Benefits of a Rain Garden

The whole reason I'm installing my vernal pool, or rain garden, or whatever it is, is to restore a little bit of natural ecology as well as reduce the amount of urban runoff into our storm drain systems.

"But, John," you protest, "how much runoff are you really eliminating with your little 110 square foot pond?"

Glad you asked. Hey, math time again!

110 square foot pool surface area
* 1 foot average pool depth
__________
110 cubic foot pool volume
* 7.48 gallons / cu. ft.
__________
822.8 gallon capacity
* 66% full
__________
543 gallons

That's right, in Friday's storm my little pond held well over 500 gallons of rainwater — not counting what it percolated during the storm — and ultimately released it back to the earth rather than letting it flow out to the gutters or dumping it out at my home's foundation, whether into a French drain or not.

543 gallons and counting? I'm feeling pretty good about myself right now.

Rainy Day Math Fun

Well, wasn't that refreshing?! Nothing like two inches of solid rain to wake up the winter, and more is coming down today. It's the perfect test of my vernal pool, which began filling around 8 a.m. yesterday and ultimately filled to about 6" below capacity within 12 hours. The water comes primarily from our roof, with about 2/3 of its downspouts tied into a central outlet at the mouth of the pool. (The other downspouts let out onto the permeable driveway, so there's some percolation, although I intend to tie those into the pool as well.) There also was some water siphoned in from drainage problem areas in the back yard, but probably less than 20 gallons.

A couple of things were really amazing to me. The first is that my math was more or less right! Assuming a maximum daily rainfall of about 1 inch (not quite the record, but also above the norm), I wanted the pool to hold all the runoff from my 1200 square foot roof. Ready for some algebra? Follow along:

1200 square feet
* (1 inch of runoff ÷ (12 inches/foot))
___________
100 cubic feet of runoff capacity needed

So I built the pool's surface area to about 110 square feet, at an average depth of about 12":

110 square feet
* (12 inches ÷ (12 inches/foot))
___________
110 cubic feet capacity

Sure enough, this storm dumped 2 inches of rain; and again, the pool collects about 66% of the roof area:

(1200 sq. ft. * 66%) = 800 sq. ft.
* (2 inches ÷ (12 in./ft.))
___________
133 cubic feet of runoff

But wait! Why didn't the pool overflow? Ah, let's not forget about the soil's absorption capacity. My soil isn't completely compacted, so it should still percolate at a rate of at least .75 inch per hour (not that I've actually tested it yet). We don't know the rainfall rates per hour; we do know the day's average was 2" over 24 hours, so let's assume it fluctuated between 1/16" and 1/2" per hour. The pool was receiving those rates (depths) of rain across its 110 sq. ft. area, as well as the 800 sq. ft. of roof.

800
+110
___________
910 sq. ft. total surface area

To determine what percentage of "its own" area the pool was receiving:

910 sq. ft. total area
÷ 110 sq. ft. pool area
___________
8.27

The pool was receiving 8.27 times, or 827%, of its own area. Multiply that by the rate (depth) of rain to find out what volume it was taking on:

8.27 * 1/16" per hour = .52" per hour (assumed minimum flow into pool)
8.27 * 1/2" per hour = 4.14" per hour (assumed maximum flow into pool)

This means at the peak periods, precipitation was outpacing percolation by about 4 inches per hour. That's why the pool filled as much as it did. But that was a small minority of the time: more often, percolation outpaced precipitation by .25 per hour... and that's why the pool never overflowed.

(Knowing that the pool takes on 8.27x its area, we can also determine the "equilibrium" rainfall rate:

.75"/hour pool percolation rate
÷ 8.27
___________
0.91"/hour precipitation rate)

(By the way, the second amazing thing to me was that, after about 12 hours of no rain, the pool had no water this morning. Let's see what the fill/percolation rates are today, now that the soil is saturated.)

This kind of math is what landscape designers and architects have to do all the time in designing drainage systems, water retention basins (like vernal pools), even landscape irrigation systems. It's a lot more complex than just this algebra, but I hope it shows a little bit of the "behind the scenes" work we do — it's not all just pretty pictures.

If you've hung on this long, you're probably doing the math yourself. Please correct me if I've missed something, or feel free to ask questions. It's been a fun rainy-day diversion.

PS: This whole episode has alerted me that I might consider renaming my little water feature. Technically, a vernal pool has the geographic feature of a semi-imperpeable "pan" beneath the grade which slows drainage throughout the winter and spring. Mine, obviously, drains fairly readily; so I suspect rain garden — although becoming nauseatingly trite — may be a better description. Stay tuned.

Joyous New Year

2007 has been a remarkable year for me: my Landscapers' Challenge victory finally aired; I was honored to lead a popular class at Gamble Garden; my practice expanded to include design modeling; and the quantity and (more importantly) quality of my projects increased even as I lost far too much sleep working toward my landscape architecture degree and license.

