Benchmarking Gauging Stations

Benchmarking Gauging Stations

There are few things as magical as drilling holes in solid rock. The bit spins and the drill pounds and a pile of fine grey powder grows around the hole. Don’t inhale! When the hole is deep enough, after only a minute or so, you can install a fender washer and a concrete nail into the rock.

I can hear you asking, “But, why? Why?!” Well, for surveying purposes, of course. The sensor, the key component of a gauging station, measures the height of water over time. Since nothing is permanent in flowing water, we need to make adjustments in our data if the sensor gets knocked around. Even a few millimetres can change the data. We need to know if the sensor has moved.

The solution is to install permanent markers – benchmarks – in really solid things near the sensor, usually very large boulders. We have several years’ worth of data for our creeks but by Provincial standards the data is not considered high quality. To achieve a higher quality rating, regular surveys are required. Naturally, we want our data to be of the highest quality possible! This is why we recently installed benchmarks at Waterloo and Mud Bay creeks.

Placing the benchmarks isn’t hard, but the next step, surveying, is trickier because it involves math. One benchmark is chosen as a starting point and a survey rod is placed exactly on top of it. The transit, an extremely precise telescope mounted on a rotating plate, is set up nearby. One person holds the survey rod while the other person peers through the transit eyepiece to locate the rod. Because the transit is so sensitive, it can be tricky to find the rod in the viewfinder. More often than not, you find yourself examining your partner’s nose hairs before the rod swings into view.

 Finally, you get a reading. This is written on your worksheet and your partner moves the rod to the next benchmark where the process is repeated. At each step, after you write down your results, you do some surveying math. After you have done all the benchmarks, you change the elevation of the transit and repeat the process. If everything goes well, your list of elevations from the first round should correspond to the elevations from the second round. If not, you start all over and continue until they match!

 Six months later, you do it again. If your new numbers are within 3mm of the old numbers, then nothing has moved, and life is good. Otherwise, you have to determine what changed and how it affects your data.      Now your data is of a higher grade. And all of this makes the cordless rock drill you bought for a garden project seem like less of a self-indulgent splurge.

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