Thanks for all the responses. The earlier thread, with lots more
details about the situation, is here: http://tinyurl.com/372uspu .
That's the novel I referred to earlier. (Yeah, it's Google Groups, but
I think that's fine as an archive.)
Harry: "laser level" ... hmm, interesting. Would a laser level in the
price range appropriate for this project ($50 yes, $500 no) be
accurate enough, and usable outside in daylight? I see some that claim
20', also reviews saying sometimes this is pushing it even inside. I
have no experience with them. My drop is about 3" in 20'. If a $20 or
$50 laser level would even put me within 1" to 4" in 20', that would
be good enough for this job. Equivalently, I need a slope of about 0.7
degree. If a level could get me within 0.25 to 1.0 degree, that would
do the job.
Hmm. One of the NDC docs that Mike links to indicates that 1/8 in 12
is enough slope ... which puts me within striking distance.
(I'm sure that if I paid a couple of grand, I could get it within a
mm. Not happening for this project.)
Hank: thin wall PVC, glued, 4". Corrugated crap is what I'm replacing.
I even considered 6", but that's probably overkill even here and would
have quadrupled the cost, not to mention being a lot harder to work
Let's see ... 1" of rain gives me about 1800 gallons of water. I
probably fairly often get rates over 1" per quarter hour (though
seldom for more than about five minutes). That's a flow rate of 2
gallons per second. 4" should handle that, but what pressure is
required? If I can answer that, I can figure out how much riser I need
below the downspouts to push the water through the almost-level part
of the drain pipe fast enough. Hmm.
I looked at the flow calculator at
It's a little hard to figure out, but I think it tells me that at 2
psi, I can get a flow of 2 gps through 80' of PVC pipe with only about
.5 psi pressure drop. OTOH, I don't totally trust this calculator,
because it will happily calculate using inputs such as 5 gps and 1
psi, which results in negative pressure at the output end. If I'm
interpreting correctly, it's telling me that 1 psi (2' risers) will be
marginal to give the 2 gps flow, and that I should probably consider
Does anyone know of a reliable flow calculator? Or how to interpret
One of the NDC documents which Mike linked to puts me in the region
that gets 4" in one hour (max in 100 years). I believe that for a full
hour, but I think I'm justified in using that rate as far more common
for five or ten minutes -- a really heavy rain here very seldom lasts
longer. The same doc also says that 4" smooth wall pipe will carry up
to about 80 gpm. But at what head? It doesn't say. I'm guessing they
mean at almost no head, so providing even 1 psi would increase the
flow. Which agrees with the calculator (above).
jamesgangnc: ground is very sandy, but also little slope, and the
slope isn't helpful. Those unroll things would only get the water a
few feet from the house, and some would be on the uphill side. The
water really needs to get to the ditch, 80' away at the nearest point.
Dry wells would be a huge task to make them large enough, not to
mention a huge disruption and a maintenance issue. And though the perc
rate is high at the surface, I don't know how far down you hit clay.
This is north Florida, and you only have to go a few miles north to
get into the famous Georgia red clay. I used to live 35 miles west of
Tallahassee, on high ground (290' above sea level! a mountain in
Florida!). Once I had to dig a 6' deep hole for a large post, and hit
solid clay and a stream of water about 5' down. So dry wells would
require engineering planning too.
This house did originally (1953) have a septic tank. In fact it still
has a septic tank -- I've located it with a probe. Within 15 years it
had been converted to city sewer -- I know this from a neighbor who
has owned his house since the late 1960s. I assume it also originally
had a well. The water and sewer lines both come out the back of the
house. Anyway, my point is that the perc rate was good enough for a
septic tank. But that doesn't necessarily tell me a lot about soaking
up heavy rain, which would be a far greater volume. A large enough dry
well would certainly help, but I think it would take a fairly large
one, and I'm loathe to dig up that much of my yard.
Una: I love the rain garden idea, but although the measurements make
it sound like I have a lot of space, I have various limitations that I
haven't mentioned despite all I've written. ;-) Although the yard is
100' wide (frontage), there's some slope, so only about the lowest 20'
would be usable. Of the 80' from house to ditch, I would need a good
space from the house, and the last 30' is city property -- I don't
hesitate to plant and do minor landscaping on that, but if I were to
plan anything requiring a permit, it would be rejected. The front yard
has two large live oaks, three large pines, a moderate size Bradford
pear, and two Chinese elms. The only place for a rain garden would be
where I already have large beds of azaleas and irises and other
flowers. (I have very little grass and I'm trying to kill what's left.
