One minor but long-standing problem with the front yard was a slightly
low spot sort of near the end of the driveway, which would puddle up a
bit in heavier rain and probably contributed to the March snow-melt
water retention that would always turn the outer part of the driveway into
a mud pit for a couple of weeks. I had already added a couple of inches
of fill across the area, but this didn't really fix anything and what
I really wanted to do was simply get rid of the water more effectively.
Inspired, no doubt, by seeing the massive drainage and infiltration
structures get installed during the
road upgrade job earlier
in the season, I believed I now knew to solve this little problem.
The drainage pits for the two gutter downspouts had been working well, but were allowed to simply overflow a little when they got full -- their purpose was to serve more like splash blocks rather than true infiltration areas, but had been doing fine for both functions in lighter rainfall. Here the water would have nowhere else to go but down, so I needed to construct something to connect the low spot above grade to the more permeable subsoil. As I had seen the fairly clear difference between topsoil and subsoil while digging the downspout pits, I surmised that an appropriately placed column of free-flowing drainage stone might do the trick.
It was time to go play in the dirt again!
The fundamental problem is that the richer organic topsoil is far less
water-permeable than what's underneath. Sure, water will eventually soak
in and nourish all the stuff that grows in that dirt, but it takes
much longer to percolate through and will become saturated fairly
quickly. From the street job and related
construction around the area we saw that the base subsoil is *very* sandy
and almost perfect for soaking up large volumes of water. In theory,
all I had to do was penetrate the top layer and access the giant
I made a larger hole about six inches down just to get a clear workspace, although as expected there was no hint of hitting any subsoil yet.
|Then I started to go straight down across a smaller diameter, and immediately hit an entertaining obstruction: the old wire that used to feed the lamp post at the end of the driveway. Which was long since gone, so this simply got cut away.|
|Two feet down and still no real sign of the sandy stuff. However, some chunks of strange spongey wood-like material began coming to light. At this point everything at the bottom of the hole went sort of soft, requiring almost no force to push the shovel in and bring up large volumes of fairly loose stuff. My best guess is that there was a rotten log or something buried here which had never decomposed completely away, but was providing almost no structure on its own. Perhaps this is why the low spot was there to begin with?|
So because of all that soft stuff which I had to remove and was still not
looking like the sandy substrate I expected to find, my hole suddenly went
from two feet to five feet deep. [Note where the shovel is sitting,
handle on the bottom.] But in the process I had finally gotten
down to what seemed like drier, sandier soil -- not quite the play-sand
quality we'd seen under the road, but likely much more permeable
than the black loam. Once I stepped back and looked at the piles on the
tarp, the difference was more obvious. But now I had a hole twice as
deep as I expected and a bit larger across the bottom, and the amount
of stone I had on hand probably wasn't going to be nearly enough. Some
of it was stuff I'd saved out from my own excavations, and some was
excess from the road job itself that had, well, fallen by the wayside
and I'd basically collected up off the neighbor's front lawn since he
didn't want any of it. The road crew had taught me the merits of using
filter-fabric for structures like this to prevent silting-in, and I had
even saved a scrap of some that they threw aside during their project.
Which I thought would be ample to line the proposed pit, but would now be
barely enough to go around most of this monster I'd wound up creating.
Now, sure, I could have just filled some of the hole back in with the same dirt that came out, but what the heck, there would be no harm in having more actual infiltration surface area and buffer volume down there as that was the only option for where water could go. I could always get more rocks, as noted during the downspout job. To fill a bit of the extra volume in the center I scrounged up some of the larger objects on hand -- heftier rocks and some dead bricks, and I still had a collection of the big blocks from that old drywell they'd dug out. At this point I couldn't reach the bottom of the hole even with both shoulders down into it, so I'd have to carefully drop my new content in and push it around with long tools.
