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RE: Retaining Wall

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> Since the area that is draining is not on the property,
> the owner has no survey or map that allows me to
> figure this area out.

Depending on the area of the country you are in, and the size of the
municipality, the City may have 2' contour maps of the area.  If not, you
should be able to obtain a 7.5' or 15' "Quad" map from the USGS.  These are
typically available from your local map store.  Using these items, you can
roughly determine the contributing drainage area to your wall.

>  1.Is this really necessary? The client is improving the situation and the
>  wall has drain rock, perforated pipe, and the v-ditch at the top. I don't
>  want to relearn this stuff and still be unsure of what I am doing.

Whether or not this is necessary is between you, the client, and the plan
checker.  It sounds like the plan checker required the information, so if
your client wants a permit, it is probably necessary.  It is pretty straight
forward stuff, you may get a civil-civil buddy to run the numbers for you.

>  2. If I do calculate Q, I assume that I divide Q by the width of my ditch
>  and the velocity or the runoff along the ditch to figure out how deep the
>  water gets in the ditch so it doesn't spill over the front of the wall.
>  Also, what is a reasonable V to assume, I was thinking 3-5 ft/sec.

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.  for the length of the wall provided, I
would assume what is called normal depth.  Also, you can not assume V.  You
calculate Q and determine V and A for a specified channel profile.  If V is
within an acceptable range, your done.  If not, change the channel profile.
5 ft/s is rather fast for an unlined channel, but that will depend on your
soil characteristics.  Try to stay between 2 and say 3.5.

>  3. Is this really necessary???


Brian K. Smith, P.E.

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