I'll be talking about how the system runs at all, on a good day, so you can understand how crippling it was to lose that one worst-case main, and then I'll dig into what I was actually somewhat expert in, why the big mains break, ever. How the system runs, I had to learn for the first time from Hugh Costello, who planned and designed the big system for his career, and Alan Beairsto, who managed the staff for the treatment plants, who are also the operators of that larger system of big mains and the pumps.

The map colours the lines by broad bands of elevation, with cool blue and green colours way up hills, like Nose Hill and Broadcast Hill, and hotter orange and red colours as you go down the Bow River valley to our lowest elevations in town, far south.

Our lowest customers are at 950m above sea leavel, our highest, way up in Spy Hill Jail, are nearly 1250, so that's 300m of elevation difference across the city. A thousand feet of water pressure, of course, is enough to blow houses out of the ground, if it were all one continuous pressure vessel, so it isn't.

You can see that the Glenmore Pressure Zone is the centre of everything; includes the two big river valleys, the downtown; water radiates out from it to everywhere else.

That the east side of Calgary, which is flatter, only has a half-dozen zones from north to south; it's all those hills on the west side that get complicated to lift water up to.

Calgary is among the luckiest of cities when it comes to good water; straight out of the mountains, high quality, and we have two separate rivers, fed by two watersheds; we have a whole backup river, if one gets contaminated.

Notice again that the east side is flat - only a couple of those 25m contours in the entire Northeast, the "North Hill" zone; the same down south in the Ogden, Foothills, and Midnapore Zones.

Some zones are totally dependent on the one plant, get water only from one river; others get a mix of waters from both, and the mix can change throughout the day, because of how these plants don't really supply people, exactly: they supply reservoirs, nothing more than huge concrete boxes we bury uphill from your house.

There have to be buffers inside treatment plants, which put water through several processes that are different machines and tanks. Skipping over the details of how we treat water, it's multiple processes, in different buildings, handing water through the assembly line.

None of these processes run at precisely the same number of litres per second, so you have tanks filling up and draining down, between them, until you get them all in balance. Once you get the whole plant working at the same number of megalitres per day, you hate to ramp it up or down. So plants, where possible, run at the same output 7x24 - which demand does not.

They are never allowed to fall below 50% full, by the way, because we not only provided drinking water, and garden water, we fight fires. The other half of that reservoir might be sucked away by a big fire or two, in a matter of hours, one hydrant putting out as much water per minute as a house uses in a day. Also, you have a safety level of water, the way aircraft are not allowed to fly further than 70% of their fuel, divers don't go to the end of their air, either.

The red lines show the free pressure control and smoothing we get from reservoirs. All the little on/off jumps in the pumping, change the pressure on your block by not one iota; they only affect how much flow gets to the reservoir, not the pressure.

That only changes by a bit, 1.42 PSI per metre, as the reservoir goes up and down by 2 or 3 metres during the day. Yes, we use the imperial/metric conversion of PSI-per-metre. We work in metres, but every pressure guage is made in the USA.

The red horizontal line, we call the "Hydraulic Grade Line" and it's not quite horizontal, because pressure is lost via flow down any pipe, but neglecting that, the big deal is how far your house is below your reservoir.

The highest house in a zone has to be nearly 30m below it, for that minimum 40 PSI shower, and can't be more than 70m, to save the hot water tank from cracking. So, as you'll see, the reservoir that serves a zone is actually sited inside the zone above it.

This is our coolest pressure zone map, shown in 3D, and all the zones look like flat shelves, because they show that hydraulic grade line, the top of the water in the reservoir box. The top of each box is already under 30m of water, so to speak, and as you go down in the box, you're going further under water. When you open a tap in your house, imagine a submarine that's 50 metres down, opening a pipe in the hull.

The one long arrow points to a tiny zone just south of the Glenmore Plant, where the rich folk overlook Heritage Park, and the Moyie in the Glenmore Reservoir. They don't get a separate zone because they're rich; they get to live on a hill with a view because they're rich, and we had to valve off a separate zone for that hill - and there was no way to put a reservoir above it.

So little Eagle Ridge is a rare, "closed zone", the pressure created entirely by pumps constantly pushing just enough water to keep the pressure even. When consumption goes up, more or larger pumps have to kick in; if flow goes too high, a relief valve opens to let water back out and reduce it.

Then the zones to the east of the Glenmore zone are actually lower than the Glenmore plant. It would be great to just make free power out of that water going downhill, and you see proposals to scavenge energy from it with microturbines and so forth. None of them are worth it. So, the plant just pushes water out into the Glenmore zone, and the lower zones just take water from it, throw away the extra energy with "Pressure Reducing Valves", i.e. the water goes through a plate with a small hole in it to turn the pressure into turbulence and heat, and the valve, like the variable pump, has a control system to keep the pressure on the downside within limits.

