How French Drains Work
practical.engineering
practical.engineering
Soil migration is the number one failure mode. This is mentioned but perhaps a bit understated in the video.
To prevent soil migration you absolutely need commercial grade geotextile fabric wrapping the gravel and pipe.
You can buy pipe with much bigger and more numerous holes than the tiny slits depicted in the video. That also obviates the need to decide what orientation the holes should be.
Void space is the most critical factor when choosing gravel. You need a lot of space between the rocks to support fast drainage. Do not use playground pebbles or anything similar. Pay close attention to the type of aggregate you’re buying.
Calculate how much water you actually need to drain. You can use the “100 year flood” values for your locale to get an upper bound on rainfall, then multiple by drainage area. This is especially important if your roof is part of the watershed.
https://www.usgs.gov/special-topics/water-science-school/sci...
Sometimes there is insufficient gradient to move the water anywhere, ie the land is too flat. In this case you may be better off building a drywell, which is constructed very similarly.
Long story short, we ended up installing a french drain on two sides of our house, putting a drain at the end of our driveway to redirect water [from just running down our driveway to our house], having the front wall foundation rebuilt and jacked up because the footer had nearly completely eroded (1970s house), and still needed to add two 24" square drain boxes in the back yard (with buried 4" PVC draining into the creek at the back of the property) to remediate all this. It was also a tree-filled lot and the drain boxes would almost immediately clog with leaves during heavy rains, so I spent a lot of time out there in ankle to knee deep water with a rake to keep the drains clear.
This still wasn't enough! We got a so much surface run-off from our uphill neighbors that it looked like a sheet of rushing water across our backyard during heavy storms. We had a creek at the back of the property but were not allowed to mess with it because of Corps of Engineers easement regulations. At the end of the day, we finally solved our problems by getting an agreement from our neighbor to let us dig a surface water drain from the low point in their yard into the creek, and then we'd build a "temporary" (cf prior easement rules against permanent walls) berm along the property line to prevent surface water from coming into our yard at all. That "berm" was 76 bags of Qwikrete stacked three high and covered with grass clippings, yard waste & mulch. Technically we could have removed it if the city had inspected, but it was very much a cement wall.
We have never since, and never will, build or purchase a house at the bottom of a hill.
There was a ditch out back but it obviously wasn't deep enough. I did the math and ended up with an astounding amount of water flowing by (something like 20,000 gallons per minute[0]) and complained to the builder about it. They sent a couple of guys out there who installed two 4" corrugated drainage pipes and said it was good enough.
When the next big rain happened, where those pipes emptied out there was a 3 foot tall fountain of water from the pressure. It was an amateur Bellagio, lol. The yard still flooded but luckily didn't come into the house again.
Since then, the landscaping came in throughout the neighborhood which controlled the runoff. But it taught me a lot of respect for getting slopes & drains right - Always use a larger pipe than you think you'll need.
[0] The formula seems to be: V = 0.408 × Q/(D^2) where V is velocity in feet-per-second, Q is the flow rate in gallons-per-minute, and D is the diameter of the pipe in inches. I estimated the diameter based on the land profile that was underwater, and timed the velocity by following twigs/leaves floating by.
To get a flood policy you would have to speak to a broker about whether you qualify for the National Flood Insurance Program. They won't issue a policy to you directly.
One of my friend works in reinsurance and his favourite joke is that every time the insurance company pays a claim, someone is getting fired. Obviously a joke but it seems to paint a fairly accurate picture of how the business is run.
Certainly, they're in business to make money, and helping customers is just part of the process, not their goal.
But --- it's in the customers' interest for the company to be accurate about accepting and denying claims. If the company accepts claims that should be denied, that means higher premiums. If the company denies claims that should have been paid, that may result in the company having to pay those claims after appeal/lawsuit and possibly with penalties; penalties get paid by the pooled insureds one way or another.
Insurance fills a need and is not just an evil company doing evil. Most insurance covers risks of loss that the insured would be unable or unwilling or at least unhappy to cover themselves. For most homeowners, if their house burns down, they would be unable to rebuild without insurance, unless there was clear liability from a person or company with big pockets.
