Permaculture Designers Manual
CHAPTER 9 –
EARTH WORKING AND EARTH RESOURCES IN PERMACULTURE
Section 9.4 –
Slope Measure in Permaculture
Slopes are measured and expressed in several ways.
First, the slope can be measured as DECREES FROM THE HORIZONTAL, or the base angles of Figure 9.2.
This is expressed as “a slope of 15 degrees”, and is sometimes used by geologists to describe the dip (angle) of rock strata.
Next, the 90″ between horizontal and vertical can be divided into 100 parts, and expressed as PERCENTACES, a measure used by engineers.
Finally, slope can be expressed in PROPORTIONS or ratios of base to height .e.g. 1:4 is a slope 1 unit high, with a base 4 units long.
This is the rise of slope over distance, often used for drains and pipes, in units used by hydrologists. Many irrigation drains use slopes of 1:500 to 1:2,000, or a fall 1m in 500-2,000m and these are referred to as “grades”. (Figure 9.3)
Stable natural foothill slopes are not straight but down-curved (concave).
When building a road, it is well to remember that they will achieve a concave curve by slump over time, so it is best to cut this curve into the slope to start with.
Natural lower slope faces in humid areas are also down-curved and should be made to curve down or to be concave. Earth banks will assume this profile in time (indicated by a dotted line in Figure 9.4). Soils have type-specific resting angles with concave profiles.
Slope angle (the dolled line) is therefore a straight-line approximation of real slopes. The notch at the top of a cutting runs water off the face to safer slopes and is a standard feature in embankment stabilization.
Average safe slopes used by engineers are:
Clay (well-drained) 1:2
Clay (wet) 1:4
To dry a road, the sun side should be cut further back and so allow sun in; conversely, to shade areas, banks can be as steep as the soil will Stand (Figure 9.5).
When we cut or “bench” into a slope, the natural erosion processes try to re-establish the original slope, and if left to their own devices will do so.
Benching at the base of sloping and unstable tilled strata will therefore bring down the whole hill and incur endless costs in road-clearing and slump removal. Slumps in unstable hillsides will carry with them or bury below them, house and people.
A great many film stars perched on unstable ravine edges in the canyon systems of Los Angeles will, like the cemeteries there, eventually slide down to join their unfortunate fellows in the canyon floors, with mud, cars, and embalmed or living film stars in one glorious muddy mass. We should not lend our talents to creating such spectacular catastrophes, but to the avoidance of obvious problems and by re-location before disasters occur. Local government often delineates areas of instability where building is not permitted due to mudslides, fire, and earthquake.
In the dry hill scrublands of California and France or on desert borders, intense fire upslope can generate sufficient heat to bake soils. When such fires occur in steep, unconsolidated shale’s and mudstones, the baked earth (healed to 6 cm deep) forms water-sealed slab surfaces (such hydrophobic crusts will no longer form crumb structure).
Valleys in such areas can then expect catastrophic mud flow unless early misty rains fall to restart vegetative growth and to that end the ashes can be sown down with oat, lupin and shrub seeds, but if heavy rains fall before plants take hold, water can penetrate below the glazed surface and mass movement (bulking) of mud and glaze will then occur.
The wall-of-mud effect and the inevitable cascade downhill may well block roads in any such mud-flood emergency, threatening resources in the valleys. Mud flows are a feature of unstable sediments in wet-dry climates.
Thus, the water run-off alter fire (as in Adelaide, Australia in 1984) can be catastrophic and hot fires can have a long-term effect on soils and soil loss.
The combination of fire in late summer, followed by a sudden autumn onset of rains is typical of Mediterranean lands, and of devastated soils.
For a designer (and town planner) areas of known soil instability should be avoided as settlement sites and kept in permanent forest; most are designated on soil maps or can be researched.