The Clarence River Valley in the north coast region of N.S.W. has a long history of flooding, especially at Grafton. In recent years, the urban areas of Grafton and South Grafton have been protected from moderate floods by the construction of levee banks. Raising of the levees to higher levels has been suggested in previous studies. As part of the investigations relating to the levee bank proposals, the Public Works Department (PWD) commissioned Weatherex Meteorological Services Pty. Ltd. in September, 1983 to estimate the probable maximum precipitation for the Clarence River catchment.

The objective of the study was to estimate the probable extreme storm precipitation over the Clarence River catchment upstream from Grafton for periods up to 5 days. The Terms of Reference for the study, as set down by the Department, covered the following points:

• Values of estimated maximum precipitation are to be derived by synoptic methods of storm maximisation, using both in-situ and transposed storms.
• In addition, the generalised method developed by the Bureau of Meteorology is to be applied, using specific catchment adjustments for the Clarence system. For this purpose, the Clarence catchment is considered to be comparable with the Queensland Coastal Zone.