Modeling Tidal Exchange through culverts and Inline Storage

[maputnam]

Hello,

I am working on modeling tidal exchange through culverts to an upstream storage area (I have attached a sketch showing what I am trying to accomplish). I have completed the construction of the test scenario shown in the sketch and have assigned a stage hydrograph as the downstream boundary condition and the downstream cross sections and culverts behave as expected and oscillate with the tide. However, the upstream storage node stays constant at the elevation prescribed in the initial conditions portion of the unsteady flow file and does not oscillate with the tide. As shown in the image below the initial elevation of the storage area is set to the starting tide elevation (2.396) so that the water surface elevations on the upstream and downstream ends of the model are the same. Is this a feasible application of the model or is what I am showing not possible in HEC-RAS?

As shown in the attached sketch the first goal of this model is to be able to model and quantify tidal exchange and stages within the swamp/creek upstream of the roadway embankment. Consequently, the next obstacle to be overcome is that the most downstream culvert crossing has flap gates installed on 1 out of the five culverts under the roadway (one of them broke off and is no longer functioning). The ultimate goal is to be able to remove all of the flap gates from the other culverts and see what, if any, difference is experienced after the gates are removed.

Thank you in advance, any help would be greatly appreciated.

Mike

Model_Schematic.pdf

[Anonymous]

This is a doable application of RAS. The boundary condition for the storage area should be inflow. This could be a flow hydrograph if there is contributing area to the storage area, or zero inflow if that is appropriate. With tidal simulations you need to “warm-up” the system by running a few tidal cycles until the flows and stages repeat, then run the storm hydrograph (if any).

[maputnam]

Michael,

Thank you for your response. I do have a couple of questions regarding the boundary conditions. At first this model is intended to be “dry weather” with rainfall added in during subsequent analysis. If I add inflow as a boundary conditions during dry weather, would I be specifying the inflow from the downstream culverts, and not letting the program calculate the inflow to the storage basin based on the culvert hydraulics? (See Image Below)

Also, when I specify the initial elevation for the storage node the water surface elevation of the storage node does not change while the model is running. If I do not specify an initial elevation the model will not run. (See Image below)

My approach thus far has been to use inline storage. However, after thinking about this would it better to use offline storage and a lateral structure for use in analyzing this reach? (See Image Below)

Finally, I still have the need model flap gates on the downstream culverts. Is there a way to do this in RAS?

Thanks in advance!

Mike

[Anonymous]

The initial elevation of the storage area must be specified, as you found out. If the water surface in the storage area is not changing with the flow thru the culvert, I suspect that there is a connection error. Check that the reach containing the culvert is connected to the storage area.
Flap gates are simulated in HEC-RAS. (See p. 8-15 of the RAS 5.0 Reference manual.) The type of gate is selected when defining an inline or lateral structure.
Michael

[maputnam]

Michael,

Thanks for all your help. The inline structure works for modeling the culverts with flap gates save for 1 of the 5 culverts under this roadway in which the flap gate broke off. Since the software will only allow a blanket application of flap gates on culverts I modeled the 5th culvert with the broken flap gate with a sluice gate. Also, I figured out what was going on with the storage area and the elevation not fluctuating. Turns out I had the wrong units for the storage volume. I had thought that I set my spreadsheet to output in Ac-ft but it was set to cubic feet instead. This made the available storage in the reservoir extremely large and as such its elevation was not changed very much with tidal influence.

One further question, Is it possible to place a storage area in-line along a reach instead of just at the top of the reach or bottom of the reach?

Thanks,

[Anonymous]

You can have multiple reaches connected to a storage area. Therefore, you can have one reach entering and one leaving a storage area.

[Author]

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