NORTH DAKOTA STATE UNIVERSITY
- FARGO, N D
As has been repeatedly demonstrated, areas susceptible to slope instability in the Red River Valley are easy to identify. However, mitigation of the problem is difficult, if not impossible -- and often too costly for the average homeowner. Hence, governmental agencies have been pressed to apply expensive engineering in an attempt to reduce damages to properties that should have never been built in such vulnerable zones.
In this section, we assess some ways in which the processes associated with bank destabilization might be reduced. Some of these methods are practical; some are impractical. None are guaranteed.
1. LIME STABILIZATION - With weak, expansive clays underlying much of the Red River Valley, this technique attempts to stabilize these sediments by reducing their plasticity. Holes are augered through the zone of failure, and into these holes calcium oxide (CaO, quicklime) is tamped. The quicklime serves to reduce clay plasticity in the immediate region of the hole, plus the quicklime is chemically transformed into a stabilizing pillar.
The technique was widely, and apparently successfully, applied on vulnerable slopes in areas such as Iowa. Attempts here in the Red River Valley have met with little success. In theory, the technique should work -- but the clays in our region may have plasticity indices so high as to be beyond remediation. In addition, slopes onto which the technique is applied are often so far into the process of failure that mitigation by lime stabilization is impossible. The technique is expensive, and the homeowner should be wary of its mixed record of success in our region.
2. REDUCING INFILTRATION OF WATER INTO SOILS - While this does not totally prevent slope failure, it can significantly reduce the rate of failure. It is the most important and most practical method of mitigation for the average homeowner.
The principal is simple. The clays that underlie the Red River Valley have a high capacity to absorb water. The more water that the clays absorb, the more unstable (plastic) they become.
Some of the worst cases of mass wasting that we have observed have been on properties where the owners: 1) regularly watered their lawns (often using free water from the river to do so), and 2) had their septic drainfields beneath the slopes. For mitigation, these owners were encouraged to move their drainfields and to entirely eliminate the lawn on these slopes, replacing it with willows and other stabilizing natural vegetation (see #3). Furthermore, we recommended that they install drain tile and other drainage structures to expedite the movement of water away from these prone slopes.
Eliminating entirely the introduction of water onto these slopes is impossible, and periods of floods can significantly enhance a slope's moisture content and plasticity. Nonetheless, the technique can buy significant time to evaluate alternative decisions.
3. PLANTING STABILIZING VEGETATION - In this mitigation technique, lawns on affected slopes are replaced by natural vegetation. Tree types include willow and cottonwood: species that have a high tolerance to floodwaters, as well as extend deep, stabilizing roots. Trees furthermore enhance the removal of water from saturated soils, thereby reducing a soil's plasticity.
The technique is often applied too late. Spring floods wash away new plantings, or slopes give way before stabilizing roots can develop.
We have regularly encountered homeowners who refuse to give up their green, well-watered lawns that sweep down to the river's edge. However, reestablishing vegetation can return benefits to these homeowners: including the potential for creating natural areas for wildlife close to their homes. The best advice is to never remove existing natural vegetation in the first place.
It should be emphasized that, even with the presence of mature trees, banks can still become unstable. See the photos in slump section for examples.
4. LOW-HEAD DAMS - Over the years, we have observed that the greatest rates of mass wasting occur: 1) when soils are saturated with water, and 2) when the river level is low. Because the Red River is a manipulated river (operations of dams upstream often control outflow), times exist when the river level at is low even at times of high soil moisture conditions.
Our maps of problem areas in the Fargo region show a related phenomenon: slopes in pool areas above low-head dams show little propensity for large-scale failure. Indeed, much of the area of east-central Fargo has shown no indication of slump development, even during saturated soil conditions.
We suspect that the dams are affecting the rates of slope instability in two ways: 1) in the pool area, a constant water level serves to induce continuous hydraulic pressure against the channel walls, slowing clay flow toward the channel, and 2) the velocity profile of the river is "evened-out" across the channel because of the presence of the dam. Because water spills over the lip of the dam at a uniform rate, erosion is no longer concentrated at the cutbank positions of meanders within the pool region.
Although dams seem to be effective structures at reducing mass wasting rates, it is unlikely that any more will ever be constructed along the Red River. Dam construction is expensive, and the placement of dams on the Red requires an approval process involving two states plus the federal government. Dams have a negative impact on fish movement. They are also dangerous (many drownings have occurred on local dams) and inhibit the development of other recreational uses for the river. In some areas, the presence of a dam could negatively impact water quality; such, for example, would probably be the case if a new dam were to be installed at the north edge of Fargo.
5. OTHER ENGINEERING SOLUTIONS - Over the years, we have witnessed the emplacement and subsequent failure of many slope stabilization structures. Most of these were doomed from the beginning: they did not address the basic processes inducing mass wasting.
One of the more intriguing, successful, and expensive approaches was the construction of a dike east of the V.A. Hospital in north Fargo. After having experienced the failure of an earthen dike positioned above the weak cutbank slope, a decision was made to use sheet piling to serve as the dike. Compared to an earthen dike, the sheet piling dike is relatively low load, and the sheets can be driven to depths where they remain stable. While not particularly esthetic in appearance, the dike at the V.A. is one of Fargo's most successful structures.
In 2001, the city and Corps of Engineers shared expenses in installing bouldery riprap at vulnerable locations (see Case Study #2). Whether this technique (which cost $1 million in taxpayer funds for three locations) is effective remains to be seen: the riprap is a high load material and is being emplaced on weak, unconfined sediments. The quality of its esthetic appearance and its effect on riparian habitats are also arguable.