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| Titel |
Monitoring the Dynamic of a Fluvial Channel after Lahar Disturbance: Huiloac Gorge (Popocatepetl Volcano, Mexico) |
| VerfasserIn |
N. Andrés, D. Palacios, J. J. Zamorano, L. M. Tanarro, C. Renschler, J. J. Sanjosé, A. Atkinson |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025506
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| Zusammenfassung |
| Volcanic eruptions generate disturbances that affect hydrological systems (Major, 2003) by
depositing large volumes of sediments in watersheds that exceed amounts common to
non-volcanic river systems (Montgomery, 2005). If the eruption releases abundant melt water,
the river system may respond immediately by forming hazardous flows called lahars. River
system recovery following eruptive and laharic impact is an important process, but it has
received little attention (Gran and Montgomery, 2005) despite the fact that Major et al. (2000)
and Hayes et al. (2002) have shown that these disruptions cause long term instability and their
effects persist for decades.
Lahar deposits resulting from interaction between volcanic activity and the glacier located
above the Huiloac Gorge on the northern slope of Popocatepetl volcano (19º02´ N, 98º62´ W,
5,424 m), have infilled the gorge (Palacios, 1995; Palacios et al., 1998 and 2001; Capra et al.,
2004; Muñoz, 2007). All of the major lahars that occurred on the volcano in 1995 (4
km), 1997 (21 km), and 2001 (14 km) have channelled through Huiloac Gorge,
and have dramatically altered its morphology and dynamics through erosion and
deposition.
The present study traces these changes in the aftermath of the laharic events that occurred
from 1997-2001. A sector of the channel, located at 3200m-3240m altitude, of 500 m long
and 15 to 20 m wide, in the mid-section of the gorge, was chosen as the control site.
Precipitation is heaviest there and is most apt to trigger secondary post-eruptive lahars.
ArcGis software was used to draw 6 geomorphic maps of the site showing spatial variations
in the landforms for the period February 2002 – February 2008. In addition, 29
cross-profiles were made of the gorge for the same time interval, excluding February 2004.
The volume of sediment eroded and deposited was calculated for each date by
comparing variations in the height of the floor and banks of the gorge depicted in the
cross-profile, and estimating the volume of erosion and deposition for a given time
interval.
Analysis of the geomorphic variations for the period February 2002-February 2008,
shows that the banks formed by lahar deposits in 1997 and 2001 at the study site receded
from 48.6% to 27.4% in favor of the riverbed, and 31.0% to 52.7%, in favor of
the terraces formed by secondary lahars. The time sequence for the cross-profiles
depicts a general widening and infilling of the gorge floor, with greater emphasis on
erosion from September 2003-February 2008 (1523 m3as compared to 387 m3of
accretion).
The changes, however, were not homogeneous during the study period, due to varying
rainfall patterns that cause fluctuations in runoff. This relationship became evident after
comparing geomorphologic and topographic alterations and heavy precipitation
(maximum/24 hrs), and variations in monthly precipitation yield and average yield for the
period 1971-2000 (data provided by the Mexico’s National Meteorological Service).
Although both lower slope recession and lahar-induced deposition on the floor of the channel
increased during the period February-October 2002, down cutting and evacuation of materials
(677 m3) greatly surpassed deposition (145 m3). The predominance of one process in relation
to another was much more acute for this period than for any other, even though it
occurred during a relatively dry rainy season with below average precipitation.
The explanation for this is attributed to the onset of lahar activity in January 2001
and high maximum rainfall late in the rainy season. This was followed by a dry
spell and a second very wet rainy season from October 2002 - September 2003.
During this time, the slopes continued to recede in favor of the riverbed, where
some down cutting and infilling occurred, resulting in a slight net gain in surface
area. During the next five months of the dry season, changes in the landforms were
proportionally greater, producing a substantial increase in surface area and laharic deposition
formations. These features are attributed to an unusually long rainy season with above
average values in October, and to atypical precipitation in the normally dry month of
January.
Modifications in the surface area from February 2004-March 2006 reached 19.2%, a
decrease from the previous two-year period (28.1%), which was caused by a slight increase in
average precipitation in 2004 and a marked decrease in 2005. Values for erosion (620 m3)
and deposition (621 m3) are similar for the period from September 2003 - March 2005, and
the slopes receded slowly as new laharic deposits increased the level of the channel floor. The
final two-year interval shows an increase in changes (31.1%) where the receding sides of the
banks affected the ridges of the levees, and the floor of the channel, marked by
areas of deposition and incision, became flatter and wider. The increased activity is
associated with the prolonged effect of El Niño (ENSO), more rainfall during the
2006 rainy season, and below reference values for 2007 for the average wet and dry
seasons. The volume of material removed and accreted was 1424 m3 and 277 m3,
respectively.
Changes occurring in the channel to recover equilibrium in the river system included the
widening and infilling of the floor, but these alterations were largely conditioned by increased
water supply that triggered secondary lahars. The short interval between deposition by the
primary lahar and the exceptionally heavy precipitation at the end of the rainy season,
contributed to increased erosion of the gorge. The time intervals that registered below average
precipitation are characterized by less surface disturbances and greater infilling of the channel
floor. The opposite is true for the rainy seasons with above average values when slope
erosion caused uneven deposition on the channel floor, or during the dry months that
registered the most significant changes in response to episodes of exceptional heavy
rainfall.
References
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Popocatépetl volcano (Central Mexico): textural and sedimentological constraints on
their origin and hazards. Journal of Volcanology and Geothermal Research, 131:
351-369.
Gran, K. y Montgomery, D., 2005. Spatial and temporal patterns in fluvial recovery
following volcanic eruptions: Channel response to basin-wide sediment loading at Mount
Pinatubo, Philippines. GSA Bulletin, 117; 1-2: 195–211.
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