Vanguard(the brokerage) is structured the same way pretty much, except it's the various mutual funds(like VTSAX/VTI) that own the larger corporation. So far the infection of c-suite salaries continually growing larger and larger hasn't gotten to Vanguard yet, but they are now a few CEO's removed from the original founder(Jack Bogle), so it's probably about time that shift starts to happen.
> The people in charge(C-Suite) have zero incentive to ever refund any annual profit.
For example, in 2020:
> USAA, the country’s fifth largest property-casualty insurer, will be returning $520 million to its members.
If what you say is true, why did they return any money at all? They could easily pay that to C-suite and senior execs in huge bonuses.
Also, to clarify, the correct term is "mutual insurance", which Wiki describes as:
> A mutual insurance company is an insurance company owned entirely by its policyholders. It is a form of consumers' co-operative. Any profits earned by a mutual insurance company are either retained within the company or rebated to policyholders in the form of dividend distributions or reduced future premiums.
This is consumer insurance, so your typical auto, home, renter's, etc. policies. If you need commercial or E&O coverage, ask your broker to check them against the AM Best rating list.
> I honestly don't think consumer insurance is worthwhile
Further - if you never have a problem the value of the inspection is zero. If you do have a problem the value of an inspection after the problem has occurred is minimal. The value of an inspection that prevents a problem is something - but how to quantify that is difficult.
Topographic maps, flood risk maps, wildfire risk maps, maps of services nearby (substations, transformer locations, storm drains etc), easements, special planning/protection zones, surrounding area zoning etc.
These should all inform your decision. I've vetoed houses because they back onto nature/wildlife reserves, are too near substations, are in flood risk areas etc
If you personally know any architects or general contractors in the residential market, they should be able to recommend someone.
Why is this a bad thing? To a naive person, it might seem like a good thing, because you get to look out into a forest rather than someone else's yard.
Essentially because nobody was allowed to trim/maintain the trees due to it being a natural habitat. It's nice to look at but when a 60m tree falls in a storm and wipes out your house it's not great. The reserve was also on the north side of the property and down here in the southern hemisphere that means that every year as the trees grow your house gets more and more shade with accompanying maintenance issues.
I should add that people may weigh that up and be happy to live there, but people should be aware of the risk/benefits before they buy. Natural light/property orientation and shading is a major issue that most people forget about.
It's all pretty silly, really, especially considering the hottest markets, where you either want a house or you don't, and if you you'll probably pay top dollar with no contingencies and possibly not even your own inspection... since the seller knows if you don't buy it someone else is in line right behind you. Most of the time, houses sold this way aren't in significantly worse shape than those that are fully inspected and repairs made before the transaction.
Overall, this has led me to believe that, in most cases, pre-sale inspections don't really benefit anyone, especially since it's so difficult for buyers to seek E&O recourse/damages against either an inspector or seller after the sale closes.
We took that list back to the sellers (who were then legally required to provide a copy of that report to other future buyers), and we simply waived the little stuff. We wanted them to fix the big stuff. They didn’t want to do that, so they came down on the price by a considerable amount. He saved us tens of thousands of dollars.
Now, given the problems we’ve had since then which the inspector didn’t identify, we would have paid back more than what he saved us — Except for insurance coverage. USAA is really good about paying out on claims, when many other insurance companies would weasel out.
We love this house, we love this neighborhood. If we had to do it over again, I think we would have pressed harder to get even the minor problems fixed or to get the seller to come down even further on price. But I think we probably would have still bought this house.
I don’t blame the inspector for anything he missed. Most of those problems were well disguised, until such time as there was an actual catastrophic failure. He couldn’t have known. If I knew his name and contact details, I would recommend him to future buyers in the Austin area.
And we’re definitely sticking with USAA.
It's particularly demoralizing to see the scope of the problem when it's all that water flooding your property and know there's nothing you can do in the short term, and even the long term is just trying different things and hoping they work, knowing that it will probably take a few tries.
Perhaps the balcony over your front door is not properly constructed and is not properly shedding rain, so one day during a torrential thunderstorm you suddenly have a flood coming in OVER your front door.
Perhaps your water heater pan is not draining correctly, and one day you have a catastrophic containment failure in your water heater, causing an interior flood.
Perhaps you change the toilet in an upstairs bathroom, and the new modern toilet is not providing enough water per flush to make the long run of the upstairs plumbing. So, it clogs up over time, then one day it backs up while you’re taking a shower, and you’re standing in multiple inches of grey water. Then the water does slowly drain, but you don’t notice that the pipe has cracked above the clog and it’s actually flooding into the room below, through the light fixture.
