Tweed Shire Council was one of the first Councils in New South Wales to undertake flood studies for the purposes of defining planning controls and determining the impact of potential filling and development. This Tweed Valley Flood Study is the first key stage in the floodplain risk management process as outlined in the New South Wales Floodplain Development Manual. The key outputs of the study, including a 2D hydrodynamic model and design flood levels, depths, velocities and flows across the floodplain, will form the basis for identifying and assessing floodplain management options as part of the subsequent Tweed Valley Floodplain Risk Management Study and Plan.

The main arm of the Tweed River flows for approximately 50 km in a general north-easterly direction through the towns of Murwillumbah (about 28 km upstream) and Tweed Heads (at the mouth) and past the villages of Condong, Tumbulgum, Chinderah and Fingal. The catchment comprises approximately 1100 km2 with the main tributaries including Oxley River, Rous River, Dunbible Creek and the Terranora and Cobaki Broadwaters. The river flows to the sea immediately south of Point Danger, close to the border with Queensland.

The study area covers approximately 230 km2 of the floodplain, including the Tweed River downstream of Byangum, the Rous River downstream of Boat Harbour, and the lower reaches of the Broadwater tributaries. The valley comprises a wide floodplain of alluvial sediments contained by higher ground of bedrock.

The townships of Murwillumbah, Condong, Tumbulgum, Chinderah, Tweed Heads and Tweed Heads South have frequently experienced inundation from floodwaters. The February 1954 flood, the largest flood on record, caused major inundation in all flood prone regions. Severe flooding was experienced in the areas downstream of Chinderah due to a combination of the ocean storm tide, a congested entrance, and flooding from catchment runoff.

A system of levees currently protects the main townships of Murwillumbah and Tweed Heads South from the more frequent floods. Other flood mitigation measures such as the installation of floodgates on creeks and farm drains, the raising of the natural levee bank in some areas, and the construction of drainage systems have also been undertaken.

Numerous flood studies were undertaken throughout the catchment from the late 1970s onwards, based on 1D hydraulic modelling of flood behaviour. In 2005, the first edition of the Tweed Valley Flood Study was published, which was the first study to incorporate fully 2D hydraulic modelling of the entire floodplain from urwillumbah to Tweed Heads. This 2009 update of the Tweed Valley Flood Study was undertaken primarily to incorporate much improved catchment topography (based on 2007 ALS data). The opportunity was also taken to update both the hydrologic and hydraulic models to reflect advances in methodology and model development in the intervening four year period.

A DEM was developed for the whole catchment based on the new ALS data together with bathymetric data from the previous 2005 Flood Study. WBNM hydrologic and TUFLOW hydraulic models were developed and jointly calibrated to the March 1974 flood, and verified against the March 1978 and April 1989 floods. The models were then used to simulate a range of design events for existing catchment conditions. The 5, 20, 100 and 500 year ARI, together with an ‘extreme’ and PMF event, were simulated for a 36 hour duration storm (the critical storm duration for the Tweed River at Murwillumbah). Catchment inflows and runoff were combined with downstream ocean and storm surge levels adopted in consultation with DECC. The 100 year ARI design flood for the catchment was adopted based on the maximum ‘envelope’ of two scenarios:

• 5 year ARI rainfall + 100 year ARI storm surge; and
• 100 year ARI rainfall + 20 year ARI storm surge.

Both digital and hard copy maps have been generated of modelled flood levels, depths, velocities and flows across the range of design events for the future purposes of floodplain management and development planning.