Updated for 2014(Originally from 2011 Flood Fight)
Many people have been asking about this information, so have created a seperate page just to provide easy access on the website. This information is also found in part of the page on Protecting Your Home:
How High Do I need to be?
That is the million dollar question. There seem to be four factors to consider:
- Estimated Water Flood Level;
- Estimated Wind Set-up(often called Wind Tide);
- Estimated Wave Height;
- Some type of a safety factor.
This website is not an authority on these items, however, the following links may help you in determining your own idea on what height you need to be at:
1. Estimated Water Flood Level – Manitoba Water Stewardship’s Highest Peak record for 2011 was 817.7 feet asl. For the most current report go to: http://www.gov.mb.ca/mit/floodinfo/floodoutlook/forecasts_reports.html#daily_flood_sheets and click on Lake Manitoba.
2. Estimated Wind Set-up(often called Wind Tide) – Manitoba Water Stewardship has provided an estimate of Wind Set-up for what a sustained North Wind of 60 km/h would do to Lake level including Wind Set-up the South Basin(Note: This is basically what we at the beach often call Wind Tide). Basically a 3 foot Wind Set-up(Wind Tide). However, the storm of May 31, 2011 provided a 3.25 foot wind tide. According to current NOAA statistics, their is a 27% chance of this type of event between now and freeze up.
3. Estimated Wave Height – In the May 18, 2011 Flood report, Manitoba Water Stewardship reported:During high winds, individual properties and dikes will be impacted by wave action differently. There are a number of factors that will change the way wind and the resulting wave action impacts properties and dikes, including: the slope of the shore, the distance from the water, obstructions that may buffer waves (roads, vegetation, etc.), the slope and height of flood protection dikes, and the depth of standing water against the dike.
• Waves will generally be higher and have more energy if the beach is shorter and has a greater slope. Waves will generally be lower and have less energy if homes are located far from the normal lake edge, have a gently sloping beach or are travelling over flooded vegetation or other obstructions. Homes with a gently sloping beach could experience waves as high as 3 ft.
• The impact of waves on dikes is dependent on the depth of standing water against the dike and the slope of the dike. With a sustained 60 km/hr offshore wind:
– Sandbag dikes close to the water’s edge may experience waves as high as 3 ft
– Sandbag dikes with water reaching the base of dike may experience waves as high as 4 ft
– Sandbag dikes with water reaching part way up the dike are likely to experience waves as high as 7 ft.(Note to readers: Sand bag dikes were useless in 2011, so this section has been strucken out. The only effective deterent found in 2011 was SuperSandBags or Stone walls).
– Clay dikes are generally built with a gentler (5:1) slope when compared to the slope of sandbag dikes (1:2), dissipating the energy of the wave and reducing wave height.
4. Some type of a safety factor. Manitoba’s Sandbag Dike brochure suggests a 2 foot of freeboard.
So an example of how all this works, assuming:
- Lake Level of 817.5 feet
- Wind Set-up of 3.25 feet
- Wave height of 3 feet
- Freeboard of 2 feet
Add it all up:
817.5 + 3.25 + 3 + 2 = 825.75 feet is height sought
Now, for this example, you can compare your estimated worst case water height to your elevation, and see if you are OK or not.
For example, if my house survey says it is at 818.8 feet, I’m pretty low compared to the 825.75 feet number, so I’d probably want to build a Dike of 8 feet to be safe.
Editor’s Note: The information above was provided prior to the May 31, 2011 storm event. The DBA was not provided with any advice or guidance from the Province as to how to protect the homes, despite repeated requests.
On May 31st, the storm destroyed the Sandbag systems which so many had worked so hard to construct, and along with it many homes. The sandbag systems proved to be useless against the power of the waves combined with debris in the water. Since that time many residents have constructed either stone wall or supersand bag defence systems. It is recommended you consider such a system if you are trying to protect your home.