Erosion and drainage are an important part of any landscape plan, but they are absolutely critical when hillside or sloped properties are considered.

Precipitation and landscape irrigation can quickly damage your carefully planned landscaping and even put your home or structures at risk if proper allowances are not made. Steep slopes offer no resistance to heavy runoff from storms, so water carves away topsoil, threatening your foundation, patios, walls and even homes below.

The techniques for handling rain runoff and sprinkler drainage are similar, although your landscape designer should take into account the worst-case scenario. It isn’t practical of course to design for 100-year flood events or hurricanes, though you can minimize damage from unusual weather events by using correctly sized components and by designing overflow capacity into your landscape drainage.

Masonry culvert replaces concrete gutter.

Proper irrigation drainage is less a problem when sprinklers, drip, and flood-type irrigations are correctly sized and designed. You’ll use less water, saving both resources and money in your pocket. (See Irrigation.)

Consider the nature of your landscaping surface materials. Heavy rain falls evenly across your property, but non-porous materials will channel water to other areas. If half of your landscaped areas are paved or covered with nonporous materials, precipitation will drain to other to drain to planted soil areas. Add a steep slope below and you have the recipe for a truly bad day.

Since roofs and awnings don’t absorb water, runoff from structures should also be a part of your landscape master plan and your landscape designer should take this into account.
One alternative that will also supplement your irrigation is by simply directing roof gutter flow into your planted box areas or other drainage-designed landscape plants. Collecting rainwater in barrels is another solution, provided that the barrels are covered to avoid breeding mosquitoes.

Plants that are deep-rooted and hardy such as the following, can help control slope erosion: Artemesia, Banks Roses, Clematis, Day Lilies, Forsythia, Juniper, Manzanita, Potentilla and Salvia. Many grasses, especially ornamentals, form obstacles or clumps that control drainage.

Hillside homes are wonderful places to live; they provide excellent views and sunsets, while slopes add privacy and visual texture. If you are landscaping a hillside home or other sloped site, have your landscaper evaluate your soil type. Rocky soils tend to withstand erosion better, although enough rain can cause even these types of slopes to slide, and when heavy or rocky material begins moving, it can cause major damage to hardscapes.

Different sized pipes channel hillside runoff into driveway culvert.

Ironically, one of the worst times for erosion is during home and landscape construction. When construction is complete, excavated areas and bare soil remain. Here are a few suggestions on how to deal with erosion and water control at the beginning:

• Schedule construction and landscaping during a dry season. Use sod instead of grass seed and take advantage of natural soil erosion inhibitors like straw and compost.

• Have your building contractor and landscaper work together during the design and building process, communicating about slope issues, drainage, site and soil conditions.

• Building placement and architecture should be "site sensitive," i.e., it should take natural slope and drainage into consideration. This is also an excellent way to reduce construction costs and costly stabilization work down the road.

• Build harmonious landscape. Preserve native plants and trees on your property if possible, and make them integral to your landscape plan. Native plants work well; their very presence proves they’re an excellent erosion control tool already adapted to the local environment of soils and slope. Make certain your contractor protects existing plants and trees during home construction.

• Avoid extensive "cut and fill" grading; existing soil is already compacted and stable, while fill must settle for a period of time. Even though fill material is mechanically compacted during site preparation it doesn’t drain as well as native slopes. Follow natural slope contours as much as possible.

If you cannot avoid precipitation during construction, or if your landscape budget means the work will have to be done in installments, use temporary erosion control measures.

Temporary measures include:

Grass-- An obvious short-haul erosion protection, grass is inexpensive and grows quickly. Ryegrass, for instance, grows particularly fast and takes hold well. For these reasons it is often planted to stabilize wildfire-ravaged hillsides in California. Your landscape designer, local farm agent and USDA soil conservation service are great sources of information on what kinds of grasses do well in your area.

Mulch-- The universal erosion prevention tool used by most landscapers is mulch. Not only do mulches like straw, tan bark, sawdust, leaves, wood pulp and fibers, clippings, and compost absorb moisture, they release it slowly without washing away your soil, providing the kind of organic starter environment growing plants love.
Mulch has also become a high tech field, with numerous types of mechanically applied matting, hydraulic mulching and composition mulch-and-seed treatments available to stabilize and plant hillsides at the same time.

Matting-- Woven fabric matting or plastic sheets work well on especially steep slopes. Non-organic materials must be removed, unless they are made of several new starch-based polymers that biodegrade over time. Otherwise, natural fibers like jute netting and excelsior matting will help plants take hold on steep slopes, slowing the flow of water.

Your landscape architect might incorporate any number of design elements as erosion control measures to reduce water flow and velocity:

Barriers and boulders-- Adding large rocks in steeper areas or perhaps small rock gardens at various points along a slope slows flow. Barriers like rails or timbers can be placed to overlap at right angles, like a zipper.

