FAQ

Flooding FAQ

Q : What happens at the ORRPB when there's heavy rain and the risk of flooding?

A : When there's heavy rain and the risk of flooding, here's what happens at the ORRPB:

When heavy rain occurs, or may occur, which could cause levels to reach flood thresholds, the Ottawa River Regulation Secretariat (ORRS) is notified by one of its member agencies.

Once the rainfall warning call is received, the ORRS contacts all members of the Ottawa River Regulating Committee (ORRC). Data is then transmitted to the ORRS from each agency about how much water is in the rivers and at the reservoirs.

The ORRS then develops a forecast of how much flooding there may be based on the above field information, and on rainfall information from dozens of rain gauges, and on river flow information from dozens of river gauges. This forecast is discussed at a conference call between the ORRS and members of the ORRC.

If this forecast predicts flooding, provincial agencies responsible for issuing Flood Advisories or Flood Warnings are immediately notified. Further a press release is sent by the ORRS to electronic media in the Ottawa River basin.

During this time, dams owned by ORRPB members are operated to reduce flooding as much as possible. Dam operations are based on all of the above information, as well as the amount of water in the system downriver and upriver of each dam and reservoir.

Q : What do the dams owned by members of the ORRPB do when we get heavy rains?

A : During heavy rain, dams owned by members of the ORRPB are operated to reduce flooding as much as possible. This is their most important job during and after heavy rain. This reduction in flooding has the result of reducing property damage and possibly, loss of human life.

Q : How do they reduce this flooding?

A : During and after heavy rain, dams owned by members of the ORRPB carry out a number of functions. The main function of the reservoirs is to hold back as much rainfall runoff as possible for as long as possible. Holding back or storing this rainfall reduces, and sometimes prevents flooding downriver of the reservoirs. It is also important to realise that the Timiskaming reservoir is the last significant reservoir on the main stem of the Ottawa River. There are a number of dams on the main stem of the Ottawa River from Timiskaming to Montreal, but they have very little storage capacity and generally pass whatever amount of water is coming down the river in periods of high flows.

Q : What else do they do after a heavy rainstorm?

A : Once the reservoirs reach their storage capacity, or become full, their function changes slightly because there isn't enough storage left for more runoff to enter. At this point the outlet or gates of the dams are opened so that the additional runoff is now released from the dams through the gates.

Q : Do you mean to say that members of the ORRPB sometimes open the dam gates during and after heavy rainfall?

A : Yes. The dams owned by members of the ORRPB have always done this, along with every other competent operator of flood control dams in the world.

Q : But if there's flooding, why would you open the gates?

A : As we explained above, once the reservoir is full or almost full, the dams owned by members of the ORRPB release an amount of water from the reservoir that is equal to the amount of water coming into the reservoir.

Q : Why?

A : After heavy rain the members of the ORRPB have no control over the amount of rainfall runoff coming into a reservoir behind one of their dams. Through the use of large floodgates, the dams owned by members of the ORRPB do control the amount of water going out of gates at the dams. However, the outflow has to be increased to equal the inflow to the reservoir once the reservoir is full.

Q : Why?

A : If the dam operators who are members of the ORRPB didn't release an amount of water equal to the amount coming into the reservoir once the reservoir is full, the water would continue to rise.

Q : What would happen if the water continued to rise at the reservoir?

A : The dam operators who are members of the ORRPB would never allow this to happen. If they did, the dams could overtop and the damage could be devastating. However, as mentioned earlier, the members of the ORRPB have no control over the amount of rainfall runoff coming into a reservoir. Therefore, this situation could happen.

Q : But what would happen, theoretically?

A : Theoretically, if the water behind a dam rose so high that the water came over the top of the dam, the results could be devastating for people living downriver. The water coming over the top of the dam structure could wash away the earthen part of the dam and cause it to collapse. The dam operators who are members of the ORRPB would never allow this to happen to one of their dams.

Q : But what if it did?

A : Theoretically, you could have a huge body of water pouring down a river valley in a completely uncontrolled way. At the Otto Holden Dam, for example, the amount of water bursting through the broken dam could be over 15 metres high (50 feet) on the leading edge. The dam operators who are members of the ORRPB would never allow this to happen to one of their dams.

Q : So when some trailer parks and low lying properties flood, its because the dam operators didn't have any more room for water behind their dams. And the dam operators released the additional water to prevent the failure of their dams, and to avoid major damage and flooding downriver.

A : That's right.

Q : What about the trailer parks and low lying properties that get flooded?

A : Trailer parks and low lying properties get flooded because they are located on the floodplain of the river. The function of a floodplain is to be flooded.

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