Swimmin' Stats

I am neither a meteorologist nor a mathematician, but I know a weather-related statistical problem when I see one.

Here’s what it is. So far, this week, Halifax’s harbour beaches have been closed every single day. That’s despite the renewed assurances from the mayor and Halifax Water that, thanks to the sewage treatment plant being back online, everything is A-Ok for swimming at Black Rock Beach and Dingle Park again.

This week’s closures have been due to this: any time there is a “heavy rain event” (and I take that term from a news release issued by the Mayor’s office in conjunction with Halifax Water) the wastewater discharged from the “combined sewer overflow chambers” will not receive full treatment, and will be discharged, after screening, directly into the harbour.

“Following the heavy rain events, swimmers should not swim in the harbour for three days,” says the Halifax Regional Municipality’s announcement.

Now, I’ve lived in Halifax for many, many years, and I happen to know that it tends to rain here fairly often during the summer months. Therefore, I have theorized that, if each time there is a heavy rainfall it means that the beaches will be closed for the next three days, the beaches are going to be closed...uh, a lot.

I am a Virgo, though, so not to go on gut instinct alone, I have collected some facts and figures to back up my hypothesis.

I called James Campbell at Halifax Water to ask what the municipality’s definition of “heavy rainfall” is, but he told me it was a difficult thing to put numbers on. “It all depends on the duration and the volume,” he said, indicating that short bursts of rainfall were more likely to overwhelm the system than an all-day rain.

Then I went to the Environment Canada website and started looking back at the climate data for the summer months in Halifax. Since we don’t really know what the tipping point for our sewage system is, I’ve looked at days with “greater than 5mm” of rain on a single day. Between 1971 and 2000, the number of days with that amount of rainfall breaks down as follows:

In the month of June, there have historically been 5.8 days with rainfall greater than or equal to 5 mm. In July, there are usually 4.5 days, and in August, we typically have 4.7 days with that much rain.

That means that, on average, 15 days out of 92 (in June, July and August) have a pretty good dump of rain. And that means that on 45 of those 92 days, the beaches (had they been subject to today’s conditions) might have been closed, depending on how intense the rainfall was. That’s not to mention the 15 days on which it actually rained, so there would most likely have been no one swimming, anyway.

My conclusion, then, is that, despite all the talk about the harbour being so swimmable, the likelihood is that even with the sewage treatment plant online, the water could still be un-swimmable for fully two-thirds of the summer.

So, I guess it might be best not to plan any beach parties at Black Rock or the Dingle this summer (or really, any summer), because the stats simply do not appear to be measuring up in your favour.