Low-Impact-Development Site Assessment: Topography, Water, Soils, and Vegetation

Topography: Understanding the three-dimensional lay of your land is a good starting point to consider your possibilities and look out for potential concerns. Your terrain will affect how water flows and drains across the property.

Be sure you note where your house is placed and how drainage and slopes might affect it.

Sudden changes in elevation might be places where natural springs occur—often seasonally.

Sunken areas may have seasonal water and might have different soils than other parts of your property.

If you have steep slopes on your site, you need to ensure that they remain stable with a healthy cover of trees and shrubs.

If your site has steep slopes, including marine bluffs, you will need to be especially careful in developing or planning LID strategies.

Avoid removing vegetation from the slope or the slope edge.

Replant any cleared areas as soon as possible with trees and shrubs to prevent erosion and intercept stormwater.

Learn how water flows and drains within the slope as well as across it before using LID strategies.

For instance, rain gardens should almost never be placed near a home on a marine bluff, and must be sited at least 50 feet back from any slope greater than 15 percent.

Water: As you begin assessing your topography, you’ll start to get a sense of how water flows and drains on your site. Understanding your water flow will help you decide what kinds of LID projects will help you and where you can place different drainage systems.

How and where your roof, driveway and other hard surfaces drain during rainstorms.

Obvious standing water (wetlands, seasonal or year-round streams, ponds).

Natural springs, which can be identified by spongy ground or the presence of moisture loving vegetation, such as skunk cabbage, buttercups, willows, or salmonberries.

Other water coming onto your property, such as from drainage ditches, swales, or runoff from developments up hill from you.

Saturated soils or seasonally wet spots, which will be spongy or mucky to walk through during winter (at least), and are usually in depressions in the ground.

In evaluating your site’s water, check with your local

water utility to see if your property is in a wellhead

protection area or near one. There are usually regulations that govern what you can do if you’re in proximity to a drinking-water source.

Soils: The soils on your property are critical for absorbing

stormwater. Your soils hold the key to many LID options, including

landscaping, pervious paving systems, and rain gardens. To do

many LID strategies, you’ll need to have some detailed but

simple information about your soil.

Shovel Test: If you can't get a shovel in the

ground with moderate force, you either have terribly

compacted soils or most of your topsoil has been

removed during development.

Hand-textural analysis or ribbon test: This simple

test helps you determine about how much clay, sand,

and silt your soil contains. This information tells you

how quickly water drains through and how

available nutrients are to your plants.

Take a peach pit amount of lightly damp soil

into your palm; form it into a ball.

Does it keep its shape?

Yes

No

Is the soil damp enough?

Yes

No

Add water.

Is the soil too damp?

Yes

No

Add dry soil.

Create a ribbon with your

soil and note:

The texture.

The length before

it breaks.

Very gritty.

No ribbon or

very short.

Sand

Sand

<1 inch.

Mostly smooth

(maybe some grit).

Silt

Smooth/slippery.

>1 inch.

Clay

Simple pit test: Dig a two-feet wide by two-feet

deep by two-feet long hole and fill it at least

halfway full of water.

If you dig your test pit during the dry

months, fill and drain the hole

three times to simulate

wet-weather conditions.

Your drainage is adequate to

do a variety of LID practices.

You can still do several LID practices,

but you may need to start by

rebuilding your soil.

Track how fast water drains.

1/2 to 1 inch per hour:

<1/2inch per hour:

Soil analysis: Check with your local conservation district

or extension office for free or low-cost soil analyses.

They’ll give you instructions for gathering samples,

and their reports will tell you more precisely

what kinds of soils you have, how much

organic matter is present, and even some

information about nutrients.

Clay: You might find ribbons of clay here and

there, which is different than wide

expanses of clay. If you have soils that

are mostly clay, you will have to add a

lot of organic matter to your soils to

be able to absorb most of your

stormwater on your property.

Test for different soils: If you have changes in topography or vegetation, you should check to see if the soils under those spots are different than other areas on your site.

Groundwater: Keep an eye out for signs of

perched groundwater or a high

water table.

Look out for rust-colored veins,

or steel-gray patches.

If you see these signs, you might want to consult with a specialist to provide details about

potential underlying drainage problems

in your soil, or the limits to on-site

infiltration of stormwater.

Depending on the depth, perched groundwater

might limit your site’s ability to absorb

stormwater, especially through a

rain garden.

If water starts pouring into your hole while you’re digging, it’s a sure sign you have perched

groundwater or a high water table, but

this might only happen during winter.

Vegetation: Documenting the existing plants on

your site is the next step in your site evaluation.

Mature vegetation: Take note of big trees and

shrubs, along with healthy layers of low-growing

shrubs and groundcovers. Places with mature

vegetation on your property will likely have

soils rich in organic matter and will already

be doing a lot to absorb stormwater on site.

Planting beds: These areas might include

landscaped areas where the soils have been

built up with new topsoil or compost. Planting

beds absorb some stormwater, but not as much

as mature vegetation. Sometimes you can

modify existing planting beds to be LID features,

such as rain gardens or more expansive,

layered planting areas.

Lawn areas: Lawns don’t usually absorb

stormwater effectively, especially where

underlying soils were removed and compacted

during construction or they have not been well

amended with organic matter. Some lawn

spaces are good candidates for creating LID

areas, such as rain gardens, pervious patios, or

expanded, layered landscaping.

Invasive plants: Invasive plants are those that

will try to take over an area so nothing else

can grow. You will need to remove these before

you can install appropriate, sustainable plantings.

Sizing root-protection zone: Forest specialists

advise using the tree’s trunk diameter at breast

height, or DBH (4.5 feet from the ground), as

a guide for protecting your trees.

For every inch of DBH, protect a minimum of

one-foot radius (for instance, a 10-inch DBH

requires a minimum 10-foot radius of protection).

Shallow, compacted, or saturated soils might

require up to twice as much room.

Deep, well-drained soils may only require

two-thirds that distance.