Perhaps the main schedule 40 PVC pipe from the water main at the street slowly develops a crack, due to the roots of a tree nearby, and one day you discover that the water pressure in your house is really low and that you have flooded the whole street.
Guess which of these have happened to us?
Building codes exist for reasons. Sometimes they seem frivolous, but often they're there for a damn good reason.
I've seen a number of homes in my area where the builder received special permission to build right where the drainage needs to go. Some were beautiful and I was tempted to give it a shot, but your story reminds me that I need to trust my gut on this issue and head for high ground.
Beat it with 2 ditches, a drain, a berm and 2 pumps.
The previous homeowner put in a DIY french drain, but either he didn't do it right, or it's too flat, or it's too small, or it is clogged. We instead get surface water flow and erosion along the same path where the french drain runs.
(It doesn't help that when our neighbors got sand delivered for an above ground pool bed, the delivery truck dumped the entire pile of sand ON TOP OF the river rock exit basin for our french drain. The sand was later removed/used but I was still upset; unfortunately it was my brother in law who ordered the sand. Since the drain is above it I don't think it went in the drain, but maybe this rock basin was supposed to be a dry well.)
I am thinking of building something like your concrete berm along my back fence line, possibly modifying the french drain to just transfer the water from the far side of the berm to the exit over the cul de sac rather than trying to absorb it from the ground.
The ditch actually speeds up the water drainage. I dug the ditches to divert the water around the property line, so the surface area of ditch in total is large, even though you can't really see it and it doesn't get in the way. I figure at least a quarter of the water drains through the ditch before it gets to the pond. This is probably helped by the fact that the soil is very rocky.
Yes he does name soil migration as the biggest problem with subsurface drains, but to be fair the video is contextualized around dams.
The spirit of my comment was intended to be helpful for folks attempting this stuff at home, not to suggest a lack of good info in the video.
I wonder, does climate change also affect those calculations, with the need to adjust those bounds upwards? I remember the (technical) term "100 year flood" getting lots of ridicule in the last years because we already got multiple "100 year floods" in the range of a decade.
Development happens faster than the flood maps get redrawn.
The 1:100 year safety level should thus be considered a bare minimum for high impact events. In the Netherlands water infrastructure is designed to withstand the 1:10.000 year storm for catastrophic events, and e.g. 1:1.000 year storm for high impact events.
But even with higher safety standards, it is key to design not only for the current climate, but for the future climate over the design life of whatever you are building. Extrapolating 1:10.000 year events from 30 years of observations might seem sketchy already, but with all the uncertainties in climate change predictions is a bit of a stretch.
Long term observed water levels are fed into a model with normal distribution, and a 100-year flood is then defined as 99-th percentile of the model. This means that there's a 1% chance of a 100-year flooding each year.
Global warming and climate change have an effect that there's a lot more water vapour in air. Same air mass can deliver more rainfall on a given territory, pushing observed water levels into higher extremes, meaning what used to be 100-year flood is now 25 (4% yearly chance), 10 (10% yearly chance) or even 5-year (20% yearly chance) flood.
https://www.usgs.gov/special-topics/water-science-school/sci...
But that got me thinking, how often do they adjust these estimates? Was a "100 year flood" in 1950 less than in 2000, say?
Obviously it'll be different for different areas, especially between countries, but still.
A proper drain requires extensive soil testing for particle size. Video goes over this along with the issues of geotextiles.
No one is doing soil tests for these unless they are installing huge critical commercial infrastructure.
We were a bit naive buying our new construction home. It's the only home on the street with a basement due to the high water table. One year after purchasing, Hurricane Ida hit and we had water up to the door handles in our basement. Our primary sump pump runs constantly for 18 minutes then off for 18 minutes 24/7. We have a backup pump on the other side of the basement connected via the perimeter drain, and a liberty water backup pump. The only addition since Ida is a whole house generator and superior window wells.
During Ida, the street was flooded, the drainage systems were backed up, and there was no where to pump the water to.
I'm assuming it's a fully-enclosed basement? I.e. no walk-out side?
The best you can likely do is waterproofing the basement walls + pumping out from inside or an inside-perimeter drain. But that's only racing pump_out_flow > seep_in_flow.
