Geoscience Reference
In-Depth Information
Assuming also (as a first approximation) that the valley cross-sectional area did not vary
appreciably along the flow path, and (for want of better information) that in the valley
ahead of the surge the effective water depth was of the order of
h
2
=
1.0 m, according to
7.4ms
−
1
; thus, it would have
taken the wave about 52 min to cover the 23 km, which in fact it did. Admittedly, this
result is obtained with unknown and therefore assumed values of
n
and
h
2
, and it would
be possible to obtain the same value of
c
s
with other equally plausible combinations. For
instance, roughly the same result would be obtained with assumed values of
n
Equation (7.7) one finds that the surge came down at
c
s
=
=
0.05
and
h
2
=
0 m, representing a smoother channel with negligible depth of flow prior to the
disaster (cf. Problem 7.1). It would require a more detailed field survey and analysis to
estimate which values of
n
,
h
1
and
h
2
would be appropriate for the dam-break event that
took place in this valley.
A notorious but less disastrous flash flood occurred on Willow Creek in the town of
Heppner in Oregon, in the late afternoon on Sunday June 14, 1903. According to Morrow
County records (see also Taylor and Hatton, 1999), it was generated by a cloudburst,
which was later estimated to amount to about 35 mm over an area of some 50 km
2
,
mainly around Balm Fork, in the hills 10-15 km south of town. The flood waters raced
into town around 1700, causing the death of some 247 inhabitants, nearly 20% of the
population, and destroying one third of all structures. The
Heppner Gazette
of June 18,
1903, reported that the flood struck, “without a second's warning, a leaping, foaming
wall of water, 40 feet in height.” This surge height may well have been an overestimate.
Without detailed information on the temporal and spatial distribution of that rainfall
event, it is hard to know exactly how the runoff was funneled into a surge in the Willow
Creek valley. Nevertheless, the suddenness and power of the flood were no doubt also
exacerbated by the presence of a laundry at the upstream end of town; combined with
accumulating debris, this structure at first blocked passage of the water somewhat, only
to give way after a short while, abruptly releasing the built-up water mass.
Other examples of this type of flood are the glacier lake outbursts caused by ice
avalanches in Peru, where they are known as aluviones (see Lliboutry
et al
., 1977;
Morales-Arnao, 1999). Typically, they are triggered with almost no warning and bring
down ice blocks, boulders and mud, leaving death and destruction in their path. In the
past three centuries more than 22 major outburst floods have destroyed a number of
towns and villages in the region. In 1941 one such aluvion, caused by the failure of a
moraine dam higher up in the Cordillera Blanca, destroyed about one third of the city of
Huaraz and resulted in an estimated 5000-7000 deaths.
7.1.2
Monoclinal rising wave or kinematic shock
While the surge considered in the previous section is obtained by considering only the
dynamic terms in the momentum equation (5.22), the monoclinal rising wave is obtained
from a balance of the other two terms in that equation, i.e.
S
f
=
S
0
(in the absence of lateral
inflow). In other words, the accelerations are considered to be so gentle that the flow
can be assumed to be quasi-steady-uniform and that the only forces of any consequence
