This beautiful beach is a great place to understand a principle of geology – the angle of repose. It is also the great place to note the differences between a “maintained” beach and a natural one. The coordinates bring you to the far north end of the maintained beach and the beginning of the “wild” beach. There is parking further west as well – with restrooms, concession etc.
Before we get into the principle of angle of repose walk the beach until you get from maintained to wild beach. You should note a few differences. The maintained beach is flatter and with finer grained sand. The wild beach has a very steep drop off – a wall of sand and fine rocks deposited by the wave action of Lake Erie. It is this steep wall we can experiment a bit with the “angle of repose”.
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. The picture below does a good job of illustrating.
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.
Sand is an excellent media to demonstrate the effects of cohesion and lubrication of the sand with water. And what better place to do it than a beach where sand and water are plentiful! As anyone who has made sand castles knows – there’s a certain amount of moisture that works best. Fine, dry sand like that on the maintained beach can only be stacked to about a 30 degree angle. Coarser sand tends to have more natural internal friction and when stacked will have a steeper angle. Add some water and you can make nearly vertical castle walls. Have too much water and you have a mushy mess. This is shown below.
Adding some water increases the cohesion of the sand – the water aids in the sand particles to stick together - and allows a steeper angle of repose. As you add more and more water and the sand becomes more and more saturated you reach a point where the cohesive forces break down and the water acts as a lubricant. Try it out yourself – the concession stand does sell buckets if you didn’t bring your own. If you prefer mud you can do the same experiment. Dry dirt will clump together a little, moist makes a nice mud ball, more water and you have a slurry.
To log this earthcache do the following:
1) Stack dry sand on both the maintained beach and the wild beach. Estimate the angles – which is steeper and why?
2) On the wild beach take a close look and the steep drop off of sand. Do you notice more than 1 angle of repose for this sand “cliff”? Try to explain why the foot of the “cliff” has a different angle than the more shear face. (hint: you may have to get your hands sandy)
3) Optional: post a pic of your day at the beach!
Email your answers to me – do not post in your log.