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How French Drains Work

refibrillator
15 replies
2h2m

I’ve built quite a few french drains in residential settings, some lasting longer than others - I’ll share a few hard earned lessons here:

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.

https://en.m.wikipedia.org/wiki/Dry_well

ejstronge
3 replies
1h41m

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

How do you incorporate this into a drain design?

ejstronge
1 replies
1h15m

Both of these links include a dry well - if one is draining to daylight, how do you incorporate the 100-yr flow rate?

toomuchtodo
0 replies
1h12m

Can you share what daylight looks like for your use case? Where is your target for the water to be managed?

daedrdev
3 replies
1h42m

The video also mentions that sometimes the geo textile fabric is not enough since it too can get clogged, though this seems important in dams and not so in simpler projects

datavirtue
1 replies
1h12m

The drain pipe is at the bottom of the drainage excavation. On top of the pipe is 3/4 aggragate, and on top of the aggregate is geotextile. Soil is filled in over the geotextile. There is no clogging risk in this design. Any soil particles that make their way to the pipe are carried away.

cityofdelusion
0 replies
26m

Fine clay and silt clogs the geotextile.

A proper drain requires extensive soil testing for particle size. Video goes over this along with the issues of geotextiles.

bshacklett
0 replies
1h25m

If you live in a place with a lot of clay, geotextile fabric can certainly be problematic for simple residential settings.

hatsix
2 replies
1h36m

Not sure you watched the whole video, he shows how soil migration causes failure (7:30), links to another video that covers that in-depth and discusses how geotextile isn't enough for things like damns (10:15). He also mentions that you can get pipe with holes all the way around (5:58).

ryanisnan
1 replies
1h30m

Definitely didn't watch the video. He also mentions hole direction and problems, as if the video didn't go at length into the topic.

refibrillator
0 replies
50m

I definitely watched it but missed a couple of those sentences so thanks for the timestamps above.

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.

xg15
1 replies
1h32m

You can use the “100 year flood” values for your locale to get an upper bound on rainfall

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.

jnwatson
0 replies
1h26m

Faster than climate change is development. The addition of impermeable surfaces like parking lots and buildings upstream causes more water to get to you.

Development happens faster than the flood maps get redrawn.

eitally
0 replies
1h4m

Our first house was one a 1/2ac lot at the end (bottom -- this will become important later) of a cul-de-sac in a great semi-urban neighborhood in Cary, NC. We learned after repeated backyard flooding and near-incursion of storm water into our patio door that our street had been built on what had been the original stormwater collection pond for the larger neighborhood. The builder got permission from the city to move the pond to build our cul-de-sac, but that didn't stop our street from still being the lowest point in the neighborhood, where all surface water ran-off to, and our house was at the very lowest point.

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.

datavirtue
0 replies
1h16m

The roof should be shedding to solid drain pipe.

zeristor
3 replies
2h50m

The coding language Julia; perhaps Ada should have been named Lovelace then?

smt88
2 replies
2h45m

Julia doesn't count because it wasn't named after a person.

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).

lucideer
1 replies
2h29m

I think narrowing the theme of the list to just inventors is more an accidental result of the author's selection rather than their intended theme - eponyms seems to be the theme.

tialaramex
0 replies
2h16m

The point is they're unexpected. Elo is a great example, people tend to assume this chess rating system is an abbreviation, but it's somebody's name.

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?

ec109685
1 replies
2h52m

I don’t think it qualifies. This drain is intentionally named after Mr. French.

sbradford26
0 replies
2h49m

The would be the whole reason of it making the list. Most people would think it was named after the country or something not Mr. French.

xg15
0 replies
54m

Spearheaded of course by the late E. P. Onymous, may he rest in peace.

walthamstow
0 replies
2h1m

Wow what a brilliant blog post. Full of OMG moments, not least Max Factor!

lucideer
0 replies
2h32m

Great site! (though Unilever seems like a stretch, & gasoline is extremely contested).

