Zen Garden Drainage Page

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Zen Garden Drainage Page

Drainage

This is should be the single most important consideration prior to any garden or structure design, second only to safety.  The reason for this is that water has awesome destructive power.  It can wash away foundations, sink structures, flood houses, drown plant or even create unsightly puddles in your yard.

Soil Permeability

Soil made up of a complex mixture of many elements.  And depending on what is in a particular soil mixture and how compact it is will determine what plants will grow or thrive in it, what plants can not survive in it and how well it drains.

In regards to drainage, there are a few major components of soil.  Sticks together well.

• Clay - microscopic and flat.  Retains water temporarily and drains very poorly

• Silt - larger than clay and irregular shaped.

• Sand - larger than silt and irregular shaped.  Feels grainy in hand and falls apart in hand.  Drains very well.  No nutritional value to plants.

• Organic mater - various sizes.  Decomposes over time.  Drains better than clay and "breaks up" soil.  Nutritional components for plants.

• Rock - various sizes.  Water will not drain through rock, but soilless gravel drains very well.  Does not compact, so can be used for foundations.

Small areas that puddle can be dug up and replaced with sand to allow water to drain through to the subsoil.  Larger areas with small drainage issues can benefit from mixing in compost or other organic material into it.

Drains

Areas hit by large amounts of water, such as water runoff, are better addressed with engineering solutions such as teraforming  your yard so that water runs downhill and away from structures and your garden.  Ideally, you will want to run this water to a better draining area such as a river, storm drain or possibly your neighbors better draining yard.

French Drain

In areas where you can't or don't desire to create large sloping areas, you can use French Drains to discretely collect water and drain it away from an area.  Ideally, you will want this water to run to daylight down hill from the area you would like to drain.  If this isn't possible, you may need to use a dry well or seepage pit to collect and store water during rainstorms and minor floods, and then slowly drain it into the subsoil while keeping your surface soil "dry".

Swale (low area between wall and road above)

Swales are a surface drainage solution that directs water away from an area without the need for drainage tile and gravel.

Open trenches are the open version of French Drains.  They drain very well, are exposed to plenty of oxygen (important to avoid an anaerobic environment) and can "dry out" when the sun returns.  They are unfortunately often visually unpleasing, require maintenance to clean out the debris that collects in them and can be dangerous, creating potential fall, trip and even drowning hazards in some cases.  

Dry Well (aka seepage pit, soakaway)

Setback vary by jurisdiction. A basic guideline requires:

from property lines	5 feet
from building foundations	10 feet
from cesspools (septic tanks)	20 feet
from drinking wells	500 feet
below the ground surface	36 inches ( to allow for utility crossings and prevent freezing)
above the high groundwater table	10 feet or more
above bedrock	10 feet or more

A common rule of thumb to determine an appropriate setback distance for infiltration facilities in general is to assume that water moves equally in the horizontal as it does in the vertical, in other words that it moves in a 1H:1V ratio. An appropriate setback can be calculated by ensuring that saturated zone of soil is outside the structural components of the building.

Note that is some places (Indiana perhaps), dry wells may be illegal and this may allow you to send polluted water directly into ground water without the normal filtration of soil.

Concrete 30 inch Drywell

Flo-Well plastic drywell

Will a Dry Well Work in my Location

The proper test to check on whether a dry well will work to drain storm water runoff at your location is a percolation test, also shortened to "perc test." To begin the perc test dig a four foot deep hole in the place you hope to put in a dry well. A posthole digger or soil auger works well for this task. If the hole fills in with groundwater, then the groundwater table is too high for a dry well to be used, because it won't drain. When the hole has been dug, pour at least 5 gallons of water into the hole. Measure the time taken for each one inch drop in water level. If the water level drops at 3 minutes per inch or faster, it is a good candidate for a dry well. If the water drains very quickly, fill the hole again check the rate that it recedes. Very dry soil of any type will absorb water very quickly. For good storm drainage the second 5 gallons should drain at a good rate also. If the water drains very slowly or the hole remains full for a long time with no perceptible drop in level, then a dry well won't work in that location.

Links:

Flo-Well Dry Well

Flo-Well Calculator

Principles of Exterior Drainage

 

French Drains

A grade of 1% (i.e., a drop of 1 foot for every 100 feet in length) is often recommended for French drains; others advise a drop of 6" for every 100'.

Building Codes

Most building codes require that the ground around a foundation slope away from the foundation at a minimum of 6 inches in the first 10 feet (5% grade) and with another foot of slope in the next 100 feet.

WOOD IN CONTACT WITH THE SOIL (Council of American Building Officials)

• CABO Building Code requires that untreated wood siding never be closer than 6 inches from the soil or mulch surface;

• CABO also requires that untreated structural wood, such as sills, joists, plates, etc., be no closer than 8 inches from the surface of the soil or mulch;

• If treated wood is used, the proper retention level or exposure condition must be chosen; above ground, ground contact, wood foundation, or salt water;

• Wooden stairs and decks and all other exposed wood should be built of appropriately treated wood and sealed with a layer of paint or water repellent stain.

International Code Council - International Residential Code 2006

Ch. 4 Foundations: Section R401.3

R401.3 Drainage

“Surface drainage shall be diverted to a storm sewer conveyance or other approved point of collection so as to not create a hazard. Lots shall be graded to drain surface water away from foundation walls. The grade shall fall a minimum of 6 inches (152 mm) within the first 10 feet (3048 mm).”

Exception: Where lot lines, walls, slopes or other physical barriers prohibit 6 inches (152mm) of fall within 10 feet (3048 mm), the final grade shall slope away from the foundation at a minimum slope of 5 percent and the water shall be directed to drains or swales to ensure drainage away from the structure. Swales shall be sloped a minimum of 2 percent when located within 10 feet (3048 mm) of the building foundation. Impervious surfaces within 10 feet (3048 mm) of the building foundation shall be sloped a minimum of 2 percent away from the building.


 

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