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| Titel |
Geomorphological method in the elaboration of hazard maps for flash-floods in the municipality of Jucuarán (El Salvador) |
| VerfasserIn |
C. Fernández-Lavado, G. Furdada, M. A. Marqués |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Science ; 7, no. 4 ; Nr. 7, no. 4 (2007-07-18), S.455-465 |
| Datensatznummer |
250004655
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| Publikation (Nr.) |
 copernicus.org/nhess-7-455-2007.pdf |
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| Zusammenfassung |
| This work deals with the elaboration of flood hazard maps. These maps
reflect the areas prone to floods based on the effects of Hurricane Mitch in
the Municipality of Jucuarán of El Salvador. Stream channels located in
the coastal range in the SE of El Salvador flow into the Pacific Ocean and
generate alluvial fans. Communities often inhabit these fans can be affected
by floods. The geomorphology of these stream basins is associated with small
areas, steep slopes, well developed regolite and extensive deforestation.
These features play a key role in the generation of flash-floods. This zone
lacks comprehensive rainfall data and gauging stations. The most detailed
topographic maps are on a scale of 1:25 000. Given that the scale was not
sufficiently detailed, we used aerial photographs enlarged to the scale of
1:8000. The effects of Hurricane Mitch mapped on these photographs were
regarded as the reference event. Flood maps have a dual purpose (1)
community emergency plans, (2) regional land use planning carried out by
local authorities. The geomorphological method is based on mapping the
geomorphological evidence (alluvial fans, preferential stream channels,
erosion and sedimentation, man-made terraces). Following the interpretation
of the photographs this information was validated on the field and
complemented by eyewitness reports such as the height of water and flow
typology. In addition, community workshops were organized to obtain
information about the evolution and the impact of the phenomena. The
superimposition of this information enables us to obtain a comprehensive
geomorphological map. Another aim of the study was the calculation of the
peak discharge using the Manning and the paleohydraulic methods and
estimates based on geomorphologic criterion. The results were compared with
those obtained using the rational method. Significant differences in the
order of magnitude of the calculated discharges were noted. The rational
method underestimated the results owing to short and discontinuous periods
of rainfall data with the result that probabilistic equations cannot be
applied. The Manning method yields a wide range of results because of its
dependence on the roughness coefficient. The paleohydraulic method yielded
higher values than the rational and Manning methods. However, it should be
pointed out that it is possible that bigger boulders could have been moved
had they existed. These discharge values are lower than those obtained by
the geomorphological estimates, i.e. much closer to reality. The flood
hazard maps were derived from the comprehensive geomorphological map. Three
categories of hazard were established (very high, high and moderate) using
flood energy, water height and velocity flow deduced from geomorphological
and eyewitness reports. |
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