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).

gilleain
0 replies
1h54m

Ah, like Arkhan Land and his Land Raider.

belval
0 replies
2h46m

Fun fact, in the French speaking part of Canada, "French drain" are fully translated to "Drain français" because of this quirk even though we don't usually translate proper nouns.

seanalltogether
8 replies
2h46m

One thing I regret after moving into my new house was not getting a detailed list of drains installed in the property. I have lots of drainage issues and don't know if there is a drain there but inadequate for the amount of water we get, or just not installed at all.

philistine
2 replies
2h37m

When I bought my house the previous homeowners legally had to declare whether or not there was a drain around the foundation. Very useful, it's the first thing we did and we never had any water breach the house.

seanalltogether
1 replies
2h33m

Sorry, I should have clarified, I meant for drainage in the yard. We live in a development that's on a hill so 2 of our neighbors are on properties above us.

pfdietz
0 replies
2h30m

I wonder if there's a way to get that mapped, say with earth-penetrating radar.

sbradford26
1 replies
2h41m

A major lesson of home ownership is that it is a continual fight against water. Keeping water away from places you don't want it, and keeping water in and available in the areas you do want it.

anticorporate
0 replies
10m

This is absolutely true. I think half of the home renovation projects I've done in my life have been to either move water, or repair the damage from where water ended up where it shouldn't be. These are never the fun projects, but in terms of protecting your property, probably the most important ones.

pfdietz
0 replies
2h40m

Does the house have a sump pump? In our current house, which we bought with full knowledge of the issue, there was seepage and no sump pump, and we ended up having one installed (after negotiating down the sale price a bit after the inspection.) This involved jackhammering through the unfinished basement floor around the perimeter and installing a drainage pipe, then repouring that part of the floor w. inspection/cleaning ports into the pipe, along with a sump and pump in one corner. Works like a charm now.

There had been a drainage pipe but it was not working properly, probably being crushed or filled at some point by tree roots.

engineer_22
0 replies
1h33m

Yea, often these improvements are not well documented, you are right that it would have been easiest to ask the prior owners while they still owned it.

bombela
0 replies
2h33m

Redirecting the gutter downspouts away from the house can help if this isn't already done.

EDIT: saw your other comment about drainage being an issue in the yard, not the house.

aristus
7 replies
3h5m

I discovered French drains while trying to dig a hole for a fruit tree a while back. The land was on a hill and about 30cm down I hit this huge pile of dirty gravel. Ok, so maybe someone filled that spot with gravel. Sunk another hole a bit farther down. More gravel, etc. It took me longer than i'd like to admit to figure it out.

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.

pfdietz
4 replies
2h44m

And, if people are fertilizing their lawns, all that lovely fertilizer runoff.

aristus
3 replies
2h42m

Oh, yes. No lawns in that neighborhood but I thought hard about what might be going into the fruits. Years on, no ill effects.

pfdietz
1 replies
2h31m

Fertilizer would be welcome, pesticides less so. Fortunately, one doesn't really need pesticides on lawns, particularly if one doesn't have a dislike of non-grass species (like clover, which helps with fertilizing anyway).

cmiller1
0 replies
2h2m

Not uncommon for people to have their lawns treated for ticks here in the NE US

phsau
0 replies
2h33m

Wouldn't it be better to keep the tree and its roots away from the drain? That growth might come at significant cost!

cityofdelusion
0 replies
23m

Tree should be far away from the drain line. The roots will eventually grow into the pipe. Tree roots are incredible at finding water sources. Even a drop-at-a-time drip from a water pipe will be completely root wrapped in a few years.

bell-cot
0 replies
12m

Turns out it was...

Might I ask how well- (or ill-) documented the location of that kinda-important drain was?

bane
6 replies
2h0m

I had absolutely no idea this was named after an American!

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.

HPsquared
2 replies
1h27m

If it's important, I guess you could add some boreholes or sensors to check the groundwater level and see if it's creeping up over the years.

falcolas
1 replies
1h20m

A simple sump (basically a hole in your basement floor), and sump pump, can help identify it before it reaches your basement floor. IIRC, this is also covered (albeit briefly) in the video.

kevin_thibedeau
0 replies
57m

High water pressure at the basement walls can result in intrusion without ground water below the floor being a problem. I have no sump/drain in my basement as it never floods because it sits on well draining soil. A neighbor's driveway directs water toward my block foundation and, with heavy rains, the localized pressure in the saturated soil along the wall would produce a jet of water coming out of a hole in the mortar. I resolved it by digging a swale that diverted that excess water around the foundation. It is much easier to maintain than a French drain that would silt up rapidly given my soil.

jnwatson
0 replies
1h22m

It was in a comment by darkerside.

throw0101a
0 replies
55m

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.

