alaska; rain gardens: a how-to manual for homeowners in the municipality of anchorage

8/3/2019 Alaska; Rain Gardens: A How-To Manual for Homeowners in the Municipality of Anchorage

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www.anchorageraingardens.com

A bauiu faur yurh adsca hat il imprvwar qali i Achrg

ai arns:R GA How-To Manual for Homeowners

in the Municipality of Anchorage


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A How-To Manual

for Homeowners

in the Municipality

of Anchorage

ai arns:

R G

Watershed Management Division

Municipality o Anchorage4700 Elmore RoadAnchorage, AK 99507

(907) 343-8084

[email protected]

For more inormation on rain gardens viswww.anchorageraingardens.com


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Glossary

Contributing area - All land area that

contributes ow to a design point.

Hydraulic conductivity - Hydraulic

conductivity is a measure o the ability

o a soil or rock to transmit water. The

rate at which water can pass through is

aected by the size and distribution o

pore space and ractures.

Impervious surace - A surace that

permits insignicant or no inltration

o runo water over the duration o a

single stormwater runo event; any

surace with little or no capacity to

transmit water.

Inltrate - The process by which water

on the ground surace enters the un-

derlying soils.

Permeable soils - Permeable soils

are those through which water readily

passes. Permeable soils tend to be

coarse-textured with large, well-con-

nected pore spaces.

Recharge - Recharge is water that

inltrates into underlying auiers.

Rainall and snowmelt are the most

common sources o recharge. The

process by which water inltrates into

the ground to become groundwater is

known as groundwater recharge.

Runo - Stormwater runo is

rainall that reaches streams,

lakes, and oceans by means

o owing across impervious

suraces. Impervious suraces

include roads, parking lots, drive-

ways, and roos.

Stormwater - Flows originating rom

surace runo o rainall or snowmelt.

Watershed - A watershed is a region

o land where surace drainage rom

rain or snowmelt drains downhill to

a single point. The watershed area

includes the streams and rivers that

convey the water as well as the land

suraces over which water drains into

those channels. Each watershed is

separated hydrologically rom adja-

cent watersheds by a divide, oten in

the orm o a ridge, hill, or mountain.

Wetland - A landorm eature so

designated under the Anchorage

Wetlands Management Plan. An area

that is inundated or saturated by

surace or groundwater at a reuencyand duration sufcient to support, and

that under normal circumstances does

support, a prevalence o vegetation

typically adapted or lie in saturated

soil conditions. Wetlands generally

include swamps, marshes, bogs, and

similar areas.

Rain garden:

A rain garden is a small depressed

area planted with vegetation that

helps lter rainwater that runs o

roos, paved suraces, and lawns

into the ground.


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4

Rain Gardens: Contributing to Cleaner Water in Anchorage

In many places around the world, people are build-

ing rain gardens. Rain gardens are landscaped areas

planted with native vegetation that help lter rainwater

that runs o our roos, driveways, sidewalks, and our

lawns. Ater a storm, the rain garden lls with this water

and allows it to naturally lter into the ground rather

than running o into the street or a storm drain system.

In most places in Anchorage rainwater does not slowly

lter naturally into the ground. Instead water enters our

stormwater collection system and ows directly into

local lakes and streams. This water can carry pollutants

such as animal waste, oil rom leaking cars, road salts,

and ertilizers. By allowing stormwater to lter into

the ground where it alls, we can reduce the impact o

these pollutants on our local lakes and streams.

I we allow this polluted water into our local lakes and

streams, we run the risk that it could harm wildlie,

including salmon populations.

In addition to pollutants carried in stormwater, dust and

dirt rom streets and driveways contribute to cloudy,

silty water and sediment build-up. This reduces the

available habitat or sh and other reshwater crea-

tures, and can negatively impact wetlands. Not only

does stormwater runo cause these problems, it also

decreases groundwater recharge which is needed or

healthy stream ows.

Despite eorts to mitigate or these stormwater ows,

reduce pollutants to streams, and control ooding,

stormwater runo continues to degrade our lakes and

streams. This presents long-term threats to the uality

o our local water resources. The good news is that you

can help by adding a rain garden to your landscape!

Why Should You Install a Rain Garden?

Even i you have a limited amount o space in your yard,

it can still be worthwhile to build a rain garden. Even

small rain gardens can reduce, lter, and improve the

water uality o stormwater runo. Your rain garden

will add to and enhance the natural eatures o your

property and when combined with other rain gardens

throughout Anchorage, will reduce runo and provide

substantial environmental benets, and reduce costs

associated with stormwater inrastructure.

Rain Gardens

Increasetheamountofwaterthatltersintothe

ground. This allows groundwater recharge that

helps to maintain healthy water ows in our

streams.

Helpprotectcommunitiesfromoodingand

drainage problems.

Safeguardstreamsandlakesfrompollutants.Theseinclude the ertilizers and pesticides we apply to

our lawns, oil and uids rom leaking cars, animal

waste let on the ground, and numerous other

harmul substances that can wash o lawns, roos,

and paved areas.

Improvesalmonhabitatbyreducingdirtanddust

entering streams.

Enhancethebeautyofyardsandneighborhoods.

Createvaluablehabitatforbirds,butteries,anda

host o other benecial animals.

Helpeducatechildrenandadultsabouttheimpact

o stormwater on local water uality.

Building a rain garden is perhaps the

single best thing you can do to improve

the water uality in your area.


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Building a Rain Garden is Easy and Fun

This manual provides an easy and economical step-by-step

guide to building a rain garden on your property. However,

it is important to realize that this is just a guide. Your own

creativity in the process will allow you to tailor the product

to meet your own needs and make the experience more en-

joyable. We hope this manual will be a rst step in design-

ing and building your own rain garden!

There are two alternatives presented in this manual: the simple solution

or the designed solution. The simple solution reuires no calculations or

sizing, can be placed anywhere in the yard to break up the ow o runo,

and reuires minimal digging and planning. However, i you want to direct

a particular water source to your rain garden we recommend developing a

design using the criteria outlined in this manual. I you choose the simple

solution, determine a location or your rain garden, skip ahead to Step 2:

Building Your Rain Garden on page 18 and start digging! The simple solution

reuires that you create a depression and use plants rom the recommended

list. I youre not sure which solution to choose, please read over the manualand the right choice or you may become apparent.

