[Note: You will need a GPS receiver that can record elevation as well as longitude and latitude. A scientific calculator will also be needed. ]
Angle of Repose
This is a term used in describing the stability of a sloped surface – like the one before you. By definition it the steepest slope angle of a pile of material relative to the horizontal that can be maintained by the material without sliding or slippage. This concept is used by people who study avalanches in snow covered mountains and those geotechnical who deal with the stability of steep slopes of soil. A familiar application of the study of stability of slopes is the sides of the Panama Canal where engineers design against landslides that would impact the canal. It is also used by gravel pit operators assessing the amount of space a pile of a particular material will occupy. Landfill operators also must consider how steeply they can build their fills. A simple natural example is the slope of sand dudes shown below.
We’ll use the concept to consider the hillsides of the Sharon Woods Gorge and the potential for landslides.
Landslides
Landslides, or the more technical term “mass wasting”, is the process by which soil and rock move down a slope under the force of gravity. When the force of gravity on the mass of the hillside exceeds its resisting force, mass wasting (landslide/mudslide) occurs. When this occurs is highly dependent on the hillside’s material strength and cohesion and the amount of internal friction of the material. So whether a landslide will occur is dependent on the angle of the slope and the makeup of the hillside. The diagram below shows an example of a rotational landslide.
Hillside Makeup
There are several factors that affect the strength of a hillside: weakening due to the breaking down of the hillside material due to weathering, water content, and vegetation cover. The latter two come into play in the area where you are.
It is pretty obvious that a hillslide covered with deep rooted trees is more stable than one covered only with grass, but the relationship with water is more complex. Water can increase or decrease the stability of a slope. A familiar example of this is a sand castle. The angle of repose changes for sand are:
Dry sand = 34 degrees
Water filled sand = 15-30 degrees
Wet sand = 45 degrees
So you can’t make a castle with dry sand, but if it too saturated, it won’t stand either. In soil, small amounts of water can strengthen it by giving it cohesion. Think of the difference a hand full of dry soil and mud ball. Too much water and it will act as a lubricant – like when you wash your hands off after the mud ball fight. The angle of repose for generic earth is in the range of 30-45 degrees.
Gorge Hillside
Now it is time to take some measurements, make some calculations, and make some observations. The waypoints listed are the places to stand to take the elevation measurements. I’ll assume we’ll use our GPS receivers for this. Due to the accuracy of GPS coordinates and the short distances involved you might want to use some waypoint averaging to improve the accuracy. Take elevation readings at the points listed and make some notes of the hillside (Also note evidence of past landslides or not and what the vegetation coverage of the hillside is).
To calculate the angle of the hill sides you’ll need a scientific calculator. To make it easier I’ll give you the horizontal distance between the points.
Reading A N39 17.079 W84 23.383 (Trail system sign)
Reading B at listing ground zero (another sign)
Reading C N39 17.010 W84 23.454
Reading D N39 17.084 W84 23.391 (corner of deck)
Reading E N39 17.022 W84 23.466
Reading F N39 17.007 W84 23.472 (near sign)
The hillside pairs distances are:
A to D = 50 feet
B to E = 55 feet
C to F = 88 feet
Referencing the diagram below - he angle is calculated using the arctangent function on your calculator where Y is the elevation change
theta = arctangent ( Y / X )
Logging requirements.
At each pair of measurements:
1) What is the slope angle of the hillside at the 3 locations? List your elevation readings.
2) What is the vegetation makeup of each hillside pair?
3) At the 3 reading sites does the hillside look like it has undergone mass wasting recently? In the past? List at least two pieces of evidence at ground zero. Consider evidence of transverse ridges and scarps.
4) Based on the averaged values what is your estimate of the angle of repose bound that hillside? Does the value seem reasonable for the surface?
OPTIONAL: post a pic of yourself in front of the hillside.