|I leveled off the bottom a bit and shoved in one ratty corner of the filter-fabric, and sort of arrayed it around the periphery as best I could from so far away. A first layer of drainage stone went down with a couple of bricks embedded, and then I managed to lower one of the drywell blocks in a ways and drop it about in the middle without screwing everything else up. Holding that up more vertical with the crowbar, I filled in more of the drainage stone around all that. The idea was to keep the solid volume-filling objects in the center and have the permeable stone around it contacting the soil, to yield as much actual infiltration area as possible.|
|Here's where a little engineering came in. The pit had widened into sort of a "bulb" at the bottom during digging, which was fine for the actual drainage purpose. I had just enough stone to fill in the rest of the lower area, tamping it lightly into place while continuing to array the filter-cloth around it. But for the shot up through the less permeable soil I wouldn't need the full width of the hole -- just enough to let water run down *to* that larger area, so to keep filling would require building a calculated arrangement of stone and dirt. To guide where the column of stone would neck down a little I fetched a piece of leftover 6" PVC pipe from the HRV duct job and stood it on top of what I had in there so far, which would then keep things separated while I continued working it upward. The pipe wouldn't stay here, it was just a tool at this point -- sort of serving as a moving cofferdam.|
|The very last of my stone supply went into the bottom of the pipe, so then it was off to the garden-center to buy more rocks. These were the cheap four-bucks-a-bag "river pebbles"; well worth it at this point since while I might have been able to dig up and clean off more rocks from an old dirt pile in the back yard, that would have taken a lot more time and I wanted to get the rest of this built.|
|I had to wash this stone anyway, as for some reason it usually comes covered in fine sand or clay of some sort which I certainly didn't want down my percolation column. I scrounged up another piece of some kind of landscaping fabric to add in on top -- weed barrier or something like that, I have no idea but it could help continue the separation layer higher. Slowly I worked the whole construction upward, alternating more rocks tamped down into the pipe, more dirt packed in around it, and raising the pipe a little at a time out of the filter-fabric "sock" so these layers could meet solidly at the boundary. underneath its edge.|
|At the end the pipe came completely out again and I had my nicely delineated column of stone sticking up out of the relatively impermeable soil, isolated against dirt intrusion by all the stuff wrapped around. Now it was time to design how water would actually get into this and not carry too much surface soil along with.|
|The top of the stone column transitioned into a small mound with a couple more remaining scraps of filter-fabric around it. This would rise slightly above surrounding ground level and serve as the catch area, with the top edge of the fabric as sort of a small dam against soil erosion.|
|So here's a conceptual as-built of what I wound up with. The rock column and "bulb" of infiltration area down into sandy subsoil, lined with filter cloth, with some extra filler volume inside but not near the edge. Or put another way, a little drawing fun with Gimp. Not exactly like the stuff that had gone in under the street, but totally the same basic idea.|
I wanted to test this, but it had been dry for several days with no
immediate prospect of rain. Another little house project had been to
complete a small general-purpose pass-through added to one of the
ex-basement-window blockoffs in front, just big enough for a hose
or a wire. Just a short nipple of PVC pipe set in place through the
sandwich, with a screw cap on either side.
Not only could I test how to rig up a water feed in
either direction, with the hose available out front I could run a bunch
of water into my new creation to see how it would do with a reasonable
volume coming in.
Fifteen minutes of full-on hose and a couple of extra bucketfuls dumped in, and the pit took all of it without any sign of backup. Was that enough to fill the volume if it had all been impermeable? Not sure.
|The final step was adaptation of the surrounding grade, and one of the chief areas that I wanted to encourage water to flow *away* from was the end of the driveway near the other side of the hosta plants. So it seemed like a cute idea to sort of stealth a little sloped trench in *under* the hostas aiming at the target area, and I could continue adjusting various ground slope nearby toward it later. I held the surrounding plants back and pulled up maybe two of its root bundles to make way for the water channel, and backfilled with a bit more stone to help keep it clear for water flow. These plants vanish completely over winter and come back with great enthusiasm fresh out of the ground every spring, so I wasn't too worried about long-term damage to them.|
|After restoring some of the original topsoil layer and trying to transplant a bit of moss to right around the rock mound, it was done! If I could get this lovely moss that's all over the backyard to actually take over in front I'd be happy, but it may simply get too sunny for it out here.|
Apres dig, le deluge
I knew my little hose test was likely not anywhere near real-life "hundred year event" flood conditions, but just a few days later it began to look like I might get my answer soon. Another "polar vortex" jet-stream event was pushing a big cold mass down into the whole eastern half of the US, giving rise to all kinds of flood warnings because it was creating a sharp boundary against some warm and humid conditions that had been there before. The moving pattern was quite obvious on national radar the morning before the storms arrived, and they were saying the Northeast was going to potentially get walloped pretty hard. It didn't look like so much on the national scale, but the thin line of turmoil was already spawning some pretty serious squalls in Texas and elsewhere.
