Consider a dam on a relatively small river that discharges water over cliff and into a much larger river. In the Canadian Dam Association Guidelines 2007, Table 2-1 for dam classification talks only about downstream consequences of failure. Yet for this dam, the downstream consequence of dam failure are insignificant.

I would be loathe to call this a low risk dam because there actually are significant risks of flooding upstream, say if the flow control equipment doesn't work. The area along the riverbanks upstream of the dam contains a large permanent population. There is very little storage capacity in the reach upstream.

So how does one go about classifying such a dam?

dam classification is for risk caused by failure of the dam and the flood consequences of the failure. the dam doesn't cause the flooding upstream, the river does. (unless you are saying that the emergency flood pool elevation is higher than the upstream town)

If the flow control system fails (i.e. we can't get the stoplogs out in time), then it's the "failure" of the dam that causes the upstream flooding, no?

dams are designed to hold water, potentially to the very top. dam failure would be overtopping of the stoplogs causing flooding downstream. flooding caused by impoundment is totally predictable and should have been considered in the design or in the subsequent management of the reservoir. Usually there is a drainage easement or right of way to the limits of the flood pool preventing development of private property within it.

I'm not sure about any flood easement---I'll have to check with the local municipality and find out. Thanks for the heads-up on that.

But still, does failure of the flow control system count as a "dam failure"? After all, some damage done by the Saguenay flood was because they couldn't get their gates open on time....
http://www.canadiangeographic.ca/magazine/ma97/fea...

It might be considered a "failure" but the potential to raise the reservoir elevation to the top of the dam is a basic dam design feature. no structures should be built within that area.

one instance of dam failure caused by flow control system malfunction would be Taum Sauk reservoir. but again the failure caused the dam to burst and the flooding was downstream.

http://en.wikipedia.org/wiki/Taum_Sauk_pumped_stor...

OK, gotcha now. Thanks.

Interesting link!

Who was there first, the structures or the dam? I know of a similar situation where the contractor messed up on the spillway elevation, so houses would flood before the reservoir even reaches the spillway crest.

after reading a portion of the article, it appears that most of the residential flooding was a result of riverine flooding and not caused by water storage behind any of the dams. I think this quote from the article sums it up nicely...

"From 200 metres above Rivière Chicoutimi, the recipe for turning an act of God into a man-made disaster seems rather easy to follow. Start with an unstable landscape, poised on sensitive clay and riven by streams and rivers. Block the waterways with concrete walls, the largest of which were conceived to contain only unexceptional levels of rainfall, the smallest of which can barely evacuate a fraction of the capacity of the dams upstream. Then alter the courses and flows of the rivers with so many dikes, embankments and bridges that you forget who owns which. Issue permits to homebuilders for hundreds of riverbank properties. Toss in tens of thousands of waterlogged pitounes, some frozen floodgates, and absentee landlords who can barely describe their dikes, let alone get to their dams in an emergency.

Then add water"

Guess what---we got buildings in the 1/100 floodplain!! No idea which came first, but probably the dam (it goes back a long time).

I'm going to ask the Consultant to figure out what response time we need to get the dam open and compare it to the time it takes to actually open it; and if there is a problem, give us some recommendations.

Any other questions anyone can think of that would be useful to find out?