Adding lids to only one cross-section.

[Anonymous]

When a lid is added to only one cross section, what is the effective width being modeled?

Say there are cross sections XS-1, XS-2, and XS-3 (Downstream to Upstream). Assume XS-2 is 100 feet from XS-3 and XS-1 is 100 ft from XS-2. If a lid is applied only to XS-2, what is the deck width being modeled? Does the lid

1. act like a thin sheet blocking the flow,
2. have a width of 100 feet since the next downstream XS is 100 feet away, or
3. have a certain minimum width?

[Anonymous]

I don’t think a lid is treated any differently than any other change in the cross section. For instance, if XS-3 has a channel that is 100 feet wide, XS-2 has a channel that is 50 feet wide and XS-1 has a channel that is 100 feet wide.

Depending on whether you are doing steady or unsteady flow and user options: There will be a velocity that is computed at each of the 3 cross sections. It will compute a contraction and expansion loss using these velocities. Using Manning’s equation, there will be a friction slope at each of the three cross sections. And from that, friction losses are computed depending on the friction method that is selected.

So RAS isn’t really assuming how wide the deck is. But to answer one of your questions, if the water hits the lid at XS-2, it will affect the computed losses from both XS-1 to XS-2 and XS-2 to XS-3.

I guess as a rule-of-thumb approximation, you could think of it extending halfway from XS-1 to XS-2 and XS-2 to XS-3. And if you wanted to more explicitly define the limits of the deck, you could interpolate additional cross sections and choose which ones to attach the lid to.

If you want to dig into it, the RAS Hydraulic manual has quite a bit of info.

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