Melbourne Water engaged DCE to assess options for the Mittagong Creek development services strategy (DSS).

The goal of the project is to ensure that development associated with the Wallan Structure Plan is protected from flooding both overland and from Wallan and Mittagong Creeks. A complex two-dimensional hydrodynamic modelling approach (TUFLOW modelling) was selected to accurately simulate the complicated hydraulics of the area that includes the confluence of Wallan and Mittagong Creeks. Detailed flood modelling of Wallan township downstream of the Mittagong Creek DSS was undertaken to Melbourne Water standards as per the Guidelines and Technical Specifications (November 2018).

The existing Melbourne Water RORB model was updated by DCE industry best practice standards, to produce outputs in accordance with Australian Rainfall & Runoff 2016. This process was undertaken in collaboration with Melbourne Water to be sure that the adopted hydrological inputs were considered suitable by all stakeholders. Peak flows in Wallan and Mittagong Creeks have been defined based on ARR 2016 methodology.

TUFLOW modelling was undertaken for four (4) different concept designs for the DSS. The concept designs considered the complex location of a Yarra Valley Water sewer pump station and the requirement to protect the asset from flood events up to and including the 1% AEP event. Alternative locations were considered for:

• SPS asset
• Wallan and Mittagong Creek constructed waterways
• Stormwater quality treatment.

Rigorous and comprehensive sensitivity checking was conducted on modelling inputs and outputs to provide certainty to Melbourne Water regarding actual on-ground conditions. Flood mapping downstream of the DSS has been completed to provide advice to an urban growth area on the fringe of the Wallan Township. The preferred solution for the DSS has been optimised to limit the amount of infrastructure required to protect development.