How to design your landscape to allow water to seep into the ground

Find out how rainwater penetrates even in difficult sites.

Sinking water, also called infiltrating and percolating water, allows water to pass through the soil of your home landscape rather than sending it downstream through pipes or storm drains. This protects water supplies, recharges aquifers and improves water quality by reducing downstream runoff and naturally purifying water.

This is the third blog in a series on the motto of sustainable stormwater design: Spread it out slowly and let it sink. First, learn how to design your landscape to slow down the water and spread it across the landscape before reading about how to make it settle.

How infiltration works

Infiltration is an important natural process that improves water quality by absorbing and infiltrating water into the soil. Filtration is a phenomenon in which water is purified as it passes through the soil, and organic matter in the soil absorbs the water. The “sink it” step involves slowing down and spreading out the water. This is because it slows down and makes it easier for water to spread across the surface.

Harnessing the soil’s natural ability to absorb water is the easiest and most direct way to “shape” water, but it becomes complicated when soil infiltration rates are low. Soils may not have the physical, chemical, or both capacities to hold or seep large quantities of stormwater runoff.

If permeability is poor, water may still sink on the site, but the soil in rain gardens and swales may need to be amended to increase the soil’s infiltration capacity. Use the soil survey site to find out your soil’s infiltration rate. When you select a property for a soil survey, you will be provided with details on the soil infiltration rate measured in inches per hour.

Use ground cover material that aids infiltration

If you have space and good soil infiltration, you can follow the “slow” and “spread” ideas described earlier and let the water settle directly into the ground as a third step. One of the easiest ways to improve the permeability of outdoor space surfaces is to use permeable materials in landscaped areas of driveways, walkways, and patios. Good permeable materials that aid penetration include:
➢ Pebble/Gravel
➢ Wood chips
➢ Permeable pavement

Also, look at non-landscaped areas of your yard and think about ways you can plant them in a way that aids infiltration.

➢ Meadows and grassy perennials
➢ Bed of water-loving plants

Green roof design for infiltration

Green roofs have the ability to slow down and stabilize water. Shallow types, like this green roof planted with sedum, absorb water during small storms, offsetting roof runoff. A deeper green roof allows more water to penetrate.

Add planting pads for infiltration

Rain gardens can effectively absorb and infiltrate a lot of runoffs, but they can require a lot of space to do so. If space is limited, water may be run directly over a series of planted beds for infiltration. Combinations of different types of plants are suitable for absorbing rainwater and can be used in a variety of locations depending on climate and conditions.

They can be planted in grassy meadows and prairies where the plants have a variety of root structures, including taproots, bulbs, deep roots and superficially spreading roots. This type of planting replaces a lawn and allows more water to penetrate in full sunlight.

➢ Dense, multi-stemmed, water-loving trees and shrubs can be planted in shady gardens or areas where privacy is needed.
➢ Ferns and mosses can be planted in soil with high organic matter content.
➢ You can plant dense, layered plants with ground cover, woody shrubs, herbaceous plants and trees. You can create this type of planting by adding more native plants to existing planting beds or by installing new tiered plantings along the edge of your property as a stunning backdrop.

Infiltration includes bioswales

Swales, which often collect and move water, can also be designed for infiltration. These types of swales, called bioswales, are fully planted to absorb moisture and have soils engineered to maximize infiltration. Exact soil specifications vary greatly depending on native soil and local regulations, but typically include coarse sand and compost.

Small space ideas

In small spaces and soils with low infiltration rates, water may have difficulty settling. Often these challenging sites require more design and planning work to increase the capacity of the soil to absorb water or to use certain plants within the water absorbing plants in a smart way.

Don’t have space for a bioswale or large rain garden? By planting an infiltration layer that acts as a small rain garden in a small space, you can allow water to seep into small spaces. The planting strip between driveways is a good example of a small planting bed to infiltrate runoff from a concrete surface.

Another small space idea is to use rain planters, which function like a rain garden inside a container, capturing and absorbing water from the roof. A third option for small spaces is to create an infiltration area underground using engineered options such as dry wells. They temporarily store water and then slowly infiltrate the water into the amended permeable soil around the well.