Typhoon Morakot struck central and southern Taiwan on August 8, 2009. The high intensity and accumulation of rainfall induced floods, landslides and debris flows. During Typhoon Morakot, the flood level was beyond the design limit of many river embankments in southern Taiwan. It is, therefore, desirable to estimate the peak flow-discharge for similar extreme rainfall events in future. In this study, the destructive flood in the Gao-ping river in southern Taiwan, caused by Typhoon Morakot, was selected as an area for case analysis. First, hydrological and geomorphological data for historical typhoons together with rainfall data were collected and analyzed. Next, the rational method, combined with a number of equations, was proposed to estimate peak flow-discharges in the river produced by extreme rainfall events. The estimated peak flow-discharges of historical events were then compared with the observation data acquired from stream gauge stations and previous studies. The results showed that the proposed method is able to estimate the flow-discharge in the Gao-ping river with reasonable accuracy if the flow rate of interest is greater than 5000 cms or the return period of interest is more than 100 years. It could, therefore, be used to determine the peak flow-discharge for extreme rainfall events.
extreme rainfall events, peak flow-discharge, rational method, Typhoon Morakot
 Kuichling, E. & Hering, R., TheRelation Between the Rainfall and the Discharge of Sewers in Populous District, American Society of Civil Engineers: New York, 20, pp. 1–56, 1889.
 Watts, L.F. & Hawke, R.M., The effects of urbanization on hydrologic response: a study of two coastal catchments. Journal of Hydrology, 42(2), pp. 125–143, 2003.
 OPW, Flood StudiesUpdate Programme:Flood Estimation in Small and Urbanized Catchments, Hydrology and Coastal Section, Office of Public Works (OPW), 2012.
 NYS, Standards and Specifications for Erosion and Sediment Control, New York State(NYS),2004.
 Debo, T.N. & Reese, A., Municipal Storm Water Management, p. 209, 1995.
 QG, Queensland Urban Drainage Manual, Vol. 1, 2nd ed., Queensland Government (QG), 2007.
 SWCB, Technical Handbook of Soil and Water Conservation, Soil and Water Conservation Bureau (SWCB), Council of Agriculture, Executive Yuan, Taiwan, 2003.(in Chinese)
 Chen, J.C., Jan, C.D.&Lee, M.S., Reliability analysis of design discharge for mountainous gully flow.Journal of Hydraulic Research, 46(6), pp. 835–838, 2008. http://dx.doi.org/10.3826/jhr.2008.3173
 Hays, D.C.& Young, R.L., Comparison of peak discharge and runoff characteristicestimates from the rational method to field observations for small basins in central Virginia. USGS Scientific Investigations Report, 2005–5254, 2006.
 Nyman, D., Hydrology Handbook forConservation Commissions, MassachusettsDepartment ofEnvironmental Protection, 2002.
 Chow, V.T., Maidment, D.R.&Mays, L.W., Applied Hydrology. McGraw-Hill: Singapore, 1988.
 Chen, J.C., Jan, C.D. &Huang, W.S., Characteristics of rainfall triggering of debris flows in the chenyulanwatershed, Taiwan.Natural Hazards and Earth System Sciences,13, pp. 1015–1023, 2013. http://dx.doi.org/10.5194/nhess-13-1015-2013
 Horner, W.&Flynt, F., Relation between rainfall and runoff from small urban areas. Trans.ASCE,101,pp. 104–183, 1936.
 WRA, Technique of Hydrological Observation and Criteria of Hydrological Analysis, Water Resources Agency (WRA), Ministry of Economic Affairs, Taiwan, 2011(in Chinese)
 Chin, D.A.,Water-Resources Engineering, Prentice-Hall, 2000.
 WRA, Planning on Gao-ping river treatment, Lab. of Hydraulic planning, Water Resources Agency (WRA), Ministry of Economic Affairs, Taiwan, 2008(in Chinese)
 WRA, Analysis on extreme rainfall and flow discharge in Typhoon Morakot, Water Resources Agency (WRA), Ministry of Economic Affairs, Taiwan, 2009(in Chinese)
 Chen, J.C. & Chuang, M.R., Discharge of landslide-induced debris flows: case studies of Typhoon Morakot in Southern Taiwan.Natural Hazards and Earth System Sciences, 14, pp. 1719–1730, 2014. http://dx.doi.org/10.5194/nhess-14-1719-2014
 SEC, Investigation on the 88 flood, Sinotech Engineering Consultants (SEC), Inc., Taiwan, 2009 (in Chinese)