Meteorological Signals for Flood Peak Flow at Cuntan and Forecast Model

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Meteorological Signals for Flood Peak Flow at
Cuntan and Forecast Model
Shuhua Yu Wenliang Gao
Yuhua Xia
Institute of Plateau Meteorology, CMA, Chengdu
emailShuhuayu_at_mail.Sc.cninfo.net
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1. Introduction 2. The characteristics of flood
peak flow at Cuntan 3. The relationship between
regional precipitation in Sichuan and flood
peak flow at Cuntan 4. Strong signals for the
flood peak flow 5. The relationship between the
HFPF and the surface rainfalls around
branches of the upper reach of Yangzi
River 6. Prediction model of flood peak flow 7.
Summary
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Introduction(1)
Jia lin River
Min River
Fu River
Qu River
Da Du River
Tuo River
Qin Yi River
Cuntan
Ya long River
Jin Sha River
The upper reach of Yangzi River is complex in
topography. Sichuan Basin is a place with rivers
converging to Yangzi River. Flood Peak at Cuntan
is closely related to large-scale precipitation
in Sichuan
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Introduction(2)
Hydrologists predict flood flow and waterline
according to real-time precipitation. In order
to predict flood earlier, in our opinion, a
flood-peak-flow prediction model based on both
meteorology and hydrology is necessary.

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Introduction(3)

Data daily flow record at Cuntan from May to
September during 1991-2001 (totally 11 years) .
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The characteristics of flow at Cuntan
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the characteristics of flow at Cuntan (1)

Practically, a more-than-30,000 m3/s water flow
at Cuntan can cause the waterline in the Yangzi
River up to or even over 180 meters, a dangerous
water level for the middle reach of the Yangzi
River. So we call this kind of flow at Cuntan as
Huge Flood Peak Flow, HFPF in this study.
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the characteristics of flows at Cuntan (3)
Features are as follows 1?The maximum HFPF each
year mainly occur in July and August (91 of the
total ). 2 ? The yearly number of days with HFPF
differ from year to year during the 11 years. In
1998 when Yangzi River encountered a
record-severe flooding, there were 54 days with
HFPF, but none in 1994.

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the characteristics of flows at Cuntan (4)
3 ? Generally speaking, the year with more floods
may occur severe floods. But the opposite is not
true. For example, in 1992 when HFPF is fewer,
both the daily and the process increase in flow
are the largest for the 11 years. This means
worse flooding may occur in the fewer- flooding
year.

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The relationship between regional torrential
rain in Sichuan and HFPF

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The relationship between regional torrential rain
in Sichuan and flood peak flow at Cuntan (1)

1 ? The relativity between regional torrential
rain and HFPF is high, 0.75.
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The relationship between regional torrential
rain in Sichuan and flood peak flow at Cuntan (2)
2 ? There were 39 regional torrential rains for
the 11 years, 29 of which caused HFPF. As for the
other 10 without leading to HFPF, 7 occurred
before rain season, in other words, waterline at
Cuntan is low then.
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Strong Signals for HFPF

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strong signals for HFPF(1)

The size of a torrential rain in Sichuan is a
sign for HFPF.

