Landscape design of a steep or sloped site can provide quite a challenge. It is rare when designing a scheme that you are presented with a perfectly flat site; and why would you want a flat site? The demands of a sloping terrain can often lead to innovative landscaping ideas and beautiful designs. A sloped site gives you the opportunity to create views, different zones and areas, and sometimes, have some fun!
How you respond to a steep, hillside or sloping site will depend on numerous factors, budget, timescales, climate, building requirement, external environment, risk of fire and much more. In this post I will try and give some ideas, along with some questions to consider when you are faced with designing the external space of a sloped site.
The questions and ideas may relate to both residential or commercial projects, so extract what you will dependent on your specific requirements.
Before starting your design
Before you get started on your sketching and planning of the space we have a few questions to consider.
What is the space for?
Is it to be viewed or used?
Can it accommodate a variety of uses?
Is the space a focal point, part of a journey, a quiet space?
Who will be using the space and at what times in the day?
Is it a safe place for everyone that will be using it?
Who will maintain the space and how will this be managed and financed?
Examining the site
It is important that slopes are protected and that erosion is controlled. If a slope is too steep it can be subject to erosion from stormwater runoff. This can be addressed with site gradings and careful planting, along with suitable drainage.
If is slope that is 2:1 drops one meter (foot/unit) for every horizontal meter (foot/unit). A slope with a gradient of 2:1 is considered the steepest slope that is able to support plants, but this can often be too steep as water runs off the slope before the plant roots can be saturated. It is also the steepest slope without structural support.
A slope of 3:1 is considered to be more suitable for plants to grow, as long as they are selected carefully to be suited to drier conditions. A slope of 3:1 is the maximum slope for a mowed lawn.
Considerations for designing with a sloped site
Consider what is already there
Before you start ripping out all of the existing shrubs and trees to start with a blank canvas, consider what effect this will have on the site. If the site is populated with a large amount of mature trees, removing them will have a significant effect on ground movement and the integrity of the slope. Although in many cases, it is not possible to preserve every tree on a site, just bear in mind the consequences of removing the trees.
Preserve and enhance the view
Often a sloped site means that the building is going to benefit from some sort of view. If this is the case be sure to maintain this view as you plan your landscaping. Consider how the view can be enjoyed. Platforms, decking, viewing points can all add to your landscaping scheme. Be aware the planting further down the slope will grow, and if larger trees are specified will they obliterate the view in time? Or can you position new trees to create ‘windows’ that frame the views.
Consider how to grade the slope
Every time you cut into the slope you can compromise the integrity of it and will therefore require more significant support. In some cases this is unavoidable and retaining walls are essential. However, by cutting smaller terraces into a slope, you can avoid the need for large retaining walls at the base and erosion control at the top, that you would get with a larger cut and fill design.
Improve the strength and stability of the slope
Slopes can be prone to mud slides if not designed and looked after correctly. With heavy rainfall, the slope can become saturated making the layers in the soil lose their cohesive qualities and slide off. Consider specifying trees that can form a network of roots that will bind the slope and hold the layers together.
Consider implementing an irrigation system into your design. Dependent on the location of your site and the climate you are faced with, it is worth looking into an irrigation system that drips a slow flow of water to the base of the plants. It is common to see brand new landscaped slopes looking fantastic at first, but with a lack of water planting can die off quite quickly, leaving a very labour intensive job or a site that becomes neglected.
Stop the rain from eroding the site away
If your design has too much bare soil exposed on the slope, over time the rain will dislodge and erode the soil particles away. A way to remedy this issue is to plant ground cover shrubs and plants, or position rocks to cover more soil and slow the speed of runoff.
Allow for maintenance
Depending on the scale and detail of your design, it is important to provide some sort of access for maintenance, be it both horizontal and vertical. If the design features heavy planting, there is no doubt this planting will require regular pruning, clearing and so on. Likewise, if water features have been specified, easy access for maintenance should be considered too.
Can the design be used by everyone? Once again, depending on the nature of the project, it is important be ensure that the site can be accessed and used by everyone. Are there sloped paths as well as steps? Are there suitable railings, balustrades or handrails where necessary. Is it safe for someone who is partially sighted? Would someone in a wheelchair be able to access and enjoy the site?
Incorporate seating and furniture
Personally I love the idea of allowing the topography of the landscape to create pockets or areas that can be used as social spaces. Stepped sites lend themselves to creating seating within the slope.
Decking, terraces, viewing platforms
Using the natural form of the landscape to create viewing areas, decking or terraces maximises the opportunities for working with this kind of site.
The retaining wall is designed to hold soil on one side and be freestanding and exposed on the other side. They are used frequently in sloping sites and come in many different forms depending on aesthetic requirements, along with site conditions and structural requirements.
In almost every case it is necessary to have the wall designed by a structural engineer to ensure suitable structural strength.
Different materials can be used to create retaining walls, ranging from timber sleepers, to stone/brick walls, concrete or gabion walls. Choice of materials will depend on structural requirement, drainage requirements, aesthetics amongst other things.
Resources for retaining walls
Slopes are often very dry when there has been no rain, but can become quite boggy at the base when there has been rain. It is important to select plants that are tolerant to dry weather for the top part of the slope, but plants that are ok in damper soil for the base of the slope. If you think that erosion may be a problem on the site, select deep rooting plants.
A good mix of ground cover, shrubs and trees will ensure that when it rains, the force of the water hitting the ground is deflected, and therefore erosion will be avoided.
Water features are a way of making use of the natural topography of the site.