Derivation of improved initial and continuing losses in design flood estimation for NSW Australia

Flood is one of the worst natural disasters that cause significant economic damage and loss of lives. Flood estimation is often required in the design and safety assessment of water infrastructure like bridges, culverts and flood control levees. Rainfall runoff model is often used in design flood estimation, which needs various inputs such as rainfall and catchment characteristics including rainfall duration, rainfall intensity, rainfall temporal patterns and losses. Loss is defined as the amount of precipitation in a rainfall event that does not appear as direct surface runoff at the stream gauge. The objective of this paper is to derive improved initial and continuing loss values using data from selected catchments in New South Wales (NSW). In this paper, a total of 253 rainfall runoff events are selected from five NSW catchments. From the analyses, the median initial loss has been found to be 17 mm which is closer to the lower limit of the Australian Rainfall and Runoff (ARR) 1987 recommended value of 10-35 mm. The median continuing loss value has been found to be 0.94 mm/h which is 62% lower than the ARR recommended value of 2.5 mm/h. The resulting design floods from the observed loss values are likely to be higher than those obtained from the ARR 1987 recommended loss values.