Soils near the Red River of the North and its tributaries are inherently weak and natural forces are always moving the river channels. These soils display weak engineering properties and when exposed to the slopes of the riverbanks and valley walls their high plasticity frequently and naturally leads to foundation shifting and pavement failure.
Despite the vulnerability, these areas have in recent years undergone extensive urbanization, leading to an artificial acceleration of riverbank slumping and instability. This natural process is often accelerated by the following homeowner activities:
Houses are often built too close to the riverbank where the soils are most susceptible to bank instability. The weight of the structure places pressure on the riverbank and increases soil hydration because of increased storm water runoff. When the riverbank begins to actively slump, the placement of the home close to the river often provides homeowners little options except to move the house off the property.
Irrigation systems and septic drain fields add extra weight and excessive water, both reducing the soils structural strength. The saturating of the soil decreases the strength of these already weak soils and is one of the biggest contributors to increased and accelerated slumping.
Adding additional weight to the riverbank with houses, structures, retaining walls, riprap, soil and fill, and extensive landscaping places greater pressures on the riverbank and can increase and accelerate riverbank slumping.
Replacing deep-rooted, native vegetation with shallow-rooted vegetation, which further weakens the soils. The shallow roots of turf grass provides little soil strength and the absence of trees diminishes water removal from the soils.
Because riverbank slumping is a natural process and often inevitable, attempts to stop it have not typically proven successful. Attempted bank stabilization techniques including lime stabilization and homeowner constructed retaining walls are not always successful. Many homeowners also confuse riverbank slumping with erosion and place riprap, concrete, or other material on the slope in an attempt to stop the erosion; the added weight of these materials on the riverbank often accelerates the slumping.
Even professionally engineered techniques such as riprap or sheet piling for single sites are not always enough and typically require application to large stretches of the riverbank to be successful. The costly design and construction associated with these techniques is most often cost prohibitive to the average homeowner.
The best solution to this natural process is not building too close to the riverbank in the areas most susceptible to bank instability. Landowners are also highly encouraged to reduce their activities that accelerate this natural process by limiting the artificial introduction of water on their property, limiting the amount of weight on the riverbank, and maintaining or planting deep-rooted vegetation.
Time Lapse Animations
Following are four instances of slumping in Cass County. Click on each photo to view a time lapse animation of each example over the course of a few years.
The following animation illustrates an example of the processes occurring.