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Stormwater drainage analysis and design in Victoria, Australia

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Victoria, Australia

Wallingford Software's asset business planning and urban drainage network modeling solution InfoWorks CS and its predecessor solutions have been used in Victoria, Australia for wastewater modeling since 1992. While dynamic modeling has been the standard planning and analysis platform for over a decade for wastewater networks, it is only in recent years that dynamic modeling of stormwater drainage systems has taken place on any scale.

The Infrastructure Group at Dalton Consulting Engineers primarily undertakes sewerage and stormwater master planning studies directly for Municipal and Water Authority clients. As client needs have become more involved, the group has provided an increasing amount of modeling support to the company's Land Development and Sporting groups with some new and innovative applications of the software.

In Victoria, most consultants and Councils still use the Rational Method in spreadsheet or static software packages to size stormwater drainage for greenfield developments. While this is a reasonably conservative design approach for sizing pipes for 1 in 5 year storm events, these static analysis systems become invalid when the pipes in a system become surcharged. This proves to be a major limitation when analyzing system performance from 1 in 100 year storms and taking account of overland flows.

Static versus dynamic

The Councils are generally aware of the limitations of static analysis and are interested in the benefits of dynamic analysis, but are cautious about it being applied as they do not have the capacity to undertake checks in the traditional manner.

There are a number of limitations to static analysis that make dynamic analysis very attractive. Static analysis is conservative, and takes no account of on-line storage. It struggles when pipes surcharge, fails when pits surcharge and cannot assess detention storage. Overland flows have to be assessed separately and a static analysis doesn't adequately demonstrate the mode or impact of failures.

In contrast, dynamic analysis, when applied to a stormwater network, allows the design to be optimized, and enables in and off-line storage to be readily assessed. This method can also analyze existing systems with multiple flow splits and convergences, deal with worst case scenarios and temporal storm patterns and enable storages to be sized. Further advantages are the ability to set-up and optimize pumps and, most importantly, provide long-term simulations as required under the WSUD (Water Sensitive Urban Design) procedures.

A number of case studies from projects around the state illustrate the advantages of dynamic analysis.

Flemington race course

At the Flemington race course, home of the world famous Melbourne Cup, the track was being protected from a 1 in 100 year flood risk from the nearby river by construction of a bund wall. This meant that existing gravity outlets from the stormwater system to the river would be sealed, and the stormwater would have to be pumped over the bund wall to the river.

The existing internal drainage network was modeled in InfoWorks CS with proposed new connections to redirect flows to the proposed pumping stations. In addition to confirming that the proposed pumping stations were adequately sized , the analysis identified the cause of a number of frequent flooding problems across the site.

Mornington race course

At Mornington race course, as part of a major facility upgrade, DCE designed a new storm water drainage system for the tracks to prevent flooding from up to a 1 in 2 year storm, as well as maximize the rainwater that could be harvested for track irrigation. The proposed drainage design was modeled in InfoWorks CS with a sump/pump/rising main designed to optimize rainwater capture from peak events. In addition an annual time series analysis was undertaken for rainwater harvesting analysis and dam sizing optimization.

Kensington industrial site

Another project at the Kensington industrial site related to an existing system with limited outfall capacity. The intention was to maximize the use of the existing infrastructure and provide sufficient storage and retention systems to cope with ten-year flows and 100-year overland flows. InfoWorks CS was used to design the underground stormwater drains, optimize the sizing of major inline storage pipes and a storage retention facility and confirm the adequacy of the 100 year overland flow paths.

Future challenges

Future challenges for development of the market in Victoria include:

  • Council asset inventories and attribute information are generally in need of significant effort to enable a model to be created; without owning the relevant software themselves, Councils are unable to verify results independently;
  • the results from dynamic analysis will show that the worst case situation occurs at different times, reflecting real life scenarios – but this is not what councils are used to seeing with static analysis tools.

DCE Infrastructure Manager Neil Moody says: “It is clear that with further urban density and redevelopment, the increases in runoff will have to be managed in the future. This will require more sophisticated analysis using dynamic solutions such as InfoWorks CS.”

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