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Useful Techniques to Control Slope Runoff

Garden Concepts

Proper drainage of residential slopes can incorporate many different techniques. Hydro seeding can be a fast and effective method of covering the slope when the rainy season is fast approaching.

In recent years residential development has expanded beyond the urban flat lands to the surrounding foothills and mountain ranges. Because of the topography, development of communities in these areas is much more costly and time consuming than ever before. A builders plans often require extensive engineering and grading to prepare home sites and new streets. Controlling runoff ensures a minimum amount of soil loss from graded home sites during seasonal rainfall. During the excavation and grading phases of the project strict erosion control management is required by law to reduce the negative effect it has on storm drains and natural ecosystems.

During the course of home construction, as the phases of development are completed, homeowners take possession of their newly built homes and must then call upon a professional Landscape Contractor. It’s our job, once again, to face a new challenge, and create a functional and aesthetically beautiful garden.

Planting a combination of ground cover plants, shrubs and trees to a bare slope lets the roots stabilize the soil.

One increasingly common challenge is dealing with the existence of sloping hillsides on residential properties. Front yard slopes , back yard slopes, and side yard slopes are common scenarios, not to mention homes built on sloping hillsides. The landscaping commonly installed on slopes by the home builder satisfies a minimum standard required by the local municipality for maintaining the integrity of a slope. Often, this is merely a handful of plants that may or may not be suitable for the slope, and a spray irrigation system.

Depending upon the homeowners desires, and their landscaping budget, sloping areas are often altered to accommodate the final landscape design plan. Over the years, we have found that most homeowners disdain slopes as a waste of land and maintenance resources.

As professional Landscape Contractors we understand that certain measures must be taken not only to beautify sloping hillsides, but to ensure that proper drainage is maintained and erosion is minimized to prevent the slope from failing. The use of drainage devices and drainage techniques are needed to remove excess gravitational water from soil by natural or artificial means to reduce the erosive force of water velocity-the single most important factor in maintaining slope integrity. This surplus water usually originates from storm water runoff or a high water table.

In the case of sloping hillsides, much of the water ends up at the lower portion of the slope along with the accumulation of salts, nutrients, chemical pesticides and herbicides. Also, excess water has a detrimental effect on plants because it reduces the oxygen available to plants' root zone. It is important to know that simply installing drainage devices alone is not a cure-all measure that will solve all long term drainage problems associated with sloping hillsides. Achieving successful results is part of an integrated plan of attack based upon careful evaluation of the soil type, climate, grade of the slope and the slope aspect.

Large slopes should include a gradual sloping culvert to collect and direct runoff water. At this site, runoff water is diverted into a natural drainage channel. Though helpful in moving water to storm drain systems, culverts are susceptible to being catch basins for debris as well.

In most new developments, slopes have been cut and graded to provide adequate surface drainage and slope stability. The face of the slope is usually straight or terraced, and the top of the slope is crowned to direct adjacent surface water away from the slope. In all cases, the crown of the slope must always be maintained and surface water must never be allowed to erode the crown and flow downward from the top of the slope. On cut and filled slopes we often find a variety of situations. Commonly, we find that soil horizons, once buried, have been exposed by excavation and grading and are now on the surface of the slope. This newly exposed soil is usually low in fertility and ranges from impervious to sandy, or a combination of both. Identifying the correct soil type is important in determining the soil's infiltration rate. Soils with a low infiltration rate will have a higher volume of runoff. Consequently, soils with a high infiltration rate will have a lower volume of runoff. Both extremes are undesirable because a high runoff rate will tend to over burden the drainage system below the slope while a high infiltration rate can cause soil saturation and significant earth movement during periods of high rainfall. The existence of impervious soil layers consisting of either clay or rock running horizontally across the slope can complicate the final solution. These layers of soil or rock have been cut during grading to create the slope angle. In this situation, water will move laterally along these impervious soil layers and may come to the surface in one or more places on the face of the slope, especially during and after periods of high rainfall.

Tall slopes are more susceptible to the effects of water velocity especially when the soil type present is easily eroded. Water velocity increases as it flows downhill. As it gains speed its volume and weight over power everything it in path. In natural areas, erosion is part of the geological process. Eroded soil, suspended in water, moves for miles down streams and rivers until it reaches the ocean and deposits it's sand on what we know as beaches. To prevent our streets from becoming rivers, as they sometimes do, slopes exceeding certain heights require aggressive measures to control erosion.

Culverts are reinforced concrete lined v-ditches that are built into and across the face of the slope. The width and depth of a culvert, and where it must be located on the face of the slope, is usually specified in landscape plans.

A headwall is installed using cinder blocks where culvert channels meet and direct the runoff into a main channel. The headwall is beneficial in preventing erosion of the culvert in case of heavy water runoff.

Culverts are engineered to carry the maximum amount of runoff during periods of heavy rainfall. They are built to a specific dimension and detail. Water flowing into the culvert is directed gradually to lower elevations and dumped into a storm drain system. Where the culvert dumps into a storm drain, high water velocity can pose a problem during periods of heavy rainfall. For this reason headwalls or stone rip rap are specified, as part of the design, to reduce water velocity and snag debris carried in runoff.

