Environmental Engineering Reference
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channel moved to the left side by about 80 m. The debris flow caused a great disaster, 14 people were
killed and many new houses were buried.
There were different reports on the rainfall intensity triggering the large volume debris flow on Aug.
13, 2010, mainly because the rainfall intensity differed greatly in short distances. No systematic rainfall
data at Wenjiagou were recorded, instead, several rain storms were measured with a rainfall meter
installed on the roof of a farmer's house. It was reported on Aug. 13, 2010 that the rainfall was 98.5 mm
at Wenjiagou, 227 mm at Xiaogangjian (7 km south of Wenjiagou), and 163 mm at Nanmugou (8 km,
south of Wenjiagou) (Yu et al, 2010). Systematic rainfall data were obtained from the Mianzhu rainfall
station, which were provided by the Mianzhu Weather Bureau. The distance from Mianzhu to Wenjiagou
is about 50 km, but the rainfall at Mianzhu was only 6 mm on Aug. 13, 2010. Some people used 227
mm/day as the rainfall intensity for triggering the large volume debris flow in Wenjiagou.
Fig. 11.68
(a) Water flowed over the dams and scoured the base of the dams causing dam failure; (b) Bed incision
increased the slope angle and resulted bank failures, which resulted in debris flow
Let
E
k
denote the kinetic energy of a column of water flowing in the gully with a unit length in the
flow direction. The height of the column is the water depth
h
, and the width is the channel width
B
. As
shown in Fig. 11.68(b), the flow scours the channel bed and the loose solid materials on the two banks
slide into the flow. Assume there is no sediment in the water column at position 1. As the flow travels a
distance ' to the position 2,
h
and
B
increase because a lot of solid materials have entered into the
column. The kinetic energy of the flow column increases (or decreases) by
'
:
k
1
(
2
2
'
E
Mu
Mu
)
(11.38)
22
11
2
in which
M
1
and
M
2
are the total mass of the water-sediment mixture in the column at position 1 and
position 2, respectively;
u
1
and
u
2
are the velocity of the water-sediment mixture at position 1 and
position 2, respectively. The kinetic energy increases due to release of potential energy of water
e
1
, and
decreases due to the friction at the channel bed and banks
e
2
; the kinetic energy also increases due to the
release of potential energy of solid materials
e
3
, and decreases due to the energy consumption in solid
particles collisions during the transportation
e
4
. Thus,
'
k
Eeeee
(11.39)
1
2
3
4
The released potential energy of water flowing from position 1 to position 2 is
e
' ' ' (11.40)
in which Uis the density of water,
S
is the bed gradient,
Q
is the discharge of water, and ' is the time
needed for the water flows from position 1 to position 2. The energy consumption due to friction at the
channel bed and banks is given by
U
ghBS
x
U
gShBu
t
U
gSQ
t
e
W
(
p
p
)
'
x
W
(
p
p
)
u
'
t
(11.41)
2
0
B
w
0
B
w
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