Effect of Data Length to the Consistency of Design Rainfall

Abstract

Design rainfall is one of the hydrology quantities that can be used for analyzing design flood. This discharge can then be applied to design and manage civil engineering structures such as drainage channels in urban areas. Generally design rainfall is analyzed using maximum daily rainfall data with a minimum data length of 10 years obtained from a rainfall observation station. In various references, the use of rainfall data with a longer range is highly recommended. This paper intends to examine the use of variations in the length of rainfall data on the consistency of design rainfall. Rainfall data from the Mutiara Meteorological Station of Palu, Central Sulawesi (1990-2019) are grouped into five categories based on the length of the data: A (10 years), B (15 years), C (20 years), D (25 years) and E (30 years). Frequency analysis was performed to predict the design rainfall of each data group using four distribution methods: Normal, Normal Log, Gumbel and Log Pearson III. Design rainfall consistency is measured based on design rainfall deviations which are calculated using the longest data. The analysis shows that the consistency of design rainfall in the study area at various return periods for all statistical distribution tends to increase in proportion to the length of the data with high category. This consistency value indicates that the 10-year rainfall data is still sufficient for frequency analysis.