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.)

gregmac
5 replies
2h34m

I love the glimpse into topics like this, where one might initially think "what is so hard? Dig a trench and fill it with gravel" but it turns out to be way more complicated:

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
pimlottc
4 replies
2h8m

I wonder how many of these stories start with him discovering one of these massive government reports. They always seem to pop up in his videos! It really is impressive how much invaluable practical information is provided for free by the various federal agencies.

engineer_22
1 replies
1h35m

A minor quibble - while the information is free to the world, those federal agencies cost the People money.

rimunroe
0 replies
48m

The fact that it costs the government money to do things is obvious to the point of pointlessness. I could understand pointing out the actual cost of producing the info if you had it, but just pointing out the fact that it cost them money to produce some report doesn't add anything to the discussion.

extraduder_ire
0 replies
1h20m

I have to assume most of these reports, standards, and other documents end up being tax-neutral or better from wider dissemination of this info and/or propping up the status quo in engineering and construction.

LegitShady
0 replies
12m

Learning the basics of designing the gradation of sand and gravel filters is Soil Mechanics 1 (think 2nd year university). The massive reports are because of massive problems caused by poorly functioning or failed filters, but the basic premise of much of these videos is "give the first lesson in some subject without any math and actually try to explain things instead of typical engineering prof standards".

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.

avgDev
4 replies
2h32m

Great video and I love this channel.

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.

finnh
1 replies
2h17m

We cut a french drain into a below-grade room a few years ago. I was definitely happy to pay someone else to do it - cutting the concrete alone was a nasty piece of work and only the start of the job. Plus the unexpected footing running beneath an interior wall they had to cross below, such hard work.

dboreham
0 replies
2h15m

That's usually called a footer drain or foundation drain. French drain is typically a trench dug from the surface, some distance away from buildings.

dboreham
1 replies
2h16m

And this is why I own an excavator, loader and a dump trailer.

avgDev
0 replies
53m

I wish I had the space, I'm too close to the city and would need to buy some land.

I feel like if software didn't pay this much I wouldn't mind having a business utilizing heavy equipment.

seltzered_
2 replies
1h45m

As someone whos been thinking quite a bit about land restoration I cringed a bit when watching the video about the Henry French's philosophy.

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)

engineer_22
1 replies
1h38m

There are many places where there is no shortage of water, and for safe hygienic habitation it becomes necessary to remove it

seltzered_
0 replies
38m

Yes but..

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...

wonder_er
1 replies
22m

I think a french drain to a pit-type depression that can hold a few hundred gallons of water and drains across hours is the way to go.

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.

hinkley
0 replies
18m

Permaculture uses very shallow ditches to force water to serpentine across the property. What doesn’t sink in shows up in the greater watershed much later in the precipitation. Not unlike urban rain gardens.

Dumblydorr
1 replies
2h1m

Anyone know how to maintain retaining wall pipes? Installed by previous owner of home, have no idea if they’re done properly or if they’re clogged or anything. Just want to keep that water moving! :D

toomuchtodo
0 replies
1h15m

What are the existing pipes made of?

zoklet-enjoyer
0 replies
1h29m

All sewers lead to Paris

uslic001
0 replies
24m

We just had French drains put in around a farmhouse we bought as it had major drainage issues. They seemed to be working well last weekend during a few thunderstorms. Given the current tropical storm pounding NC I am glad we did it before all this rain.

turtlebits
0 replies
28m

This applies to houses as well, ideally you should have an air gap behind your siding to let moisture out (to prevent rot) - some common methods are strapping or ventilated rainscreens.

aynyc
0 replies
42m

As someone who battled water in basement in the last few years:

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.

ChrisMarshallNY
0 replies
32m

This drives home the power of water.

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.