Bivariate Flood Frequency Analysis using Copula with Parametric and Nonparametric Marginals Subhankar Karmakar Slobodan P. Simonovic The University of Western Ontario The Institute for Catastrophic Loss Reduction
4th ISFD Toronto 2Simonovic May Presentation outline Introduction Objectives of the study Study area Data and flood characteristics Marginal distributions of flood characteristics Parametric and nonparametric estimation Joint and conditional distributions using copula Conclusions
4th ISFD Toronto 3Simonovic May Introduction Flood management (design, planning, operations) requires knowledge of flood event characteristics Flood characteristics Peak flow DurationVolume
4th ISFD Toronto 4Simonovic May Introduction
4th ISFD Toronto 5Simonovic May Introduction
4th ISFD Toronto 6Simonovic May Study objectives To determine appropriate marginal distributions for peak flow, volume and duration using parametric and nonparametric approaches To define marginal distribution using orthonormal series method To apply the concept of copulas by selecting marginals from different families of probability density functions To establish joint and conditional distributions of different combinations of flood characteristics and corresponding return periods
4th ISFD Toronto 7Simonovic May Study area Red River Basin 116,500 km 2 ( 89% in USA 11% in CDN) Flooding in the basin is natural phenomena Historical floods: 1826; 1950; 1997 Size of the basin and flow direction No single solution to the flood mitigation challenge
4th ISFD Toronto 8Simonovic May Study area - data Daily streamflow data for 70 years ( ) Gauging station ( ) - Grand Forks, North Dakota, US Location - latitude 47°55'37"N and longitude 97°01'44"W Drainage area - 30,100 square miles Contributing area - 26,300 square miles
4th ISFD Toronto 9Simonovic May Study area – flood characteristics Dependence between P, V and D P and V highly correlated All the correlations positive
4th ISFD Toronto 10Simonovic May Marginals - parametric
4th ISFD Toronto 11Simonovic May Marginals - nonparametric Nonparametric kernel estimation of flood frequency Orthonormal series method
4th ISFD Toronto 12Simonovic May Results
4th ISFD Toronto 13Simonovic May Results P follows gamma distribution (parametric) and V and D follow distribution function obtained from orthonormal series method (nonparametric) Mixed marginals
4th ISFD Toronto 14Simonovic May Results Second test
4th ISFD Toronto 15Simonovic May Copula An alternative way of modeling the correlation structure between random variables. They dissociate the correlation structure from the marginal distributions of the individual variables. n - dimensional distribution function can be written:
4th ISFD Toronto 16Simonovic May Copula
4th ISFD Toronto 17Simonovic May Results – joint distributions Peak flow - Volume
4th ISFD Toronto 18Simonovic May Results – joint distributions Volume - Duration
4th ISFD Toronto 19Simonovic May Results – joint distributions Peak flow - Duration
4th ISFD Toronto 20Simonovic May Results – conditional distributions
4th ISFD Toronto 21Simonovic May Results – conditional distributions
4th ISFD Toronto 22Simonovic May Results – conditional distributions
4th ISFD Toronto 23Simonovic May Results – return period
4th ISFD Toronto 24Simonovic May Conclusions Concept of copula is used for evaluating joint distribution function with mixed marginal distributions - eliminates the restriction of selecting marginals for flood variables from the same family of probability density functions. Nonparametric methods (kernel density estimation and orthonormal series) are used to determine the distribution functions for peak flow, volume and duration. Nonparametric method based on orthonormal series is more appropriate than kernel estimation.