Water runoff and permeabililty

Water runoff and what to do with our wet Pacific Northwest winter water.

The Convergence Zone is a common term heard amongst Pacific North Westerners. It refers to the weather pattern we experience for 9 months of the year which brings rain, wind and dark skies. The Pacific Northwest typically receives 30-32 inches of rainfall between September and May. That is a lot of storm water and as we continue to urbanize, there is much more surface water run-off that we need to address.

The water cycle in natural and in suburban/urban areasAccording to the above graph, less surface water is evaporating and even less is absorbing into the ground. With less trees and more hard surfaces, water runs off at a higher rate and with consequences. When water can permeate into the ground, benefits include balanced soil conditions, natural filtration of storm water, cleaner streams and lakes, improved air quality and so on.

There are multiple ways that we as individual homeowners can do to improve storm water infiltration and begin the process to improve our soil and air. In effort to simplify the information, I have broken it down to 4 easy steps: Reduce, Recycle, Re-use and Replenish. Each of these topics is explained below.


The first step is to reduce the amount of storm water run-off that occurs on our properties. The roof, the hardscape and outbuildings on our properties contribute to the storm water run-off. Installing surfaces that allow the storm water to absorb is more beneficial than sending it through sewer drain lines.

Installing Plants

Plantings that are native or acclimated to our conditions, respond better to our natural sub-climatic conditions. These plants can tolerate the wet winter rains offset by dry summer months. Planting the right plant in the right place promotes low maintenance, less irrigation and overall healthier plants. Plants naturally absorb storm water, contributing to photosynthesis. And as water is absorbed into the ground, it is naturally filtered by the soil, roots and organisms thus sending clean water run-off to our streams, lakes and rivers.

Permeable Pavers

Driveways and patios have commonly been constructed with solid concrete surfaces, causing excess water to run-off. By replacing these solid surfaces with permeable surfaces, storm water can infiltrate into the ground. A gravel sub-grade is built up and pavers are set with spaced joints that are filled with fine pebbles. This allows water to seep through the joints and gravel into the soil. It is important to properly install a permeable paver surface to ensure function and longevity.

Installing French drains

A French drain in the landscapeExcess water run-off can be controlled with French drain lines. A French drain is typically created with perforated pipe surrounded by drain rock. They are meant to absorb water and direct it to another location. French drains are effective at the edge of hard surfaces, under drip lines of eaves, and in planting beds.


Recycling storm water run-off can be done in many ways throughout the landscape. By repurposing, storm water in the landscape is slowed down, absorbed and filtered before excess water runs off.

Rain water Harvesting

The roof surfaces of buildings create the most water run-off. Downspout drains can be directed into rain barrels or tanks for use in the landscape. Depending on your use and water collection, the reservoir should be appropriately sized to meet your needs. Remember, we typically get 30-32” of rain a year. An average roof can produce 750 gallons per inch of rain!

Rain Gardens

Storm water from can be collected and directed into a garden swale or rain garden. These are usually depressed areas in the landscape that allow for rain water collection and the natural absorption into the ground. Plants are installed that can tolerate the wet winter rains and the dry summer months.

Green Roofs

With the onset of urbanization, there are more solid roof surfaces contributing to excess water runoff. By turning a roof surface into a living space, the storm water can be absorbed and filtered and reducing excess run-off. Green roofs are constructed with a drainage membrane and lightweight soil with low maintenance plants. It is important that a green roof is constructed to withstand the weight conditions and properly waterproofed.


water storage tank in use Storm water can be re-used in many ways in the landscape and the home. No matter how you re-use your storm water, it requires a reservoir and a pump to move the water along with a secondary water source in case of drought.


Harvested storm water run can be connected to a landscape irrigation system. In the PNW region, we need to consider the excessive amount of rain we collect in the winter compared to the summer. A large capacity tank is required to supplement landscape irrigation throughout the summer. A secondary water source may be required to support irrigation throughout the season.

Non-Potable Water Use

Water that is collected in tanks can be connected to the household and used for non-potable water. Non-potable water is non-drinkable and should only be used for means of non-consumption. These uses include toilet water, laundry and washing the car. A supplemental water source is required.

Water Feature

What a better way to use the storm water in your landscape, than in a water feature. Water features add sound and beauty to a landscape as well as a welcome for birds and wildlife. A simple pond to a flowing water fall can be created from water run-off. Year round enjoyment of the water feature will require a pump and secondary water source during dry periods.


To ensure the soil and the plants can absorb the storm water naturally it is important that your soil allows water to permeate and is rich in nutrients.

Soil Amendments

A high nutrient and balanced soil will grow healthy plants and allow water infiltration. It is a good practice to test your soil so you know what it needs. Clay soil needs organic material to break it up and allow water to permeate. A sandy soil needs organic material and peat for nutrients and to hold moisture longer.

Organic Mulches

Annual application of organic mulches is highly recommended. It replenishes the soil of nutrients that are consumed by vegetation and washed away. Organic mulches hold moisture and allow plants to absorb water slowly. Organic mulches break down into the soil and keep landscapes rich in nutrients.

We hope that you will adapt one of these best practices to use storm water within your landscape and reduce the amount of run-off. Excess run-off that is unfiltered has effects on the wildlife and people who use the water. The more we can do to increase the amount of storm water that is absorbed into the ground, the better off we will be.

If you would like more information, please contact Environmental Construction Inc at (425) 803-9881 or by email info@envconst.com.

Category: Landscape Ecology

Environmental Construction, Inc.

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