|Most of the day's precip seemed to split to the right or left of where I was, but later in the afternoon a sharp and intense line of rain advanced across the area and *amply* made up for any prior lack.|
|Wham! It was a total cloudburst, rain heavier than I'd seen it for quite a while around here, and instead of hunkering inside while it passed I shouldered up an umbrella and the camera and went outside to patrol not only my new pit but all the various drainage items of interest as things intensified.|
|So far so good on my structure ten or fifteen minutes in, as the ground around it began to saturate and a healthy flow was arriving via the little trench under the plants. I could see a nice volume of water dropping in at the edges of the central rock mound, hopefully *not* carrying too much silt with it.|
|The trench, in fact, started receiving enough volume to completely cover the rocks in it because of the pseudo-dam formed by the filter cloth. But anything that came over that, the main column was still handily taking in without any sign of backing up. In the past, this whole area would have been a 3 inch deep puddle by now.|
I walked around to inspect the other items. The new berm across the end
of the driveway was mostly doing the right thing, sending street water
off toward the main catch basins, although a little flow seemed to be
escaping before where the berm really began and trickling into the
[I did add a bit more fill across there later, using dirt conveniently recovered from further end-of-driveway-area regrading efforts as they took place.]
|The neighbor's berm was doing okay, although I still don't understand why they curved it off like this instead of continuing right across in front of the grass to reach the actual catch basin. It was at least still a little higher than the grate level, and thus kept the main flow out of his driveway.|
|My downspout pits were doing what they're supposed to, filling up fairly quickly and sending the rest overboard. I keep pieces of nylon window-screen weighted down with rocks over these now to help keep crud out, which works quite well.|
|Then I went back over to the new pit, and found that it was *just* starting to back up! I wasn't sure if I should have been surprised by this or not. A *lot* of water was clearly going into it, after almost half an hour of steady lashing downpour.|
|As I watched over the next minute or so the area filled up a little more, so at this point my pit was evidently beyond maxed out. The trench from the driveway area was a lake by now. But the puddle around everything here didn't actually expand too much, which it would have almost instantly if this had been a hard stop on accepting water. Another hint that downward flow was still pretty healthy was given by the small collection of organic detritus nicely centered on top of the rock mound -- a little telltale as to where flow was still going.|
|Back out at the street, the "uphill" side catch basin was a raging torrent as water thundered into it from all sides. I stood here for a while watching closely, wondering if this would be enough storm to back up *that* massive system. This was the side of the street where most of the yards uphill from here slope down toward the roadside, as opposed to the other side where some of the water can actually spill off into the land alongside [which is why the "neighbor's berm" flow was substantially less]. Since it's all connected in parallel under here it doesn't really matter, but it probably does mean that this side's basin will generally crap up faster than the other one. Hopefully those guys with the little dump truck and the big metal claw would keep on top of this.|
|Overall it was a solid drencher lasting 30 or 40 minutes, easily dropping an inch and a half of rain before slowly moving on. It finally started to slack off, and as it did I was pleased to see that my new pit recovered very quickly and cleared before any of the surrounding stuff could even finish dumping into it. So this was a nice brute-test of my excavation fun, and now I had a pretty good idea of its limits. It does what it's supposed to -- drains the low spot, and I was confident that it would help handle a lot of the nearby snow-melt flow as well. It would be a while before I could observe that.|
Mitigation and the Melt
Soon after this page was originally created, the driveway got completely
redone with a permeable-pavers job. Story is here.
This would pretty much mitigate the spring mud-pit issues, but the fact
remained that the low spot in the front yard still needed to drain to
somewhere so the pit would continue to serve a useful function.
Fast-forward to the Great Snowy Winter of 2015, with its seemingly endless digging-out and ice-busting. The key thing was that it stayed very cold for quite a while, which had some interesting effects. At one point in early March I shoveled a short path from the driveway and dug out the area over the pit, thinking that it might be worth being able to watch the effects of the Big Melt which *had* to come along sometime.
I was right. A couple of warmer days with some rain generated a lot of
melt water, and I soon observed that the pit wasn't accepting *any* of
it. This was odd because while the ground was fairly saturated
by then, it shouldn't have been *so* waterlogged that the pit would
effectively be completely full again. But there the water
sat over the hole, completely unmoving.
The answer came about a day later, as the warm spell continued. All the backed-up water suddenly disappeared. I can only conclude from this that the "neck" of the rock column had become choked up with ice maybe a foot or more down, and it took an extra day or so for the warming to penetrate into the frost layer enough to open it up. Once it was clear, it all stayed high and dry as the melt continued. Phew, the thing was still doing its job as designed.