For those of you as excited about the changes that are being demonstrated in the North Saskatchewan over the last 24 hours, from low to very high flows, as I am – which I quite reasonably imagine is pretty much all of you – here are a couple of graphs of both water depth and flow (up until 3:10 pm August 24, 2016), compared to the last 5 days, and a few thoughts on them.
Graph 1: In the last 24 hours, there have been huge changes in both flow and water depth in the river! Not surprisingly, increases in water depth and flow track each other very closely, i.e., the deeper the water, the greater the flow. Yes, obviously that’s how these things work. If you look closely at the two different vertical axes of the graph, you’ll notice that the depth – the red line – has increased from about 3.2 meters to about 6.5 meters, or approximately twice as deep (depth is shown on the right-hand vertical axis). However, if you look at the blue line and the left-hand vertical axis (for flow), you’ll see that flow has increased from ~200-225 m3/s to ~1,350 m3/s, or by approximately 6 times. What this means, if you think about it, is that flow in the river increases exponentially, relative to depth.
Graph 2: One of the things that we at EMSD monitor in rivers is flow. A simple way to do this is to install a monitoring station that has a float in it that moves up and down as water levels rise and fall, and a datalogger that constantly tracks the changes in the position of the float over time; the position of that float is compared to measured water depth in the river, and with a simple conversion you get the continuous changes in water depth in the river, based on the data of the float position and movement (or you can use something like installed pressure transducers that calculate depth based on water pressure, or other techniques). Hydrometric monitoring staff may install such a float monitoring system, but then also have to go out to the site where river depth is being measured and actually measure flow in the river at various times when flow is low, high, and a bunch of points in-between (using something like an acoustic Doppler current profiling system, or even by going old-school with a hand-held rod with a propeller that spins when it’s submerged in water that’s moving and converting the prop spin speeds to water velocity… and by repeating that across the width of the river or stream to create a transverse water speed profile from which they can calculate flow rates at that point in the river). By then plotting a graph of the measured flow data versus the water depth that was present whenever they went out and measured flow, they can generate the line that best fits that relationship between water depth and flow rates, and determine the mathematical formula of that line. That’s generally called the “rating curve”. Using that rating curve formula, if you are collecting water depth data, then you can easily convert them to flow data. That’s at least part of what EMSD hydrometric monitoring staff do when they go into the field: install permanent flow monitoring stations (*although they may actually be measuring water depth), calibrate them to river depth, measure flow a bunch of times, create rating curves of water depth and flow, and then go back regularly to those stations to make sure that they’re still working and the calibrations and rating curves are still valid. Graph 2, which shows the exponential mathematical relationship between water depth and flow in the North Saskatchewan River, is basically what a rating curve looks like. Although in this case I’ve cheated by creating it using the same flow and depth data shown in Graph 1 – what’s cool about what’s happened in the last 24 hours is that we get to see a large range of the relationship between flow and water depth