ABSTRACT
The photograph shows the sediment disasters that occurred at a resort area on the Caribbean coast of Venezuela in December 1999. Cities that had grown up on the fans formed by ten rivers penetrating through the zonal area along the coast for about 20 km suffered catastrophic damages due to debris flows runoff from the rivers. The particular city taken in this photograph is the largest one; Caraballeda that is on the fan of the San Julian River. The low-rise houses existed among the high-rise buildings and in the midst of the fan were flushed out or buried under the runoff sediment of about 1.8 million cubic meters that covered an area of 1.2 km2
with a deposit about 5m thick containingmany boulders as large as 5m in the largest diameter.
I have done many field investigations on the actual conditions of flood and sediment disasters, starting with the occurrence at Okuetsu, Japan, in 1965. I have studied not only the physical phenomena but also the issues of psychology and social science. Because the disasters are so interlaced with complicated factors, there still remain a number of problems to be solved. However, by piling up and preserving the questions that arose in the surveys, I, helped by the achievements of others, have progressively found a number of approaches to solve the problems. In this chapter, I will look at some examples of my field surveys and, using the basic investigations discussed in the previous chapters, try to reproduce them by the numerical simulations. In fact, the sequence of the development of the investigations is reversed; the fruits of efforts to generalize the particular phenomena discovered in the fields are the basic physics of debris flow described in the previous chapters. This chapter shows how the basic investigations can be used to understand the actual phenomena, but it also introduces new problems that cannot be explained by the mathematical models introduced in this book. This is an indispensable step for the level up of investigations.
