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SLOPES, DRAINAGE, AND PLANT SELECTION

In recent years residential development has quickly expanded beyond the urban flat lands to the surrounding foothills and mountain ranges. Development of communities in these areas is more costly and time consuming due to the topography and inherent soil conditions. Master plans for housing developments require extensive engineering and grading to prepare home sites and new streets. Controlling erosion during construction is a high priority as it 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 down stream.

During the course of home construction, as the phases of development are completed, homeowners take possession of their newly built homes and now it’s time to call the professionals to create a functional and aesthetically beautiful garden. In recent years, 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. Plants commonly installed on slopes by the home builder satisfy 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. Over the years, we have found that most homeowners disdain the intense maintenance required on slopes, and dismiss slopes as a waste of valuable land. Depending on a homeowners’ desires and their landscaping budget, sloping areas are often altered to accommodate the final landscape design plan.

Professionals in the landscaping industry 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 techniques are needed to remove excess gravitational water from soil by natural or artificial means. This reduces the erosive force of water velocity-the single most important factor in maintaining slope integrity. This surplus water usually originates from rainfall, or a high water table. On slopes and hillsides, most of the water ends up at the lowest portions of the slope along with an accumulation of salts, nutrients, chemical pesticides, and herbicides. Also, excess water has a detrimental effect on plants by reducing the oxygen available to the plants root zone. It is important to know that simply installing drainage devices alone is not a cure-all measure that will solve 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 ratio of the slope (slope angle), and the slope aspect (the direction it faces).

In many developments, especially in the west, slopes have been cut and graded to provide adequate surface drainage and stability. The face of the slope is usually straight or terraced, and the slope is crowned at the top to prevent adjacent surface water from running down the slope face. In all cases, after landscaping improvements are completed the crown or berm at the top of the slope must always remain in place. On cut and filled slopes a variety of situations may be present. It is common to find that soil horizons have been exposed by excavation and grading. Recently exposed soil is often low in fertility and ranges from impervious to sandy. Identifying the soil type is important in determining the infiltration rate of the soil. Soils with a low infiltration rate will have a higher volume of run-off. Conversely, soils with a high infiltration rate will have a lower volume of run-off. Either extreme will tend to overburden the drainage system at the base of the slope. Also, a high infiltration rate will cause soil saturation and soil 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 a project. 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.

In natural undisturbed 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 sand on what we know as beaches. When additional runoff from development is added to this process it clogs waterways and reduces their ability to carry large volumes of water during heavy rainfall and also inhibits the natural cycle that creates beaches. To prevent this, soil must be protected from erosion whether flat or sloping. Slopes exceeding certain heights require aggressive measures to control erosion. Tall slopes are more susceptible to the effects of water velocity, especially when the soil type is easily eroded. Water velocity increases as it flows downhill, and as it gains speed its volume and weight overpower everything in its path.

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. Culverts are engineered to carry the maximum amount of run-off 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 drainage design plan to reduce water velocity, and snag debris carried in run-off.

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. Large rock or stones used loosely, or cemented into place to dissipate water velocity are referred to as rip rap. On the surface of the culvert outlet stones are 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 run-off. 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.

The drainage method often employed in residential properties is the installation of solid plastic drainage pipe. This method is used to intercept run-off at the bottom of slopes and behind all structures built into, or on top of slopes. Drainage, backfill, and weather proofing requirements for structures and retaining walls on slopes are usually specified by the local city or county building and safety department. Drainage system installation involves the use of solid perforated plastic or PVC pipe, a fabric sock or sleeve liner, ľ" crushed rock, and landscape fabric. Normally, 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 run-off slopes. The perforated drain pipe must be higher than the main drainage system it will empty into. The bottom of the trench is lined with 2- 4 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 and debris suspended in run-off; of course, the long-term effectiveness of this system is dependent 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. Jute mesh netting should be installed prior to planting on slopes with soil prone to erosion, and on slopes with a grade ratio of 1:1 or 2:1. Additional work may be required to stabilize slopes where special circumstances are present.

A properly planted slope must include a combination of plants, (ground covers, shrubs, and trees), suitable not only for their aesthetic value, but their ability to contribute to the long-term stability of the slope. Plants considered suitable share one common trait-roots that are able to penetrate through soil horizons and spread laterally to bind soil. Ground covers and shrubs that grow laterally cover exposed soil and reduce erosion during seasonal rainfall. Trees 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 pipes, or topple over during periods of high winds and heavy rainfall. Large, fast growing trees also deplete the soil of water and nutrients, and create excessive shade; both of which will eventually cause plants to decline in vigor. Another critical point to consider during plant selection is that plantings located at the upper portion of the slope will always have less water available to them during the course of their life than plants at the bottom of the slope. Select plants suited to the soil moisture conditions present where they will be located on the slope. Massing plants is preferred to a "one here one there" approach. Achieving this is more difficult than it seems, and professional help or consultation should be sought to successfully complete the planting project. Once the slope has been planted, a pre-emergent can be applied and the slope should be mulched with a composted and shredded wood by-product heavy enough not to be blown away by wind or easily carried off by surface water.