MAY 2020: Please note that this blog post was written in 2016 when the snowpack was lower than this year. The latest report from the BC River Forecast Centre has the Fraser snowpack at 110% of normal.
Original 2016 Post:
Flood is not top of mind for the 315,000 people who live in the Lower Fraser River Floodplain. Should it be? It has been many decades since the last extreme event, and years since the last big scare. The lack of a recent event means apathy for the residents and regulators of the Lower Fraser River floodplain. In this blog we explore why we should banish apathy and instead calmly prepare for flooding. There may not be extreme amounts of snow in the mountains, but that doesn’t mean we won’t get wet.
The Fraser River has what is called a nival flow regime, this means that the peak flows are generally seen in the spring when snowmelt is a major contributing factor to water volumes. This is also known as the freshet. For the francophones amongst you – the french term for this is beautifully descriptive: La vidange hivernale, or “winter emptying” in English. Yes, snow volume is a major contributing factor to the flow volumes – there are however many other factors that complicate the math. First, there is the rate at which snow melts, which is dependent on weather (amongst other things). Hot weather melts snow, warm weather melts snow slowly, and freezing weather maintains the snowpack. Rain also plays a role – rain falling on snow makes it melt faster and also adds water volume. These rain-on-snow events are commonly what cause high winter flows on the smaller rivers and creeks in the Lower Mainland. So, albeit a little simplified, the peak flow on the Fraser River can be considered to be a function of snow volume (or snowpack), temperature and precipitation and not just snow volume.
With this year’s snowpack hovering around normal we wanted to get an idea of what the chance is of this turning into a flood. For this investigation, we enlisted the help of the fabulous hydrologists and forecasters at the BC River Forecast Centre. They provided us with their historic dataset of the April 1st Snow Water Equivalent (SWE) Index for the Fraser Basin. This index gives us an idea of how much snow there is in watershed and is based on measured information from the network of snow pillows. The April 1st part of the data description just means that the number describes the volume of snow on April 1st; this is nominal date when the snowpack is at its peak. The SWE is a measure of how much water is stored in the snowpack, which is of course, what matters for flood and drought forecasts. Some snow holds more water than other snow falls; picture the light fluffy snow of Christmas cards – this will hold less water than the heavy wet flakes we tend to see down in Metro Vancouver. Simply put, the SWE Basin Index calculated by the RFC gives us an idea of how much water is stored up in the mountains.
Figure 2 below charts the April 1st SWE over time, starting in 1953 (the first year we have records for). The mountains represent the snowpack (we had fun with this graphic :)). We did some quick stats (distributions, standard deviations and other math best described somewhere else) on the data set to pull out the the biggest snowpack years – these are marked. The biggest year on record is 1974, when the snowpack was 145% of normal, closely followed by 1999, with 144% of normal snowpack. In the same image – the river represents annual peak flows at Hope (more fun with graphics for us). If you follow across the graphic you can see that in most cases, big flow years line up with big snowpack years (1964, 1967, 1972, 1974, 1997, 1999, 2007, 2012). But what about the red and purple years? The purple years are big snow normal flow years. More interestingly for this year (when our snowpack is hovering around 100%) are the red years. In 1955, 1986 and 2002 – the snowpack was also hovering around 100% of normal, but because of the other factors (weather, precipitation, etc) we actually ended up with high peak flows. In fact, 1955 with a snowpack of 99%, is the 4th highest recorded flow for this period. Of course, we must note that the period of record for SWEs doesn’t start until after the two largest flows on record (1894 and 1948).
Another way to look at this is to plot April 1st SWE against peak flows for each year (see Figure 3; this figure includes some statistical magic which we are happy to share with anyone interested). Looking at the dataset as a whole, if you lean your head to the left, you can see a sort-of-trend going from the bottom left of the graph (very low snowpack and very low flow) to the top right (very high snowpack and very high flow). But there are also a lot of points that don’t fall on this imaginary line. And, because at Ebbwater we focus on floods, we are most interested in those points that fall to the right of the graph. Again, the ones of greatest interest are the high flow years that came about despite a normal snowpack (in red).
So what about this year? So, as mentioned above, the SWE Basin Index is 96% for April 1st this year. It’s also melting fast – all that summer-like weather has been heating the mountains as well as the beaches. The hardworking folks at the RFC produce 10-day forecasts (their best guess given snowpack estimates, and weather forecasts) for dozens of sites around the province. You can watch the Fraser River here.
So – for us, the important message here is that we can still expect high flows with a normal snowpack. And, that it is important that we aren’t apathetic, and that we prepare. It could flood in any year, and the best way of reducing the impacts of a flood is to be ready for it. We’ll focus in on this in our next blog. For now, the US has some great resources to get you started.
Special thanks for this blog post goes to Dave Campbell of the BC River Forecast Centre. We couldn’t have done written this without your data 🙂