The MOA Watershed

Management Services would like

to thank the U.S. Fish and

Wildlie Service or the grant

which unded the development

and distribution o this manual.

We would also like to thank the

University o Alaska Fairbanks

Cooperative Extension Service,

the Alaska Chapter o the

American Society o Landscape

Architects, the Department o

Natural Resources, the Plant

Materials Center in Palmer, and

the Alaska Department o

Environmental Conservation or

contributing to this manual.


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Q. Who should use this manual?A. Any property owner who wants to design andbuild a rain garden (or gardens) on their residential

property in Anchorage can use this manual. Guidelines

in this manual can also be used to treat runo at com-

mercial and industrial sites. However, the manual should

not be used to design rain gardens or parking lots,

busy streets, and other heavily used paved areas. These

areas reuire more complex design in order to meet the

demands o higher water ows, the larger amounts o

sand and particles, and to meet the potential need or a

higher level o water treatment.

Q. Does a rain garden orm a pond?

A.No. The design o a rain garden is intended to al-

low the water to soak into the ground so the rain garden

will become dry between rainalls. (It is important to

note however that some rain gardens can be designed

to include a permanent pond, but that is not covered by

this manual.)

Q. Is a rain garden breeding ground ormosuitoes?

A. No. Mosuitoes need 7 - 12 days to lay andhatch eggs, and the standing water in a rain garden will

only last a ew hours ater a storm. It is more likely that

mosuitoes will lay eggs in bird baths and storm sewers

than in a sunny rain garden. Rain gardens can actually

attract dragonies and other wildlie that eat mosui-

toes!

Q. Do rain gardens reuire a lot omaintenance?

A. Rain gardens are like other types o gardens inthat their level o maintenance varies depending on

their design. Generally, they can be maintained withlittle eort ater the plants are established. During the

rst two years, weeding and watering may be needed as

the plants establish themselves. Thinning in later years

may be needed as the plants mature. Some gardens

may reuire you to rake allen leaves beore snowall to

allow easier inltration o water in the spring.

Frequently Asked Questions


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Q. Is a rain garden expensive?A. It doesnt have to be. Your amily and a ew riendscan provide the labor, and perhaps you can nd some

o the native plants recommended in your yard or your

riends yards. I that is not an option, the main cost will be

in purchasing the necessary plants and soils. In addition to

providing you with this How-To Manual, the Municipality o

Anchorage is oering to reimburse homeowners who install

rain gardens 50 percent o the cost o installation (up to $750) based on

availability o unds. To apply or this mini-grant, go to

www.anchorageraingardens.com or call 343-8084.

Q. Why are rain gardens important?A. Two very important environmental issues that Anchorage is acing arewater uality and stormwater control. Rain gardens enhance local water ual-ity by allowing water to naturally lter through soil instead o running through

the stormwater system and out to our streams, lakes, and ponds. A simple and

eective way to enhance water uality and control stormwater is through the

creation o rain gardens.

In addition to providing you with

this How-To Manual, the

Municipality o Anchorage is

oering to reimburse homeowner

that install rain gardens or 50% o

the cost o installation (up to $750

based on availability o unds.

To apply or this mini-grant go to

www.anchorageraingardens.co

or call 343-8084.


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This section o the manual covers the basics o

building a rain garden. This includes: where to put the

rain garden, what size it needs to be, how deep to dig it,

and what type o soils and slopes are best. As you ollow

the instructions in this section, youre on your way to a

successul rain garden!

Perhaps you already know the size and location o your

new rain garden. I thats the case, skip ahead to the

section about building your rain garden on page 20.

However, we do suggest that you take the time to

review the inormation provided in this manual about

location, working with slope, and sizing your rain

garden. For instance, i the location o your uture rain

garden has a slope greater than 12 percent, it is best to

pick a dierent location. Slopes greater than 12 percent

reuire a good amount o extra work to create a level

garden.

Determining the Location o Your Rain Garden

Your home rain garden could be built in many locations:

near the house to collect runo rom the roo, arther

out on the lawn to collect water rom both the roo and

the lawn, or by driveways and walkways. Consider the

ollowing points beore deciding.

Thenexttimeitrains,observehowstormwater

is carried o your property. Watching these ows

might tell you where your rain garden should go.

Youdontwantwaterseepingintoyourhouse,so

make sure the rain garden is at least 10 eet rom

your house.

Donotplaceitoveryoursepticsystem.

Itmaybetemptingtoputtheraingardeninapart

o the yard where water already ponds DONT! The

goal o a rain garden is to encourage inltration,

and the places where your yard ponds water show

where inltration is already too slow. Instead try to

direct ponding water to an area where water soaks

in more rapidly.

Buildyourraingardeninfullorpartialsun,not

directly under an established tree. This helps to dry

out the garden between rains, but it also removes

the risk o damaging your trees roots.

Puttingtheraingardeninaatpartoftheyard

will make digging easier and the rain garden may

operate more efciently because you wont have

to compensate or the slope o the surrounding

landscape.

Considerhowyouremovesnowonyourproperty.

Sands and salts rom roads and driveways can

damage plants and compact soils in the garden.

Consider the Overall LandscapeWhen considering the placement o your rain garden,

keep the end result in mind and give some thought to

how the rain garden will be integrated into your

landscape. Not only should you consider how it looks

rom various points in your yard, but also think o how

it will look rom inside your house. Think about how

close or how ar it will be rom your outdoor gathering

spaces or play areas. One suggestion is to locate your

rain garden near a gathering area to take ull advantage

o colors and ragrances in the summer months. Figure

1 shows how a rain garden can be located in your yard,

whether it is in the ront or the back o your yard. The

shape o the rain garden can be crescent, kidney,

teardrop, or any shape you would like to see in your

yard. Figure 1 shows a cross-section view and Figures 2

& 3 shows a view rom above.

Step 1: Sizing and Siting Your Rain Garden



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Dont damage your trees!

