WORKING WITH GRADES IN LANDSCAPE DESIGN
by Chris Grampp, Landscape Architect (#2499, California)
Landscape Architecture Program Director, Merritt College
Oakland, CA
My teacher, employer, and mentor Edward Janelli used to say trees and grades were the two most important parts of a designed landscape. I agree with Edward. In their most primary state landscapes have two shapes: vertical and horizontal. Trees represent the vertical realm, and among the numerous things they do for a design, they establish spatial definition. The ground represents the horizontal realm, and is basically the floor upon which everything in a design comes to rest. Once trees and grades come into harmony with one another, a substantial part of a design is accomplished.
Edward also used to say that the best grading design is the one that you’re not even aware of. What I took him to mean was that good grading design was so integrated with all the other parts of a project, that it didn’t stand out as an entity in and of itself.
In this light, here are some ways that I’ve approached grading, and some suggestions for how you might also do so:
• when you start a project, get as much information about the lay of the land as you can. Ideally you’ll want to shoot elevations with a zip level, or have someone do it for you, and create a contour map. Without this information it’s going to be very difficult to assess drainage, estimate cut and fill quantities, calculate wall heights and stair layouts, and much more.
• always pay attention to how water moves, and grade according. Water movement can make or break a design, and points to the need for a contour map or at least key spot elevations to determine high and low points. You can predict water movement pretty well with a contour map: just draw arrows perpendicular to the lines all over the map; where the arrows point is where the water wants to go.
• color your topo map according to steepness of slope. A colored may will allow you to assess site potential quickly. Here how I do it (see accompanying diagram below):
Green: flat or nearly so (1-5%; conducive to gatherings, work areas)
Yellow: gentle (5-15%; a few stairs, walls probably not necessary)
Orange: moderate (15-33%; stairs, low-medium walls, rising installation costs)
Red: steep (33%+; high walls, erosion, high installation costs)
Blue: depression (rain garden, pond, wind shelter)
A colored contour map as described above. These allow you (among other things) to quickly assess site potential.
• learn to identify basic landforms and assess their design potential (see notes at the end of this piece):
- Ridge and swale (these are opposites)
- Peak and depression (these are opposites)
- Concave slope and convex slope (these are opposites)
- Uniform slope (engineered; a uniformly tilted plane)
- Saddle (sits between two peaks)
- Summit (the highest point on a project)
• don’t block water against a house or structure. The results can be very damaging. Most building codes stipulate that grades need to drop at 2% minimum (1/4”/foot) away from houses and buildings for at least four feet.
• along the same lines (as above) keep drywells, rain gardens, bio-swales, and dry creeks at least 10’ from houses, buildings, and structures. They collect water which can undermine nearby foundations.
• try to balance cut and fill. Importing and exporting soil is expensive, and bad for the environment when done in large quantities. If you need to remove soil in one area to create a flat terrace, try to find another place on-site to put it.
• there is no such thing as a literally “flat” slope. When designers talk about flat slopes they usually mean surfaces that slope 1.5 – 2%, the minimum generally needed to prevent water from ponding.
• avoid little fussy mounds. Isolated mounds, sometime called berms or “mice under the carpet,” do little to improve a design while potentially creating all sorts of visual and technical problems. If you are going to use mounds, make them a consistent design them and tie them together.
• create continuity in your overall approach to grading a site. Avoid creating piecemeal solutions to specific areas that don’t relate to the overall feel of the site.