And that's a bad game to be in, because water under pressure (which it will be a story below ground) finds a way.
I'd make the basement flood tolerant (limit mold-hosting materials and places water can be trapped) + pump it out quickly after it floods + buy a dehumidifier, and accept it's going to happen.
French drains around the perimeter help, but you're literally fighting water from all directions... including directly underneath.
Have a perimeter French drain but a crack in the slab? Enjoy watching water bubble up from that crack once things get saturated.
There isn't a "perimeter" per se, because water is literally coming from depth (in most locals / soil types).
So drain/pump just creates drier 3d soil areas. Whether that's enough to attract all the surrounding water (at a high enough rate) that would otherwise be infiltrating depends on soil permeability (e.g. sandy or clay?).
A lot of the "handyman"-type "dry basement" shops have no idea about any of this, toss a generic solution in there, and then blame the gods if it doesn't work (and they've billed for time-consuming and expensive excavation).
It gets dicey applying internal barriers, because one needs to be thoughtful about making sure there's a place for water to go when it's actually too much (you don't want an internal barrier to be a dam holding back all the pressure), but it can be an option.
They sell them in membrane form or paint-type form.
I wonder what building codes require, ie from engineering and inspection POV, like can the things you mention even be built today? If so isn't this a failure of the planning departments?
Yes definitely, just depends on area and your local geology / codes.
The article seems to be quite clear that, for critical applications, one should not use geotextile fabric at all — instead one should use (appropriately selected!) sand.
How do you incorporate this into a drain design?
Incidentally, our roof had gutters/drainage originally. We put in our own french drain after about 8000 hours of research, and decided to re-route the roof drainage to right above the new french drain (separate pipes) because the roof drainage broke at some unknown point and was dumping water around our foundation piers (yay...). Well, our roofers came and installed gutter guards after all this, which caused the gutters to shed 60% of the water into the drainage area because of the way they're angled. If we hadn't put in a french drain, it'd be a real problem, but as it is now, it's a mild annoyance that I keep telling myself I'll fix.
My parents home is on a mountainside. All the rainwater collects higher up the mountain and then moves down the mountain where their house is in the way. The soil is thin, mostly clay, and you hit bed rock very quickly. When it rained, water would have little place to go under the soil and would surge up under pressure in areas to the surface. One of those areas was that house.
They purchased the house new and the builder neglected to consider this water reality, so for the first couple years the basement would flood from the bottom, then later water would come rushing into the basement windows.
They spent a small fortune with heavy equipment digging out underground trenches in this rocky environment and laying french drains around the perimeter of the house, and then also in a long diagonal in front of the house as a sort of catchment. The collected water was routed underground and around the house to an exit further down the mountain.
This stopped all the flooding immediately, so much so that my parents finished the basement and turned it into a massive master suite where it stayed dry for decades.
A handful of years ago, my father passed on, and early the next year when the rains started, the flooding starting up again. My mother, absolutely distraught had me come out, and the water had destroyed flooring, drywall, window treatments, furniture, books -- it looked like the aftermath of a hurricane. During one rain I watched waterfalls come in the windows...clearly something had gone wrong with the drainage system.
We had engineers come out, concerned that the water had cracked and penetrated the foundation, but no, it turns out the French drains had failed in someway. Another small fortune, they were dug up, replaced, redone, and now....no flooding again.
This is a great video that may have explained exactly what happened.
Generally speaking you want some kind of inspection/clean-out port so you can snake a camera through the system to see what's going on:
* https://www.drainbrainllc.com/what-you-should-know-about-the...
(This is for both the sewer drain and French drain systems.)
They're just called land drains or field drains elsewhere.
Yeah, I feel like French drains should be added to: https://news.ycombinator.com/item?id=23888725
The FAQ mentions that it's owned by CuriosityStream, which I've heard about before, founded by John Hendricks who also founded the TV channel/network Discovery (now Warner Bros something something).
As far as I've seen, CuriosityStream is mainly about "popular-science" stuff and over-dramatized American "documentaries", is Nebula something in the same vein? Would be weird to see Grady over there if so.
Creators often also post exclusives or early releases.
