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How to Deal Effectively With Run-off and Erosion in the Built Landscape

By Alrie Middlebrook

This Italian historic garden utilized a gravel pave system for a pervious solution to the hardscape areas. Also featured are used bricks on sand and downspouts connected to underground perforated pipes that extend to the landscaped areas. Existing rainfall is retained on site through hardscape solutions and finished grade.
Photos courtesy of Middlebrook Gardens

The simple phrase, "retain rain water on site" packs a powerful punch when it comes to water conservation and watershed protection. Unfortunately, for years the solutions we have found for paving, parking and walking surfaces have created the opposite effect.

Constructing impervious hardscapes speeds the flow of water and focuses large volumes of water where it is undesirable--into the storm drains. In Northern California where I practice, this freshwater eventually flows to the San Francisco Bay, often loaded with contaminants from automobiles and home-based products that run the gamut from paints and solvents to herbicides, pesticides and phosphate-based fertilizers.

This is the after shot of an eco-engineered waterfall with a redwood riparian plant community. The flowing water oxygenates an existing pond. No concrete was used here, nor is there a retaining wall. Boulders, native plants and jute netting combined to naturally stabilize the slope.

The Solution

If you want to be a part of the solution and keep rainwater away from the storm drains, the simplest and most economic approach is to design the solutions into the project initially.

Taking a walk in a natural area on a rainy day has taught me more about sustainable solutions for slowing down the flow of water than most textbooks and lectures. If I were to choose one word that describes how to design effective solutions for water in the built landscape, that word is TRICKLE. If you can creatively design features on site that allow water--including large volumes of water during short intervals--to trickle, then you are finding the most sustainable solution. The natural landscape allows water to trickle and therefore percolate over a greater area at a slower rate. Soil structure acts as a natural filter to dissipate contaminants and particulates before surface water reaches the underground water table.

All downspouts including those venting from air conditioners, should be tied into the landscape, buried at least 18 inches and directed into the planting areas.

When I first visit the site of a new project, I ask, "How many different methods can I utilize to slow the flow of water?" Depending on slope, sometimes the solution may be a retention basin. Other times it may be a seasonal creek feature, or both devices may be used on the same project. In most cases finished grade, including subtle and slight changes in topography, will result in myriad ways to disperse water over a greater area at a slower rate.

Simple Solutions to Utilize on a Single Site

  1. Tie all downspouts into the landscape by attaching flexible perforated drain tubing to the spouts at lengths of l2-feet or more and direct them away from the structures and into the planting areas. Bury the tubing at least l8 inches and install at a two percent grade.
  2. All hardscape surfaces should be graded and installed to drain into the landscaped areas, not to the street or driveway.
  3. Create specific natural features to accommodate excessive run-off. Some of my favorites are: dry creeks that during the rainy season may become seasonal wet features. In these creeks we install water loving native California rushes that prefer wet areas but can also take prolonged periods of drought. Dry creeks can be built at the edges of parking lots and other large hardscape areas. We also build vegetative swales to retain rain water and allow it to slowly percolate. These swales can be very decorative. Native bunch grass meadows with undulating surfaces are a very effective way to retain and disperse rainwater. The more water-loving wildflowers can be seeded in the low areas. The drought tolerant species will prefer the ridges and slopes. If the site is a steep slope, a seasonal pond may be a design feature, or even a waterfall. Capturing run-off by mimicking how nature solves the problem is usually the most effective solution, and most economical as well!
  4. Select pervious materials for hardscape solutions. Gravel with Gravelpave, grass with Grasspave, brick or concrete cobble on sand, stone cobble on sand, decomposed granite, concrete tiles on sand or sports turf. I prefer any of these solutions to interlocking pavers since pavers are 90 percent impervious. The more that water is allowed to drain through the hardscape, the better the solution. How to Deal (Continued from page 72) Many people are not ready to give up this control. Unfortunately, the more control people want, the harder the surface and the less water is allowed to dissipate. If clients can begin to think of outdoor solutions as different from indoor solutions, that it's necessary to give up control outside and find solutions that mimic nature, it will be infinitely easier to practice sustainability.

