The next layer in the design process that we will look at is design for slope. We've already looked at the permaculture zones and sectors, and both of those design tools use somewhat of a regular pattern: the zones being a concentric circle pattern and the sectors expressed as pie wedges on the sector compass. But the fact is that hardly any sites are actually so regular that these geometric templates fit neatly over them. The truth is that the topography of the earth is infinitely varied, and each site has unique variations of climate, microclimate, soil, aspect, slope, water flow, hydrology, wind direction, etc etc. It goes on and on. Each site is unique. So the permaculture design process continues on beyond zones and sectors, and the next design layer that we’re going to look at is slope. On a compass that is 360 degrees all the way around, one quarter of that is 90 degrees. A slope that bisects that is 45 degrees. That 45 degree slope represents an equal rise and an equal run. So if a completely flat slope is a 0%, and the straight up and down is 100%, then half of that is a 50% slope, which is the same thing as a 45 degree slope. Now, the third way to express this same slope is as a ratio, H:V or horizontal to vertical. So this slope has an equal horizontal to vertical measure, so it is a 1:1 slope. Now, all of these different ways are used to express slopes. It really varies by country, by discipline, and by situation. But for this lecture and throughout the course, I will primarily use percentages. I personally find it the easiest to conceptualize a slope is in percentage. I’m going to draw a grid now to do my drawing on top of so that we can better conceptualize the slope for this module. We’re going to start by looking at how people practice different land uses on different gradients of slope. This is a 75% slope, and is about the maximum slope that we see in the world that anyone has cultivated for agriculture. In some areas of China, Vietnam, the Philippines, we find terraced rice paddies built on these slopes at a great expense of effort, as well as tea plantations, when we look around. Terraces on this kind of slope also exist in Peru in the Andes Mountains and probably other places; I haven't looked everywhere. I'm not totally sure about all of them. In order to terrace a slope like this, you need a really tall terrace wall to get a small growing area. This is an incredible amount of work for not a great deal of space, as you can see. So only civilizations who have really limited arable lands with great population pressure and surplus of labor and resources would undertake such a project. It’s actually pretty mind-blowing to think of the civil engineering involved in these hand-built projects, because there’s a huge risk of destabilizing the slope with work like this. It’s a lot more common to find paddies and terraces at more like a 50% slope. That’s still considered very steep, but I'll show you what that looks like. And you can see here that the ratio of wall height to terrace width gets better as the slope becomes more gentle. Now, you wouldn’t have a terrace wall that was completely vertical anyway, you’d have it on a slight slope; but I’m just drawing the geometry of it, not an actual construction document here. So people are cultivating extreme slopes all over the world, but that doesn’t mean that it’s ideal when we look at the whole watershed system and what the optimum uses of different slope gradients are. Now, in the Permaculture Designer's Manual Bill Mollison advocated that any slope steeper than 20% be kept in permanent forest, for permanent soil stability and for a number of other reasons. I've drawn a 25% slope for ease, you can see it's 1 to 2. You can see how the 75% slope, for the same rise of terrace you get one unit of horizontal space. On the 50% slope, for the same rise of terrace you get two units of space. And then, on the 25% slope, for the same rise of terrace you get four units of horizontal space. So, permanent forest does not mean that it’s an unproductive landscape by any means, and I’d be really curious about what Bill Mollison's prescription would be in a land-scarce and people-rich area where extreme slopes are currently being cultivated. So now we’re going to look at a typical foothills landscape profile in the temperate or humid environment, with rounded hills and relatively gentle slopes. So, at the top of the hill here we have a gentle slope. That is a 10% slope right here. Then, as we go down from the top of the hill, we have a steeper 30% gradient. This then breaks right here into a gentle 15% gradient, which then hits basically the slope at the bottom of the valley, which flattens out to 5%. So, I’ll add some irregularity to make it a little bit more realistic. So at 30% Mollison says we want to keep this in permanent forest. This has several benefits. First off, this is the upper watershed, so the flow of water is slowed and absorbed, to slowly percolate downslope and build the subsurface water table. The water table gets deeper as you go up the hill, and more shallow