How run-of-the-river stations work during the spring freshet
May 1 2018, 13:41 PM
NB Power’s hydro facilities are located along the Saint John River system. They are “run of river” facilities with very little storage capability. Storage is measured in hours, unlike larger facilities like Churchill Falls in Labrador which can store water for months. Water coming from upstream into the headponds must be used for generation at that moment, or must be allowed to bypass the dam. Put simply, the water that flows in must flow out.
The Mactaquac Generating Station at full load passes water through at 80,000 cubic feet per second. Any flow greater than that must pass through the spill gates. Currently, the Saint John River flows at Mactaquac are more than 300,000 cubic feet per second. As a result, water at Mactaquac is passing through the spillways. At high flows, above plant generating capacity, the water coming in must be released immediately to maintain the proper slope on the headpond to allow the river to flow downstream.
Essentially the river returns to its natural state during high flow events. In order to maintain the natural flow of the river and allow the water to pass the facilities safely, NB Power has very specific operating guidelines.
Water naturally runs downhill. Increased water flow requires there is adequate slope on the river or headpond to continue the natural flow of the river. In order to accomplish this, NB Power lowers the Mactaquac headpond level at the dam to maintain this slope, thus allowing the passage of natural river flow. When the river flow decreases, the headpond level will return to normal levels.
In the lower Saint John River Basin the Reversing Falls in Saint John creates a natural barrier in the river system that is essentially the narrow end of the funnel. With the current river flows being greater than 300,000 cubic feet per second, approximately only half of that water can pass through the falls at low tide. As a result, a bathtub effect is created in the lower basin whereby the water that is not able to pass through the falls backs up and cause flooding. This is compounded during sustained high flows like New Brunswick is currently experiencing.
Higher than average snow fall in North Western New Brunswick and Northern Maine coupled with rain events has resulted in these sustained high flows.
NB Power is constantly observing and communicating river and station conditions with the goal of operating facilities with the least possible impact on the natural flow of the river while doing everything possible to keep its infrastructure and people safe.
Categories How it works