Noosa River Entrance Channel Dynamics

Abstract

Noosa has become a tourist mecca that attracts large numbers of interstate and international tourists. These visitors are largely attracted by the climate and beaches that have proven popular with a variety of beach users. Noosa Main Beach is located between the Noosa headland and the Noosa River entrance. The river entrance was trained in 1978 with a single rock jetty, and a rock groyne created midway between the river entrance and headland in 1983. Erosion of Noosa Main Beach has been an ongoing problem, with historical records indicating that storm events have caused cutback of the beach for as long as historical records have been maintained. Prior to the mid 1960s though, the beach and river system essentially remained in a state of dynamic equilibrium. The sand dune was able to accommodate storm cutback and the beach seemed to respond without any major concern. However, large scale development witnessed in the mid 1960s led to modification of the beach system and resulted in the construction of the protective rock seawall, training of the river entrance and construction of the rock groyne. The seawall and rock groyne failed to address the erosion concerns and resulted in the formation of a new control point for the beach and river system. Beach nourishment activities have been performed in recent times in an attempt to maintain a useable beach. However, the modifications to the beach system and atypical weather conditions in recent years have resulted in a beach that remains in a severely eroded state for the majority of the time. Erosion concerns have led to the placement of approximately 40 000 m3 of nourishment sand being placed on the beach annually. The sand reserves from the ‘inactive’ zone adjacent to Noosa Sound have almost been exhausted and restrictions have been imposed by the Environmental Protection Agency (EPA) on dredging locations due to the existence of a fish habitat on the northern side of the entrance. It is now proposed that nourishment sand be obtained from the ‘active’ zone consisting of tidal deltas and deltas inside the river entrance. A number of solutions to the beach erosion problem have been investigated, including extension of the rock groyne, realignment of the seawall and construction of an artificial reef. All these options have been rejected for a variety of reasons and nourishment has continued in order to maintain a useable beach. This report focuses on the behaviour of nourishment sand entering the system and the likely consequences should the proposed new dredging scheme proceed. It has been concluded in view of the findings that continued nourishment is not a suitable beach management strategy and that dredging of the active zone will be of limited effectiveness and poses an unacceptable risk to the fish habitat on the northern side of the river entrance. More available data and better continuity of data is required before an accurate conceptual model of longshore transport and delta response can be developed. Once this is available, detailed modelling of the beach and river system will provide a more thorough understanding of the nourishment sand transport processes and the long-term consequences of the proposed dredging activities. These factors will then assist in determining an effective and longer-term beach management strategy for the region.