Trees are great water

puriers. Large amounts o rain and

snow are intercepted by branches,

leaves, and needles and never

reach the ground. Trees also pump

huge uantities o water up rom

their roots and out into the air

through their leaves.

Be careul not to damage tree

roots during rain garden

construction so they can continue

to do their job. Avoid digging or

changing grade within the critical

root zone o a tree. This is

generally a circle 1 oot out rom

the trunk or each 1 inch o the

trunks diameter as measured 6 inches up rom the ground. For example, ithe trunk diameter is 8 inches, do not disturb the roots within 8 eet o the

tree.

What Size Should Your Rain Garden be?

The size o your rain garden depends on how much space you want to

dedicate to it, and the time and money you want to invest. Any reasonably

sized rain garden will be benecial. Typical residential rain gardens range

rom 100 to 300 suare eet. Rain gardens can be smaller than 100 suare eet,

but very small rain gardens will have less space or a variety o plants. I a rain

garden is larger than 300 suare eet it will take more time to build, and will be

harder on your budget.

Some actors that aect rain garden size include:

Characteristicsofsurroundingsoils

Volumeofstormwaterrunodirectedtothegarden

Existingslopesinproposedraingardenlocations


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The inormation provided here, along with other sizing

actors rom Tables 1 and 2, will be used to determine

the surace area o the rain garden. You can use the

worksheet provided at the end o this booklet to help

guide you through the steps.

To design your rain garden ollow these seven easy

steps:

1. Choose the ideal location or your rain garden

based on your observations about how water ows

in your yard.

2. Measure the slope at your potential rain garden

location.

3. Determine the soil type at the bottom o the dug

out hole where treated soils will be placed.

4. Estimate the suare ootage o the areas in your

yard that contribute water to your rain garden.

5. Calculate the Water quality Volume (WqV).

6. Figure out the number o suare eet that your raingarden will need to be to handle this volume o

water.

7. From this suare ootage, plan the length and width

o a rain garden that will provide the size you need,

and will work with your yard and desired design.

Please remember the guidelines presented in this

manual are not hard and ast rules! The sizing

reuirements outlined here are intended to lter 100

percent o the runo or the average rainall in

Anchorage while keeping the size reasonable. A goal o

collecting 100 percent o the runo will help balance

some o the errors that tend to creep into the design

and construction o any rain garden.

I the guidelines in this manual are ollowed and the

calculated surace area is too large or your yard, just

make it smaller. Any rain garden is better than none, and

it will still control some o the runo. The rain garden

could be made deeper to accommodate the reduced

surace area. Similarly, it is ne to make a rain garden

bigger than these guidelines indicate. However, i you

choose to make the rain garden larger than the

guidelines, consider plants that survive in drier

conditions.

Depth o Your Rain Garden

A typical rain garden in Anchorage should have a

ponding depth (depression) o about 6-8 inches deep.

A ponding depth more than 8 inches might hold water

or too long, might look like a hole in the ground, or

could present a tripping hazard.

A ponding depth less than 6 inches will reuire

additional surace area to provide it with enough waterstorage to handle larger storms.

An optional layer o mulch can be placed on the

ponding layer. I you are ltering ne sediment through

your rain garden, a 1-2 inch layer o mulch will lter

sediment and prevent build-up or clogging in the

rain garden. The mulch is easy to replace i it becomes

clogged with sediment.

Regardless o the size or placement o the garden, the

goal is to keep the rain garden as level as possible. The

planting soil reuired will depend upon the soils

available on your property. As a general rule the

recommended soil type should be 50-60 percent sand,

20-30 percent compost, and 20-30 percent topsoil.

Create this mix by adding the desired amounts o the

components needed during excavation, and removing



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a portion o the existing soils. This layer

will be around 18 inches in depth.

In order to get the drainage youll need

or your rain garden, dig down deeper

than the nal rain garden will appear.

We recommended that you dig down 2-3

eet to make room or drainage materials.

At this depth you may consider installing

a gravel under-drain that is about 12 inches deep and consists o pea gravel, and

above that a 6-inch optional layer o sand (Figure 4).

For inltration o snowmelt and on sites with less permeable soil types, consider

building in a gravel trench or under-drain pipe that empties onto your lawn

or street gutter at the bottom o your

garden to provide additional storage

or larger storm events, and snow-

melt. The under-drain pipe or gravel

trench will ensure inltration into

underlying soils.

Determining Slope

Your yard should slope toward your rain garden, but the planting section in

center o the rain garden should be kept as level as possible. Rain gardens are

simplest when installed on slopes o less than 12 percent. A rain garden on a

steep lawn will reuire extra topsoil to make sure the down-slope rim is up to

the same height as the up-slope rim o the garden.

The slope o your lawn and contributing suraces (roos, driveway, etc.) will

determine the size actor when calculating the area o your rain garden. Finding

the slope o your lawn can be done by the ollowing method (Reer to Figure 5

to see how the stakes and strings should look).

1. Pound one stake into the ground on the uphill side o your proposed rain

garden site, and pound another stake in at the downhill side. The stakes

should be at least 15 eet apart or this to work properly.


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2. Tie a string to the bottom o the uphill stake and

run the string to the downhill stake.

3. Using a carpenters level, tie the string to the

downhill stake so that the string is level between

the two stakes.

4. Measure the length o string (in inches) between

the two stakes.

5. Measure the height (in inches) on the downhill

stake between the ground and the string.

6. Divide the height by the length and multiply the

result by 100 to nd the lawns percent slope. I the

slope is greater than 12 percent, it is best to nd an

alternate location or your rain garden or consult a

proessional landscaper, as signicant excavation

may be reuired.

Example or Determining Slope

I the length o the string between the stakes is 180

inches long, and the height at the downhill stake is 9

inches, you would divide the height by the length and

multiply by 100 to nd the lawns percent slope:

(Height Length) x 100 = % slope

(9 in. 180 in.) x 100 = 5 % slope

Determining Soil Types at Your Rain

Garden Site

Determining the soil type at your garden site is a very

important step or calculating the size reuired or your

rain garden to unction properly. Your soils may drain

readily, but it is also possible that they may need to be

amended to provide good drainage, or may not be suit-

able or a rain garden at all. Accurately determining your

soil type will prevent any unwanted ailures.