    This ridge features a dry creek bed that drains into a pond water feature. The plantings are in the grassland meadow community with the more drought tolerant plants, such as Idaho fescue, at the top of the ridge.
  5. If there is concrete on site, break it into large stepping stone-size pieces and relay it on a bed of decomposed granite. We may choose to back-fill the spaces between with soil for planting, pea gravel or other decorative small pebbles. Each of these solutions allows water to percolate through the hardscape. We also avoid trips to the landfill by recycling the concrete to create a new garden feature. This type of hardscape solution is called urbanite.
  6. Try to avoid using new concrete. It is a totally impervious material, unless you use pervious concrete. Pervious concrete is a relatively new product that is being used effectively in highway construction and has landscape application. It can be colored as well and can make a smooth transition between decomposed granite pathways to parking areas. However, it must be noted that the manufacturing process for concrete is the number two contributor to "greenhouse gases," and thus global warming.

    Broken concrete, or "urbanite," was installed with grey gravel as stepping stones in this Asian themed woodland garden. Some areas have soil or Corsican mint as groundcover between the stones. California wax myrtle is in the right front and back. Mid-ground is Siskyou blue, California rush and Idaho fescue. At the top left is China doll and Western azalea.
  7. Select native plants that naturally occur together according to their ecological plant community. Determine which community to select according to the dictates of the site and the existing soils. When planting, use the principles of succession. All garden sites in urban areas are disturbed. To reclaim the site, it is necessary to plant grasses, annuals, and fast-growing perennials and woody shrubs that act as colonizers in a disturbed landscape. One of my favorites is coyote bush. This is a great plant for erosion control on steep slopes. Cover all planting areas with a 3-4 inch layer of weed and seed free organic material. These types of plants can do more to dissipate and slow down the flow of water than expensive retaining walls and elaborate, over-engineered drainage systems. Native bunch grasses have fibrous roots that grab water and allow it to trickle into tiny crevices of the grass' root system. These grasses grow very quickly and in a few months begin to change the composition of the soils. Planting young trees with an understory of native bunch grasses will assure long term soil stabilization.
  8. Irrigate with underground drip lines. Use overhead spray minimally for turf and meadow areas. If your client wants turf, suggest sports turf made from recycled plastic and black bits of pulverized, recycled tennis shoes and sand. Water flows through the surface and it requires no irrigation to stay green. Most professional sports fields use this solution. Sports turf is particularly good if the client has children and/or dogs, and it's a very natural looking, forgiving surface that's easy to walk and play on.

This plant community is an oak woodland/grassland meadow featuring a natural rock outcropping with buckwheats and lilac verbena. The plantings capture the water and allow it to trickle down through the soil before it reaches the driveway or the stormwater drains in the street.

All of these solutions share one common goal. They seek to understand and replicate a natural system. The closer our solutions come to copying nature, the less human energy is expended to maintain or manage the system and the less negative impact we will have on our environment. It is ultimately about giving up control. When our species can be become comfortable with that solution, we will begin to have more control over the things in our lives that are really important, like family and friends, because we'll have the time to spend enjoying them instead of spending so much time fixing problems that could have been avoided in the first place. So, let your rain water return back to your soil, trickling along the way...

This pervious paved path of California gold gravel using the "Gravelpave2" system from Invisible Structures was installed on two inches of compacted base rock. It is a pave system using interlocking honeycombs of injection-molded plastic and is totally pervious. It will hold the weight of a fire truck if necessary. The meadow is planted with Idaho fescue, purple needle grass and California melic. The annuals and perennials in the meadow are Clarkia, Queen Fabiola, California poppies, globe gillia, yarrow, lizard tails, iris, blue-eyed grass, lupines and desert blue bells.

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February 17, 2020, 1:36 pm PDT

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