Isopleths for Rainfall Data in Sabah

Isopleths are lines along which rainfalls have the same specified intensity. These isopleths form the basic information needed for the design of systems needed for roof drainage, urban drainage, water supply, irrigation and flood control. Hence they are very important for ‘water management'. For the design of roof drainage systems, rainfall data for short durations such as 2 minutes, 5 minutes, 10 minutes & 15 minutes are needed for return periods of 50 and 100 years. In Sabah State of Malaysia (Part of Borneo Island), from 1960 to 1984, only few stations were recording rainfall and that too only daily rainfalls. So, the minimum duration data available was only for 1 day. From 1985 onwards, the data for 5 minutes, 15 minutes, 30 minutes, 60 minutes & 120 minutes have been recorded along with daily rainfall. The available rainfall data is only for 19 years from 1985 to 2003. Based on these available data, rainfall data for longer return periods such as upto 100 years are predicted using multiple regression analysis of exponential type. The equation used is P = KT^at^bT₁^cP₁^d where P is the rainfall in mm, T is the return period of P in years, t is the duration of P in minutes, T₁ is the return period of P₁ in years, P₁ is the daily rainfall in mm and K, a, b, c, d are constants. After completing the Intensity — Duration — Frequency (IDF) curves for 52 stations & using these data, isopleths for storms of duration 5 minutes for return periods of 20 years, 50 years & 100 years are plotted. Using these isopleths, it would be possible to estimate the design rainfall for any location in Sabah.