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

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• 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.

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