To determine how well a rain garden will perorm in

your yard the soil type must be identied as sandy, silty,

or clayey. Sandy soils have the astest inltration, and

clayey soils have the slowest because the pore spaces

in clayey soils are so small that it is difcult or water to

pass through them. Rain gardens in clayey soils must be

larger than rain gardens in sandy or silty soils. One way

to determine the soil type is to eel the soil in your hand.

I the soil eels very gritty and coarse, the soil is probably

sandy. I the soil is smooth but not sticky, the soil is

probably silty. I the soil is sticky and clumpy, it is

probably clayey.

Simple Soil Tests

Two tests can be done to determine i your soil is

appropriate or a rain garden. You will have to deter-

mine soil hydraulic conductivity using one o the two

tests below. I you perorm Soil Test #1 the appropriate

hydraulic conductivity is ound in Table 1. Soil Test #2 is

a test that directly measures the hydraulic conductiv-

ity. I you perorm Soil Test #2, your measurement is the

hydraulic conductivity o your rain garden.

Soil Tests: Test #1

Test #1 has two steps. The rst involves digging a hole 8

inches wide and 8 inches deep to see how long it takes

a bucket o water to sink into the soil. The level o the

water should go down at a rate o about 1 inch per hour.


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Table 1: Hydraulic Conductivity

Based on Soil Type rom Test #1

Soil Type Hydraulic Conductivity (i

Sandy 0.43

Silty 0.26

Clayey 0.04

*Source: Adapted rom Rawls, Brakensiek,

and Miller, 1983

I it takes longer than that, you will need to improve

inltration by digging a deeper hole or installing a

gravel under-drain.

The second part o Soil Test #1 involves taking a

handul o soil and dampening it with a ew drops o

water. Knead the soil between your orenger and

thumb, and sueeze it into a ball. I it remains a ball,

roll it into a ribbon o uniorm thickness. Allow the ribbon to emerge and

extend over your orenger until it breaks rom

its own weight. I the soil orms a ribbon more than an inch long beore it

breaks and it eels more smooth than gritty, the soil has too much clay and is

not suitable or a rain garden.

Table 1 will help you determine the rate at which storm water will inltrate

into the ground on your property. This characteristic o your soil is known as

the hydraulic conductivity. I your soil type is clayey a rain garden may not be

right or you.

Soil Tests: Test #2

Soil Test #2 is the most accurate test, and will directly measure the hydraulic

conductivity that is specic to your yard. This involves purchasing a PVC pipe

about 1 oot long and 4-10 inches in diameter. Dig a hole to the desired rain

garden depth (approximately 3 eet deep), and pound the pipe into the soil

to a depth o about 4-6 inches. Then ll the pipe with water and measure

the depth o the water in inches within the pipe. Measure the depth o the

water every 6-12 hours to calculate hydraulic conductivity. For example, since

hydraulic conductivity is measured in inches per hour, i you measure the

initial water height at 6 inches, and you check back in 12 hours and the water

height has dropped to 3 inches, you can calculate the conductivity by divid-

ing 3 by 12. The hydraulic conductivity in this example is 0.25 inches/hour.

I your soil type is clayey, a rain garden may not be right or you. You may

be able to compensate or the lower inltration rate by digging the garden

deeper and adding a oot or more o gravel or pea gravel. I you are unsure


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about perorming these tests please contact the Munici-

pality o Anchorage, Watershed Management Services

at (907) 343-8084 or view our website at

www.anchorageraingardens.com.

Other websites outside Alaska are listed below:

http://www.ci.minneapolis.mn.us/cso/rain-garden.asp

http://www.raingardens.org/Index.php

http://www.dnr.state.wi.us/org/water/wm/nps/rg/links.htm

http://www.arc.govt.nz/library/f43363_2.pdf

http://www.dnr.state.wi.us/org/water/wm/nps/rg/index.htm

Determining the Area Draining to Your

Rain Garden

The next step or determining the size o your rain

garden is to nd the area that will be draining into it. Asthe size o the drainage area increases, the size o the

rain garden should also increase. There is oten some

guesswork involved in determining the size o a

drainage area, especially i a large part o the lawn is

upslope rom the proposed rain garden site. The

suggestions below can help to estimate the drainage.

I your proposed rain garden is more than 30 eet rom

the house and you do not wish to have a swale or a

downspout cutting across your lawn, you can run a PVC

pipe underground rom your downspout to the rain

garden or install a gravel lled-trough.

Rain gardens less than 30 eet rom the water source:

1. In this case, where the rain garden is close to the

source (Figures 1 and 2), almost all o the water

collected will come rom the roo downspout,

sidewalk, driveway, or patio. Walk around your

house and gure out what areas will contribute

water to your rain garden. This includes estimating

the amount o roo area that will shed water toward

your garden. I you have a peaked roo, not all o the

water that hits your roo will enter your rain garden.

2. For your roo, estimate the amount o area that it

will contribute. You can gure this out by measur-

ing your houses length and width. Multiply the two

together to nd the approximate area o your roo.

3. Finally, multiply the roo area by the percent o the

roo that will contribute water to your rain garden.

When added to the other areas that provide water

(driveways, sidewalks, patios, etc.) this number will

be the drainage area used to size your rain garden.

I there is a signicant area o lawn that also eeds

the rain garden add this lawn area to your total.

Rain gardens more than 30 eet rom the watersource:

1. I there is a signicant area o lawn uphill that will

also drain to the rain garden (Figures 1 and 2), add

this lawn area to the drainage area o the roo/

driveway/patio/sidewalk. First, nd the contributing

drainage area by the suggested method above.

2. Next nd the area o the lawn that will drain into

the rain garden. Stand where your rain garden will

be and identiy the part o the lawn that is sloping

into the rain garden.

3. Measure the length and width o the uphill lawn,

and multiply to nd the suare ootage o the lawn.

4. Add the lawn area to the roo drainage area to nd

the total drainage area.


