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Flood hazard map of the Becho floodplain, Ethiopia, using nonstationary frequency model

Tola Sintayehu Yadete, Shetty Amba

Acta Geophysica

2024

Vol. 72, no. 2

1079--1095

Flood estimates based on stationary flood frequency models are commonly used as inputs to flood hazard mapping. However, changing flood characteristics caused by climate change necessitate more accurate assessments of the probabilities of rare flood events. This study aims to develop a flood hazard map based on the nonstationary flood frequency using a generalized extreme value distribution model for the Becho floodplain in the upper Awash River basin. The distributional location parameter was modeled as a function of rainfall amount of different durations, annual total precipitation from wet days, yearly mean maximum temperature and time as covariates. The one-dimensional Hydrological Engineering Center River Analysis System (HEC-RAS) hydraulic model with steady flow analysis was used to generate flood hazard map input, depth and velocity, and inundation extent for different return periods. The result indicated that the model as a function of rainfall, such as monthly rainfall (August) and annual wet day precipitation, provided the best fit to the observed hydrological data. Rainfall as a covariate can explain the variation in the peak flood series. The developed hazard map based on depth alone and the combination of depth and velocity thresholds resulted in more than 70% of the floodplain area being classified as a high hazard zone under 2, 25, 50, and 100-years return periods. The current study assists water resource managers in considering changing environmental factors and an alternative flood frequency model for developing flood hazard management and mitigation strategies.

Przepływy i odpływy maksymalne w środkowej Polsce

Jokiel Paweł

Geography and Tourism

2016

Vol. 4, No. 2

7--21

In the article presented above the seasonal and multiannual variation and spatial differentiation of a few parameters were studied. These specifications were obtained on the basis of maximum discharges registered for 71 water-gauges in central Poland in the second half of the 20th century (1951-2002) (Table 1). In particular, seasonal and spatial regularities of maximum outflows, maximum discharges (WWQs and WWQs) and Françou-Rodier Flood Ratings (K) appearance were established. The greatest number of WWQs (WWQs) in central Poland was registered during the cold half-year (months: January and March) and in the summer (July and August). In autumn and early winter months (from September till December) maximum outflows were not registered. In central Poland the maximum unit runoffs are not very differentiated as they range from 23 to 228 dm3∙s-1 km-2. These values can be assumed as the representatives of the regional extremes. Regression equations between catchment surface areas and maximum discharges (unit runoffs) were also calculated - equation 1 (Fig. 4) and equation 2 (Fig. 6). In the catchments of the region, the maximum unit runoffs values (WWQ) decrease very slowly with increasing catchment areas (compare with Fig. 6). Spatial differentiation of Françou-Rodier Flood Indexes (K, equation 3) was under investigation. Moreover, the regional equation of the relationship between WWQ and K (equation 4, Fig. 8) was computed. Due to the Françou-Rodier Flood Indexes (K), the risk of precipitation and snowmelt/rainfall floods in the small and large central Poland river valleys is of minor importance. In general, some previously identified patterns, which describe the relationships between WWQ, WWQ, K and A in the global, continental and national scale are also visible considering the regional scale (central Poland area). The aim of this study was also to investigate the multiannual variability of the Flood Indexes (IWW K) – equation 5 and Fig. 9, which were calculated for central Poland area. In the second half of the 20th century the IWW indexes, which were determined in this region were continuously declining. It proves that the scale and the frequency of floods in the rivers of the particular area are getting reduced. The relationships and linkages between annual North Atlantic Oscillation Indexes (NAO) and Flood Indexes (WWQ) were also studied, and it can be stated that they are unclear and statistically insignificant (Fig. 10) and cannot be treated as simple statistical correlations.