A headwall is basically a cinder block wall installed where culverts intersect and dump into a main channel. A headwall prevents water from eroding and undermining the culvert if the water volume is too great for the culvert to handle. Rocks are used loosely or cemented into place to dissipate water velocity; they are often referred to as rip rap.

Storm drainage areas tend to be the final destination for trash and other debris. Sand bags are often used to collect the debris.

On the surface of the culvert outlet, stones are often used cemented into place half buried, pointed upward and on their ends. The stones must protrude at least six to eight inches above the surface, and at a specified distance from one another, to be effective. Rocks or boulders used loosely are also of a specified size and weight so that they remain in place and are not carried off by water. They are normally used in high flow situations where water dumps into a natural stream or river.

In residential situations, installing culverts is a costly method of controlling runoff. On many home sites, culverts are required to be installed by the home builder and included in the cost of the home. This solution is more commonly used along highways or streets that are adjacent to sloping areas.

When slopes are not planted early enough, water runoff may carry soil into the storm drains.

The drainage method often employed in residential properties is the installation of solid plastic drainage pipe. This method is used to intercept runoff at the bottom of slopes and behind all structures built into or on top of slopes. Drainage, backfill, and weather proofing requirements involving structures and retaining walls on slopes are usually specified by the local City or County building and safety departments.

Drainage installation involves the use of solid perforated plastic or PVC pipe, a fabric sock or sleeve liner, 3/4-inch crushed rock and landscape fabric. A trench is dug at the foot of the slope to a minimum width and depth of eight inches by twelve inches. Wider and deeper trenches are needed to remove water from high runoff slopes. The perforated drain pipe must be higher than the main drainage system it will empty into.

A newly planted myoporum parviflora ground cover is not much bigger than a quarter when planted, but within six to eight months, a quarter becomes dwarfed.

The bottom of the trench is lined with two to four inches of crushed rock bed. The perorated pipe with the sock liner is installed on top of the rock with the pipe holes pointing downward. Allow for at least three inches of space for rock backfill between the sides of the trench and the pipe. More space is required for larger diameter plastic pipe.

The trench is then back filled with crushed rock to the top of level grade. Landscape fabric can also be used to line the bottom and sides of the trench as an interface between the soil and the crushed rock. The fabric liner helps reduce the long term effects of silt accumulation in the perforated pipe and in the down stream drainage system. It is also recommended that catch basins be installed at the lowest points in the system to trap any silt carried in runoff. Of course, the long term effectiveness of this system is dependant upon the installation of plants and the stabilization of the slope as part of the overall project.

Once the drainage system is in place the slope is raked clean of loose rock and debris. Washouts and slumps must be repaired, and loose soil compacted. At this time it may be necessary to eradicate burrowing animals from the slope to prevent future damage and costly repairs. Several treatments may be necessary to fully control infestation. Evaluate the usefulness of existing plants and keep those that are well suited for the slope.

Installing a permanent landscape is the final step in completing the project. If the rainy season is approaching hydro seeding can be beneficial in establishing a quick cover crop. Certain hydro seed mixtures increase the soil fertility by fixating nitrogen as well as holding soil in place until a permanent landscape can be installed. To increase soil stability jute netting can be useful on poorly compacted slopes with loose soil, and on slopes with a grade ratio of 1:1 or 2:1. Properly planted slopes include a combination of ground cover plants, shrubs, and trees. Plant selection must meet certain criteria to be considered suitable for slope planting. Plant's roots must penetrate deeply into the soil horizons and spread laterally to bind soil.

A 50-foot-high slope behind a residence in Temecula, Calif. has become problematic after only four months. The soil has been disturbed by gopher fans and rivulets which indicate erosion problems.

Ground cover and shrubs should have lateral growth which covers exposed soil and reduces the erosive effects of seasonal rainfall. Trees planted on slopes should be small to moderate in size with a narrow form. Avoid large, fast growing trees that have shallow invasive root systems which can damage drainage pipe or topple over in high winds and heavy rainfall. Large trees will also create excessive shade that will eventually cause ground cover and shrubs to decline.

Once planted, a pre-emergent can be applied and the slope can be mulched with a composted, shredded wood by-product. One last point to consider when planting a slope is that plants located at the upper portion of the slope will always have less water available to them than plants at the bottom of the slope, so select plants that are suited to these conditions.

An established slope with myoporum parviflora and trees can prevent erosion and allow the roots to solidify the slope.

As with most projects, improving the drainage and appearance of residential slopes can have varying degrees of complexity based on the landscape design and the homeowners wishes. In all cases, careful analysis of the slope condition is the single most important factor in determining the correct course of action and in making sure that excess water does not threaten to erode your reputation.

James Garcia is a graduate of Cal Poly University with a B.S. degree in Landscape Architecture, and a minor in Ornamental Horticulture. He is a Landscape Designer and owner of Garden Concepts in Temecula, California.

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June 18, 2019, 8:58 pm PDT

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