The River Always Wins

In a floodplain with a wide meander belt and a highly laterally moving river, the preferred approach to bridge design is to span the entire width of the geomorphically active floodplain or, as it is often referred, the active channel migration zone.  In this valley, the meander belt width is 650 m yet the bridge opening is a fraction of this size.   The following reports emphasize the best practice:

 

  • In a January 5, 2015 report from the City of Calgary to the Deputy Minister of Transportation, the City advises, “Bridge crossing spans shall be sufficiently wide to accommodate a reasonable amount of river morphological change (i.e. 1.5 to 4 times the Elbow River meander belt width) to accommodate some channel migration within the bridge span”.  This means that the bridge should span the valley; 
  • The City of Calgary Transportation Plan (2009), “A clear span bridge is usually the preferred type of crossing because it typically causes less impact to watercourse and flood plain functions”;
  • The Klohn Crippen Berger report (Nov 2015) further reinforces this notion and they state, “ Further designs should accept and expect river mobility as much as possible. The design philosophy should be to protect the infrastructure (i.e. road embankment, bridge piers and abutments, and stormwater ponds) to an appropriate level, rather than to attempt to control the river”;

 

  • Alberta WaterSMART in their “Room for the River Pilot in the Bow River Basin, Advice to the Government of Alberta” report (Dec 19, 2014) state, “Revise the SWCRR plans to include a wide span bridge, preserving room for the Elbow River” and “Consider alternatives to optimize room for the river considerations at this location”; and
  • The Deltares report  (Oct 2015) regarding the Springbank and McClean Creek Dry Dams also states, “The province should continue to pursue the multiple layers approach flood mitigation as outlined in previous work on Room for the River, structural mitigation is only one element. Programs like wetland restoration, floodway regulations and removal of obstructions should continue.”

The designers of this bridge have ignored this advice from the “experts”.

Environmental and watershed management groups have also weighed in on this issue.  Their comments include:

  • The Alberta Wilderness Association in a report entitled, “A closer look at Bow Basin flood mitigation proposals” (2015) states, “The overwhelming funding preference for building flood mitigation infrastructure gives the impression that watershed ecology is only of symbolic importance to the Alberta government. Ideally, more attention should be placed on developing and maintaining river sinks that will not disrupt natural habitats. They are very good long-term flood mitigation solutions. This is the message AWA continues to deliver to our elected officials”; and
  • In a statement on its website about ring road concerns, the Weaselhead/Glenmore Park Preservation Society (WGPPS) notes that limiting the meandering of the Elbow into a new channel could “alter the ecosystems, biodiversity, water quality, fish habitat and more, while reducing forest rejuvenation and increasing risk of forest fires over the long-term.” It also cites concerns about negative effects on water quality and on parks, the local community, and wildlife. “I don’t think they [Alberta Transportation] understand how unique the Elbow River is,” WGPPS’s Dahlseide said. She added that designers may be underestimating the power of the river, especially in times of flood. “I don’t think that’s wise.”

This advice from environmental groups that under the watershed has also been ignored.

Figure 1 below demonstrates what happens when a river meander (bend) is cut off by a new channel.  This abandoned channel creates what is know as an oxbow lake.  

Meander_short_cut.png

As can be seen in figure 2, the channel realignment as proposed by Alberta Transportation will eventually create an abandoned channel (an oxbow lake), when high-velocity flood waters take a short cut. 

Given that this meandering river concept is widely understood by geoscientists and river morphology specialists, we are not sure why Alberta Transportation is investing tax dollars in a realignment that will fail.  If they are to realign the channel, then the best practice would be to dredge the inevitable  “short cut” and widen and deepen this new channel, which will slow down the water velocity.

 Appendix B of the Calgary Transportation Plan, “Principles and design considerations for river crossings” also states:

  • All transportation crossings of rivers and creeks require the construction of culverts, piers and bridges, and have the potential to affect riparian areas and river and creek habitats. For these reasons, the need for river and creek crossings must be balanced with impacts to the environment and be treated with the utmost environmental sensitivity.
  • It is essential to balance the need for expanded infrastructure with the significance of the environmental areas and communities that may have to be crossed. When a crossing is deemed necessary, these facilities should be designed and constructed to protect the rivers, creeks and other natural ecosystems that will be affected.
  • A balanced triple bottom line framework should be used to assess the social, economic and environmental implications of the crossing and the corridor it serves and all alternatives, including the option of doing nothing.

What is the value of Calgary’s most biodiverse river valley?  Has this been taken into consideration? We don’t think so.

This notion that the proposed channel realignment is being engineered for failure is reinforced in the Transportation Association of Canada’s Guide to Bridge Hydraulics (2004). Figure 5.18 in this report, a figure from 1903, recognized that the river will take the “expected short cut”.

The Transportation Association of Canada’s Guide to Bridge Hydraulics (2004) also states, “Channel realignments or diversions were widely used in the past to simply bridge crossing layout and to reduce the potential for channel shifting and erosion. Concern over environmental impacts has resulted in realignments and diversions receiving critical scrutiny from environmental authorities and the public”.

Paul Finkelman, President of the Weaselhead/Glenmore Park Preservation Society stated at an October 7, 2014 open house at the Cedarbrae Community Centre, “A lot of people don't know but they are re-routing the Elbow River by a kilometre to accommodate a small bridge, which we think is ridiculous. It won't even withstand a flood like the one in 2013” 1

The Alberta Government report “Stepping Back from the Water a beneficial management practices guide for new development near water bodies in Alberta’s settled region” (2012) sums it up:

WATER ALWAYS WINS.

The illustrations below from the 2013 flood demonstrate the destructive nature of flood waters.  Bridges that were designed in accordance with Alberta Transportation standards failed due to scouring at the bridge foundations and road embankment.  This concern with bridge site scouring and erosion is also raised by the Transportation Association of Canada in their 2004 Guide to Bridge Hydraulics report.   Sites susceptible to scour include:

  • Bridges in erodible streambeds – especially sand or fine gravel – with scour-vulnerable design features
  • Sites on frequently shifting stream channels, or where flood currents strike piers and abutments at severe angles of attack  

An Alberta Transportation presentation, "Geotechnical Considerations from the June 2013 Southern Alberta Flood" provides an overview of the power of the 2013 rainfall event:

  • 174 landslides, debris flow and debris flood events were documented.
  • This was in addition to 157 road washout locations, 135 major erosion sites and more than 200 blocked culverts. 
  • 30 bridges were severely damaged and had to be closed for repairs while 90 other bridges required minor repairs that did not require closure.

Given this very high bridge failure rate, we concur with the Weaselhead/Glenmore Park Preservation Society's assessment, i.e. we don’t think they [Alberta Transportation] understand how unique the Elbow River is and we believe that designers may be underestimating the power of the river, especially in times of flood. Failed_bridges_June_2013_copy.png