Protective measures5 August 2004
Following the devastating floods in the Red river basin in Canada and the US in 1997, planning began for the Community of Grande Pointe flood protection project
IN 1997, the Red river basin of Canada and the US experienced the largest flood in recent history. Many communities within the valley, from Fargo, North Dakota to Winnipeg, Manitoba were inundated and attempts by residents and communities to protect their homes were, in many cases, futile. Economic damage in the two countries approached US$5B. The economic and social impact of the flood was significant, and it was recognised that a permanent solution to protect many of the communities in Southern Manitoba from similar floods was required. Immediately after the flood, the Province of Manitoba began assessing the need for protecting the impacted communities from a flood of the same magnitude as the 1997 incident. Acres International was awarded the contract to provide design and construction administration services for the Community of Grande Pointe flood protection infrastructure. Background
The Community of Grande Pointe consists of approximately 170 homes and businesses and is located immediately south of the City of Winnipeg. The Red River Floodway, a large diversion channel which diverts flood waters from the Red river around the City of Winnipeg, is located immediately north of Grande Pointe. The Seine river that flows from the east bisects the community. The Red river is located approximately 7km to the west. Provincial Trunk Highway (PTH) 59 runs north-south along the eastern boundary of the Community. The area around Grande Pointe is mainly farmland and is nearly flat.
During the flood of 1997, the community was inundated by overland flow from the Red river. Flood waters from the Seine also contributed to the situation, but not to the same level as the waters from the Red river. The flood protection system had to be designed to prevent flooding from both the Seine and the Red river; while still allowing the Seine river under normal conditions or during small floods to flow through the community.
Early in the design of the flood protection, Manitoba decided to twin PTH 59 from the Floodway bridge to south of Grande Pointe. This provided a unique opportunity to mesh the flood protection project and the twinning of PTH 59 project to significantly reduce the total combined cost. This was accomplished by using the north bound lane of the twinned PTH 59 as the eastern flood protection dyke for the Community. In addition, a third project was underway involving the creation of two gaps in the Floodway East Embankment; this to allow water to enter the Floodway more efficiently. The material from the gap excavation was used in the construction of the western dyke, resulting in a significant cost savings to the project.
The total cost of the three projects was approximately C$18M (US$13.7M), and the work was completed under 10 separate contracts (Acres administered six construction contracts). The collaborative effort between Manitoba Transportation and Government Services, Manitoba Water Stewardship, and Acres resulted in the successful completion of the flood protection project on-schedule and within budget in the fall of 2003.
Some of the design challenges that the team encountered and the innovative approaches to addressing these challenges are summarised below:
Providing protection from both a Seine river flood and a Red river flood
In reality, the Community of Grande Pointe experienced two floods in 1997: the first flood occurred from the Seine river; and shortly later, the Red river flood further inundated the Community. Clearly any flood protection design would have to protect the Community from both flood conditions. Observations made during 1997 indicated that the two floods could be physically separated by extending the dyke south to tie into higher ground (the 1997 Red river flood water levels overtopped PTH 59 to just south of Grande Pointe). Separating the Red river flood from the Seine river flood enabled the crest elevation of the dyke to protect the Community from the Seine river flood to be lowered, thus saving construction costs. Therefore, the design involves an east dyke (at a lower elevation to the Seine river flood level), a south dyke and a west dyke. A north dyke is not required as the existing floodway south embankment forms the northern protection.
Protecting the community from a Seine river flood (East Dyke)
As stated previously, flood waters from the Seine river had to be prevented from entering the community; however, flow through the community was required during normal flows or small flood events. Furthermore, the flood waters could not simply be held back as this would cause overland flooding upstream. An innovative solution involved a self-regulating gated culvert control structure at the north bound lane of PTH 59, which will be open for small flood events, but partially or completely closed for larger flood events (greater than 1-in-10 year events). Flood waters will then be diverted through a diversion channel, or ‘mini-floodway’ to a drop structure at the Red river floodway. A dyke was also constructed between the diversion channel and the community to prevent the floodwaters from large flood events from reaching the community. This dyke comprised the northbound lane of the twinned PTH 59.
The diversion channel was excavated adjacent to the new PTH 59, and the material excavated was used to raise the northbound lane to the required flood protection elevation. The crest elevation of the highway was lowered near the floodway bridge to accommodate the grade change from the bridge and an envisioned future interchange in the area. An earthen dyke tying the highway to the floodway embankment was used to complete the flood protection dyke.
Three municipal roads were cut-off from PTH 59 when the diversion channel was excavated. To provide access to PTH 59 from the east, two roads were realigned to a new bridge that was constructed over the diversion channel. An existing dirt road was upgraded to a municipal gravel road, which then allowed traffic from the third road access to the bridge.
An energy dissipating or drop structure was required to pass water from the diversion channel into the floodway without causing erosion damage to the floodway channel. The drop structure is designed to operate with the floodway in operation (i.e. high tailwater level) or dry (no tailwater). While most of the pre-existing drop structures along the Floodway consist of a culvert drop structure, Acres proposed a baffled apron drop structure and, providing increased energy dissipation, efficient operation at all tailwater levels, at a reduced construction and maintenance cost. The Grande Pointe drop structure is the only large baffled apron drop structure in Manitoba, and one of the largest in Canada.
Providing flood protection from a Red river flood
The Community is protected from overland flow from the Red river by a 15km earthen dyke that runs from the floodway embankment south to Mondor Road, then east along Mondor Road to PTH 59. The dyke has two openings (or flood closures) which will have to be filled in during significant flood events. The closures occur where the dyke crosses the CP Rail tracks and at Richardson Rd.
The dyke was designed to provide flood protection to the water levels observed during the 1997 flood. Standard practice throughout the Red river valley was to apply 0.6m of freeboard above the 1997 flood level to allow for wind and wave action. This standard 0.6m freeboard was not considered sufficient for this dyke, due to the water depth and the potential for significant wind set-up and waves. Acres determined the acceptable freeboard allowance for the dyke was 0.9m for the dyke from the floodway embankment to the CP Rail Tracks and 0.6m for the remaining dyke.
The material to construct the dyke was obtained from three sources; a ditch adjacent to the dyke; the Floodway embankment as part of the Floodway Embankment Removal project; and from a drainage swale. A total of 410,000m3 of material was required to construct the dyke. Constructing a drainage swale from the dyke to the Seine river eliminated the need for a pumping station, which offered a significant capital and maintenance cost advantage. All the internal drainage within the dyke area flows into the Seine river. The additional drainage ditches provides an added benefit to the farmers, as it allows them to improve the drainage of their fields.
Project management challenges
The Grande Pointe flood protection project overlapped with two other projects: the twinning of PTH 59; and the floodway embankment removal project. A significant cost saving was achieved by combining the three projects; however, this complicated the scheduling of the various construction activities due to the interdependence of the various tasks. This interdependency between projects resulted in 10 separate tenders being required.
Strict project management procedures were implemented and frequent meetings were held to ensure timely completion of designs, tender documents and construction activities. Critical path tasks were identified early in the design stage and measures were implemented to avoid potential scheduling conflicts. In particular, scheduling conflicts during construction could have had significant cost impacts due to the seasonal nature of some of the activities. The cooperation and the willingness to share information between Manitoba Water Stewardship, Manitoba Transportation and Government Services, and Acres was paramount in the successful completion of this project on-budget and on-schedule.
The authors are Warren Gendzelevich, Acres Manitoba, Steven D Topping, Manitoba Water Stewardship and Phil Pantel, Acres International. For more information, email: [email protected]