Construction - The Last Major Dirt Work (Cont'd some more) - Rain Gardens

Anal retention alert:  "Rain gardens" are not exactly on the minds of most people so the detailed information about rain gardens here and in the previous post will undoubtedly test most folks' indulgence.  But I am basing the need for information on my own ignorance about the important sustainability role that rain gardens play.  Despite building into the side of a hill, they were never part of our original design.  It wasn't until a few years ago that our membership in the Wild Ones organization made me realize that our property would be the quintessential beta site for rain gardens.  And, since my green thumbed wife, Dorothy, and I were both hopelessly uninformed, I am assuming that most page-viewers here, despite their interest in sustainability, could be uninformed as well.

When people ask, "Why rain gardens?", I say that our intention is that every drop of rain that falls on our property leaves it underground and purified rather than on the surface carrying silt and pollutants.  What follows is the description of the initial dirt work involved with realizing this goal.

The treatment of rain gardens in the last post was of a generic nature.  The discussion here is about the five rain gardens that we need to control runoff from our hilly property, where the grade falls at least 30' from the highest elevation behind the house to the lowest elevation in front of the house.  At the time of this writing, the gardens were roughed in for a trial run before final contouring and topping off with the rain garden mix in time for planting with native plants next spring.  Out of the five gardens roughed in, two of the berms (dams) failed at the first torrential rain and had to be reconfigured.  

(Remember: the pictures can be enlarged by clicking on them.)

Encircled are the two highest rain gardens next to the
house in which we live

Rain Garden Siting For Our Project

The next highest is beside the garage of the new house
and is one of two that failed

The rain gardens at the highest elevation are on the property where we live east of the building site. They are modest and intended primarily to catch the runoff from impervious surfaces -- house and garage roofs and driveway --  before it can erode the hill sloping down towards the building site.  The next highest is alongside the new house garage but down-slope enough not to threaten the AGS system*. Its perc rate was a non-issue because it lies directly over and is drained by the east-most gravel-filled French drain that catches any deep water flowing towards the AGS conduits.  However, initially the garden failed -- it did not have the intended capacity and had to be dug deeper and wider and the dam raised.  At the time of this writing, it has held against several heavy rains

The two largest rain gardens lie further down-slope. 

The garden in the foreground is the one discussed in the
next section and the second one to fail;  encircled
is the berm that separates it from the street;encircled
 in the distance is the former retention pond

One, with the help of a continuous berm paralleling the street, catches all of the runoff from both properties on the east side of the new driveway. It, too, failed, perhaps due to the excess water from the other failed garden above.  It was modified and has withstood several heavy rain events..  The overflow from it passes through the driveway culvert into last garden that is merely a shallower version of the retention pond that was maintained during construction to keep runoff from leaving the property.  The pond can be seen in the Google Earth photo in the previous post.  Any water the last rain garden cannot handle leaves the property as it did before construction began -- through a culvert under the street and into the neighbor's lake.  (Edited note: as described in the 2019 update below, another rain garden was added downstream between the converted pond rain garden and the culvert.)

The overflow channels for all of the gardens will be
rip-rapped to control flow rates and to add interest;
large river gravel/stones would be more attractive than
 than rip-rap but are beyond our budget

Rain Garden Construction and a Battle with Glacial Till (an anecdote) 
Unfortunately, rain gardens for our property were not as simple of scooping out a shallow depression then adding a berm and overflow 

Over-excavation for a rain garden (4-5' deep); the
 big chunks to the right are glacial till

outlet on the downhill side.  I already knew going in that the gardens were likely to lie over the same glacial till (hardpan) that we had encountered throughout construction and during final grading.  Sure enough, hardpan was situated immediately below the topsoil at the most critical site.  Knowing that the hardpan layer would be only a few feet thick, I continued digging with the trackloader until I was below the layer.  The length of the ramp that was necessary for safe digging with the loader, coupled with a reasonably sized flat bottom in the hole, created a long, wide and deep cavity.  In the bottom, I dug perc test holes but an overnight rain rendered them moot.  The deepest part of the excavation filled with a foot of water that had drained away the next time I looked at it, a few hours later.  The water from an inch and a quarter rain a week later was gone reasonably fast as well.

