10.3.2 Debris Flows- these occur at higher velocities than solifluction, and often result from heavy rains causing saturation of the soil and regolith with water. They sometimes start with slumps and then flow down the hill forming lobes with an irregular surface consisting of ridges and furrows (figure 9). They contain a mixture of rock fragments, soil, vegetation, water and, in some cases, entrained air that flows downhill as a fluid (figure 10). Debris flows can be further classified as mudflows and earthflows depending on the ratio of water to soil and rock debris. Lahars are a special form of debris flow caused by volcanic eruptions. Some debris flows are very fast. In areas of very steep slopes, they can reach speeds of over 160 km/hour. However, many debris flows are very slow, creeping down slopes by slow internal movements at speeds of just 30 to 60 centimeters per year.
Figure 9: Debris flow in hill slope area
Figure 10: debris flow in the forest and city.