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Table 2: Size Factor Based on Slope

Landcover Slope Slope Slop

0-2% 2-6% ove

Lawn 0.17 0.22 0.35

Driveway or

Sidewalk 0.75 0.80 0.85

Gravel 0.25 0.30 0.35

Roo 0.85 0.86 0.87

*Source: Adapted rom MOA Design Criteria M

Drat 2006

Example or determining drainage area:

I your house measures 60 eet by 40 eet, the roo area would be

(60 t x 40 t) = 2400 s. t. I you estimate that only 25 percent o the roo area

contributes water, you would multiply 2400 suare eet by 0.25 to get the

downspout drainage area o 600 suare eet. I you have planned to place the

rain garden more than 30 eet rom the downspout, the lawn runo will be

contributing to the garden as well. I the lawn upslope o the proposed garden

location is 40 eet by 40 eet, the lawn area would be (40 t. x 40 t.) = 1600 s. t.

Summary: Roo Area = 2400 s.t.

Lawn Area = 1600 s.t.

Using the Rain Garden Sizing Criteria

Ater estimating the drainage area, and soil type you can determine how much

water will be directed into your rain garden. This volume o water is known as

the Water quality Volume (WqV). Table 2 is used to determine which multiplier

is used or the WqV based on characteristics o your contributing drainage area.

The euation to determine WqV is:WqV = (Rainall) x (Size Factor) x (Contributing Area)

The size actor you will use comes rom Table 2, contributing area comes rom the

amount calculated in the previous section, and the rainall amount or calculating

WqV in Anchorage is 1.25 inches (0.104 eet). The WqV is the storage volume re-

uired to treat approximately 90 percent o the average annual stormwater runo

volume. The amount o rainall used or the WqV calculation comes rom analysis

o rainall records (Municipality o Anchorage, Watershed Management Services,

Anchorage Intensity-Duration-Freuency Curves Update, Nov. 2006).

The ollowing steps must be ollowed to calculate the WqV:

1. For each type o surace contributing water to your rain garden, nd the

size actor that is listed in Table 2 or the slope you calculated or your pro-

posed rain garden location.

2. Multiply these size actors by the contributing area o that surace type.

3. Add the results or each surace type together, and multiply this sum by

0.104 eet. This is the WqV!


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Example or Determining Water

quality Volume

I the contributing area is 600 suare eet, and the

estimated slope o your roo is 5 percent, your size actor

rom Table 2 would be 0.86. Multiply 600 suare eet by0.86 to get 516, and then multiply by 0.104 eet. to get a

nal WqV result o 53.66 cubic eet.

600 s. t. x 0.86 = 516 s. t.

516 s. t. x 0.104 t. = 53.66 cu. t. or WqV

So the WqV or the roo is 53.66 cubic eet.

We also need to add the WqV or the lawn. Since the

contributing area or the lawn is 1600 suare eet, the

slope is 5 percent, and the size actor is 0.22, we calcu-

late the lawns WqV by ollowing the steps below.

1600 s. t. x 0.22 =352 s. t.

352 s. t. x 0.104 t. = 36.6 cu. t. or WqV

So the WqV or the lawn is 36.6 cubic eet

To nd the total WqV add the two calculated values

together.

Summary: WqV (roo) = 53.66 cubic eetWqV (lawn) = 36.6 cubic eet

Total WqV = 90.3 cubic eet

Determining the Ideal Size o Your

Rain Garden

To determine the area o your rain garden you will

need to have calculated the Water quality Volume

(WqV), and either determined your soil type to

nd hydraulic conductivity rom Soil Test #1 or

Soil Test #2.

The calculation or the rain garden area ollows the

ormula below.

Area o Rain Garden = [0.46 x (WqV)] Hydraulic

Conductivity (Table 1)

*Note: The multiplier (0.46) is derived rom values

determined to be site-specic or the Anchorage area

and conversion actors, provided by the Municipality o

Anchorages Watershed Management Services.


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Determining Length and Width o Your Rain Garden: Choose a

Rain Garden Size that is Best or Your Yard

Remember that these are only guidelines. The size o your rain garden will also

depend on how much money you want to spend, how much available space

you have, and how much o the total runo you want to control. Again, you can

reduce the size o your rain garden and still control a portion o your runo.

I the sizing calculations suggest that your rain garden should be 200 suare

eet, but you only have room or a 140 suare oot rain garden, that will be

ne. A smaller rain garden will usually work to control most stormwater runo,

although some bigger storms might overow your garden. To increase the

eectiveness o a smaller garden, a gravel under-drain will help to control more

water (Figure 4).

Beore building your rain garden, you need to think about how it will catch the

water. The water entering the garden should be spread out evenly across its

entire length. In order to do this, the rain garden must be as level as possible

so water will ll it evenly, and not pool at one end or overow at a low point on

the rim.

The longer side o the rain garden should ace up-slope. This means that the

length o the rain garden should be perpendicular to the slope and the source

o water (Figure 2). This ensures that the garden catches as much stormwater as

possible. Still, the rain garden should be wide enough or the water to spread

evenly across the bottom and should also provide ample space or a variety o

plants.

A general rule o thumb is to build the rain garden about twice as long (perpen-

dicular to the slope) as it is wide (parallel to the slope).

Think about the slope o your lawn when determining the width o the garden.

Rain gardens that are wide or rain gardens on steep slopes will need to be dug

deeper on one side to make sure they are level (Figure 6).

Example or Determining

Rain Garden Area

We will use the total WqV value rom th

previous example o 90.3 cubic eet. I

your soil test determined you had silty

soils then the Hydraulic Conductivity ro

Table 1 would be 0.26 inches/hour.

Using the calculation provided above

Multiply 0.46 by WqV

0.46 x 90.3 cu. t. = 41.6

Divide that result by the Hydraulic

Conductivity o 0.26 inches/hour.

41.6 0.26 = 160 s. t.

The area o the rain garden should

be 160 suare eet!


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I the rain garden is built too wide, you may need

additional soil to get the downhill edge to the right

height. Oten, making a rain garden about 10 eet wide

will be a good compromise between slope eects and

how deep the rain garden should be. The rain garden

should have a maximum width o 15 eet, especially on

lawns that have more than an 8 percent slope (Figure 2).

