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.
How do you incorporate this into a drain design?
https://web.archive.org/web/20240807170330/https://www.ndspr...
https://web.archive.org/web/20240807170407/https://urban-wat...
Both of these links include a dry well - if one is draining to daylight, how do you incorporate the 100-yr flow rate?
Can you share what daylight looks like for your use case? Where is your target for the water to be managed?
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
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.
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.
If you live in a place with a lot of clay, geotextile fabric can certainly be problematic for simple residential settings.
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).
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.
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.
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.
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.
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.
The roof should be shedding to solid drain pipe.