Just another invasive exotic, and it doesn't even grow very well due
to the shade.) This would also be directly over the water, gas, and
sewer lines -- though likely no problem since I recently replaced the
gas and sewer lines, so probably many decades before it has to be dug
Also, my goal is more than just getting it away from the foundation.
The house to the south (low end) is on a slab and the floor is
probably 18" lower than mine. So allowing a flood on that end is not a
good idea. If I owned that lot and it were vacant, doing the rain
garden thing there might be a great idea. But since there's so little
slope from the house to the ditch, if I don't get the water all the
way there, it'll come back, and the rain garden will stretch from the
end of my house to the front door of the next house. Yeah, an hour
later the water will be gone. Usually. When the ground is really
saturated, it'll take longer. (That 20" from TS Fay that you mention
was only a few miles from me.) And an hour is too long when it's
pooled against two houses.
And although the figures you lay out would hold 1" of runoff from the
roof, when it overflows, it would be into the yard next door. The fish
would go with it. That's if the fish survived that long. Your plan
includes a small pool for the fish to live in when the pond is
otherwise dry. Small enough for the raccoons to get the fish. Raccoons
got into my porch last year and killed my goldfish.
I'd love it if all the water falling on my property soaked in. The
other 17,000 sq ft is indeed soaking up all that falls on it, in situ.
If I could persuade the water falling on the 3000 sq ft of roof to
magically disperse uniformly underneath the foundation, I'd be fine.
Preaching at it hasn't worked, and if you don't think I can preach,
you haven't read this far. It's the concentration of this 3000 sq ft
of collector along its edges that's the problem. (I have about 1000 sq
ft of paved driveway, but the runoff from that seems to be absorbed
with no problem.) What I put into the ditch, eventually joins a large
drainage ditch which passes (frighteningly) directly underneath the
house of a friend about half a mile away. Would not do it if I thought
I had a choice.
Mike: thanks for the NDS links, very useful. That long doc in
particular has good info about slope and flow which indicates that my
situation is marginal but OK. (1/8 in 12 slope min, 80 gpm in 4" pipe
at unspecified head.) As far as having it easy, I don't really have
any complaints. I'm high enough that I can get the water away, and the
ground doesn't stay squishy for eleven months of the year, and plenty
of houses even in my neighborhood have such problems. But I do need to
get the water all the way to the ditch, or it's going to come back at
me. That's based on what happened before anything was done (eight
years ago) to remove the water.
I'm using thin wall. Wasn't willing to go with the extra cost or
weight of the heavier stuff, and anyway I'm kind of committed by now.
(I'm confused by terminology, since it all seems to be called schedule
40 these days -- don't see schedule 20 any more. I suppose schedule 40
is a document that describes various grades of pipe?) Perhaps I should
do something to protect the parts that aren't up against the
foundation. PT 1x6 laid on top? I know it will deteriorate (not ground
contact rated), but would still provide protection for a long time. Or
pour a thin layer of concrete? I would guess that even 1/4" thick by
4" wide would deter damage from digging. At that rate of application,
2 cubic feet would cover my entire run. I also plan to photograph the
entire layout before I fill over it.
I have screens over my gutters. Some junk will get down, but it'll be
small stuff. If I thought I could discharge into the yard, the popup
emitters would sound really good. The lot is 200' deep -- if the house
had been built somewhat farther back (and thus up), I'd have a lot
hr(bob): no freezing. The last hard freeze was on February 13, 1899.
That was the day it hit -2F in Tallahassee. When the official
temperature is 12F (which happens every few years), the temp near my
house is probably 20F, partly due to all the trees. Much of the pipe,
including all the downspouts, is under the eaves, keeping it even
warmer. It's an unusual night when the frost line is as much as 1/4"
deep. The NWS calls 25F a hard freeze here, but it's no danger to
pipes in my neighborhood. When I lived farther out of town and had 1"
PVC risers to faucets, and a well with exposed pipes, I had to be more
Jeez, have I written another novel? Well, at least all this discussion
has led me to a better understanding of my situation. Thanks again.