Terraced hillsides-- Terracing techniques have been used for centuries in mountainous areas. You can adapt these to both control runoff and help planted areas retain water better. Be sure terraced areas are wide and deep enough and that the forward edge is protected from erosion.

Riprap-- Boulders or loose stones placed in a hollowed out culvert area protect soil underneath from erosion, while slowing water flow. Use stones that mimic a natural streambed, although granite’s blues and grays can make a striking visual impact.
Especially steep slopes call for landscape design and careful engineering, requiring that local building codes be consulted. Note that property owners can be held responsible for damage downhill from their property, so make sure that your erosion control and drains don’t carry water to adjoining surrounding property.

Grates drain low point in driveway.

Controlling and draining water is the second part of the weather and irrigation equation. Your landscape designer will calculate slopes and check soil types to size and plan runoff and drainage control for your situation.

Make sure your drainage plan slopes away from homes or other structures to protect against flooding and moisture damage.

Hardscapes should all have sufficient slope to avoid runoff stagnation or puddles that can damage tree roots and pavement. Large patios should have integral drains or proper slope design that will carry water to a low point drain. Planters should have clog-guarded internal drains or a sand or gravel base to allow water to go back into the soil.

Sloped driveways and walkways can act as culverts, so baffling and regular drain intakes will reduce the problem.

Tightly packed plants and turf can also shed water, instead of absorbing it. Make sure that plants have adequate spacing to allow root structures and soil to absorb runoff. Note, too, that your grass areas, if regularly dethatched, will do the same.

Here are some drainage design components your landscape contractor can use to control and divert water:

Roof gutters and downspouts-- Gutters collect roof runoff and channel it to downspouts. A downspout should drain into a gravel sump or other overflow device; or, it can drain to your landscaping in order to protect foundation and planted areas adjacent to your home.

Underground tubing-- Drain the water away from your home to a better location using subterranean tubing. Some roof gutter exchange downspouts for baffle type designs break up the stream of drained water, so that it does not erode the ground below.

Slotted pavers-- Cover patio drain culverts with slotted pavers, adding square footage of usable space while water underneath drains away.

PVC perforated piping-- Help water percolate back into the ground with PVC perforated piping placed underground with a surface drain or drains feeding it.

Rock filled sumps-- Some homes at the bottom of swales or low points are natural water collection sources. A sump can catch large amounts of rain and allow it to naturally percolate. Sumps use the weight of water and gravity to help soil’s natural percolation. A sump pump can drain a low-lying area and divert water to another location where it can drain properly.

Trenches-- Trenches can also act as sumps or diversion pits that can aid soil in absorbing rain and runoff.

French drains-- Repeated storms may saturate soil. French drains help avoid waterlogged soil that can damage plants and structures by collecting water and drain it downhill to an outlet.

Passive devices like diversion ditches and gentle slopes, (grades of 1:100) can also work to drain low spots or poorly draining soil to other areas.

Aboveground and underground drains are important, but what about the ground itself? How well water moves down through your soil determines drainage design and affects landscape health.

Your landscaper should test your soil as part of the landscape design process. From that information, he or she will be better able to control where water goes and how it is controlled.

Silt, clay and adobe often form solid barriers that shed water instead of absorbing, so consider adding sand, mulch or other absorption enhancing materials to low-porosity ground.

Sandy soil, on the other hand, has excellent drainage, but sand can drain too well. Consider adding mulch or another commercial soil amendment that aid water retention for plants and trees.

Catch basins can help you deal with unusually large amounts of precipitation and rainfall, and in fact, most building codes call for them when large or heavily sloped properties are involved.

A catch basin is simply a low point that is large enough to fill during rain events without overflowing. They utilize the natural draining ability of soil to return water to the ground.
Internal drainage can be enhanced by reducing the amount of impermeable pavement in your hardscaping plan. Consider pavers with gaps that can be planted. Reducing the size of patios or other paved surfaces will also aid internal drainage.

Turf and sod drainage is critical, according to experts, especially if your grass areas will see heavy utilization.

The U.S. Golf Association has come up with some excellent ideas for turf design and drainage that might be useful to you and your landscape designer.

Commercial golf courses utilize a subgrade of combined layers of gravel and sand. Several inches of coarse sand, spread on top of a shallower layer of gravel, helps provide an excellent water retention/drainage balance. Sand also provides a firmer, less muddy surface underfoot, while aiding growth, as long as proper aeration and topdressing are part of your maintenance plan.

Peat and sphagnum make a good organic surface for turf that will retain water and nutrients. A mixture of sand and peat can also be used to aid growth and drainage.

Dense, thatched grass prevents proper drainage, robbing roots of needed water and wasting irrigation. Maintaining oxygen and water to plant roots is the result of good aerification and top-dressing of turf.

Top dressing, adding soil and sand that trickle through grass and ground cover plants is not just necessary for proper drainage, but for plant and grass health.