Can you put a stop at end of hec-ras channel to force all flow over the lateral weir

[Aileen Gleeson]

Hi Please can someone help me.

I have a spill way channel that flows through the channel and also laterally over a weir into a storage area. By the end of the channel I have increased the height to put a blockage in the channel, however the WS keeps on building up until it breaches this blockage as I increase the height the WS increase’s.

The channel is designed so that all flow will flow over the weir, eventually.

Even when I remove the storage area and allow to flow out of the system the WS height is still the same this cannot be right.

Please Help

[Lonnie A]

You might try tying the end of the reach into a storage area that has a small volume, then have that storage area tied to the dam with a connector.

[Aileen Gleeson]

Thank you for that I am trying to add a small storage area equivalent to the drive in assess to the channel.

Another question is what are the units for the storage area, maybe this could be why my model is not working. My view does not tell me the correct units, I have tried to research but cannot find it.

[Aileen Gleeson]

Apologise I have a question that is possibly going to sound very stupid, but how can you connect the end of the reach to the storage area at the end, the only was to connect the reach to the storage area is laterally, can you finish a reach in a storage area.

[Lonnie A]

units are 1000m2 or acre

To connect reach to storage area use the Edit->Move points/objects and drag the last point of the reach into the storage area. RAS will then ask if you want to connect it.

If you find stability issues once you get it connected:

You will likely need to set the initial conditions of the SA to be close to the elevation your channel WSE (without the connection) at the initial model start time.

You may find increasing you SA area a little can help with stability as well.

[Aileen Gleeson]

Thank you Lonnie,

I finally got it run correctly.

I am finding the height of water flowing over the weir higher than originally anticipated based on hand calculations, roughly 100mm higher, over a 85.5m long weir that appears quite high. I ended up using the optimising tool so Hec-Ras calculated the actual flow leaving over the weir.

When I change storage area sizes it actually has zero change on the WS which seems very strange, I have gone to extremes to change the storage areas. Could there be a reason for this.

I entered the Storage area elevations for the Process water dam as the height that is constant in the dam and for the storage area that the reach enters into I set as the ground level in the storage area. Is that correct.

Apologise for all the questions I am really new to all of these functions in Hec-Ras.

Thank you again

[Lonnie A]

Aileen,

I would have to see the model to determine if there is something wrong with the model and advise on potential changes. You are running this in unsteady correct? Changing volume should of had a impact in unsteady but wouldn’t have if ran in steady.

your model schematic doesn’t look very large so you should be able to upload it here or if you want email it to me.

Lonnie

[cameron]

What are you using for your weir coefficient on the lateral structure?

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