Nejc Bezak

Comparison between the peaks-over-threshold method and the annual maximum method for flood frequency analysis

Abstract Flood frequency analysis can be made by using two types of flood peak series, i.e. the annual maximum (AM) and peaks-over-threshold (POT) series. This study presents a comparison of the results of both methods for data from the Litija 1 gauging station on the Sava River in Slovenia. Six commonly used distribution functions and three different parameter estimation techniques were considered in the AM analyses. The results showed a better performance for the method of L-moments (ML) when compared with the conventional moments and maximum likelihood estimation. The combination of the ML and the log-Pearson type 3 distribution gave the best results of all the considered AM cases. The POT method gave better results than the AM method. The binomial distribution did not offer any noticeable improvement over the Poisson distribution for modelling the annual number of exceedences above the threshold. Editor D. Koutsoyiannis Citation Bezak, N., Brilly, M., and Šraj, M., 2014. Comparison between the peaks-over-threshold method and the annual maximum method for flood frequency analysis. Hydrological Sciences Journal, 59 (5), 959–977.

Analyses of suspended sediment loads in Slovenian rivers

ABSTRACT Suspended solids are present in every river, but high quantities can worsen the ecological conditions of streams; therefore, effective monitoring and analysis of this hydrological variable are necessary. Frequency, seasonality, inter-correlation, extreme events, trends and lag analyses were carried out for peaks of suspended sediment concentration (SSC) and discharge (Q) data from Slovenian streams using officially monitored data from 1955 to 2006 that were made available by the Slovenian Environment Agency. In total more than 500 station-years of daily Q and SSC data were used. No uniform (positive or negative) trend was found in the SSC series; however, all the statistically significant trends were decreasing. No generalization is possible for the best fit distribution function. A seasonality analysis showed that most of the SSC peaks occurred in the summer (short-term intense convective precipitation produced by thunderstorms) and in the autumn (prolonged frontal precipitation). Correlations between Q and SSC values were generally relatively small (Pearson correlation coefficient values from 0.05 to 0.59), which means that the often applied Q–SSC curves should be used with caution when estimating annual suspended sediment loads. On average, flood peak Q occurred after the corresponding SSC peak (clockwise-positive hysteresis loops), but the average lag time was rather small (less than 1 day). Editor M.C. Acreman; Associate editor Y. Gyasi-Agyei

Climate variability impact assessment on the flood risk in Slovenia

Abstract Assessment of climate change or climate variability is a significant topic in most geophysical disciplines. In this study, the flood frequency approach was selected to analyze changes in flood series. Discharge data from 55 gaging stations in Slovenia were used. The annual maximum method was applied to define the samples. The data sets were divided into 30-year periods based on a 10-year moving window. For each part of the data-set, the flood frequency analysis was performed. Changes in the estimated design discharge values with a 10-year return period, which is commonly used in engineering design, were observed for the two selected 30-year periods, namely 1961–1990 and 1981–2010, and the results were compared with the Mann–Kendall (MK) test. The results indicate that no uniform pattern can be found in the differences between estimated design discharge values for the two selected periods. The same applies to the MK trend test results, which were positive and statistically significant with the chosen significance level of 0.05 only for approximately 5% of stations. However, our comparison of the results of the flood frequency analyses among different 30-year periods showed considerable changes in the design discharge for some stations.

Analysis of flood events in Slovenian streams

Abstract The detailed analysis of individual flood event elements, including peak discharge (Q), flood event volume (V), and flood event duration (D), is an important step for improving our understanding of complex hydrological processes. More than 2,500 flood events were defined based on the annual maximum (AM) peak discharge from 50 Slovenian gauging stations with catchment areas of between 10 and 10,000 km2. After baseflow separation, the stations were clustered into homogeneous groups and the relationships between the flood event elements and several catchment characteristics were assessed. Different types of flood events were characteristic of different groups. The flashiness of the stream is significantly connected with mean annual precipitation and location of the station. The results indicate that some climatic factors like mean annual precipitation and catchment related attributes as for example catchment area have notable influence on the flood event elements. When assessing the dependency between the pairs of flood event elements (Q, V, D), the highest correlation coefficients were obtained for the Q-V pair. The smallest correlations or no correlations were observed between the Q and D variables.

