The Alberta Flood Recovery Task Force has proposed the building of three flood control berms with dry ponds behind them at the headwaters of the Elbow and Highwood Rivers within K Country, and a second set of berms in the foothills outside of K Country: one on the Highwood and one on the Sheep.
The idea is that as stream flow increases during flooding, water would build up behind the berm forming a lake. The water would be released gradually at a maximum flow rate set by an outlet constriction until the area behind the berm is dry again and the stream confined to its regular channel. The proposed berms alone would not be enough to mitigate against all flooding, but they’d probably be able to bring the flow down to 2005 levels.
The initial spin is that a conduit would be installed in a river channel and a berm built over it. There would be a constriction in the conduit that would allow normal flow to pass through, but in flood conditions back water up in a dry pond. The water would continue to flow through the constriction at the designed rate of flow. Berm, conduit, dry pond. Sounds like no big deal!
However, this is far from the case. Perusing the limited information available to the public, it turns out these are massive structures 45–54 m high, 170–1135 m long and will store 12–84 million cubic metres of water (the Glenmore Reservoir stores about 17 million cubic metres). They are major engineering projects, either earth-filled berms in wider valleys or rock-filled dams in narrow canyons. While they don’t need a leakage monitoring system, they still need an impervious core because of the sudden build-up and draw-down of water. They will need a large amount of compacted fill over a wide base to ensure stability during extreme flooding when the dry pond may be full. The berms will also need overflow protection from waves and an emergency spillway to ensure the dam does not fail if water spills over the top. They will need significant (read concrete) outflow structures to control the flow as a single pipe is unlikely to work, and some form of protection at the inlet to prevent the inlets being clogged by debris. A spillway structure will be needed immediately downstream of the dam to reduce turbulence at the outlet.
So where are these berms likely to be built, how will they change the landscape, where will the fill for the berms be obtained from, and what will be the environmental consequences for the rivers and valleys above and below the berms?
It appears, interpreting the small map provided by the task force, that the berm on the Sheep is on private land outside of K Country. On the Elbow two berms are proposed, one a short way up Canyon Creek, the other in the narrow canyon east of Cobble Flats. The Highwood has one berm below the Bull Creek Hills downstream of Gunnery Creek, the second one is outside of K Country.
The effect on the ecology of the valleys will depend largely on the maximum flow rate that is set for the outflow. During any storm additional water would be added to the system through run-off downstream of the berms. Adding to the complexity is the time it takes for water to travel from the headwaters to the communities downstream. If the goal is to restrict the flow to 2005 levels at Bragg Creek, Turner Valley or High River, then the flow rate at the outlet will need to be set below 2005 levels, so we can expect that the dry ponds will accumulate water on the average every five years or so.
The term “Fetch” is usually defined as the longest straight line across the impounded water from the face of the dam, and has implications for the height of waves that might pound or break over the dam during a storm. I suspect “Fetch” in this case means the longest distance the water will reach upstream when the dry pond is full (2.5 to 6 kilometers).
How much water accumulates behind the berm will depend on the flow rate and duration of the storm. Silt will be deposited across the valley floor and in the existing river bed. River banks will gradually collapse, taking trees and vegetation with them. If the valley floor is flooded too often it will become an unsightly mess of cracked mud and dead trees. If it is only flooded occasionally the area will become revegetated providing browsing for ungulates.
Downstream of the berm the river will continue to flow at the set rate. It will not receive the occasional flushing-out we are told is essential for a healthy river system and the accompanying riparian zone.
In the event of a severe mountain rain-on-snow event and/or extensive storms in the foothills such as we had in 2013 we could expect the dry pond to quickly accumulate water. The flow rate downstream from the berm would stay the same, but additional water added downstream would increase the flow to above the 2005 rate. In theory, the communities’ local protection would then kick in. Rivers downstream would get flushed out and there would likely be damage to infrastructure. Ecologically this would be little different from the 2013 flooding.
Upstream, thousands of cubic metres of silt would be deposited behind the berms as well as large tangles of roots and downed trees. In a worst-case scenario the dry pond would fill up and overflow the berm. Problems with debris would most likely reduce the planned flow at the outlet, putting significant stress on the berm’s embankment and outflow structure.
Large amounts of fill will have to be found near the construction sites and blasted rock may also be needed for some of the berms. Whether it is taken from the valley behind the berm to increase capacity or from adjacent hillsides it’s going to look ugly unless extensive restoration (revegetation and tree planting) is undertaken. Trees cannot be planted on the dam itself as the roots can weaken the structure. It may also be necessary to clear trees in the area to be flooded upstream of the dam to reduce the amount of debris carried downstream during the early stages of the flood.
The dam would require annual inspection and maintenance. Without maintenance there is increased risk of failure during extreme flooding. Think what would happen if the proposed dry dam on the Elbow were to fail catastrophically. Seventy million cubic metres of water would be released in a matter of hours—a rushing tidal wave of debris-laden water descending on Bragg Creek. Unthinkable!
The Task Force’s stated intention is to get these projects underway as soon as possible. Fortunately, environmental assessment is mandatory for structures over 15 m high and reservoirs with a capacity greater than 30 million cubic metres. Approval is needed under various provincial and federal regulations. In my opinion the Task Force has seriously underestimated the cost of these structures and has given little consideration to the watersheds immediately upstream. It is likely that if these project are given the go-ahead the Alberta government will try to fast-track the approval process.
If these berms go ahead we need to ensure that we have input into their approval, planning, construction and landscaping.
Note: A 1969 Prairie Farm Rehabilitation Association study found the EQ1 site on the Elbow River to be a geologically unsuitable location for a permanent dam. Source Alberta Watersmart: Download A new report on historically identified detention and diversion sites Jan. 2014.
Check out the blogs on the Alberta Water Portal site for excellent information on re-wilding our rivers.
Updated February 2014