1?A large-scale torrential rain in Sichuan could
pose a HFPF. Isolated rains are not likely to
cause a HFPF. (42/45).
1
2
8
7
3
9
6
4
5
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strong signals for HFPF at Cuntan (2)
2? The interval of two torrential rains and the
intensity of rain are useful indicators to HFPF.
If two torrential rains are less than 11 days
apart, they would lead to a HFPF When the
interval is two days longer, the rains could not
pose HFPF unless there is another precipitation
between them.
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strong signals for HFPF (3)
3? Short after a HFPF, even a small-scale
torrential rain or a large-scale heavy rain could
increase the flow. The reason is simple. After
a HFPF, the waterline at Cuntan is high enough to
have another HFPF as long as a little more
rainfall added.
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strong signals for HFPF (4)
4? More than 13 days but less than 25 days after
a HFPF, a large-scale torrential rain, or a
little more severe torrential rain, or a
large-scale two-day persistent torrential rain
could pose a HFPF 25 days after, only a
large-scale severe torrential rain or two
larger-scale torrential rains can cause a HFPF.
In short, the longer apart from the former
HFPF, the more excessive rain is necessary for
the next HFPF.
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strong signals for HFPF (5)
5?the location of a torrential rain is a another
useful indicator. It enables us to know not only
whether but also how many days later a HFPF will
hit Cuntan.
The torrential rain over the northwest-western-so
uthwest of the Basin would pose a HFPF 3 or 4
days later
fig 1a
Figure 1 sketch map of location of torrential
rain in Sichuan
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strong signals for HFPF(6)
The torrential rain over the west-southwest of
the Basin would pose a HFPF 2 days later
fig 1b
The torrential rain in the northeast -central-east
-north of the Basin would pose a HFPF 3 days
later
fig 1c
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strong signals for HFPF(7)
The torrential rain in the area of
northeast-southwest of the Basin would pose a
HFPF 2 or 3 days later
Fig. 1d
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strong signals for HFPF at Cuntan (8)
In one word, the location, size and intensity of
a torrential rain in Sichuan could be a strong
signal for prediction of HFPF.
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The relationship between HFPF and the 9 River
Basin Surface Rainfalls.
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The relationship between the HFPF at Cuntan and
the 9 River Basin Surface Rainfalls (1)
The River Basin Surface Rainfall is relevant to
HFPF in four ways.
1. One RBSR is greater than 45mm(11/13) 2. 2 or
3 RBSRs are greater than 30mm and 20mm,
respectively(9/10) 3. 4 or 5 RBSRs are more than
10mm with 2 of them more than 25mm and 20mm
respectively 4. At least 6 RBSRs are more than
10mm. In brief, the larger River Basin Surface
Rainfall, the smaller rain area is needed to give
rise to a HFPF.
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The relationship between the HFPF at Cuntan and
the 9 River Basin Surface Rainfalls (2)
As to the first HFPF, a two-day persistent
torrential rain over a river basin area is
necessary in addition to one of the 4 situations
appearance.
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Prediction model of HFPF
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Prediction model of HFPF(1)
4 situations about the River Basin Surface
Rainfall and size as well as its location 1 ?
One of 9 RBSR is greater than 45mm with the rain
area stretched in north-south direction 2 ? 2
or 3 of 9 RBSRs are greater than 30mm and 20mm,
respectively with a near square-shaped rain area
3 ? 4 or 5 of 9 RBSRs are more than 10mm, with
three of them more than 15mm or 2 of them more
than 25mm and 20mm respectively and the rain area
stretched in west - east direction 4 ? 6, at
least, of 9 RBSRs are more than 10mm with a rain
area stretched from west to east in a narrow
shape. In sum, the larger in size, the smaller
in rainfall.
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Prediction model of HFPF (2)
On the basis of the 4 situations, the river
basin rain is ranked according to the rainfall
and its size (The intensity degrades
downward) River Basin Severe Torrential Rain
four situations occur together River Basin
Torrential Rain one of the four situations
described before appears River Basin Heavy
Rain 3 of 9 surface rainfalls are more than 10mm
with a more-than-25mm rain area in it, or 4 of 9
surface rainfalls are greater than 10mm with a
more than-20mm rain area in it, or 5 of 9
surface rainfalls more than 10mm with a
more-than-15mm rain area in it.
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Prediction model of HFPF
Flow at Cuntan (104m3 /s)
Waterline is high enough
a River Basin Heavy Rain
3.0
a River Basin Torrential Rain
a River Basin Severe Torrential Rain
a two-day persistent River Basin Severe
Torrential Rain
waterline is high
1.4
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1
10
15
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(Day after a HFPF)
waterline is low
The first River Basin Torrential Rain is not
likely to cause the first flood peak, unless
another two-day persistent River Basin Torrential
Rain or River Basin Severe Torrential Rain
following
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Summary (1)

Waterline in a river and rainfall are two key
factors to flooding in the upper reach of the
Yangzi River, with any one indispensable.
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Summary (2)

The location, size and intensity of torrential
rain in Sichuan provide strong signals for
prediction of HFPF.
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Summary (3)
The prediction model of HFPF is constructed by
three factors, namely, the water current flow,
the days apart from the former HFPF as well as
the intensity of the River Basin Surface
Rainfall.
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Thank you