Regrettably, I have been following Nebula from its early years, so I don't have the elevator pitch to offer other than "videos on YouTube are dictated by YouTube, videos on Nebula are dictated by their creators" plus the obvious ad-free experience. This is some inside baseball from the channel that I recall introduced me to Nebula https://www.youtube.com/watch?v=Alqt6RCEWdM
As for your popular science concern, in general my answer is "no," it's not marketing drivel designed to inject ads into your eyeballs. However, since it is just a video platform (err, and also classes, news, and podcasts) the content quality is ALL OVER the place. That was actually why I didn't subscribe for such a long time - it was like those "web 3" video platforms - only fringe stuff was on it, so it wasn't worth my time or dollars. Since then, they have gotten a lot more content that I value so I pay by the year now
Merely as a frame of reference, here are some of the channels that I follow
https://nebula.tv/thethoughtemporium
https://nebula.tv/strangeparts
https://nebula.tv/breakingtaps
https://nebula.tv/minutephysics
https://nebula.tv/extrahistory
https://nebula.tv/extracredits
https://nebula.tv/reallifelore
https://nebula.tv/realengineering
https://nebula.tv/citynerd
https://nebula.tv/notjustbikes
https://nebula.tv/citybeautiful
https://nebula.tv/rmtransit
https://nebula.tv/stewarthicks
https://nebula.tv/tfc
All channels also have RSS which is a bonus.
It also highlights another aspect of a non-adtech owned platform: I can watch videos without fear that they're going to poison my feed and cause some topic that I was merely curious about to suddenly fill my homepage
Turns out it was the main drainage for the whole neighborhood. Heh. Moved my tree to the side and it thrived on all that lovely water.
Life finds a way.
Might I ask how well- (or ill-) documented the location of that kinda-important drain was?
That might depend on where you are. In my part of the US, fertilizer runoff is causing very serious problems. This is mostly from farms, of course, but domestic lawn care is a major source as well.
The push around here isn't to get people to stop using it entirely, but to follow proper practices that minimize the runoff.
There's a more encompassing worldview perspective that should be taken where rather in terms of thinking about 'superfluous water' there should be an inquiry around what relation one wants with water - where are the areas you want to perhaps concentrate water (e.g. at a pond/rainwater harvesting) and dryness (e.g. where you have a human settlement structure), and how one can 'slow, spread, sink' water to have a healthier relationship to the greater watershed and relationship to atmospheric water vapor.
Courses like https://waterstories.com and writings like https://climatewaterproject.substack.com/ may help with expanding with thinking this way, along with Brad Lancaster's books ( http://harvestingrainwater.com/ ) on rainwater harvesting. From a farm perspective Chris Jones is also worth listening to for his critique of drainage tile ( https://www.youtube.com/watch?v=Svwjd3FwNCY&t=5668s )
(Bias: I've partly taken the water stories class, and an investor in the upcoming climate water project book)
We have to also think about how waters being removed from land. Is it being removed so quickly that we end up with low rivers and risk of saltwater intrusion during drought conditions [0]? Is water drainage not getting buffered by say, growing vegetation?
My argument isn't against French drains outright, but whether we're thinking more broadly about the impact of human relations with water.
[0]: new orleans saltwater intrusion scare from 2023: https://www.axios.com/local/new-orleans/2023/09/28/louisiana...
Surely that's a mitigating factor that makes it much more acceptable to geoengineer water removal?
At the least I'm trying to advocate for say, pairing a French drain design with a rain garden or something to slow water down. In some areas they actually incentivize planting trees close to sewer drain areas since it helps manage water during flood events.
Not trying to be pollyanna and advocate for a sponge city approach evrywhere either, I'm aware of the issues with permeable pavement potentially leading to sinkholes and such (ref: https://x.com/waterhodl/status/1660307940451000320 )
Of all the natural disasters out there; volcanoes, wildfires, earthquakes, tornadoes, etc., the one that does the most damage, and kills the most people, is water.
Floods, tsunamis, and storm surges are absolute blockbuster bombs of destruction.
Up here, we had Sandy. It wasn't even that "powerful" a hurricane (I think Cat 2 or 3, by the time it got here), but the water that it brought with it, caused a huge amount of damage and death.
My basement flooded recently, I'm going to rent some equipment and dig drains/grade. Previous owners spent money on fixing cracks and painting foundation with stuff that always fails.