There were also a few achievements, mostly personal, that didn't quite get achieved. My vernal pool is still being formed; my narcissus bulbs still aren't in the ground; my back yard is little more than turf and a wall. But, I'm told, it's normal for the shoemaker's children to run barefoot (how cruel does that sound this time of year?!), and while my neighbors who told me they wanted to replace their lawns with water-wise plantings have installed… more lawns, I'm reducing or even eliminating the lawn in my own yard and my clients'. Score one small moral victory.

I've also had lots of opportunities to think and talk about why I do the work I do. And it's been gratifying to know that in the end, it's all about the moral victories. Every square foot of lawn I don't spec saves a little bit of water for the rest of the planet. Every child-friendly garden I design inspires another generation to get involved with the natural world. Every time I get to include a little bit of art in a garden, or make it a unique reflection of its unique owners, I help create happiness. Not a bad way to make a living.

So here's wishing you a new year that's filled with joys small and large. May nature surprise and delight you, and may we all make this planet just a little bit nicer, for our children if not for ourselves.

Happy 2008!

Fall into Winter

Foliage-wise, this has been one of the best autumns I can remember. And now that all those lovely leaves are coming down to earth, we're reminded that winter is just around the corner (here it comes at about 10:08 pm Friday night).

The gorgeous displays of reds, oranges and golds is especially striking this year because [a] it's been a very cold and dry season, both of which stress trees into withdrawing their green chlorophyll earlier and more quickly (thereby revealing the natural anthocyanins and carotene colors), and [b] the lack of rain and wind has left those vibrant leaves on the trees longer. Now that the storms are arriving, we're getting more of what we're used to:

Autumn gardens are among my favorites, not only for their foliage colors but because of the way the winter light plays on details such as leaf margins and seed heads, such as the tall Miscanthus and Calamagrostis grass plumes that look splendid all golden and backlit. Other specimens, such as the coral-bark Japanese maple and the river birch, start showing some of their best assets that are hidden, or at least overshadowed, most of the year. But there's no denying that colorful foliage is the star of the season, especially when set against a backdrop of dark evergreens such as pines, redwoods or even magnolias. Some of my favorites include:

Pyrus calleryana (Callery Pear) — different varieties have different colors and forms

Ginkgo biloba (Maidenhair Tree) — venerable forest giant with clear yellow to gold color; leaves all seem to drop within a few days

Lagerstroemia hybrids (Crepe Myrtle) — a three-season star, with gorgeous winter bark and bright summer flowers to complement fiery autumn foliage

Viburnum cultivars — small to large shrubs with bright red to purple fall color

Hydrangea quercifolia (Oakleaf Hydrangea) — orange to red leaves even as the flower panicles hold on

Blueberry cultivars — yes, the edible blueberry bush will give an amazing fall show if you dare withhold water for a while

Ceratostigma plumbaginoides (Dwarf Plumbago) — Fast growing, shade tolerant ground cover colors bright crimson; a nice surprise at ground level

Vitis californica 'Rogers Red' (California Wild Grape) — A great choice to slipcover a garage or obliterate a fence within a few seasons, stops traffic when it turns wine red

As you're out and about over the next few days, I hope you'll make it a point to notice and appreciate the colors before they're brought back to earth by the rain and winds. If you see something particularly stunning, feel free to send it in. And if you have favorite autumn plants of your own, please share!

Pardon the Interruption

Where the hell have I been for the last two months?! Let's just say this has been one of the busiest autumns I've ever had, and unfortunately doing the work to my usual exacting standards has taken precedence over musing about it. But I've still had a lot on my mind. For starters:

I'm proud to announce Verdance now offers design modeling as one of our premium services. In the last two years I've transitioned from hand-drawn plans to the enhanced precision and visualization of CAD and SketchUp-rendered designs. Now, I'm also able to replicate our designs to scale, whether to show the look and feel of an entire site or create a prototype of a specific feature. Of course I offer this service to my clientele; and I'm available as well to consult for other landscape designers and architects in bringing their design ideas to life.

Back in the virtual world, my series on Christmas trees, real or fake or organic or toxic, is due for a new chapter; the G Living Network was kind enough to do the heavy lifting for me.

After ordering about 1,000 narcissus and tulip species from Brent and Becky's, I've been hurriedly preparing my front yard to receive them and ultimately have concluded I'll have to plant them in shallow graves and unearth them for a proper replanting next fall. That's efficient.

The time for getting native plants into the ground has come and is going... and have I gotten my own vernal pool (aka billabong) prepared? Nooooo..... but I will at least have some "in process" photos here soon.

I've certainly missed having the time to write here; of course there's also some wondering whether anyone is reading (or not reading) what I do write. If there's anything you'd like to hear me wax on about in the coming months, I hope you'll let me know.