TXT-tool 2.386-1.1 Intensity-Duration-Frequency Curves for Rainfall-Induced Shallow Landslides and Debris Flows Using Copula Functions

Geological hazards such as debris flows or deep-seated landslides can cause major economic damage and endanger human lives. In order to warn people living in a hazardous area during extreme meteorological events that may trigger debris flows or landslides, in many parts of the world different early warning systems (EWS) have been developed and installed. Empirical rainfall thresholds can be applied as a part of advanced early warning systems. This study combines two concepts: (i) empirical rainfall thresholds and (ii) intensity-duration-frequency (IDF) curves, that both include information about rainfall duration and rainfall intensity. The IDF curves were constructed using a copula function. Post-event analysis of several extreme events in Slovenia, Europe, in the last 25 years was carried out. The triggering conditions responsible for the initiation of the extreme events were investigated, and several empirical rainfall thresholds were tested. It was found that different meteorological conditions were responsible for triggering deep-seated and shallow landslides. A comparison between empirical rainfall thresholds and IDF curves indicated that some thresholds correspond to a return period significantly smaller than two years. If the IDF curves are incorporated into the early warning systems, this will mean that warnings can be issued with a corresponding probability.

Rainfall erosivity in Slovenia: Sensitivity estimation and trend detection

ABSTRACT Slovenia is one of the EU countries with the largest values and largest amounts of variability in rainfall erosivity, with maximum annual values exceeding 10,000 MJ mm ha‐1 h‐1 yr‐1. Five‐minute rainfall data was analysed from 10 Slovenian rainfall stations with data‐length availability longer than 25 years with a maximum data length of 69 years and a total data‐station length equal to 443 years. Trends in the rainfall erosivity R‐factor were detected for four different sub‐samples using monthly, half‐year, and annual rainfall erosivity values. The results indicate that rainfall erosivity trends for the selected Slovenian stations are mostly statistically insignificant, with the selected significance level of 0.05. However, a larger share of identified trends are positive than negative. The maximum annual rainfall erosivity values were obtained for one specific mountain station. Furthermore, a sensitivity analysis regarding the rainfall erosivity factor R calculation showed that the rainfall threshold parameter (12.7 mm) that is used to remove the small‐magnitude rainfall events in order to reduce the computational burden can attribute up to 10% of the average annual R‐values in cases where this threshold is not used. Other parameters have, on average, a smaller impact on the calculated rainfall erosivity. Furthermore, the application of local kinetic energy equations resulted in, on average, about 20% higher annual rainfall erosivity values compared to the equation that is proposed by the Revised Universal Soil Loss Equation (RUSLE) manual and was not developed specifically for this region. HighlightsRainfall erosivity for 10 rainfall stations located in Slovenia was evaluated.Rainfall erosivity trends are mostly positive while mostly statistically insignificant.The maximum rainfall erosivity is obtained under Alpine climate.Monthly rainfall erosivity was computed.Sensitivity analyses were carried out.

Does it really rain more often on weekends than on weekdays? A case study for Slovenia

The article presents the results of precipitation and aerosol (PM 10 ) data analyses in Slovenia. Analyses of rainfall data for some areas of Slovenia, such as the Ljubljana Basin, Zasavje, Salek Valley, Celje Basin, and the coastal area, suggest that rain on the weekend is more frequent than on weekdays; nevertheless, these deviations are not statistically significant. A comparison of three separate decades showed that the pattern of weekly rainfall in the 21st century is different than that in the last period of the 20th century. The weekly cycle of PM 10 for the considered stations is quite similar to that of some of the more contaminated regions of the world. Furthermore, the connection between PM 10 and precipitation in most of the analyzed cases was statistically significant.