Melbourne Water engaged Alluvium to investigate the stormwater management infrastructure required to meet the Development Services Schemes (DSSs) requirements for the Sunbury growth area.

A challenge in delivering the DSSs in the study area was its physical form. The majority of greenfield development within the Port Phillip and Westernport region has been on the relatively flat plains, in contrast, the geological features of the study area include a basalt cap over highly erodible marine sediments punctuated by remnant volcanic cones, deep river valleys and steep basalt escarpments.

The drainage network is highly fragmented and dominated by steep tributaries that support a plethora of geomorphic and ecological values, including rare River Styles classifications and threatened flora and fauna communities. With many of these tributaries highly erodible. The erosion risk of the waterways and loss of values under post-development conditions is high. The creation of new urban developments in this landscape presents a significant engineering challenge, and Melbourne Water’s business as usual approach was identified as being insufficient to meet their waterway protection objectives.

These fragile waterways would be unable to cope with the increased runoff from the new developments, and a ‘business as usual’ design would have led to significant erosion and sediment export to Jacksons and Emu Creek, which pose risks to habitat and water quality and would be costly for Melbourne Water or Council to address retrospectively.

Alluvium responded with an approach that had not previously been used for DSS planning. The industry standard, ‘business as usual’, approach based on peak flow management has been advanced with the inclusion of the deterministic, Erosion Potential Index analysis, which considers flow patterns and landscape challenges unique to a waterway. By enabling a tailored, waterway-by-waterway approach that takes into account the local catchment characteristics, waterways in the Emu and Jackson Creek have been protected from channelisation and accelerated erosion. By rising to the challenges, rather than simply relying upon ‘past best practice’, an innovative approach to manage urban flow patterns in a way that protects fragile waterways has been developed. The proof of concept not only provides waterway managers with confidence in the feasibility and cost of this type of sustainable drainage system, and demands other designs to seek out opportunities to go beyond best practice in stormwater management.