So now the issue became how best to fill 

Same cavity after addition of a truckload of sand

the cavity back to rain garden depth without compromising percolation.  Thanks to input from friend Charlie Pitts, a Certified Naturalist, who had helped us lay out the gardens in the first place, we arrived at a solution for such an atypical situation.  We filled the excavation with a tandem truckload of sand to within +/- 3' of the original grade and plan eventually to top it off with a "Rain Garden Mix", i.e., a special soil formulated from topsoil, sand and compost that is available in bulk from a local supplier.  The remaining question is whether the distribution of the plants as described in the previous post will survive in a rain garden filled this deep with sand.  Maybe a different choice of plants will be necessary.  

The rain garden mix will be added to all of the gardens as soon as we know (a) that the gardens are functioning as planned, (b) the cover crop of grass on the denuded hillsides is holding the soil in place and (c) whether siltration from hillsides occurring before the grass is established has either filled the gardens to the proper depth or overfilled them to the extent that partial re-digging is necessary. 

Heavy Rainfall

The garden beside the garage was one of two that that
failed 

Rain gardens are not designed to catch all of the runoff from "frog strangler" rains.  Heavy downpours or worst yet, a series of downpours in a short period of time, produce more water than the rain gardens can handle so an overflow must be incorporated into the downhill berm (dam).  Thanks to Charlie, we learned that the overflow does not have to be especially wide or deep because the water passing over it is typically more like that from a gutter downspout than through a roadside ditch.  Accordingly, after the berms were finalized, we sculpted shallow troughs through them which we covered with weed barrier fabric which will control erosion in the short term and weeds later.  At the time of this writing, we had not yet covered the fabric with fist-sized stones (rip-rap) that will slow the flow and prevent erosion downhill.  

Another Month of Dirt Work Should Finish the Job
In the St Louis area, January and February are the two months with the least amount of precipitation.  Our warmer winters with less

The berm has been reconfigured for another test run

frozen ground seem to allow more dirt work but winter cloudiness and what freezing and thawing we do get cause muddy conditions that complicate things. Nevertheless, my goal is to finish the dirt work behind the house in time to plant grass seed in March or April before the spring rains.

The garden beside the garage presently drains half of the
 area north of the house because the 
insulation/watershed umbrella has yet to be installed
 behind the house; after it is in place the final grade
 will direct runoff to the north instead of curling 
around toward the south and into the garden

The remaining dirt work involves mostly the insulation/watershed umbrella and final contouring of the grade north of the house. The goal of the grading is to force as much drainage as possible northward towards a creek valley instead around the ends of the house where it might overload the rain gardens in front of the house. 

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Update - Late Fall, 2019 (nine months after the original posting)

The last of four major rain gardens showing the spillway
to the culvert (arrow)

I am happy to report that the rain gardens have been thoroughly tested.  With the help of a few weeks of warm weather late into the autumn and with the use of ground fabric where necessary, we were able to get the last of the new grass established before any major rains.  Then in early November, we had several inches of rain that filled the gardens brimful.  The last garden in the series, situated at the corner of

The rain garden next to the garage with water entering
its spillway to an intermediate garden shown in the next
photo which, in turn, overflows into the two last gardens
seen in the background here and in the photo above

the property where the water traditionally left the property via a culvert under the street, also filled but not enough to overflow onto the spillway leading to the culvert.  And, as rain gardens are supposed to do, they held the water for only a couple of days before it soaked into the soil, meaning that all of the runoff left the property underground in the water table and purified instead of on the surface carrying pollutants.  When they are planted with the proper native plants this spring, the soak time for the gardens will be even faster.  Also, at the time of this writing, the waterways between gardens had been protected from erosion with rip-rap. 

The last major rain garden project was at the end of the slope behind the house that drains a large area northward.  I had previously used excess dirt, after all necessary backfilling had been done, to build a

The denuded surface is newly-added rain garden mix
will eventually support native plants that like wet feet.

berm across the back of the property to keep water from cascading down a steep embankment into a wet-weather creek.  Despite it being too late for seeding, I refined the berm, dug a new rain garden and created a spillway.  As described above for one of the other rain gardens, I had to penetrate through glacial till, making the garden so deep that it will have to be partially refilled with topsoil and rain garden mix before it is planted next spring.  Immediately, a big rain tested it -- despite its depth and the huge amount of runoff it could hold, it failed to overflow and its soak time was also only a couple of

North rain garden roughed in with berm on the left,
spillway in the distance and, on the right, termination
of the long slope originating at the back of the house.

days. 
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*  The passive solar system for the house is called Annualized GeoSolar.  For information, click on "Featured Post" in the column to the left.