Many mountainous debris flows only occur once in a few years or decades, and often break out suddenly, it is difficult to directly observe its ins and outs, therefore, it is an important part of the debris flow investigation to investigate the traces of debris flows and visit the local people about the occurrence and development of debris flows.
Through the investigation, visit and identification of debris flow traces, the formation process of debris flow is reconstructed, the scale of debris flow at that time is estimated, and the development trend of debris flow in the future is predicted, so as to take corresponding prevention and control measures.
Caution should be exercised in identifying traces of historical debris flows because:
1. After the debris flow erupts, after a long time, the traces may remain incomplete and difficult to identify;
2. Because mudslides are mostly sudden and ferocious, people flee in panic, and their memories are inevitably different.
Therefore, the staff must repeatedly conduct on-site investigation and visits, and in the spirit of seeking truth from facts, analyze and check the information obtained "to remove the rough and refined, remove the false and the true, from this and the other, from the surface to the inside", so as to ensure that the results are true and reliable, and the conclusions are in line with objective reality.
1. Investigation and visit of traces of debris flows
1. The time, scale, number of times, duration of debris flows, and the fluctuation process of previous debris flows;
2. The location and height of the debris flow traces (distinguish the height of each debris flow trace as much as possible);
3. The situation of the eruption process of previous debris flows, such as the time and place of the occurrence of the debris flow, the characteristics of the flow process, whether there is a phalanx, whether there is a "dragon head" (including the height, material composition and movement characteristics of the "dragon head"), and the movement characteristics of large stones;
4. The dredging of the debris flow trench bed, whether there is blockage, and the material composition of the trench bed;
5. The season in which the mudslide occurs (earliest, latest, general);
6. Precipitation before the debris flow eruption, whether there have been earthquakes, landslides, collapses and other phenomena;
7. Hazards of debris flows.
2. Field investigation of debris flow traces
Some mudslide ditches, located in sparsely populated mountainous areas, generally lack historical records of the materials described by the inhabitants. In this case, we need to carry out field investigation and identification of debris flow traces. On the basis of the above-mentioned preparatory work, a detailed survey work outline is drawn up, the survey route is selected, and in-depth and detailed observation and research are carried out in the formation area, circulation area and accumulation area of the debris flow, and the debris flow of different causes, types and scales is identified according to the investigation and identification of traces such as solid substances carried (transported) by the debris flow, the flow process, the accumulation characteristics and the strength of failure.
When identifying debris flow traces, pay attention to collecting the following information:
1. After the outbreak of a debris flow, there are debris flow materials and the silent marks of the impact on both sides of the trench bed through which it flows, in the topographic changes, in the stone crevices, in the branches, among the weeds and on the buildings.
2. After the debris flow, the signs of physical, chemical and biological action caused on both sides of the bank generally have special color and morphological characteristics.
3. Debris flow trench bed characteristics.
4. Traces left by the change of curve height, rush height and silt during the movement of debris flow.
5. In the area where the trench bed changes greatly, it is necessary to find out whether the debris flow is full of troughs, and whether the traces of the debris flow are increased by the impact when the main stream rushes to the bank.
6. In areas with large changes in erosion, when it is difficult to determine the traces of debris flows, the width of the ditch and the depth of the water can be investigated, and then supplemented by other data to verify.