spatio-temporal description of the rainfall in the andean city of

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spatio-temporal description of the rainfall in the andean city of
E-proceedings of the 36th IAHR World Congress
28 June – 3 July, 2015, The Hague, the
Netherlands
SPATIO-TEMPORAL DESCRIPTION OF THE RAINFALL IN THE ANDEAN CITY OF MANIZALES
(COLOMBIA) FOR STORM DESIGN
(1)
DAVID FELIPE RINCÓN , JORGE JULIAN VÉLEZ
(2)
, PHILIPPE CHANG
(2)
(1)
Universidad Nacional de Colombia Sede Manizales, Graduate student Facultad de Ingeniería y Arquitectura. Civil engineering Department,
Manizales, Colombia.
(2)
Universidad Nacional de Colombia Sede Manizales, Facultad de Ingeniería y Arquitectura. IDEA. Civil engineering Department, Manizales,
Colombia
[email protected], [email protected], [email protected]
ABSTRACT
This study investigated a spatio-temporal description of various rainfall events in Manizales (Colombia) using nine
meteorological stations located in various parts of the city. The chosen stations were those with the greatest number of
available historical data. The selected time period from January 2006 to July 2014, ensures homogeneity of the data.
Raw data was extracted, processed and analyzed over intervals of five minutes with a minimum time period between
rainfall events of fifteen minutes. The location and topography of the city are responsible for the climate variability, in
particular due to the Intertropical Confluence Zone and the El Niño South Oscillation effect. High intensity and
cumulative rainfall in the region are responsible for triggering landslides. Hence, a detailed description and
understanding of the rainfall pattern is required to enable decision makers to reduce the risk associated with floods and
landslides, which are the most important issue in the Andean region. This rainfall analysis also improves the early
warning systems; the identification of vulnerable zones and the design methods for protective civil works. The results
indicate a high spatio-temporal variability, especially in the spatial distribution of the rainfall. The dimensionless
temporal distribution of the observed rainfall does not meet the recommended standards given by hydrology manuals
and the observations for the region indicate a uniform distribution of rainfall over time. Such information is required by
engineers for proper storm design. The rainfall reduction factor analysis indicates that the daily rainfall used to estimate
the design storm for different durations are highly variable in the entire city.
Keywords: Storm design, rainfall, spatio-temporal description, dimensionless temporal distribution.
1.
INTRODUCTION
Manizales is located on the Andes mountains in the west
Central Range of Colombia, the municipality has
implemented a network of meteorological stations to
improve the knowledge of the rainfall behavior in the city.
It is important to understand the behavior of the various
climatological variables in a given area and in particular
the intensity and duration of the precipitation and its
spatial variability. Such information allows for better
decision making and establishes the required criterion for
the design and stability of hydraulic works. The major
cause of landslides in the city are high-grade slopes as
well as significant rainfall, which leach into the soil and
constantly saturate it until failure occurs.
The condensation of water vapor in the atmosphere
precipitate to the earth's surface as rain, snow or hail.
One of its main features is its intensity, which is the
equivalent column of water falling over a given area per
unit time, usually measured in mm/hour.
For the present study the rainfall’s temporal reduction
factor (TRF), a coefficient to estimate the average rainfall
at each point of the study area was used for different
durations. It also includes the rainfall’s temporal
distribution analysis in order to describe the spatiotemporal distribution in Manizales.
2.
METHODOLOGY
The first question that arises is how much rainfall is to be
considered as a single event? This question is difficult to
answer but the proposed hypothesis is to consider that
the amount of rainfall of each event is based on its
duration, in other words, longer durations are considered
with higher rainfall thresholds. There are different
definitions for “event” and “extremes” which gives rise to a
subjective understanding of a given event (WMO, 2008;
Karl, 1996 and Groisman, 1999).
In this study, a rain event was considered as a rainfall
occurring for longer than 5 minutes with a 2 mm
minimum precipitation. This threshold was increased
every 25 minutes by 2 mm, i.e., a 30 minutes rainfall
would be considered as an event if it was greater than 4
mm, a 55 minutes event would account for a precipitation
greater than 6 mm, and so on. An actual event was said
to begin when no rain occurred 15 minutes before or after
the event, the event duration was thus determined..
The description of rainfall is performed by: 1) estimating
the TRF and its spatial distribution over the city, and 2)
estimating the temporal distribution for different
durations.
2.1 Temporal Reduction Factor, TRF
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E-proceedings of the 36th IAHR World Congress,
28 June – 3 July, 2015, The Hague, the Netherlands
The TRF is defined in this study by dividing the rainfall in
mm of each event (PP) over the cumulative total of the
day (PPT) also in mm. Eq. [1].
[1]
When an event occurred over a two day period, the total
precipitation was considered to be the average between
the two days.
In this first approach and considering 8.5 years of data
series available, a frequency analysis would have to
investigated further in future studies as it was not
considered here. This study provides an estimate of the
TRF considering only rainfall duration. The final result will
show the spatial distribution of TRF over the city for
different storm durations.
