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. 

*     *     *     *     *     *     *     *     *     *     *
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.

Construction - The Last Major Dirt Work (Cont'd) - Final Grading, Loess Lessons and Rain Garden Design

The previous post on The Last Major Dirt Work emphasized the retaining wall construction that needed to be done before the final grading, topsoil replacement and seeding could be done.  This post covers two more sections of the insulation/watershed umbrella as well as the grading, top-soiling and seeding. It also introduces the subject of rain gardens by looking at their generic design. The next post will explain how rain gardens were used specifically for our property.

Reminder: clicking on any photo enlarges it for better viewing.

(Nearly three months have elapsed since my last post.  I forsook the computer for a tool belt full time as we family members rallied to rehab my sister-in-law and her husband's Texas home that was flooded by Harvey. Their homelessness ended the first weekend in January so I am home and blogging again.)

More Insulation/Watershed Umbrella

Umbrella installation behind the garage; notice the plastic
 sheeting, two layers actually, emerging from the bottom of
 the retaining wall to be shingled over the sheeting in the
 umbrella; similar flaps lie behind the wall waiting to be 
shingled under the umbrella behind the house when it is
 installed

At this juncture, the umbrella around the
house is more than half done.  Part of it lies under the concrete floors of the screened porch and garage and the rest that is done lies in front of the house. The two umbrella sections mentioned here lie behind and in front of the garage as well as in front of the main entrance to the house.  The usual layers of sand, insulation, sheet plastic, recycled carpet and topsoil were used as described in detail in the previous post on the umbrella installation in front of the house.

Final Grading
The dirt work discussed herein lies in

Topsoil over the umbrella behind the garage ready for
 seeding; the last major section of umbrella yet to be
 installed lies beyond the retaining wall, i.e., behind the
 house

front of the house and east of the garage.  Since we own (and live in) the old farmhouse east of the building site, we have the option of carrying the grading onto that property as far as necessary to flatten contours and funnel surface runoff from both properties into a series of rain gardens.  After moving superfluous soil out of the way of smooth contours, berms and rain gardens, and, after laying down +/- 3" inches of topsoil, we were able to plant grass seed in time for germination before cold weather -- barely.  Grass is an interim crop to hold the soil then, as discussed in a

Friend Kerry helps with the umbrella installation in 
conjunction with setting forms for the concrete
 sidewalk and garage apron

previous post on native plants, the grass will be replaced almost entirely by environmentally-friendly (no mowing, no fertilizing, no watering) plants that are native to the Midwest.

Harnessing the Impetuous Loess
Building in the Mississippi River bluffs has been a mixed blessing.  The rough terrain provided the south facing hill that we needed for a passive solar build and, quite unexpectedly, within a community rather than out in the country, but the wind-blown loess soil that covers the bluffs has taken us to school.  We have learned that it is quite stable when left undisturbed and covered with vegetation because the grains of soil are elliptical in shape and, when lined up like soldiers by wind action long ago, were locked together. There are places nearby, for instance, where cuts through loess for interstate highways have shown very little erosion.  When disturbed, however, the loess (aka silt) is unstable in both wind and heavy rains --  it blows like talcum powder and erodes like lumps of sugar.  Controlling erosion during construction has been a major challenge; the retention pond that catches runoff before it can leave the property has been re-dug and cleaned out at least once a year since construction began. Parenthetically, plants thrive in the loess despite its being almost plaster-of-Paris-hard when dried out. 

View of building site before footings
 were poured; the denuded areas are
where dirt was excavated; the
somewhat denuded areas north of
the house are topsoil and subsoil
storage piles; notice the solar
collector in front of the house; the
denuded area surrounded by 
greenery in the middle bottom of the
 photo is the retention pond catching
 runoff during construction that was
 converted into one of the rain
 gardens 

Unavoidable Site Disturbance
Unfortunately, it is impossible to build an earth contact, Annualized GeoSolar* conditioned house without moving a lot of dirt, which violates one of the major tenants of green building -- that of minimizing building site disturbance.  Not only was it necessary to excavate into the side of the hill for the house itself, we also had to install the insulation/watershed umbrella that extends sub-grade +/-20 from the floor of the house in all directions.  We had to use soil-disturbing measures to amend the water table so that it did not compromise the AGS system and we had to install the large solar collector and conduits for the system.  And the final grading has been extensive in an attempt to direct runoff into strategically located rain gardens so that it does not soak into the soil where it would compromise the AGS system or stay on the surface and carry silt and contaminants onto the street or into the neighbors' lake.  The solution to both problems is a series of strategically-place berms and five rain gardens.


How Do Rain Gardens Work?
In a nutshell, the purpose of rain gardens is to corral surface runoff long enough for it to soak into the soil and, if flowing, flow underground where it is purified instead of on the surface where it carries pollutants. The plants in and around a garden are tiered such that those liking wet feet, such as sedges, occupy the bottom, those who can tolerate a wet soil, but do not like wet feet, rim the slope of the garden and those liking it a bit drier form the periphery along the top edge of the garden and often spilling over onto the surrounding grade.  In our case, the rain garden plants will be native rather than cultivars and non-natives.  For an authoritative resource on rain gardens, link to the Missouri Botanical Rainscaping Guide.


Percolation Test
In order to avoid stagnation that would kill plants and incubate mosquitoes, a percolation test is done before digging to be sure that the retained

Downloaded photo of a rain garden containing
what looks to be mostly sedges in its deepest
section; notice the graveled overflow to the left

 water will soak away fast enough.  At least two post-hole-size holes are dug 24" deep a few feet apart in the lowest area of the proposed rain garden, usually just inside of the proposed berm that will serve as the dam.  The fresh holes are filled with water and the drop in water level -- the "perc rate" -- is timed over the span of a few hours.  Then the process is repeated the next day.  The first test simulates rainfall on relatively dry soil; the second simulates rainfall on soil that is already saturated by prior rain.  A perc rate of 1/2" per hour for the second test is considered minimum for a functional rain garden. The rate for the first test is typically faster.

The perc rate per se is not the entire story however.  A good share of the water collecting in a rain garden is utilized by the plants before it can soak into the deeper soil making the water disappear much faster than if the water-loving plants were not present.

The exact manner in which rain gardens are incorporated into our landscaping is the subject of the next post.
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*Click on "Featured Post" in the column at the left for information on AGS