closer to the surface, down in the valley. As we have fog and humidity move through the low atmosphere, these forested hills and ridges will actually serve as moisture collectors when water condenses on all of the surface area of their leaves and needles. This serves to add more moisture into the upper watershed, which finds its way down through the system. The forested slope also has the effect of breaking up the cold air that flows down the hill at night. But cold air does flow down, and at the very bottom of the slope at the bottom of the valley there will be a frost line where frost can settle in winter in a temperate climate, where you have freezes. Another thing to note at this time is that soils are shallow on the slopes and get deeper as we descend down into the valley. This is because soils weather over time and drift down to deposit at the bottom of the system. You can see how this point right here where the slope breaks is where soil starts to deepen. This inflection point where the slope goes from steeper to more gentle, right here, is called the keypoint, which is a term coined by P.A.Yeomans, the Australian inventor of the Keyline Design System. When we look back at our earlier drawings of the terraces on steep slopes, and how as the slope becomes more gentle the terrace width becomes greater with the same height of terrace wall. We have one unit on the 75%. We have two units width on the 50%. And we actually have 4 units width when we get to a slope that is 25%. This same theory is true for when you're building dams and storages of water. The keypoint here where the slope breaks is a really efficient place to store water in the landscape. because of the sudden gentleness of the slope. This keeps the water at a greater height above all of these areas, which means this water stored up at this elevation can be distributed by gravity. So this area here on the mid-slope, below the forest and above the frostline, is considered the best area for horticulture, and hence the best area for housing and zone 1 activities. Now, as we move down into the flat valley bottom where we have a 5% slope, which could feel quite flat if you're actually there, but let’s consider now the efficiency of water storage at this point on the slope. You can see here how the same sized embankment holds back so much more water on a more gentle slope. This place in the landscape does not have the same potential for gravity distribution that a higher location has, but it certainly has a higher storage volume. So the types of cultivation practices that happen in this lowest zone, that is below the winter frost line, would be elements that are not susceptible to late and early frosts. These may be more zone 2 & 3 activities, like trees, animals, and irrigated horticulture. At the very bottom of the valley is where we find some sort of water way, whether just a ditch, a creek, or river. This waterway is called a riparian zone and it is best for it to be vegetated with shrubs and trees for perennial wildlife habitat. Let's not forget about the very top of our system here either. At the top of the hill is the best place for a high storage of water. Water collected and stored at the high point in the landscape profile will have lots of pressure when it is piped down to lower elevations. This can be a pond, or a water tank, and can be pumped up from lower ponds using wind or solar energy. Another important aspect of this perspective on slopes that I’m offering is the solar aspect. In most situations in a humid or temperate environment, that has a cold season and dramatic changes in the angle of the sun throughout the seasons, being on the sun-facing slope is optimal. This means South-facing if you’re located in the Northern Hemisphere, and North-facing if you are in the Southern Hemisphere. This is assuming that you are living in a climate where heat and light are at a premium in the Fall, Winter, and Spring. But sometimes there are other needs that are more important, like water, for example. I know of farms in more arid climates that intentionally site themselves on the north slope, this is in the northern hemisphere, in order to have a more assured water supply running off of a North-facing slope, where there is less warmth and evaporation from the sun, and more water flowing down slope and soaking into the water table. So, it really depends on your site, and I guess I would just say this again and again. Each site has a unique set of conditions. So, do you want to be on the sun-facing or the shadow-facing side of the slope? In the tropics, orienting towards the sun for solar gain is not even a consideration, where the priority for site orientation may be towards breezes for air flow and cooling. The point is, every site is unique, and the needs are different for each climate zone. So, I've presented to you here some very general concepts and guidelines for designing based on slope, and your individual site will have its own needs and considerations. So, I hope you enjoyed the presentation, and I look forward to talking to you again. Thank you!