In places, the distribution system has to have closed valves, because across the street from some 100PSI houses are the houses in the next zone down at the top of that one, at 40 PSI. One guy opening a closed valve can cause main breaks and blown-up water tanks across the street. Which has happened, I'm told. He isn't working here any more.

Notice the "Glenmore Island" - inside the Ogden Pressure Zone, but up on a hill, so that there's this separate network, valved off from the rest of Ogden, and connected by a feed pipe over to the main Glenmore Zone, to give it the Glenmore pressure it needs - on this map, it looks like a desert mesa.

The pressure-reducing valves are like firewalls - water goes through only one way, and the pressure on the other side is protected by assault from uphill.

The pump stations are clearly routers, connected to two networks, the lower pressure network of pipes on the downside, the higher pressure on the other.

I will, however, be showing you in coming slides that hydraulic network performance is absolutely not improved by "squeezing the hose a little bit"; quite the contrary, as you'd expect.

My own GIS shows the pipe widths proportional to diameter, because I think about pipe repair and replacement, not operation.

I also set larger pipes to be grey, instead of white, and huge pipes to be bright red, to call out the Bearspaw South Feedermain that broke. The red pipe to the west of the plant is the huge raw water pipe from the Bearspaw Dam that feeds the plant itself.

The two main water-output pipes, besides the giant one, are just called the North Feeder, straight off to the blue-square, inside the Big Hill West Pressure Zone, and therefore serving the zone below that, Spy Hill West. The Northwest Feeder, straight uphill to the Reservoir that's in Nose Hill Zone, and therefore serves the zone below that, Big Hill West.

Hugh ran the calculation for what amount of flow you could get, with a given amount of pressure, for every large main. The line width isn't quite proportional to that, because some of the pipes would cover too much of the map if they did. The draftsman cut down a few, especially the 2-metre Bearspaw South Feeder, the thickest line in blue, with some artistic license, because the line's thick enough to make the point.

The flow capacity is not like an electrical resistor, where the current a cable can carry goes up with cross-section, with the square of diameter. It's closer to the cube, in the range of diameters we use. The equation actually has a fifth power on diameter, there are just other complexities that reduce that to about a cube.

We'll get on to that, but the full-city map shows the huge simplification Hugh's colour choices provide: rather than dozens of pressure zones, and pump stations, and detail, there's really just three problems getting water to Calgary: the blue network is the base, the Glenmore zone, and all the zones that depend upon it to the east, the ones that are guaranteed water if you can just keep Glenmore full, they drain from it through those pressure-reducing valves.

You'd think that meant that all those zones didn't really have to conserve water during the outage, kilometres up north of Glenmore Plant. You'd be wrong.

That pipe doesn't take about half the plant output, as it appears; more like two-thirds, or three-quarters.

The capacity symbology, otherwise, shows how much less important other mains are. The little red mains off to the west of the plant just serve small zones on the hills. And, above all, the connections between Bearspaw-served mains in green and red, off to the east side of the city, are like paper drinking straws; the red and green networks just can't provide anything worth mentioning to the orange one.

The big service to the orange pipes, the whole North Hill area and multiple zones, is about a third of the output from that giant south feeder, grabbed by the Shaganappi pump station near the Trans-Canada Highway, and pushed up the North Hill, with a little dodge around McMahon Stadium, up to Our Lady of Perpetual Pressure.

The Shaganappi pump station that gets water up to it is arguably even more important, but North Hill is the Grand Central Station for the whole north, moving water uphill, sideways between zones of equal pressure, and even sometimes downhill, into far-eastern zones that need extra pressure because of distance.

Then the orange network goes through various zones, but all of them depend on water getting pumped up here, to Our Lady of Perpetual Pressure. A certain amount can be pumped up from Glenmore out east, but most has to come up this pipe at Shaganappi. Get it to our here, and the whole North and Northeast is covered.

And then there are a few brightly-coloured, complicated zones we just looked at, right above the Bearspaw Plant. They aren't that much of the city; the Blue and Orange networks are a good 75% of the whole city.

There's just paper straws from the green and red pipes to the west; just paper straws connecting the east side of Glenmore up the hill.

Normally, a fair fraction of Bearspaw's flow keeps going east from Shaganappi; Bearspaw water feeds downtown, goes into the "crosstown" main through downtown to points east; helps flow southward into Glenmore.

All that, the Glenmore plant had to serve on its own; and, much more, it had to send flow to all the pump stations supplying the North Hill, and they may have had to run water backwards up to Shagannappi, to supply water to this lime green main that goes under the river and up Broadcast Hill.

That, in one slide, is why nearly everybody in Calgary had to conserve water. The southern plant had to deprive its normal customers to serve the northeast, because Bearspaw just had no links over there, except that one, very critical, main.

We'll get back to how Calgary system planners backed themselves, and everybody, into that corner, where there was a single point of near-complete failure. But now I'll start my story, of how that failure slipped past all my plans to prevent it.