Thanks to the Cass County Soil Convervation District and Riverkeepers for the excellent tour of the many river bank failures on the Sheyenne, Red, and Wild Rice Rivers.
The attendees included Fargo planning commissioners, three of us city commissioners (The media was notified and present) and others that are working on extending our setbacks to prevent any new homes being built too close to rivers and drains. Â Besides the vulnerability to flooding, the instability of the soils on riverbanks in our area is even more of a reason to back away from rivers here.
We had a good turnout despite the rain. Â Hereâ€™s Riverkeepers chief Bob Backman explaining the dynamics that make riverbanks slump with our unique clay soil conditions on the Red, Wild Rice, and Sheyenne Rivers in our Fargo area.
This is NDSU professor of geology Dr. Donald Schwert explaining how the clay soils in Fargo react with the rivers here. Â The more weight from buildings and removingÂ vegetationÂ near the riverbank, the more the chance of severe slumping as we saw in many housing sites on the tour.
Todayâ€™sÂ articles on the high cost of buyouts of vulnerable homes too close to rivers is one of the primary reasons I initiated a Fargo setback of 450â€² from rivers or 100â€² from the floodway whichever is further to prevent theseÂ high costs to taxpayers.
Fargo buyouts cost The Fargo buyout average reflects a large disparity between mid-range homes that have been getting a bit above assessed value (closer to Moorheads 108% above assessed value average) and the the high end homes that have been paid well above assessed value. Â Iâ€™ve asked city staff to address this disparity as future buyouts are considered and a moratorium on building on river lots now includes a 450â€² setback or 100â€² (whichever is further away from the river) along the Red and Wild Rice Rivers. Â I believe the Sheyenne River also needs to be addressed in the same manner as allowing new homes and building too close to that river even with the West Fargo diversion is not in taxpayers interest either.
Hereâ€™s what experts show us about why itâ€™s vital to back away from rivers especially with our soils in our Fargo area:
Back away from rivers with new construction
Soils near the Red River of the North and its tributaries are inherently weak and natural forces are always moving the river channels.Â 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.Â 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.Â The weight of the structure places pressure on the riverbank and increases soil hydration because of increased storm water runoff.Â When the riverbank begins to actively slump, the placement of the home close to the river often provides homewoners 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.Â 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.Â 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.Â Attempted bank stabilization techniques including lime stabilization and homeowner constructed retaining walls are notÂ always successful.Â Many homeowners also confuse riverbank slumping with erosion and place riprap, concret, 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 pilingÂ for single sites are not always enough andÂ typicallyÂ require application to large stretches of the riverbank to be successful.Â 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.Â Landowners are also highly encouraged to reduce their activities that accelerate this natural process by liimiting 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 fourÂ instances of slumping in Cass County.Â Click on each photo to view a time lapse animation of eachÂ example over the course of a few years.
The following animations illustrate the processes that are occurring in the instances described.