This post is the fourth in a series that describes the state of flood hazard mapping in British Columbia. If this piques your interest you can also check out:
In the Ebbwater Flood Map Project, we are exploring the availability and usability of existing of floodplain maps in BC. In our previous blogposts, we looked at the publicly available, provincially designated floodplain areas in BC and found that they are all more than 16 years old, with a median age of 26 years. Other posts explored which parts of the province had the most outdated information and where the greatest number of people live on floodplains.
In this blog post we will explore more why it actually matters that our maps are old. One very important reason is that we are not using the full hydrometric record (a hydrometric record describes water level or flow data recorded over a period of time). While most of the floodplain maps were made in the 1990s, the Water Survey Canada hydrometric stations have kept recording data (well many of them at least). This means that there are 20 to 30 years of flow records that are not being used! This is particularly important in a non-stationary climate where we might find larger ranges of river flow than previously experienced.
We carefully analyzed the length of the hydrometric record used in the making of the “Provincially Designated Floodplain” maps of BC by reviewing the 47 design briefs associated with the provincial mapping. In some instances, more than one station was used in the hydrologic analysis. We included all of these stations in a database and compared the length of record used to develop the floodplain map to the total years of available data through Water Survey Canada (Figure 1). It is obvious that even though some hydrometric stations were discontinued, we are missing a lot of available data. On average, 30% of the available hydrometric record has not been used.
Figure 1: Length of hydrometric records used and available, but not used in Designated Floodplain Maps of BC
Why does it matter if we are not using the full hydrometric record?
The longer the record, the better the statistics; short records yield big uncertainties. And, with climate non-stationarity it becomes even more important to look at recent data. It matters because we are only using the observed peak flows from the period up to when the original analysis was completed to develop design flows. Design flows are then used as input to hydraulic models, which are themselves used to make floodplain maps. Design flows are also used for the design of flood protection structures such as dikes. Having a design flow that is wrong has implications . For example, a design flow that is too low can mean that dikes are also too low. It doesn’t take a lot of imagination to figure out the implications of this.
How is the hydrometric record used to calculate design flows?
Design flows are estimated by first building a flood frequency curve. Standard methods, like those defined by the USGS, exist to do this – and are mostly based on the largest observed flows in each year of record. The annual exceedance probability (AEP) for all observed peak flows (i.e. how likely it was in the past years that a specific flow occurred or was exceeded within a year) are then plotted. A curve is then fitted through these observed data points and extrapolated. Design flows, defined by an AEP, are then calculated based on the extrapolated curve. In British Columbica the design flow is generally the 0.5% AEP event (sometimes described as a 200-year flood), except on the Fraser River – where the 1894 flood-of-record defines design standards.
What if an extreme event (higher than any peak flow observed earlier) occurred after the floodplain map was made?
The exclusion of recent hydrometric data, especially if large events were recorded, can greatly impact the calculated design flow. A curve updated to include new extreme flows will be steeper than the old outdated curve. This means that when we extrapolate to the 0.5 % AEP event, we will likely end up with a higher river flow than originally calculated. A higher design flow means a larger defined floodplain!
Case Study: Lillooet River near Pemberton
As an example, we looked at the hydrometric record of the Lillooet River near Pemberton (WSC 08MG005) in more detail. The Designated Floodplain Map for this area was developed using hydrometric data from the Lillooet River station up to 1995. The three highest (> 1,000 m3 s-1) annual (daily) peak flows on record for the hydrometric station at the Lillooet River near Pemberton are:
– 1984: 1,110 m3 s-1
– 1991: 1,260 m3 s-1
– 2003: 1,370 m3 s-1
The 2003 flow is much higher than the previous maximum observed flow observed. What does that mean for the estimation of the design flow? We used a flood frequency program, HEC-SSP 2.0 (Log Pearson III distribution) to calculate the 0.5% AEP event for a) the data recorded until 1995 and b) for the entire data record available for download on the Water Survey Canada website (at the time of our analysis there was verified data available to 2010) (Figure 2). The methodology assumes climate stationarity for simplicity, whereas in a more complex analysis, non-stationarity of climate should be considered.
We found that the 0.5% AEP flow was for a) 1,285 m3 s-1 in contrast to b) 1,385 m3 s-1 . That’s an increase of 8%. It might sound small, but could be the difference between dikes holding and dikes breaching. Further, if a floodplain map was made today with consideration of the longer hydrometric record, the extents of the floodplain would likely be much larger than in the old floodplain map.
Figure 2: Impact of record length on design flow used for Designated Lillooet River Floodplain Map
So, we know our maps are out-of-date and that this has serious implications for how floods are managed in BC. The need to update them is clear. Fortunately, there is hope on the horizon. The 2015 Federal National Disaster Mitigation Plan highlights the need for mapping across Canada, and provides some base funding in Stream 2. There may yet be an opportunity to update BC’s flood maps!
Ebbwater Flood Map Project
We’ve just started our gap analysis and thought project on flood maps in BC and have a whole host of other analyses that we want to complete. We’d love to hear from you if you have a good idea on how to use or present the data, we’d also love to hear from you if you know of or have a map that’s not in our database – we’ve started our project using the Province of BC dataset, but know there are other maps (newer and better ones!) out there and would like to include them. You can contact us by email: BCFloodMapProject@ebbwater.ca or feel free to call or drop by our office.