I’ve been hearing talk about dykes being used for flood protection. What exactly is a dyke? Is it just a pile of dirt? Why does it not just wash away?
As a direct definition, a berm is a mound or wall of earth. When being used for flood protection, a berm is typically referred to as a dyke. A dyke typically is built using a clay material that can be packed tightly together to ensure no water can easily flow through it. For areas that might see faster flows, some sort of erosion control such as vegetation or rock would be required.
Can putting a dyke in one area of the city affect another area of the city?
The work being considered is not being looked at as several separate projects but rather as one big flood mitigation plan. Using very accurate hydrologic modeling software, we have looked at the impact each of the projects have on the river both upstream and downstream. Any changes to the rivers depth and speed as a result of the berms have been negligible.
Will efforts be made to make the dyke look “natural”?
Yes efforts will be made to blend the dykes into the community landscape. The dykes will be seeded with dry land grass similar to what is seen in each of the surrounding areas.
Trees cannot be placed within 3 m of the toe of the dyke because they pose a severe risk to the structural integrity of the dyke for two reasons:
- The first is that floodwaters can rip the trees out of the ground during the event. When this happens the root ball is usually pulled up with the tree and the hole that is left can be very large and affect the structural integrity of the earth embankment. Failure of an earth embankment that has been compromised in this fashion can be sudden and result in a massive rush of water through the hole as it is rapidly widened by the entering floodwaters. It is a very quick and dangerous type of failure.
- The second failure mechanism is from the roots and occurs by a phenomenon called piping. This happens when the river is in flood and the floodwaters have enough force to seep through the embankment along the paths of the roots following the root/soil interface. If roots extend into the embankment it can make a clear path for the water to the other side. The real problem with this seepage is that it can carry the fine soil particles with the water and very quickly erode tunnels in the embankment. As the seepage paths widen, more water seeps and further erodes the tunnels. The tunnels can get very large, very quickly and eventually result in the washout of the embankment. This type of failure is not as instant as the embankment failure described above but it could occur within the time span of one flood event (a few hours). The risk of this happening is much higher if the tree has died (or been cut) and the root structure has begun to rot and shrink. This leaves clear paths for the water to go and can greatly speed up the piping phenomenon and eventually failure.
The need to keep the trees a minimum 3 m away is because the root structures can extend very far. Even 3 m would mean roots will eventually extend under the embankment but the reasoning for allowing them this close is that the risk of damage from uprooting or piping is much less.
Will there be a leisure trail on top of the dyke?
Specifics of each project can be found in the project information area of the web page. The goal will be to include a leisure trail on top of the dyke where possible.
Impact to a residents view will depend on the individual house. The height of the dyke varies from project to project and even along the length of a project.
How will my lot drain if a dyke is placed in the drainage path?
Drainage of storm water off of an adjacent property is just as important to this project as the protection from river/creek flow and this will be taken into account during detailed design. Options for ensuring continuous drainage include the installation of culverts through the dyke embankment complete with backflow prevention valves. During a flood event these valves will remain closed preventing flood waters from entering properties on the dry side of the berm. Should a rain event occur during a flood event, large pumps will be brought in to pump the overland drainage over the berm and into the river.
The City of Medicine Hat has received funding through a provincial grant to install storm gates in the outfalls that lie within the flood fringe. The gates will help prevent the backup of river/creek flow into the storm system.
How is this project being funded?
The City has received a total of $14.1 million dollars in funding from the provincial government through the Flood Recovery Erosion Control Program (FREC). Applications for additional funding through the Alberta Community Resilience Program (ACRP) have been submitted. Any funding shortfalls will be covered by the City.
What will be the first overland flow protection project?
The first overland flow protection project to be completed was the Water Treatment Plant Project which was substantially completed in Fall 2014.
The City of Medicine Hat has a flood response plan that would be implemented. This response plan includes the installation of over 8km of temporary flood protection products including Barricage, Hesco CART, and Muscle Wall. A detailed plan is in place that indicates when and where these systems are to be deployed to help protect the residents of Medicine Hat.
How long will it take to complete all eight of the overland flow protection projects?
The overland flow protection projects vary in size, cost, and complexity. It is not known at this time how long it will take to complete all projects; however the target is a fall 2016 completion date.
|Water/Power Plant||2014 (completed in September)|
|Storm sewer backflow protection||2015 (Summer)|