A slope is the angle, fall, rise, steepness, gradient, or inclination of the land surface. When driving down on a highway, you might have seen a road sign reading “Steep Grade” or “6% Grade”.
The grade of the road in question is the slope of the road. Slopes are essential as well as a concern, especially for those who enjoy farming. It is always vital for farmers and irrigators to point out the land slope before starting their farming to prevent negative factors such as soil erosion.
Of course, you don’t want your fertilizer or nutrient-rich topsoil of your farm to be swept down the hill. For those trying to find a place to build that dream house, land on a slope will require a little bit more expertise and effort to level the inclination.
Either way, slopes are naturally occuring and can have both advantages and disadvantages depending on where you stand.
The slope is a fundamental landscape metric and can be measured as the rise of a land surface over the run. For instance, a good number of available geographic information systems (GIS) can analyze and survey the digital elevation data, such as digital elevation models, elevation points, and contour lines.
The information is then used to derive aspect and slope data sets. Some of the examples where sloping gradients are applied include modeling wildlife risk, describing landform, classifying soils, modeling surface runoff, assessing the possibility for development, and characterizing habitats. The different types of slopes that exist include;
- A gentle Slope that has a relative amount of inclination.
- A Steep slope is generally a hillside with a greater vertical rise over 100 feet of horizontal run
- Depositional slopes which are slopes formed because of the building up of deposits to form inclined hills, mounds, and surfaces
The contour lines in a gentle slope are far apart from each other, whereas the contour lines in a steep slope are closer to each other. If you’re wondering what contour lines are, these are the lines on a map that joins identical heights below or above sea level are what are referred to as contour lines.
Due to their steep grades, areas with steep slopes are undesirable for development purposes. Besides the presence of bushes or trees on the surface, slopes differentiate themselves in different ways based on shape, aspect, angle, and the course of the fall line. All these factors are important because they determine the decision you make when maneuvering a slopy area.
The shape of the slope
To understand the shape of a slope, you need first to know the form of a slope. The transformation or change from a steeper terrain to flat terrain and vice versa is what is referred to as the form of the slope.
External processes like sedimental transport mechanism control the shape of a slope, creep and slope wash, as well as other internal processes. When the terrain transitions from flat to steeper, the risk of slab avalanches taking place increases.
This is caused by wind depositing the drifting snow or a thinner snowpack with the weak layer exposed to the surface, thus increasing the possibilities of striking one. Based on shape, the shape may either be a convex or concave slope.
Convex slope segments
This type of slope normally forms on the upper section of the slopes in response to rain splash erosion and soil creep. The slope bows outwards and the snowpack is greatly stretched, increasing the possibilities of setting off a slide. This type of slope segment normally forms when the slope in question is below the threshold during rapid mass wasting.
Concave slope segments
These are transportation or depositional slope segments that are formed near the basement of slopes. The slopes form even in the absence of waste removal. These slopes usually bowl inwards, and the snowpack is mostly supporting the existing snow on the slope above.
Straight slope segments
These are mid-slope segments that are formed because of the removal of debris at a uniform rate or transfer of the same. The slope segments normally form at the lower part of slopes where debris removal from the base occurs.
These are types of slopes with a series of segments. This is more like a combination of the concave and convex slope with vegetation and soil in it. The upper section of a composite slope has a convex slope segment, the mid-slope is straight, and the base is concave.
Angle of the slope
The slope angle is a very significant aspect since slab avalanches occur on 30-degrees slopes or steeper. Nevertheless, how steep are 30 degrees? How is the measurement done? Think about that and try to get an idea about it.
Did you think of anything? Maybe yes, maybe no. Do not worry; to get the angle right, you need lots of practice. The slope angle is a frequent subject of discussion among snowboarders and skiers. ‘Well, I skied a 65 percent run, man!
That is steep! Thirty degrees is not a thing. Do you get it? When you go to the mountains, it is always important to have information about the angle so that you can tell when you’re in avalanche terrain.
We measure the angle of a slope in degrees and not in percentages. However, for those who love it in the percentage format, you can always do the conversion. For instance, 35.1 degrees is equivalent to 78 percent, and 38.25 degrees is equivalent to 85 percent.
A 45 degrees run is equivalent to a 100 percent run of the same. We can tell the steepness of a slope given the number of degrees.
- A 30 degrees slope is an average slope.
- A slope between 30 and 35 degrees is a steep slope.
- A slope between 35 and 40 degrees is a very steep slope.
- A slope more than 40 degrees is an extremely steep slope
Aspect of the slope
The aspect of a slope is the direction that it faces. Some of the tools you can use to determine the aspect include the compass or a combination of the sun’s position and your watch. You can also get the aspect through reading on the maps.
With a compass measuring the aspect of slope does not need any rocket science. With the arrow of the compass always pointing north, stand by the slope with your back facing the slope. Use the arrow to locate north and ensure ‘N’ is also pointing in a similar direction. You now know which aspect the slope in question is facing.
Using the sun and time, you can also determine the aspect if you understand the direction and time when the sun rises, is at its highest point, and sets. The sun descends in the west and ascends in the east.
It is at its highest point at around 1 PM (13.00). Once you understand that, you always get started. You just need to look at the time and establish the slope on which you are standing.
The slope processes
The slope process is generally the way slopes are formed. Through slope processes, slopes can be classified as either primary or secondary slopes.
- Primary slopes: These are slopes that are formed by processes that advocate for relief.
- Secondary slopes: These are slopes that are formed by processes that suppress relief. This type of slope usually evolves from the modification of primary slopes and soil erosion.
Slope movement involves the following factors:
This involves the structure and slope composition that commands the detachability of materials of a slope at a specific process. Under geology, we have rock slopes and soil slopes.
- Rock Slopes: A slope is controlled by the strength and structure of the rock. Rocks of high strength result in the formation of a free face, while rocks of low strength result to flatter slopes.
- Soil Slopes: With these types of slopes, the slope shape is controlled by processes. Erosion can take place because of water or mass wasting. Water erosion is influence by the erodability and permeability of vegetable cover or the slope materials. Erosion by mass wasting is influenced by the characteristics of sediments (cohesiveness, angularity, sorting, and grain size), as well as the structure and the degree of consolidation.
These are factors that promote or accelerate the rate of mass-wasting. These factors include man’s activities, proximity to the shoreline or stream, and the rate of incision and uplift.
The climate influences mechanical weathering and the intensity of chemicals in the soil. It also controls water and vegetation content. Arid areas that lack vegetation cover are prone to soil erosion. In such areas, slope formation is influenced by the lack of vegetation cover, which increases the efficacy of wind and water in the process of erosion.
How to calculate the steepness slope index of a mapped area
To carry out a proper calculation of the percentage slope of the mapped area, you first need to get the run and the rise of the area from the contour maps. The run is the horizontal distance or breath between two points.
The rise, on the other hand, is the variance in elevation between two points. The slope’s elevation can be determined using the contour line interval and the printed elevations from the map.
Alternatively, instead of counting the contour lines between two points, just find the elevation at each of the points and do the subtraction. This will minimize double counting and rounding up of errors.
On the other hand, a slope’s run is determined by measuring the map distance between two points and converting it to similar units as the elevation. Assuming the elevation difference is measured in meters, the distance also should be measured in meters too.
The stages involved for the above calculation include:
- Calculate the mapped area in KM2
- Put 10 x 10 grid overlay on the mapped area.
- Compute the percentage slope for each of the square grid
- Present the results of the area as median slope or area average