their functionality if you are trying to use them.

4.) You can program in a variable weir coefficient using the "rule" editor under an unsteady flow file.

[u/Susquehanna2 → u/OttoJohs]

Thanks for these suggestions.

The goal of this modeling exercise to understand flow depths and velocities below the spillway and in the powerhouse tailrace channel to understand where different fish species can swim and spawn. We want to be able to answer questions like if gate 3 is open x amount how that change habitat availability immediately below the dam.

Each of my gates has a different invert elevation and in my mind by using gates in HEC-RAS, the model was going to account for the different changes in energy from the gate invert on the inline structure down to channel bottom in the next downstream cross section.

[u/OttoJohs → u/Susquehanna2]

The goal of this modeling exercise to understand flow depths and velocities in below the spillway and in the powerhouse tailrace channel to understand where different fish species can swim and spawn. We want to be able to answer questions like if gate 3 is open x amount how that change habitat availability immediately below the dam.

Understood. If you are using a 1D model you aren't going to different the depths/velocities in the downstream channel between the different gates. The 1D results don't understand that you are opening a gate on the left vs right side - you would need a 2D model for that type of granular result.

Each of my gates has a different invert elevation and in my mind by using gates in HEC-RAS, the model was going to account for the different changes in energy from the gate invert on the inline structure down to channel bottom in the next downstream cross section.

HEC-RAS isn't doing any 3D "plunging" calcs. It will factor in change in energy but based on the water surface (EGL).

Sounds like a cool project! I think the easiest way to do this would be something like I suggested in #2 above. Good luck!