HECRAS XSECTION

[kechha2060]

How do I restrict flow only on the main channel for low flow condition without a leeve? It seems "Bank" is used just for "n" value. Shouldn't the bank station recieve any flow if the main channel overtops? Please help

 

[francesca]

I'm not sure I understand the problem you're having, but check out Ineffective Flow Areas.

 

[kechha2060]

Most upstream section is attached. Retaining wall is on the left bank. flow appears at the bottom of the wall?

 

[francesca]

HEC-RAS puts the flow at the lowest elevation. If you have somewhere lower than your channel you need to use ineffective flow areas if water will not flow there at all (i.e. enclosed pond), or a levee if water will only flow there after spilling out of the channel.

You seem to have an interesting condition in that your water cascades down an embankment and is actually lost from your system if it overtops the channel. Is this a diversion ditch at the top of an embankment and you're calculating the 100-year flow in the ditch to make sure it doesn't overtop? If that's the case, I wouldn't even bother including the retaining wall in the model and just stopping the x-section at the top of the retaining wall. HEC-RAS will throw an error message if your flow goes higher than the limit of the x-section.

 

[kechha2060]

Thanks franc,
A bulding pad is proposed at the bottom of the wall.
what is the recommended value for riprap? any idea.

Thanks

 

[francesca]

Hmm it seems a bit late to be checking that the ditch is sufficient now! The Nashville Storm Water Manual has a formula somewhere to calculate your Manning's value...

http://www.nashville.gov/stormwater/regs/SwMgt_ManualVol02_2000.htm

Chapter 3-17... n = 0.0395(d50)^(1/6) where d50 = diameter of stone for which 50%, by weight, of the gradation is finer, in feet.