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| Titel |
Influences on flood frequency distributions in Irish river catchments |
| VerfasserIn |
S. Ahilan, J. J. O'Sullivan, M. Bruen |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 16, no. 4 ; Nr. 16, no. 4 (2012-04-05), S.1137-1150 |
| Datensatznummer |
250013254
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| Publikation (Nr.) |
 copernicus.org/hess-16-1137-2012.pdf |
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| Zusammenfassung |
| This study explores influences on flood frequency distributions in Irish
rivers. A Generalised Extreme Value (GEV) type I distribution is recommended
in Ireland for estimating flood quantiles in a single site flood frequency
analysis. This paper presents the findings of an investigation that
identified the GEV statistical distributions that best fit the annual maximum
(AM) data series extracted from 172 gauging stations of 126 rivers in
Ireland. Analysis of these data was undertaken to explore hydraulic and
hydro-geological factors that influence flood frequency distributions. A
hierarchical approach of increasing statistical power that used probability
plots, moment and L-moment diagrams, the Hosking goodness of fit algorithm
and a modified Anderson-Darling (A-D) statistical test was followed to
determine whether a type I, type II or type III distribution was valid.
Results of the Hosking et al. method indicated that of the 143 stations with
flow records exceeding 25 yr, data for 95 (67%) was best represented by
GEV type I distributions and a further 9 (6%) and 39 (27%) stations
followed type II and type III distributions respectively. Type I, type II and
type III distributions were determined for 83 (58%), 16 (11%) and 34
(24%) stations respectively using the modified A-D method (data from 10
stations was not represented by GEV family distributions). The influence of
karst terrain on these flood frequency distributions was assessed by
incorporating results on an Arc-GIS platform showing karst features and using
Monte Carlo simulations to assess the significance of the number and
clustering of the observed distributions. Floodplain effects were identified
by using two-sample t-tests to identify statistical correlations between the
distributions and catchment properties that are indicative of strong
floodplain activity. The data reveals that type I distributions are spatially
well represented throughout the country. While also well represented
throughout the country, the majority of type III distributions appear in
areas where attenuation influences from floodplains are likely. The majority
of type II distributions appear in a single cluster in a region in the west
of the country that is underlain by karst but importantly, is characterised
by shallow of glacial drift with frequent exposures of rock outcrops. The
presence of karst in river catchments would be expected to provide additional
subsurface storage and in this regard, type III distributions might be
expected. The prevalence of type II distributions in this area reflects the
finite nature of this storage. For prolonged periods of rainfall, rising
groundwater levels will fill karst voids, remove subsurface storage and
contribute to recharge related sinkhole flooding. Situations where rainfall
intensities exceed karst percolation rates also produce high levels of
surface runoff (discharge related flooding) that can promote type II
distributions in nearby river catchments. Results therefore indicate that in
some instances, assuming type I distributions is incorrect and may result in
erroneous estimates of flood quantiles at these locations. Where actual data
follows a type II distribution, flood quantiles may be underestimated by in
excess of 35% and for type III distributions, overestimates by over 25% can occur. |
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