Articles - THE CHARACTER OF A MUDFLOW

The flowage of earth materials in response to gravity occurs in fragmental earth materials of just about any size. However, only flows of such materials that develop suddenly are considered to be a type of landslide. Flows are described according to the character of the material they involve. This gives rise to such names as "mudflow," "earthflow," and "soil flow." The higher the initial ground water content, and the greater steepness of the originating slope, the more rapidly and farther the mass can flow, and together with the volume of the mass, the greater the force it can exert. Probably most common is the mudflow, and that term is used hereinafter in the generic sense to refer to all landslides that occur as flows. The mudflows is the most dangerous type of landslide because of the suddenness with which it occurs and the fact that it can originate in slopes that have fairly low gradients and therefore do not appear threatening.

REDUCING THE MUDFLOW RISK

No method for determining the safety factor of a mud flow has been established, because the forces causing flow to develop cannot be estimated as can those of a shear landslide. Therefore, the best way to deal with the risk of a mudflow is to note the possibility of such an occurrence in terms of mass, slope, and ground-water development, and on that basis recommend steps either to arrest the flow if it has begun or to prevent its development by maintaining a low ground-water level. One advantage in this regard lies in the fact that the mudflow, being somewhat fluid, is directed by the topography and therefore tends to move generally normal to slope contours. Consequently, the direction of movement is more or less predictable.

The force that can be exerted by mudflow debris is due its impact on some obstruction such as a house. To a first approximation, the impact force is simply the ratio of the change in the momentum to the period during which the change occurs. The momentum of the mudflow debris is simply the product of its mass and its velocity, and the change in the momentum is that due to the change in velocity because of the impact. In equation form:

F = (mv1 - mv2)/ Δs

where F is the impact force, m is the mass, v1 and v2 are the velocities before and after impact, and Δs is the period of the change in momentum. It is to be noted that force of impact is inversely proportional to the period of the impact. In other words, the shorter the period during which the momentum changes, the greater the force exerted. This is the principle that more and more frequently is being used in the design of barrier fences that protect such facilities as roadways. When these fences, commonly reinforced by cables strung and anchored in certain ways, receive masses of landslide debris, they deform by stretching. This increases the period of momentum change and consequently reduces the force of the impacting mass.

The same principle can be used to protect, say, a residence built at the base of a slope the surface of which is underlain by soil or other relatively loose material that could fall or flow. Ordinary chain-link fencing erected along slope contours, even if they deform by stretching, can either prevent the initiation of flowage or act to slow it if it has begun. The placement of such fences, conveniently referred to as "debris fences," is done in an arbitrary manner based on the character of the site. The slope gradient and the thickness of the material that potentially can become mudflow debris dictate fence design. The observations of the geologist are therefore critical to the determination of fence design.

REDUCING HIGH GROUND WATER

Generally, the higher the ground-water level in a mass of flow-prone material, the more likely it will fail as a flow. Probably the best means to prevent the development of a critically high ground-water level in a flow-prone mass is the installation of shallow drains arranged in such a manner as to intercept the ground water and direct it off the slope as controlled surface flow. A system of perforated pipe or slotted well screen, in either case with openings oriented upward and set as close to the base of the flow-prone material as is practical, is most effective. Drains of this kind are situated along contours normal to the direction of ground-water flow, and are conveniently referred to as contour drains. PVC or ABS plastic pipes with diameters of 2 or 4 inches for ease of cleanout are practical. As in the case of debris fences, the field experience of the geologist can be especially helpful in the design of contour drains.

* * *