Jan
06

Keep it Fresh

I try to put off publishing on this site until I have some substantial breakthrough to discuss.  Alas, the process of making substantial breakthroughs has slowed in recent months.

Back in September, I wrote about finding ways to measure slope along Renick Run, a river that cuts through the limestone bedrock of Natural Bridge, Virginia.  I was intent on deriving highly accurate slope data so that I could test for a relationship between the power of water flowing through particular cross sections of the river and the strength of the rocks at those cross sections.  We’ve hypothesized that powerful water flow keeps fresh bedrock coming to the surface, and that places that see less powerful flows undergo weathering, or the rock weakening.

In September, I was struggling to derive this high resolution slope data.  We ultimately abandoned using GPS data to derive slope (see previous post) and switched over to more direct measurements.  We used surveying equipment to measure the local slope at each of our selected locations:

In the bottom right is a surveyor’s level, which I use to read the elevation on a pole held by my field assistant (in the background). By taking these measurements at several locations, spaced at known distances, I calculate slope by dividing the total change in elevation by the change in distance.

 

Even this method has some inherent subjectivity to it.  For a number of reasons, some of the slope values we measured seemed off.

To keep moving through the semester with some momentum, we recently switched over to looking at our rock strength measurements in a new way.  You may remember from previous posts that we test rock strength with a somewhat finicky tool called a Schmidt hammer.  The Schmidt hammer measures the rebound strength of a substrate by shooting a plunger at the substrate surface.  It records the relationship between the initial plunger speed and the rebound plunger speed.  This relationship is expressed as a “Q-value,” which ranges from 13 to 90.

For a Schmidt hammer rock strength measurement to be meaningful, we need to take 50 measurements at any one location.  To analyze these data, I’ve generally looked at Q-Value averages.  By doing that I ran into problems like this:

Where, for one reason or another, we were forced to take rock strength measurements at positions in each of our cross-sections that don’t necessarily correspond with the same flow stage.  Each black dot represents the average of 50 Schmidt hammer measurements taken at an elevation relative to the lowest point in the cross section.  How then, do we compare rock strength at the lowest elevation in each cross section when one is at .25m above the channel center and the other is at .5m above the channel center?

We began to look at our data like this instead:

 

Where rather than looking only at the average rock strength measurement, we looked at the distribution of rock strength  at each location.  These two plots come from the exact same data set as those plotted above (forgive the poor titles and formatting, I’m currently lacking the software to make them look better).  Only this time, I’ve plotted every single Schmidt hammer measurement taken (all 50) at each location.

This has been quite an illuminating way to look at our data.  The “strongest,” and thereby we infer “least erodible” segments of our stream range in Q-value from about 50 to 80, often with a Q-value average of about 60.  Weaker rocks, those with an average Q-value in the 40’s, showing a Q-value range from about 30 to 50.  Rock strength values consistently hit an upper limit of about 70 (remember the Schmidt hammer records values as high as 90).  Perhaps we can use a distribution of rock strength values, then, to determine the strength of fresh bedrock and use that to determine the extent of weathering at other locations in the channel.

In order for this idea to really work, it would be ideal to have a sample of the strongest rock possible, some really fresh sample.  That way, all of our comparisons of rock strength can be made in reference to “fresh rock.”  I tried to find “fresh rocks” at various road cuts near my study area in Natural Bridge.  I came to the realization that these road cuts exposed bedrock when the roads were made; in the Natural Bridge area, road cuts date to the 1930’s (I even found a blast core leftover from the road’s original construction).  What’s interesting is that the highest rock strength values  I found at these road cuts were much lower than those I measured at some locations in the stream.

I took 50 Schmidt hammer measurements within the box pictured on the right. Note the blast core in the middle of the frame. It was most likely made during the construction of this road (Rt. 11) in the 1930’s. The average Schmidt hammer value here was 46, and I rarely saw values exceed 60.

In my mind, this suggests that some of the freshest bedrock around Natural Bridge occurs in its streams.

If we go on assuming that rock strength is consistent along Renick Run, I can then potentially correlate stream power (previous post) with absolute rock strength.  But again, this  brings us back to the problem of attaining accurate slope measurements.  We’re continuing to think about ways to get high-accuracy slope measurements along a crazy rock stream.  But for the time being, this idea of absolute rock strength seems to be going somewhere.