Application of peaks over threshold method in flood frequency analysis in Seyhan Basin

Taşkın frekans analizi; nehirlerdeki belirli tekerrürlere veya olasılıklara karşı gelen akım miktarlarını elde edebilmek için kullanılan bir yöntemdir. Bu yolla hidrolik yapıların projelendirilmesi ve ekonomik olarak değerlendirilmesi sağlanmaktadır. Bu çalışmada Seyhan Havzası’nda yer alan 1801 no’lu istasyon için eşik üstü pik değerleri kullanılarak taşkın frekans analizi gerçekleştirilmiştir. Eşik değer; eşik değer seçme grafiği, ortalama kalıntı ömrü grafiği, yayılım indeksi grafiği ile ortalama ve standart sapmaya bağlı yöntem olmak üzere dört farklı yöntemle belirlenmiş ve elde dilen eşik üstü piklerin bağımsız olup olmadıkları otokorelasyon analizi ile sınanmıştır. Bir sonraki adımda, kısmi süreklilik serilerinde sıklıkla kullanılan Genelleştirilmiş Pareto dağılımı uygulanarak gözlenmiş değerlere uygunluğu sınanmıştır. Genelleştirilmiş Pareto dağılımının parametrelerinin ve hesaplanan 2, 5, 10, 25, 50, 100, 200, 500 ile 1000 yıl tekerrürlü taşkın debileri üzerinden güven aralıklarının belirlenmesiyle belirsizlik analizi yapılmıştır. Son olarak farklı tekerrürler için hesaplanan taşkın debilerinin, yıllık maksimum değerler üzerinden elde edilen aynı tekerrürlü taşkın debileri ile karşılaştırılması gerçekleştirilmiştir. Elde edilen sonuçlara göre eşik üstü piklerin analizi ile elde edilen taşkın tahminlerinin yıllık maksimumların analizi ile elde edilen taşkın tahminlerine kıyasla düşük tekerrür aralıklarında daha büyük farklar gösterdiği, büyük tekerrürlerde ise (500 ve 1000 yıl) nispeten yakın sonuçlara ulaşıldığı görülmüştür.Flood frequency analysis is a statistical method that is used to obtain the amounts of flow corresponding to certain recurrences or probabilities in rivers. In this way, the design and economic evaluation of hydraulic structures is provided. In this study, flood frequency analysis was performed for the station numbered 1801 and located in Seyhan Basin by using peak values over an assigned threshold. Threshold value was determined through four different methods that include threshold selection plot, mean residual life plot, dispersion index plot as well as the method based on the mean and standard deviation; and autocorrelation analysis has been tested to determine whether the peaks obtained above the threshold are independent. Generalized Pareto distribution which is frequently used in partial duration series was applied and tested against the historically observed series. Uncertainty analysis was performed by determining the parameters of the Generalized Pareto distribution and the confidence intervals over the calculated flood flows of 2, 5, 10, 25, 50 100, 200, 500 and 1000 years. Finally, flood quantities with the recurrence intervals of 2, 5, 10, 25, 50, 100, 200, 500 and 1000 years were computed and compared to the corresponding quantities obtained from the annual maximum series. According to the results, it was observed that, the differences between the flood estimates obtained by peaks over threshold method and the analysis of annual maximums were bigger for the smaller recurrence intervals and the differences were relatively small for large recurrence intervals (500 and 1000 years)

Estimating flood exposure potentials in Turkish catchments through index-based flood mapping

Flooding is widely believed to be the most common natural disaster in Europe, and the changing climatic conditions are estimated to increase its adverse impacts. Effective flood strategies require thorough consideration of the factors underlying the flood generation mechanism and a widened display of mitigation priorities for spatially exhaustive assessments. Flood potential maps generated herein for indicating potential flood areas prove to be among powerful tools for comprehensive flood assessments. In the presented study, a countrywide characterization is achieved in this context by analyzing catchment units, which constitute the river basin systems in Turkey, through a series of spatial indices adapted from different factors effective in flood generation. The study aims to contribute to depicting priorities for in-depth flood assessments and to the re-orientation of subsequent control measures. The flood potential maps obtained for river catchments and designating individual locations with comparably higher flood potentials are expected to set light to the selection of case studies for local flood research in Turkey while contributing to decision making and policy implementation on flood control at the macroscale