The ollowing steps can be ollowed to determine the

length o the rain garden.

1. Pick a width (perpendicular to the slope) that best

suits your lawn and landscaping or your rain

garden.

2. Divide the predetermined size o your rain garden

by the width to calculate the length (parallel to the

slope) o the garden.

Example or Calculating the Length

o Your Rain Garden

I you have decided to make your rain garden

10 eet wide, and the area o your rain garden

calculated earlier was 150 suare eet, you would divide

150 suare eet by 10 eet.

This means the garden would have a length o 15 eet.


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Noe:


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Step 2: Building Your Rain Garden

CALL BEFORE YOU DIG! You can reach the Alaska digline

at 278-3121 in Anchorage, or Statewide at

1-800-478-3121. A locator will come to your home and

mark all the underground utilities in the area o your

proposed rain garden ree-o-charge.

Beore you begin digging make sure you have all the

appropriate tools:

Tapemeasure

Shovels

Rakes

Trowels

Carpenterslevel

Woodstakes(atleast2feetlong)

String

2x4board,atleast6feetlong(optional)

SmallBackhoewithCaterpillarTreads(optional)

Tarptoplaceexcavationsoilson(optional)

I you are building a rain garden into an existing lawn,

digging time can be reduced by killing the grass rst. To

kill the grass place black plastic over the area until the

grass dies. The best time o year to build the rain garden

is in the spring when the ground is easier to dig and

plants are most likely to thrive. In Anchorage the ground

may be rozen until mid-May. This is the ideal time to

start digging and planting.

Now that the size and location o the rain garden have

been established, its time to start digging. I you work

alone it could take up to 6 hours to dig out an average

sized rain garden. With the help o riends it can go

much aster, and possibly take only an hour or two.

Digging Your Rain Garden

You should mark the edge o your garden to give you

a guide or where to dig. One way to do this is to lay a

string around the perimeter o your rain garden. The

berm will go outside the string. Now is also a good time

to put stakes along the uphill and downhill sides o the

garden, and line them up so that each uphill stake has

a stake directly downhill rom it. Place one stake about

every ve eet along the length o the garden. These will

be used to check that the rim o your garden is level.

As you dig out the depression or your garden, place

the soil you remove so that it orms the berm that will

become the rim o your garden. The berm is a low wall

around three sides o your rain garden that will orm the

depression that holds water in during a storm event.

On a steeper lawn the lower part o the rain garden can

be lled in with the soil rom the uphill hal.Depending on your site, you may need to bring in extra

soil to create a berm o the reuired height. Figure 7

shows how the berm should be placed at the gardens

edge. Ater shaping the berm into a smooth ridge about

a oot across, you should stomp on it. It is important to

have a well compacted berm that keeps the water inside

the garden, so stomp really hard! The berm should have

gently sloping sides so that the rest o the garden is

somewhat integrated with the surrounding lawn, and is

less susceptible to erosion.

To ensure the garden is at and retains the appropriate

ponding level, start at one end o the garden and tie

a string to the uphill stake at the ground level. Tie the

same string to the coinciding downhill stake and make

sure that the string stays level. Working in ve eet wide

sections with one string at a time is a good way to start



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digging. I you try to dig with too many strings in place at

one time, the strings may become an obstacle.

Start digging at the uphill side o the string. Going down

rom the string, dig until you reach the depth you want

your rain garden to be. For example, i the rain garden

ponding depth is eight inches deep, then dig eight inches

down rom the string (Figure 6).

I the lawn is almost at, you will begin digging at the same depth through-

out the rain garden and the soil you remove will orm the berm. I the lawn is

steeper, the high end will have to be dug out noticeably urther than the lower

end. Some o the soil rom the upper end can be used in the lower end to make

the rain garden level. Continue digging, lling one section at a time across the

length o your rain garden until it is as level as possible.

I you are adding the gravel under-drain the soils will need to be removed to

add the gravel, and then replaced evenly over the top.


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Now that you have your berms level to create a rim and

have the depression you need, you should amend the

soils within the garden i needed. A roto-tiller can make

mixing easier, but is not necessary. I you are going

to add to the existing soils the garden should be dug

slightly deeper. In general the soils under the ponding

depth should be loosened or amended down 2-3 eet

with the recommended soil mixture: 50-60 percent

sand, 20-30 percent compost, and 20-30 percent topsoil.

The outcome you are looking or is a at planting bed

in a depressed area where the uphill side allows water

to pour into it rom adjacent suraces. I you build your

garden into a mound o soil that is above the lawn level,

water will be diverted around the garden and will not

be contained within it to soak into underlying soils.

Leveling Your Rain GardenOne way to check i your rain garden is level is just to

eyeball it. I you do not trust your eyeball then the

ollowing steps can be used to increase accuracy.

Whenthewholeareahasbeendugouttoabout

the right depth, lay a 2x4 board about 3 eet long in

the rain garden and place a carpenters level on top

o it. Find all the spots that are not at, and ll in the

low places, and dig out the high places.

Movethe2x4tovariouslocationsindierent

directions, and ll and dig as necessary to make the

surace level.

Whenyouhavemadethegardenaslevelasyou

can get it, rake the soil smooth.

Once you have nished moving your soil, shaping it into

shallow slopes and compacting the berm, you will need

to prevent soil erosion by covering the berm with mulch

or planting it with grass. Planting with grass or covering

with mulch will ensure that weeds dont establish

themselves while the berm is bare. I you choose to

plant the berm with grass, use a light layer o mulch to

cover the seeded berm and protect it rom erosion while

the grass takes root.

I you dont want to plant grass or mulch over the berm,

you could use drought-tolerant plants such as

Bugleweed (Ajuga reptans), Creeping Jenny (Lisimachia

nummularia), Aleutian Speadwell (Veronica grandifora)

and Wooly Thyme (Thymus pseudolanginosus). You

should still use mulch to protect the berm rom erosion

as the plants establish themselves.

