How is breach discharge calculated?

classic Classic list List threaded Threaded
3 messages Options
Reply | Threaded
Open this post in threaded view
|

How is breach discharge calculated?

Márcio
Hi,
I'm trying to check the breach discharge calculated by HEC-RAS 5.0.6.

From reading the User's and Reference Manuals, I understand that HEC-RAS uses the weir formula to do that:
Q = sub * C * B * h^1.5 ,  where:
Q = breach discharge = 2277 m³/s
sub = submergence reduction = 0.72 for a submergence of 95.8%
C = weir coefficient = 1.44
B = breach width = 78.99 m
h = energy head above breach bottom = 9.15 m

According to page 8-13 of the Reference Manual, submergence is defined as depth of water above the weir on the downstream side divided by headwater energy depth above the weir on the upstream side. From the same paragraph, the submergence reduction would come from figure 5-8. That's how I got the submergence reduction above.

Now HEC-RAS gives me two results for the discharge:

Table a) "View -> Profile Summary Table -> Standard Table: Inline Structure" : Q =-2967 m³/s

Table b) "View -> Stage and Flow Hydrographs -> Type: SA/2D Conections -> Tab: Table" : Q = 1655 m³/s

I would like to understand the reasons for the difference between these tables and why I cannot reach the same value in my calculations.

Another data:
-Stage HW = 254.15 m (from Table b)
-Stage HW = 253.77 m (from Table b)
-Breach bottom el. = 245.00 m (from Table a)
-Breach side slope = 0H:1V (from Table a) = retangular breach

I'm using upstream water level (W.L.) instead of upstream energy head (E.G.), but the difference would be small and does not justify the discrepancies I see.

I also tried Q = Area * sqrt(2g*DH), instead of the weir formula, and got better results but still not the same given by HEC-RAS.

More information, such as the (negative) velocity through the breach, can be found in the attached spreadsheet.

Any insight is welcome!
Thanks,
Márcio



HEC_forum_breach_discharge.xlsx
Reply | Threaded
Open this post in threaded view
|

Re: How is breach discharge calculated?

jarvus
The Stage and Flow hydrograph is the answer that RAS computed during Unsteady flow.  This is the flow that is used during modeling.

The profile summary is the more detailed answers that the post processor computed based on the Unsteady flow output.  For the given WSE and flow it may in one sense be more accurate but it isn't the result that was actually used.

If the Unsteady is reporting the stage HW to be higher than the stage TW but the flow and velocity are being reported as a negative number, then I would be suspicious that Unsteady flow is going unstable and you are not getting good results.  It is reporting that it is going to the maximum number of iterations?
Reply | Threaded
Open this post in threaded view
|

Re: How is breach discharge calculated?

Márcio
Hello Jarvus, thank you for the answer.
The global behaviour of the model seems consistent to me, there were no errors and the simulation reaches the end.
I also noticed that "Profile Output Table" seems buggy, since the displayed values are one column displaced from the column titles.
Please see attached image.


jarvus [via HEC-RAS Help] <[hidden email]> escreveu no dia segunda, 1/04/2019 à(s) 17:30:
The Stage and Flow hydrograph is the answer that RAS computed during Unsteady flow.  This is the flow that is used during modeling.

The profile summary is the more detailed answers that the post processor computed based on the Unsteady flow output.  For the given WSE and flow it may in one sense be more accurate but it isn't the result that was actually used.

If the Unsteady is reporting the stage HW to be higher than the stage TW but the flow and velocity are being reported as a negative number, then I would be suspicious that Unsteady flow is going unstable and you are not getting good results.  It is reporting that it is going to the maximum number of iterations?


If you reply to this email, your message will be added to the discussion below:
http://hec-ras-help.1091112.n5.nabble.com/How-is-breach-discharge-calculated-tp7355p7362.html
To unsubscribe from How is breach discharge calculated?, click here.
NAML

Hec_Qtest_outputscreen.png (524K) Download Attachment