PARK–structural considerations
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Following is a list of structural considerations we have to solve
- How will the rain barrels/cisterns be supported above ground level?
- Indeed, what sort of rain barrels/cisterns?
- How are we going to keep the living wall plants and growing medium vertically in place? Check out the links on the right for ideas.
- The distance between southwest corner of the roof to the garden areas is a horizontal span of 100ft. We need to establish what we’re using for down spouting, the incline and how it will be connected to the wall.
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Here is round one of my comments, I’m still working on this and would appreciate ANY feedback, corrections, better assumptions….
Structural Considerations all depend on how much water we need to store and distribute, then we can decide on type of storage and how, where to attach it.
Need to drain system for winter, and for cleaning.
How much water is needed to sustain the garden each week (less in winter…)
– Lawn areas
Calculations should be similar to those for Living Wall, need to know area (sq ft)
of grass to water. This looks like a much larger area to cover.
Anyone know how much water grass consumes?
– Trees/shrubs (4 columnar fast growing trees for shade and height)
Depends on trees and depth of roots.
Assume we can run a dedicated drip line to each tree.
– Living Wall (36ft long and 4ft tall)
First approach:
Rain garden should be 15-20% of the the size of the drainage area
(per OSUE Rain Garden Guidelines)
Given this, a 36′ X 4′ garden needs 720-960 sq feet of roof drainage
(144 sq ft / 0.15 = 960 sq ft roof, 144/0.20 = 720 sq ft roof).
To give a quick idea of this size, 960 sq ft is a 50′ X 20′ area, or 36′ X 27′
Using 3.1″/month rainfall (based on chart from city-data.com for Coshocton),
this is 248 cu ft of water (3.1/12 * 960).
248 cu ft * 7.48 gal/cu ft = 1855 gallons/month
or 1855/4.2 = 442 gallons/week.
Second Approach:
“In general, plants need 1″ of water per week”(per OSUE Rain Garden Guidlelines)
Assuming wall is packed full of plants,
144 sq ft * 1″ = 12 cu ft of rainwater per week for wall.
12 cu ft * 7.48 gal/cu ft = 90 gallons of water per week.
Quite a difference in these answers, 442 vs. 90 gal/week!
(did I get something wrong on First Approach – were they
talking about drainage from a roof or road, or from
a nearby grassy/planty area that will absorb a lot of the
water before it even gets to the rain garden?)
Plus, there will be some lost through drainage at the bottom, and flow may not be
even, so we’ll need more water. For estimating, allow 1.5 times as much.
So worst-case is 660 gallons per week needed (for WALL ONLY).
How are water needs distributed over time?
This is ONLY FOR THE WALL for now:
If we got a good rain once each week, we would
only need to store 660 gallons.
If we want to allow for 2 weeks without rain,
we’ll need to store 1,320 gallons.
Let’s assume we need 1,200 gallons for storage.
(expect this to be closer to 5000 or 7000 once grass & trees are included)
More later!
Update to my last post (earlier on April 15):
Estimate for water needed by sod
Assuming 3500 sq feet and 1″ of rain / water per week,
we need 3500 * 1/12 = 291.667 cu ft of water.
291.667 * 7.48 gal/cu ft = 2,182 gallons per week.
That brings the total so far, without the trees, to:
2,182 gpw for sod
660 gpw for living wall
——–
2,782 gpw total, a bit less than I expected.
If trees need, say, another 1,218 gpw, then we’ll need 4,000 gallons per week total.
If we store 1.8 weeks of water, that would be =7,200 gallons capacity in storage system.
(1.8 is just another assumption based on previous post where I went from 660 gpw to 1200 for storage (1200/660 = 1.8) for the living wall).
Do we want more for use in washing off decks and platform, etc.?
Part three of my thoughts on water requirements and capacity and other structural considerations…. Trying to get some thoughts out quickly so we can get discussions going! I’ll be revising these numbers as we learn more.
How much rainfall do we get each week? – is there enough, do we need all of it, some?
Should we also include water from other neighboring building? (we may NEED to do this, see below)
Monthly average precipitation (estimated from graph on
http://www.city-data.com/city/Coshocton-Ohio.html)
Jan 2.5 Feb 2.5 Mar 3.0 Apr 3.7 May 4.0 Jun 4.1
Jul 4.3 Aug 3.9 Sep 3.1 Oct 2.8 Nov 3.2 Dec 3.0
Assuming we don’t want to worry about the plants from Oct through Apr, the minimum is 3.1″/month.
Coshocton Antique Mall & Good News Bookstore roof will give 3,500 gallons of water per inch of rainfall.
3,500 gal/in * 3.1 in/month = 10,850 gal/month from the roof in the driest growing-season month.
Rainwater from the roof per week: 10,850 / 4.2 = 2,583 gallons per week.
Required water: 4,000 gallons per week.
We won’t have enough water in driest months, even if we capture ALL of it.
In wetter months there would be about 4″ rain per month, and the math becomes:
3,500 gal/in * 4 in/mo / 4.2 wks/mo = 3,333 gal/wk, still short of the 4,000 gallon target.
I recommend building for more water than we can get, and supplementing it with hydrant water when needed. We may want to trim back the “1.8 weeks of capacity” assumption but it’s a tradeoff between using most of the roof runoff or losing more of it when we get heavy rain for a few days close together.
Also, the grass and trees will get whatever would naturally fall on them anyway, it would just be SUPPLEMENTED by the water collection system. I’ll try to work out some revised numbers (again!) taking this into account.
We had a suggestion to use large water pipes for storage. If we work with 8″ diameter PVC pipes, in 12′ lengths, we would need 229 pipes! Here’s the math:
volume = area * length, area = pi * radius * radius, and radius is 4″ or 1/3 foot, so we get 3.1416 * 1/3 * 1/3 * 12 = 4.1888 cu ft. 4.1888 cu ft * 7.48 gal / cu ft = 31.3 gallons per pipe. 7,200 gal / 31.3 gal/pipe = 228.79 pipe sections! This will drop when I exclude water already falling from the sky to the grass and trees, but will still be a lot of pipe sections.
The rain garden (i.e., living wall), “if properly designed… should hold water for a maximum of 24 hours” – neo raingarden manual.pdf
Are we diverting water from some other beneficial purpose?
Water Sculpture?
Can flow down from the roof go through any mechanical gadgets (spinners, water wheels, etc.)? I’ve seen water sculptures and they’re fun but I don’t know if many people would come out in the rain to see this one (if we built it) in action.
Ideal depth for soil in the Living Wall is 4″ to 8″ -neo raingardens manual.pdf. We need something at least that deep that can hold weight of that much soil (wet) plus plants.
How would we get uniform distribution of water across the top row of panels?
What happens to the water draining out of the bottom panel? It could drain into grass/tree flow lines.
Long term considerations:
- Parts of this will need to be replaced if it’s outside (sun, water and ice break down woods, plastics, metals), so are we looking at a 3-to-5-year lifespan then replace it all, or something that can be maintained a little at a time for a longer period?
- Same for the contents, do we refill dirt/sod/plants every spring? Good chance to clean out system.
- Need a way to clean out system – build-up of slime, scale, dirt over time. May want filtration at point water enters system. Something like a pool filter could be used, but we would need a way to “backwash” this kind of filter. A wire mesh might work, but we need to keep leaves and bird feathers out of it or the mesh will plug up the system.