The Clarence River Valley in the north coast region of NSW 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. The levee bank is expected' to provide protection up to approximately the 1 in 100 year level at Grafton, and the 1 in 20 year level at South Grafton. Raising of the levees to higher levels has been suggested in previous studies (Ref. 7). As part of the investigations relating to the levee bank proposals, the Public Works Department (PWD) commissioned Willing &Partners Pty. Ltd. In February 1984 to estimate the maximum probable flood at Grafton.

The objective of the runoff study was to determine the maximum probable flood hydrograph at Grafton by runoff routing methods, using probable maximum precipitation (PMP) data supplied by Weatherex Pty. Ltd.

This report describes the studies undertaken to fulfill the requirements of the Terms of Reference, and presents the conclusions and results reached in this study. The report includes descriptions of the rainfall-runoff model used, and the procedures adopted to set up, calibrate and verify the model for the Clarence River catchment area. This work involved detailed site inspections and a review of previous hydrological studies of the catchment in order to ensure that the model gave a realistic representation of actual catchment behaviour.