I your downspout is located a ew eet rom the entry to

the rain garden, make sure the water is directed into the

rain garden by digging a shallow grass swale or

attaching an extension to the downspout. Try toencourage the water entering the garden to be directed

evenly along the ront by incorporating rock or

widening the entrance channel. I the ow rom your

downspout is ast or heavy, place some rocks in its path

to lessen its orce and reduce potential erosion.

Tips or Designing an Attractive Rain Garden

While a rain gardens rst unction is to help protect

water uality, it will also unction as an attractive eature

in your yard and neighborhood. In designing your rain

garden think o the garden within the context o your

overall existing or proposed landscape design.

The ollowing sections outline some tips or designing

an attractive rain garden.


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When choosing plants or your garden, it is important to

consider the height o each plant, bloom time, color, and

overall texture. To create a continual, aesthetically pleasing

garden, use plants that bloom at various times throughout

the season to create a longer owering season. Heights,

shapes, and textures should be mixed to give the garden

depth and dimension. This will keep the rain garden looking

resh and interesting when owers are not in bloom.

When laying out plants or the garden, use individual

species in groups o 3-7 (using an odd number o plants is more pleasing to

the eye in inormal gardens). By planting in groupings, your garden will have

a greater overall visual impact and result in a more cohesive design. Use only

single plants when you want to accent or create a ocal point.

Try to incorporate a diverse mixture o owering species along with grasses

and sedges i possible. This creates the necessary root competition or plants

to ollow normal growth patterns and not out-compete other species. You may

also want to consider plants that add structure to your garden during thenon-growing season to add variety to your winter landscape. In natural areas,

plant diversity adds beauty and a thick underground root matrix that keeps the

plant community in balance. Once the rain garden has matured and roots have

been established there will be less species change in the coming years.

Consider enhancing the rain garden by incorporating local or existing rocks,

ornamental ences, trails, garden benches, or other decorative elements. This

helps give a newly planted garden an intentional look that provides a eeling o

neatness your neighbors will appreciate.

The outcome you are looking or is

at planting bed in a depressed are

where the uphill side allows water t

pour into it rom adjacent suraces.


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Planting the garden is the un part! Planting designs and

a list o suggested plants are at the end o this manual.

There is no single best way to plant a rain garden, so be

creative and show your individuality! I you are an

experienced gardener you should have no problems

planting a rain garden, but i you are not experienced,

look or planting tips in the ollowing sections.

Planting Your Rain Garden

When you are buying plants, be sure to select plants

that have a well-established root system. In general,

bigger plants will have better root systems that help a

rain garden activate its water ltering unction more

uickly.

Make sure to have a rough idea as to which plants will

be planted where, and lay out the plants in your garden

according to your plan. Many plants purchased romgarden centers will have labels indicating, among other

things, the recommended spacing. While laying out

the plants, keep them in their containers until they are

planted, and remember to keep your plants watered to

prevent them rom drying out.

Dig a hole or the plant wider than the root-ball and

deep enough so that the crown o the plant is level with

the existing grade. Fill the hole by rmly tapping around

the roots to get rid o air pockets. I the roots are visible

at the sides o the container, try to gently break them

up with your ngers and spread them so they point out

into the soil as you plant them.

Next, apply mulch evenly over the bed at a depth o 1-2

inches without burying the crown o the new

transplants. Mulching is not necessary ater the second

growing season unless you use mulch as a weed control

or i you like the look o mulch.

As you plant, place the plant labels next to the

individual plants or groupings. This way you can readily

identiy your young plants, and work around them as

you weed your garden. You may also want to make a

map o your rain garden in case the labels get lost.

As a general rule, plants need at least 1 inch o water

every week (an inch o water is about 6 gallons per

yard). You should water immediately ater planting

and continue to water at least twice a week until the

transplants establish themselves (unless it rains enough

to soak the soils). You can add transplants at any time

during the growing season as long as they are getting

enough water.

Maintaining Your Rain Garden

You will need to weed your garden by hand on a regular

basis or the rst couple o years. Weed by hand, and

remove only the plants you are certain are weeds. When

removing weeds, be sure to get all o the roots as

sometimes root pieces let behind will grow into a new

weed. Weeds may not be as much o a problem the

second season, depending on the variety and

tenacity o weeds present and how closely you spaced

your perennials when planting. In the third year and

beyond, the species planted should begin to mature

and out-compete the weeds. Occasional weeding will

be needed or the lie o your garden.

I you preer a tidy garden you can cut back dead stems

in the all. In the wild, dead stems are overgrown by

new growth in the spring. You can also choose not to let

Step 3: Planting and Maintaining Your Rain Garden

Step 3: Planting and Maintaining Your Rain Garden


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plants grow too close together by controlling the size

and mulching in between.

Lawn grass may encroach into the bed o the garden.

You may nd it useul to install an edge around the

garden to prevent grass rom encroaching into the

garden.

Cost o Building a Rain Garden

The cost o installing a rain garden will vary depending on who does the work

and where you obtain your plants. I you grow your own plants, or get them

rom neighbors or riends there will be very little cost. I you do all the work,

but purchase plants, the garden will cost approximately $3-5/suare oot. I a

landscaper does everything, it will cost about $10-12/suare oot.

It might seem easier to sow native wildower seeds over the garden, but seed-

ing a rain garden this way can cause problems. Perennial plants are slow to

establish rom seed and the rain garden would reuire a lot o weeding untilthe plants are established. This could take 2-4 years. Most wildower seed

mixes contain very ew species native to Alaska. These non-native mixes also

usually contain a large number o annuals and to look best, they would have to

be replanted every year. A rain garden needs to be planted with perennials to

work properly.

Growing plants rom seeds sown indoors can take one to two years to properly

establish themselves or planting outdoors unless they are a really aggressive

species. Dividing up plants already established rom you or your neighbors

yard is much easier. I you decide to grow plants yoursel, start them about

one to two years beore you plan to move them into the rain garden. When

the roots have lled the pot, and the plants are healthy, they are ready to be

planted in the rain garden.

In general it is not recommended to plant annuals in a rain garden except or

decorative purposes, or to add color early in the season i desired.

In general it is not recommended

to plant annuals in a rain garden

except or decorative purposes, or

to add color early in the season i

desired.


