Landscaping a Slope

Landscaping a Slope

How to Stop Wash Outs and Expensive Maintenance

It is rare for a landscape architect to be given a perfectly flat canvas to work with and very few would want one anyway – slopes and hills add character and natural lines.

Although a well landscaped hillside can be the highlight of your project, it also presents some big technical challenges.

Controlling erosion and the ongoing maintenance of hillside landscaping can rapidly inflate costs and add a lot of complexity to the average landscaping project [1].

Due to these challenges, slopes are often underutilized or forgotten in landscaping plans, to the detriment of the overall project. With the right material choices and some forward planning, you can have beautifully landscaped sloped areas that will not break the bank when it comes to installation costs or ongoing maintenance.

Before revealing the best ways of managing hillsides in your next project, lets just quickly review the major problems faced when landscaping hills.

The Challenges of Landscaping on Slopes

Hills and sloped blocks are known to increase the cost of landscaping significantly due to the challenges they present. The major ones are detailed below [2].

Erosion Control

The first issue that must be grappled with when landscaping a hill is erosion.

Gravity ensures that water will want to flow down any sloped ground, with the speed increasing with the gradient. For any plants to grow on a slope, this must be controlled as any consistent flow of water will quickly remove the nutrient filled topsoil or destroy any garden beds that have been created [3].

The introduction of any hard surfaces, such as paths on a slope can also significantly increase run-off rates and contributes to the velocity of water flows, making the installation of paths difficult [4].

The hallmark of a poorly landscaped slope are washouts, patches of dirt where plants can no longer grow and hard ground that has been stripped of the necessary topsoil [5].

Drainage

With the increased velocity and volume of run-off from a slope comes the issue of drainage in the areas directly below the hillside.

Unless you want the landscaped areas at the bottom of elevated areas to become swampy after every rainfall, then the installation of the correct drainage is a must. There is also the risk of environmental damage due to uncontrolled run-off [6].

Installing drainage with the capacity to deal with the run-off from even a small hill can add many thousands of dollars to a project due to the complexity and challenge of designing these systems [7].

Maintenance.

As you can imagine, landscaped areas on a slope are much more susceptible to damage due to surface runoff and generally require more regular maintenance.

Topsoil often needs to be replaced due to erosion and washouts filled. Particular attention must also be paid to paths and other hard surfaces, as any undermining of their subbase or subgrade can lead to costly repairs down the line [8].

Despite these challenges, there are ways to landscape a slope that are both easy to maintain and cost effective.

Popular Solutions For Hillside Landscaping

Here are some of the most popular solutions, including one of the newest and most cost effective methods that an increasing number of landscape architects are using.

1. Retaining Walls

The traditional method for dealing with moderate to severe slopes are retaining walls, where the slope is terraced and backfilled. The terraces are then supported by retaining walls, typically constructed from wood or concrete blocks [9].

These represent a significant engineering challenge, are expensive and also require significant drainage to avoid water building up behind the retaining walls.

2. Trench and Terrace

The trench and terrace system can be used on light to moderate slopes. It also involves the creation of flat areas on the slope for planting, but not to the extent where retaining walls are necessary [10]

Trenches are created at each level to facilitate drainage and slow the velocity of surface runoff down the slope [11]. These trenches require less construction and maintenance than retaining walls, but still must be checked regularly as a buildup of eroded material can render them ineffective.

3. French Drains

A well constructed French drain can be an effective way to remove excess water from sloped areas and ensure that soil on your slope does not retain too much water. The key here is the correct installation, as French Drains are surprisingly complex and require professional installation [12].

The first step of installing a French Drain is the digging of a trench, where a perforated pipe will be laid. The slope of this trench is crucial, as gravity will be doing the work of removing water for you. A minimum of 1% slope is recommended and the trench can be anywhere from 1 foot to 6 feet deep [13].

French drains are often laid at the base of slopes, but they can also be an addition to the trench and terrace system mentioned above if more drainage is needed.

4. Surface Drains

Surface drainage is created by grading an area to have a slope that feeds toward the desired area, which may be a more complex stormwater drainage system, or an area that can cope with the excess water.

To be effective, the area must have a consistent gradient of 2% when covered in grass or shrubs and 1% when paved [14]. The difficulty with this method is correctly calculating the gradient and keeping it consistent. Any slopes exceeding 2% are likely to increase water velocity to the point where erosion occurs [15].

Because of the chance of erosion, you should periodically recheck the gradient of your surface drain to ensure they are still working effectively.

5. Culverts

A culvert is a short length of conduit – often concrete – that is laid underneath a slope to transport large amounts of water that would otherwise flow down the slope causing erosion [16]. They are also commonly used under roadways to avoid large amounts of water flowing over vulnerable low points.

