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RE: Retaining Wall, drainage issue

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to me, the open channel flow is way more complicated than Q=AV or Manning
equation or any other single equation.
in this case we have a "Spatially varied flow" with increasing Q. please
refer to Open Channel by Chow chapter 12.
but the fact is plan checker is most probably not interested into this
detail. He probably wants to see the tributary area, max. discharge and
simple Manning equation.
if you just want to get the permit, you can go ahead and
1) specify the tributary area
2) estimate the time of concentration
3) specify the precipitation (I in/hour) based on time of concentration.
4) specify the Runoff coefficent (C it depends on your soil type, slope,
vegetation, ...)
5) calculate the Q=CIA
6) assume a section for a ditch
7) calculate your min. speed using Manning equation and min. slope along the
ditch profile.
8) calculate your ditch capacity Q=AV
9) if greater than the Q in step 5 you are OK, if not enlarge your ditch or
try to increase the slope.
this is the procedure for couples of counties in CA. I do not know where
your project is, but you can check the Hydrology Manual for the area.

I hope this helps


Henrik Nazarian, P.E










-----Original Message-----
From: SDGSE(--nospam--at)aol.com [mailto:SDGSE(--nospam--at)aol.com]
Sent: Tuesday, July 11, 2000 11:00 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Retaining Wall


In a message dated 7/11/00 7:52:03 AM Pacific Daylight Time,
smithegr(--nospam--at)bellsouth.net writes:

<< Q = V*A.  In order to determine V, you must know the area, A.  Area is a
 function of the depth, so as you will quickly find out, there are infinite
 combinations of V and A for every Q. >>

I hate do disagree, but isn't Q=AV for flow in a pipe under pressure rather
than an open channel.

Oshin Tosounian, S.E.
Los Angeles, CA

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