Here is more detailed information on the above:
Landform Terminology And Design Considerations
UNIFORM SLOPE (no variation from top to bottom):
-rare in nature over any great area; often implies engineering -provides regular surface for outdoor social use
-used to sheet drain terraces, planting beds and lawns
CONCAVE SLOPE (depressed in middle):
-"collects" space, as sunken center projects inward -often found near the bottom of slopes
CONVEX SLOPE (bulged in middle):
-"repels" space, as domed surface projects outward -often found near the tops of slopes
RIDGE (linear edge away from which ground slopes to either side): -is a divider or separator between areas
-in nature, marks the boundary of a watershed (see below) -especially prominent in profile: objects located on it will stand out -is a linear element: like a swale, directs the eye
-often makes a good path
SADDLE (low point between two peaks) -resembles a saddle for horseback riding -objects located here have "nestled" quality -often marks the beginning or origin of a swale
SUMMIT (high point on a site):
-a natural focal point: things located here will stand out -provides overlook to rest of site (outward focus)
DEPRESSION (low point of a site):
-self-contained low spot or a pond; good area for a pond or water-loving plants -a collector (expresses gravity, stability, weight)
-inward, internal focus (like a concave slope)
SWALE (a rounded, often shallow, channel which collects water)
-in nature, suggests a seasonal stream or gully; often filled with vegetation -is a linear element: like a ridge, directs the eye
-like a depression and a concave slope, a swale is a collector
VALLEY (a wide swale, well worn from eons of erosion) -a "protective" place, embraced by slopes on either side -a flat place suited for social uses
-often a river is found in the middle
WATERSHED (a distinct area of land defined by ridge lines):
-collects water from surrounding land and funnels it into swales, streams, rivers, and lakes -watersheds often have unique ecology, as their particular combination of topography, runoff, soils and micro climate set them apart from one another
UNIFORM FLAT PLANE (a completely level surface): In landscape design, this feature is rare, as almost all exposed landscape surfaces have some pitch for drainage.
-may be a concrete pad for a spa, pool, or other feature that needs to be completely level. -if you use such a surface for paving, be sure that it is well-covered, since it will not drain.
Some slope guidelines:
1% (.01)
Minimum slope for concrete paving; useful minimum for drain lines. Converts to about 1/8"-per-foot drop or rise. Only noticeable by trained professionals.
1.5% (.015)
Minimum slope for brick or flagstone on a concrete slab, ideal minimum for concrete (you can go less but puddling is likely).
2% (.02)
Minimum slope for turf and planting beds, and brick and flagstone on sand. Converts to about 1/4"-per-foot drop or rise. The slope at which soil should be pitched away from a house (for at least 4’) to stop water from pooling near the foundation.
2.5% (.025)
The point at which most people (e.g. your clients, who are usually untrained to notice such things) begin to perceive a sloping surface.
3% (.03)
Workable maximum for a paved patio: use only when necessary and limit to small areas, as pitch will be visible.
5% (.05)
Good workable maximum for pea-gravel paths and similar granular surfaces, since such surface start to get slippery at this point.
6.67% (.067)
The maximum slope for a wheelchair ramp without a handrail requirement.
8.3% (.083)
The maximum slope for a wheelchair ramp with a handrail.
10% (.10)
A good workable limit for a ramp or an inclined path. You will encounter steeper ramps or paths, and they may work for short distances; longer runs, however, are difficult to manage above 10%, and it is often a good idea to add stairs.
12.5% (.125)
The workable limit for a high-volume grass swale.
20% (.20)
Workable maximum for a residential driveway (and difficult to drive up or down; 10-15% is much better).
25% (.25)
A good limit for lawns (there are many exceptions; you will often see 3:1, but they are hard to irrigate because so much water runs off).
33% (.33)
The maximum for a landscape fill slope (unless the soil is compacted to engineering standards). Good slope for sitting. The maximum slope for operating a drivable lawn mower. Slopes that are 3:1 and steeper can also represent higher construction and planting costs, and increased irrigation runoff.
50% (.50)
The maximum for a cut slope. Difficult to work on (installation costs will get much higher, irrigation is less effective, erosion control mechanisms are critical.
65% (.65)
The maximum angle of repose for most soils (33 degrees). Don’t even think about doing anything other than the most modest alterations on a slope this steep!
100% (1.0)
45˚. Really steep (ratio of 1:1)! Great for extreme-skiing slopes. In landscape settings, terrain this steep is almost invariably stabilized by rock outcroppings.