Figure 2. SCS 24-hour rainfall distributions. US Soil Conservation
Service (1986)
2.2 Temporal distribution of the rain
The temporal distribution of the rain is the dimensionless
graph of storm duration vs precipitation as proposed by
Huff (1990) and shown in Figure 1.
3.
DESCRIPTION OF THE STUDY AREA
Manizales is located at an average altitude of 2150
m.a.s.l. and has a high topography, the climate is tropical
and is influenced by the Intertropical Confluence Zone
ITCZ and with a strong climate variability due to El Niño
South Oscillation, ENSO (Poveda et al., 2011).
Nine climatological stations were taken into account
(Aranjuez, Bosques, El Carmen, Emas, Enea,
Ingeominas, La Palma, Posgrados, Yarumos), located at
strategic points in the city, which provided the necessary
data for the study. The available records analyzed were
from January 2006 to July 2014 to ensure homogeneity in
the data.
4.
RESULTS
The results of the estimation of the TRF vs storm
durations for the 9 rainfall stations distributed over the
city area are shown in Figure 3.
Figure 1. Distributions of heavy rainstorms, Huff (1990).
The US Soil Conservation Service (1986) proposed for
the United States the distributions shown in Figure 2. This
paper shows the distribution for the city of Manizales in
order to understand the storms’ behavior.
Figure 3. Temporal Reduction Factor vs storm duration.
The estimate of the temporal behavior for every station
was done with the rainfall information for each event
duration and its dimensionless graph of rainfall vs time
for all events. The estimate for the Posgrados station is
shown in Figure 4. Where P is the accumulated
precipitation event every five minutes, PT is the total
rainfall event, t is the cumulative time every five minutes
and T is the event’s total duration.
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E-proceedings of the 36th IAHR World Congress
28 June – 3 July, 2015, The Hague, the Netherlands
Figure 5. Dimensionless temporal distribution of the rainfall weighted
for all the stations.
These behaviors were compared to the dimensionless
temporal distributions found in the literature (Huff, 1990;
Huff an Angel 1992 and US Soil Conservation Service,
1986). Manizales has a singular behavior different from
those proposed in the hydrological literature.
The next step in this study was to show the spatial
variability, which is represented through the variations of
rainfall TRF with different durations. The interpolation
along the city was performed using the Inverse Distance
Weighting IDW, due to its similarity to Kriging (Tabios
and Salas, 1993).
Figure 4. Dimensionless temporal distribution of 25 minutes rainfall
at Posgrados meteorological station.
For Manizales all stations show a very similar behavior,
the average values of all stations are shown in Figure 5.
In Figure 6 can be seen that shorter rainfall occur
mainly to the north and south of the city and as
duration increases rainfall events move westward. In
the southern part of the city long and short rainfalls
are constantly present.
The Figure 7 show the standard deviations of the rainfall
TRF for the same duration interpolated previously
citywide.
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Netherlands
E-proceedings of the 36th IAHR World Congress
28 June – 3 July, 2015, The Hague, the
Figure 6. Spatial distribution of dimensionless TRF along Manizales for durations of 5 minutes (top-left), 90 minutes (top-right), 240 minutes
(down-left) and 480 minutes (down-right).
Figure 7. Spatial distribution of standard deviation of estimated TRF along Manizales for durations of 5 minutes (top-left), 90 minutes (topright), 240 minutes (down-left) and 480 minutes (down-right).
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Netherlands
5.
CONCLUSIONS
The spatial distribution of the rainfall varies as its
duration increases in Manizales, an Andean town where
high spatial variability of rainfall is observed.
The data available for the study lapsed 8.5 years, hence it
is limited and it was not possible to obtain conclusive
results. Ideally over 30 years of data would be required
for a more detailed study. This analysis will be performed
as more information becomes available.
E-proceedings of the 36th IAHR World Congress
28 June – 3 July, 2015, The Hague, the
US Soil Conservation Service (1986). Urban hydrology for
small watersheds. US Department of Agriculture,
Technical release 55, Washington D.C.
World Meteorological Organization (2008). Guide to
hydrological practices, WMO, 168.
The temporal distribution indicates that the downpours in
Manizales do not match Huff’s second quartile, which is
currently being used by the city, and are very different
from those suggested by the SCS.
An accurate knowledge and understanding of temporal
and spatial aspects of rainfall events will help in
improving the design of hydraulic works in the city of
Manizales.
The further aspect to this study would be to estimate the
area reduction factors with the information currently
available.
ACKNOWLEDGMENTS
We thank the Universidad Nacional de Colombia Sede
Manizales and the Facultad de Ingenieria y Arquitectura
for their support. The Unidad de Gestion del Riesgo UGR,
Alcaldia de Manizales and IDEA for providing the data
required for this study.
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HUFF. F.A. (1990). Time distributions of heavy
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