Journal of the Japan Society of Erosion Control Engineering, Vol.60,No.2,2007
Probabilistic flood discharge analysis in small]scale river basins in Hokuriku
region
Hiroaki NAKAYA
Abstract
Examination and estimation of design flood discharges in upper river basins are
critical for better river and erosion control planning. Annual maximum flood series
were analyzed by 13 fitting methods in terms of SLSC goodness]of]fit criterion
at 30 small]scale [mostly less than100 km2] dam reservoirs in Hokuriku region
where inflows are influenced by snow]melting but deemed unregulated. The inflow
discharge data belong to the same hydrological regime and show mutual independency
and proper randomness as a set of statistical sample. The analysis of fitting
methods show that log Pearson Type III distribution is the best common probabilistic
distribution for the sample annual maximum flood series in the region, which allowed
us to apply regional flood frequency analysis method described in the U.S. Water
Resource Council guideline Bulletin 17B. 30 dam reservoirs are located within
about one]hundred]mile radius and their flood series are pooled for the regional
flood frequency analysis. Confidence intervals were estimated to become significantly
smaller as regional information was incorporated than when estimated separately
at each site. Concurrent comparative analysis by an index]flood method and a scaling
method indicated that higher statistics such as skew coefficient at each observing
station are indispensable in addition to mean, standard deviation, and coefficient
of variation for further improvement of the estimation. Bulleting 17B method,
which adjusts site skew by regional information, seems promising to apply to small]scale
non]regulated river basins in Hokuriku region. Further research is necessary to
clarify how river basin characteristics are related to site skew in maximum annual
flood series.
Key wordsFregional flood frequency analysis, Bulletin 17B, annual maximum floods,
dam inflow
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