This is one of the most extreme and expensive drainage options discussed here. The installation of A culvert is quite an undertaking and requires a large excavation and precise calculations on its location and gradient.

The Baha’i Gardens in Haifa, Israel

Choosing the Right Travelled Surface

Before looking at how to manage the water flowing down the slopes in your next landscaping project with expensive drainage systems, first consider ways that the volume of water can be limited.

One area to pay particular attention to is your material choice for travelled surfaces. Paved areas, particularly when using impervious materials are a major source of high velocity surface runoff that can cause erosion on sloped areas if not effectively managed.

Of course, such surfaces are necessary and cat be eliminated, but by choosing a more porous hard surface, such as permeable concrete, permeable asphalt or stabilized aggregates you can limit the amount of runoff created.

Stabilized aggregate is a particularly compelling choice due to its low cost, ease of installation and high durability even during sustained rainfall. Traditional aggregates would not be suitable for use in sloped due to their susceptibility to erosion, but the use of stabilizers such as Organic-Lock™ have eliminated this weakness.

During laboratory testing, aggregate stabilized with Organic-Lock™ performed 97% better than unstabilized aggregates, while maintaining near natural rates of water infiltration. Even if the aggregate does become damaged, maintenance is a breeze and can be completed with simple hand tools.

The installation of a stabilized aggregate hard surface serves the dual function of reducing erosion, while also slowing the velocity of surface runoff [17]. These factors significantly reduce the chance of washouts occurring, lessens the need for expensive drainage systems and can lower the impact of your landscaping project on the local environment.

Also, as long as you source your aggregate in a local and sustainable manner, you can count your stabilized aggregate installation towards your projects LEED certification, which can qualify your client for significant tax breaks and incentives [18].

Make the Right Choice for your Next Hillside Landscaping Project

The key to getting your landscaping on slopes is striking the right balance between limiting surface runoff and installing drainage to cope with the expected flows of water.

If you make the right material choices elsewhere in the project, you will find that you need to invest far less in expensive and complex drainage systems. Not only does this reduce the initial cost of the project, it also reduces long term maintenance costs and could qualify your project for tax breaks based on environmental certifications.

With some thought and forward planning, you no longer need to look at projects with large and numerous slopes with dread.
References

[1] [2] Sponzilli, J. (2016). Landscape Design Problems and Solutions. Sponzilli Landscape Design Group. https://sponzilli.com/landscape-design-problems-solutions/

[3] [4] [5] Liu, Q & Chen, L (2001). Influences of Slope Gradient on Soil Erosion. Applied Mathematics and Mechanics. 22. 510-519. DOI: 10.1023/A:1016303213326

[6] [7] Chithra S.V., Harindranathan N., Amarnath A., Anjana N.S. (2015). Impacts of Impervious Surfaces on the Environment. International Journal of Engineering Science Invention, 4(5), pp. 27-31. http://www.ijesi.org/papers/Vol%284%295/E045027031.pdf

[8] Bakhsh, K., Zollinger, D. (2014). Faulting Prediction Model for Design of Concrete Pavement Structures. Pavement Materials, Structures, and Performance, GSP 239. https://ascelibrary.org/doi/pdf/10.1061/9780784413418.033

[9] LAMPL, J. (2019). Using Terraces or Retaining Walls to Control Erosion. Growing a Greener World. https://www.growingagreenerworld.com/using-terraces-or-retaining-walls-to-control-erosion/

[10] Baryła, A., Pierzgalski, E. (2008). Ridged terraces ‐ functions, construction and use. Journal of Environmental Engineering and Landscape Management, 16:2, 1-6. https://www.tandfonline.com/doi/pdf/10.3846/1648-6897.2008.16.104-109

[11] Huang, Y., Chen, X., Li, F., Zhang, J., Lei, T., Li, J., Chen, P., Wang, X. (2018)
Velocity of water flow along saturated loess slopes under erosion effects. Journal of Hydrology, 561, pp. 304-311. https://doi.org/10.1016/j.jhydrol.2018.03.070.

[12] [13] Asdit, R. (2010). The Fallacy of French Drains. Stormwater Solutions. https://www.estormwater.com/drainage/fallacy-french-drains

[14] [15] Grounds Mag (2019). Surface Water Drainage and Slope. Grounds Maintenance Magazine. http://grounds-mag.com/drainage/surface_water_drainage/

[16]

[17] [18] USGBC (2019). Encouraging Building Energy Improvements Through Tax Incentives. https://www.usgbc.org/resources/encouraging-building-energy-improvements-through-tax-incentives