dealing with lack of slope

[Edward Reid]

OK, continuation of my gutter drain issues, but on another stage now. I'm slowly digging the new trench. (I've never been fast on this kind of thing, and a cold and a trip haven't helped.)

I finally did what the drainage contractor didn't: checked the elevation change between the house and the road (using a hose filled with water). From the low front corner of the house to the low spot just short of the ditch, there's a drop of about 10". I didn't measure the rest, but it looks like it comes up a few inches (that's the berm next to the ditch, if you read the novel I wrote about this stuff a few days ago) and then drops into the fairly shallow ditch -- I estimate another 6" drop from ground level at the measured spot to the bottom of the ditch. Not only is it about 80' from the nearest corner of the house to the ditch, the ground at that end of the house is basically level, so add another 30' to the run.

That's about a 16" drop over 110' -- about a 1 in 80 slope, or a little over 1/8" in 12". Not only is this less than anything I've ever seen recommended for any purpose, it's highly unlikely that I'll be able to set the pipe so that all sections have the slope. I'm just not that good.

There's still no question I need to take the roof runoff away from the house, so the question is what to do.

Reservoir, either drain field or sump and pump? I don't think either is feasible, and would be costly even if possible. My roof area is about 3000 sq ft, so 1" of rain produces about 1800 gallons, and I certainly will sometimes get more than 1" in a day. It would take a large drain field to perc that much, and that wouldn't get it as far from the house. Don't even know if it would be legal. That would be a lot of water to pump, and there's no good place to put a holding tank/pond. Either one would be expensive.

I could have some of the drainage pipe above ground. After all, the water is starting on the roof, so in theory it could be caught about

8' higher. But boy, talk about ugly. And although most of the aboveground part would be against the house walls, some of it would cross places that really need to be kept open for walking, and would create a barrier around the heat/AC unit.

Or I could just do the best I can, laying the pipe with that 1 in 80 slope -- but at the downspouts, bring the PVC drain pipe up above ground, perhaps 2' or even 3' at the lowest parts, and enough to reach the same level elsewhere. The front and back of the house have some slope and would not need to extend as far above ground. I would figure the pipe would not entirely drain dry and I would just have to accept that. The only way in would be at the outlet or through the downspouts, minimizing mosquito breeding. Since it's a gutter drain and not a sewer, I don't have as much concern about solid matter buildup. Rain will usually produce a strong flow, washing out what debris gets past the gutter guards. (I've also laid out the drain pipe to minimize sharp turns -- only one 90 degree elbow, and it's a long bend kind.) Most importantly, the high rise at the inlets would create some pressure if they started to back up -- 3' of head creates about

1.5 psi -- so if the run to the street was running slowly, a hard rain would create pressure to make it run faster.

You may guess that I'm leaning strongly toward the last proposal ...

Is there anything obviously wrong with this idea? Any other ideas on how to handle it?

Edward

[Hank]

Sounds like you have it figured out pretty good. Are you going to use PVC and glue it together, or the corrogated stuff? Also, make sure you use the biggest pipe you can (4" instead of 3"). Good luck!

Hank

[jamesgangnc]

What is you ground material? How about those gutter extensions that unroll themselves? I've never tried one but they seem like they would be handy for your situation. If th ground is fairly absorbent you just need to get the water a little ways away form your foundation. Otherwise I'd be tempted to try dry wells instead of digging 80 ft.

How many places do you have downspouts? Are they located such that a couple drywells could serve all of them. Any chance of getting a small piece of machinery to dig with in there?

[Una]

In Florida garden ponds require waterproof liners to stop the water from draining into the ground.

Ed, it seems to me your goal is just to get rainwater away from the house foundation. Where it goes is not so important, but getting it to the roadway ditch is somewhat difficult. So the best strategy is to get it into the ground ASAP, not remove it from the ground. For this strategy a French drain is not appropriate. A rain garden may be the fastest and simplest solution.

A rain garden is simply a scooped out swale where rainwater collects and you have semiaquatic plants. It is unlined and the water swiftly percolates into the ground. To prevent mosquitos breeding there, in the deepest part of the rain garden install a shallow waterproof pool liner, a small one, and stock it with some kind of native fish that eat mosquito larvae. This will create a small reservoir pond with water year round. When water is high the fish will spread out into the entire rain garden; when water is low they will retreat to the reservoir. Grade the yard so that excess water will run off to the ditch by the street.

See? You'll make a shorter, steeper grade from your foundation to the water surface in the rain garden. Most of the year the water there will be low, except immediately after storms.

1" rainfall on a 3000 square foot roof equals 250 cubic feet of water. A rain garden a mere 1 foot deep and 10x25 feet in area will hold that much. You have a huge front yard with room for a rain garden that could hold many times more water. Plant it with cattails and iris and elephant ears and cycads... It won't hold the accumulation of another Tropical Storm Fay, but it would handle more normal tropical storm accumulations.

(Tropical Storm Fay dropped over 20 inches of rain in parts of Florida;

has an animated map.)

Una

[Una]

More about rain gardens. Here is a highly relevant book:

And here is a long thread about them on a GardenWeb forum: Una

[mike]

This has all you need to know and then some:

Just maintain a minimum slope in your trench, and end with a pop-up emitter. Have seepage provisions (holes, gravel, geotextile fabric) near the pop-up emitter to allow the system to slowly drain after rain events.

You really have it easy. I'd keep the start of the pipe a minimum of about 5 or 6 inches below ground to allow weeding and aeration without damaging your pipe. Don't use thin wall pipe. Schedule 40 ABS will take a lot of abuse.

[jamesgangnc]

Sch 40 is a pretty expensive solution to go any distance. And it's not clear that he has the slope to do those popups.

My issue with rain gardens is that unless you can turn it into a permanent wet garden feature it's just a weed factory. Most places have too long a dry spell. You end up with a shallow weed patch. Now maybe in those situation you can create a shallow spot and plant grass in it.

Dry wells are still is at the top of my solution list. But we need to know his soil composiition at the surface and subsurface.

[hrhofmann]

Where are you located? As long as there is no freezing involved, a little water in low spots in the drain pipe are not a big problem, the new water will flush things out. I don't know if mosquitos would hatch in a pipe that was 60feet long, that't a long way for them to go to drop their eggs and then get out again.

[Una]

A rain garden can be as simple as an imperceptibly shallow swale in a lawn, covered with turf grass. The OP is in north Florida on a flat parcel at the bottom of a short bluff. That means the soil is deep sand, sand, sand; hard freezes are rare; and annual rainfall is about 60 inches (5 feet).

Una

[mike]

Several inches of drop is plenty of slope. A pop-up emitter will work with very low hydrostatic pressure. As long at the emitter is lower than the receiving end, he's fine. If you're worried about debris, just make sure your filtering devices take out leaves before the pipeline.

A downspout can fill up a dry well in no time and overflow the system. Pop-up emitters don't have that limitation. There's nothing to fill up.

If he wants to go cheap on pipe, I don't care. I don't think he needs to make such a long run, however.

[Edward Reid]

Thanks for all the responses. The earlier thread, with lots more details about the situation, is here:

'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 with.

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

4' risers.

Does anyone know of a reliable flow calculator? Or how to interpret this one?

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 up again.

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 more choices.

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 careful.

Jeez, have I written another novel? Well, at least all this discussion has led me to a better understanding of my situation. Thanks again.

Edward