**5.1 Characteristics of debris ow after the earthquake**

1. Clear relation to the earthquake

A large number of debris flows occurred in the earthquake affected areas. For example, there are 46 debris flows found in Beichuan area. The distribution of these debris flows is shown in Fig. 22 in which the red line indicates the main fault of the quake, blue lines indicate rivers and the numbers indicate the locations of the debris flows. It can be seen that these debris flows are distributed along the rivers on the two sides of the earthquake fault. Therefore, the debris flows are highly related to the earthquake.

Fig. 21. The numeric simulation of debris movements by the extended DDA

Strong earthquakes not only trigger co-seismic landslides but also they affect subsequent rainfall-induced debris flows over a long term because these co-seismic landslides greatly increased the amount of sediment material for potential debris flows (Lin et al.,2006; Tang et al.,2009; Khattak et al.,2010). After the 2008 Wenchuan Earthquake, the earthquake affected areas experienced two rainy seasons till 2010, and a large number of debris ows occurred, which claimed as many as 450 fatalities. It makes the restoration and reconstruction much

In this section, at first, the characteristics of debris flows in the earthquake affected areas are summarized. And then, an approach of simulating debris flow is proposed for disaster mitigation. Finally, a large scale debris flow is simulated so as to show the effectiveness of

A large number of debris flows occurred in the earthquake affected areas. For example, there are 46 debris flows found in Beichuan area. The distribution of these debris flows is shown in Fig. 22 in which the red line indicates the main fault of the quake, blue lines indicate rivers and the numbers indicate the locations of the debris flows. It can be seen that these debris flows are distributed along the rivers on the two sides of the earthquake fault.

**5. Debris flow arising from the earthquake** 

**5.1 Characteristics of debris ow after the earthquake** 

Therefore, the debris flows are highly related to the earthquake.

more difficult (Xie et al.,2008).

1. Clear relation to the earthquake

the proposed.

#### 2. Large surge peak discharge and huge volume

Since the material sources of debris flow got much richer after earthquake, it is easy to form large scale debris flow. For example, the surge peak discharge reached 260 *m*3/*s* in the debris flow occurred in Beichuan town on Sept. 24, 2008. The volume was too large to a basin with the area of 1.54*km*2. The cover of debris is so thick that it buried the fourth floor of some buildings. Another example is Sanyanyu debris flow. The volume of the debris reached 144.20 million *m*3. The debris flow carried many huge stones and destroyed houses and bridges (Tang et al., 2009).

Fig. 22. 46 debris flows in the Beichuan area(Picture from Tangchuan et cl., 2010)

Many preventive structures designed based on the standard of conventional debris flow were also destroyed by the large scale debris flows after the earthquakes. For example, 19 check dams were destroyed by the Wenjia debris flow occurred in Mianzhu Qingping town area on Aug. 13, 2010. The Fig. 23(a) shows one of the check dam destroyed by the debris flow. The extreme large scale and destructive impact of the debris flow seems beyond imagination.

Earthquake Induced a Chain Disasters 409

The thicknesses of all the debris deposits and the geological and geotechnical behaviors

Grids are required for solving equations with finite different method. A DEM map can be converted to a raster image using GIS for the drainage area. The grids can be obtained by

The debris and water mixture is assumed to be a uniform continuous, incompressible, unsteady Newtonian fluid. The following Navier-Stokes equations are used for debris flow

*uu g p u <sup>t</sup>*

 

The so-called depth-averaged model as shown in Fig. 24 is used. And then the following equations are used instead of Eq. (11). They are solved by finite difference method (FDM).

> <sup>0</sup> *hMN*

() () ( ) cos tan 

*M MU MV H M M gh gh t x yx x y*

*txy* (12)

 *x*

(13)

2 2 2 2

 0 

*u u*

 

viscosity; *g g* (0,0, ) , *g* is the gravitational constant and *t* is time.

Fig. 24. Definition of coordinate system for 2D governing equations

 2

(11)

is dynamic

 

is the mass density; *p* is the pressure;

2. Make field investigations.

3. Generate the grids using GIS.

saving the raster data. 4. Solve the equations.

governing equations:

where *u uvw* (,, ) is velocity;

should be investigated in this procedure.

3. The low critical precipitation for triggering debris flow

The critical precipitation for triggering debris flow got decreased obviously after the earthquake. For example, the critical precipitation of 37mm became lower after the earthquake in Zhouqu County areas. A 22mm rainfall could trigger a debris flow during the past 3 years. According to preliminary analysis by Tang et al. (2009), the critical cumulative precipitation has been reduced about 14.8%-22.1%, the critical rainfall intensity per hour about 25.4 %~31.6% in Beichuan County area.

Fig. 23. (a): The destroyed check dam in Qingping debris flow; (b): Ming river blocked at Yingxiu town by Hongchungou debris flow ( photographs from Tang chuan)

4. River blocking

The disasters chain induced by the earthquake is very significant. The earthquake induced landslides caused debris flows which blocked rivers, and flooding disasters occurred. For example, Jianjiang River was blocked at 3 locations and half blocked at 8 locations by debris flows during the rainstorm on Sept. 24, 2008. Mianyuan River was blocked at 2 locations and half blocked at 11 locations by debris flows occurred on Aug. 13, 2010. Ming River was blocked at 1 location and half blocked at 5 locations by debris flows occurred on Aug. 14, 2010 (see Fig. 23 (b)).