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Rain Garden Suggested Plant ListsThe ollowing is a plant list or rain gardens in Anchorage. Remember that the design possibilities or your garden are

almost limitless. Many greenhouses sell non-native orget-me-nots that are very aggressive, and should be avoided.Be sure to check whether you are getting the species rom this list i you wish to use plants that are native to Alaska.

Latin Name Common Name Bloom Time Bloom Color Height Spacing

SHRUBS

Aronia melanocarpa Black Chokeberry 3-5 eet 4 eet

*Cornus sericea Red-twig Dogwood Early Summer white 5-8 eet 5 eet

Cornus sericea aviramea Yellow-twig Dogwood Early Summer white 5-8 eet 5 eet

*Viburnum edule Highbush Cranberry Spring white 4-8 eet 3 eet

**Willow Willow Spring varies

Myrica gale Sweet Gale white 3-4 eet 3 eet

PERENNIALS

**Auilegia Columbine All Summer varies 6-36 in. 12 in.

*Aruncus dioicus Goats Beard Early Summer ivory white 5 eet 24 in.

**Aconitum delphiniolium Monkshood purple 3-4 eet 2 eet

*Dodecatheon pulchellum Shooting Star Late spring pink 12 in. 12 in.

*Geranium erianthum Wild Geranium Late spring blue/violet 24-36 in. 18-24 in.

*Oplopanax horridus Devils Club Spring white 3-10 eet 2-3 eet

*Athyrium elix-emina Lady Fern 30-36 in. 24-30 in.

*Frittilaria camschatcensis Chocolate Lily Spring purple/brown 18 in. 5-6 in.

**Dodecatheon Shooting Star Spring violet 12-18 in. 12 in.

*Dryopteris dilitata Wood Fern 30-36 in. 24-30 in.

*Geranium erianthum Cranesbill Geranium Spring/ Summer purple/white 12-24 in. 12 in.

Hemerocallis stella de oro Stella de Oro Daylily All Summer yellow 12 in. 12 in.

*Iris setosa Alaska Wild Iris Early Summer purple/white 18-30 in. 18 in.

Iris psuadacoris Iris Early Summer yellow 18-30 in 18 in

Ligularia (x2) stenocephala/

przewalskii The Rocket/ Ligularia Late Summer yellow/ gold 36-60 in. 24 in.

*Matteuccia struthiopteris Ostrich Fern 36-48 in. 24-30 in.

*Mertensia Bluebells Spring blue/ purple 18-30 in. 18 in.

*Myosotis alpestris Forget-Me-Not Spring/ Summer blue/pink 4-12 in. 12 in.

**Polemonium Jacobs Ladder Summer blue 12-36 in. 12 in.

Thalictrum Meadow Rue Summer pink/white/purple 36-48 in. 18 in.

Trollius Globeower Spring/ Summer yellow/ orange 24-36 in. 12 in.

Filipendula Meadowsweet Late summer white/pink 18-60 in. 24-36 in.

GRASSES AND SEDGES

*Carex gmelini Native Sedge 36 in. 30 in.

Miscanthus sinesis Red Flame Grass Late Summer Silver/white 3-4 eet 24 in.

*Deschampsia cespitosa Tuted Hair Grass Late Summer Gold/ Silver/ Purple/ Green 2-3 eet 1-2 eet

**Elymus mollis Wild Rye 1-3 eet 12 in.

* Indicates Native Plant Species ** Indicates Native or Non-Native Plant Species


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Rain Garden Layout Example


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1. Chose a location or your rain garden

2. Determining slope

(Height Length) x 100 = % slope( in. in.) x 100 = % slope

3. Determining soils typeSoil Test 1: Rate and texture

Dig an 8-inch x 8-inch hole and ll with a bucket o wa-ter. The water level should go down 1 in./hr.

Your hole drained water in./hr.

Ribbon test:Does your soil orm a ribbon?

More than one inch?

How does your soil eel?smooth sandy gritty other

Soil Test 2: Measuring the hydraulic conductivity

Measure the initial water depth within the pipe.

in.

Measure the water depth at 6 or 12 hours.

I you measured it at 6 hours:in. 6 hrs. = in./hr.

I you measured it at 12 hours:in. 12 hrs. = in./hr.

4. Determining drainage area

Estimate the percent o your roo area shedding watertoward your garden. %(Remember to divide by 100 beore multiplying the percent

with the roo area.)

Houses (length) x (width) = area o root. x t. = s. t.

(Roo area) x (% o roo contributing) =

Actual area o roo shedding waters. t. x % = s. t.

Roo: t. x t. = s. t.

Other areas providing water to your gardens:

Driveway: t. x t. = s. t.Patio: t. x t. = s. t.

Sidewalks: t. x t. = s. t.Lawn: t. x t. = s. t.

5. Rain garden sizing criteria(Rainall) x (Size Factor) x (Contributing Area) = WqV(Anchorage rainall is 0.104 eet)

Roo WqV:0.104 t. x s. t. x s. t. = cubic eet

Driveway WqV:0.104 t. x s. t. x s. t. = cubic eet

Patio WqV:

0.104 t. x s. t. x s. t. = cubic eet

Sidewalk WqV:0.104 t. x s. t. x s. t. = cubic eet

Lawn WqV:0.104 t. x s. t. x s. t. = cubic eet

Total WqV:

(Roo WqV) + (Driveway WqV) + (Patio WqV) + (Side-walk WqV) + (Lawn WqV) = Total

t3 + t3 + t3 + t3 + t3 = t3

6. Determining the ideal size o your rain gardenArea o your rain garden=

[0.46 x (WqV)] hydraulic conductivity (Table 1)

[0.46 x cubic eet] = s. t.

7. Decide on the length and width o your rain gardenArea width = length

s. t. t. = t.

Rain Garden Worksheet: Sizing and Siting the Rain Garden


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Watershed Management Division

Municipality o Anchorage4700 Elmore Road

Anchorage, AK 99507(907) [email protected]

For more inormation on rain gardens visit:www.anchorageraingardens.com

alaska; rain gardens: a how-to manual for homeowners in the municipality of anchorage

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