Clarifying the mechanisms of movement of a pyroclastic flow is important in studying how to prevent or minimize damage due to pyroclastic flows. Factors presumed to affect the movement of pyroclastic flow include sediment discharge from the flow, the degree of flow liquefaction due to volcanic gas, and topographical conditions such as the flow or sediment gradient.
Basic experiments were conducted to analyze how channel gradients and ascent speed of air affect velocity, sediment length, width, and shape of pyroclastic flows, depending on the degree of liquefaction. Air ascent speed, channel gradient, gradient of the sediment section, and amount of sediment were controlled so that results could be generalized.
The following tendencies can be inferred from our tests.
(1) If the gradient of the flow section was smaller than angle of dynamic friction, the velocities of pyroclastic flows are strongly affected by the channel gradient, the air ascent speed, and the volume of sediment.
(2) If the gradient of the flow section was larger than angle of dynamic friction, the velocities of pyroclastic flows are not affected by the air ascent speed.
(3) If a pyroclastic flow is very liquid and the channel gradient is steep, both the sediment length and width are large and the sediment is thinly dispersed over a large area.
(4) If the channel gradient is steeper than the angle of dynamic friction, the air flow does not greatly affect the sediment length width, and shape of a pyroclastic flow. Instead, these are affected by the channel gradient itself.
(5) If the gradient of the flow section exceeds a certain value with reference to the gradient of the sediment section, the sediment suddenly begins to spread laterally and be deposited. Furthermore, the sediment is likely to be dispersed and deposited for, at least, twice the width of the channel.