The best way to keep your basement dry is proper grade and if unable to achieve proper grade drains. It is literally that simple. This can help prevent formation of cracks and foundation problems as there is less pressure on them that way.
I feel like if software didn't pay this much I wouldn't mind having a business utilizing heavy equipment.
> It is hard to overstate the importance of properly filtered drains for dams. If you don’t believe me, take it from the Federal Emergency Management Agency in their 360-page report, Filters for Embankment Dams: Best Practices for Design and Construction. If that’s not enough, try the Bureau of Reclamation in their 400-page report, Drainage for Dams and Associated Structures.
To the point that:
> A civil engineer could spend an entire career just thinking about subsurface drains
If you google "Gradation Design of Sand and Gravel Filters" you'll find Chapter 26 of the USDA's national engineering handbook, which will give the basics of how its done with some simple examples.
This is baby engineer type stuff, for the most part. If you're designing for big structures or complicated situations it becomes a much more complicated exercise.
https://notes.rolandcrosby.com/posts/unexpectedly-eponymous/
This reminds me, my partner recently bought me some "Coffey Whiskey" because I like coffee, which led to my own discovery of another one of these (albeit one requiring a spelling oversight).
Even then, neither of those would fit the theme of the list above because they aren't named after their inventors (even though Ada is technically an eponym).
It's not unexpected that Ada is named after Ada. It is unexpected that PageRank is named after a person named Page rather than web pages. It may seem less unexpected if you happen to know who Larry Page is, but the same could be said for Glen Bell, if you knew who he was then it's "obvious" why the business is named Taco Bell right?
We have so many similar highly charged but largely meaningless holy wars in our industry that it was fun to see the same thing in a different one.
1. Make sure your house is not at the bottom of a hill or swamp or whatever. French drain will not stop surface run off from uphill. You need to divert that from the top. Which is almost impossible since it's most likely someone else's land.
2. Make sure your gutter is cleaned and properly installed. And Make sure your gutters drain into a solid PVC pipe and channel that solid pipe away from your house at a pitch as far as you possible can. Most likely to the street, but that kinda mess with downhill and town drainage.
3. Have sump pump(s) in your basement and make sure they work and have back up power.
4. If your yard has flooding issue, then try french drain and/or dry wells. I found dry well work better on flat ground.
Just a side note, most french drains will fail at some point depending your soil and installer.
- Increase airflow in your basement or crawl space. Add a pest proof window that you can open and close. Or operate a fan based on humidity or temp, similar to a gable/attic fan.
- Add a properly sized dehumidifier that pump water out of your house to keep the humidity low.
- Keep plants away from your house if your house is swamped by plants. Plants hold ton of moisture and it's great for your air, but terrible at humidity.
Whole-house dehumidifiers. One popular brand in the US:
* https://www.santa-fe-products.com
Search Youtube (and generally) for the topic and you'll get a bunch of building science stuff on the topic.
Just because your thermostat says the temperature is fine (and so does not run AC or heating) does not necessarily mean your internal environment is (totally) "fine". Perhaps, to start with, buy a (few) humidity metre (hygrometer) and see what various rooms / floors are in your house:
* https://www.amazon.com/ThermoPro-TP50-Digital-Thermometer-Te...
Ideally they should be 50±5% (though ±10% can be fine).
And the french drain can be simply a slight trench. It doesn't need to be underneath anything. This worked for me and water runoff management around an old house. The lot had a funky grade, and all I wanted to do was get the water away from the structure. A little light pickax/trenching work got me what I needed.
I was curious with all the talk of drywells in the comments, so I had to calculate it. Here in Texas my roof produces 68 gallons per minute of runoff during a typical heavy thunderstorm.
No wonder the drainage ditch gets so full after a storm!
There had been a drainage pipe but it was not working properly, probably being crushed or filled at some point by tree roots.
EDIT: saw your other comment about drainage being an issue in the yard, not the house.
This drain disperses water under some sort of front yard (it's the countryside...) where the grass consequentially has a very high growth rate...
Anyway I suppose that such concrete underground structures are often built on the cheap, and by the very nature of their content they're hard to inspect, so they're bound to develop a tendency to leak over the prospect of decades.
I have wondered if there is a cheaper, stopgap solution while I gather the money, something like aerating the soil along a path, the path that a drain would take, such that the water flows preferentially through the aerated soil.