Complying with stormwater quality targets using music

In 2009, Brookfield Multiplex was contracted to develop and construct the Hilton Hotel and Residences: the 33-level Boulevard Tower and 57-level Orchid Tower at Hilton, Surfers Paradise on the Gold Coast.

To comply with stringent Gold Coast City Council (GCCC) stormwater quality requirements, the development had to meet set targets of a reduction in pollutants of 80% Total Suspended Solids (TSS), 45% Total Phosphorous (TP), 60% Total Nitrogen (TN) and 90% Gross Pollutants.
 
Brookfield Multiplex engaged multi-national advisory and design consultancy Hyder Consulting Pty Ltd to perform structural engineering to ensure compliance. The site, its engineers found, presented multiple challenges.

“Land for the development was highly impervious, with limited opportunities to implement bioretention systems. The constraints were to minimise the sizes of stormwater treatment devices and maximise the efficiency,” says Ben Frost, Undergraduate Civil Engineer within the Property Team at Hyder Consulting.

“Some catchments were larger than others, making it difficult to balance large catchment areas with areas for treatment opportunities and to fit required treatment devices into smaller catchments. The stormwater management plan had to be altered to redirect stormwater flow and at the same time try to avoid creating a burden for detailed designs.”

To help with the work, Hyder Consulting used eWater’s urban stormwater modelling software, music (model for urban stormwater improvement conceptualisation) to develop a conceptual stormwater management plan for the Development Application (DA) process for the Hilton Hotel and Residences.

music is one of the few software tools available that can accurately simulate real-time water sensitive design. It is designed to help urban stormwater professionals visualise possible strategies to tackle urban stormwater hydrology and pollution impacts. Since first developed in 2001, the software has been used by thousands of professionals working in private practice and in state, regional and local government agencies throughout Australia.

The software is allowing planners to assess the pros and cons of various engineering systems for improving stormwater quality, including biofilters, swales, wetlands and other measures.

It includes major advances to the science and enhances the ability to model new stormwater technologies like porous pavements. It also includes raw rainfall data for 50 major population centres in Australia.

For the Hilton Hotel and Residences site, Hyder Consulting’s Property Team applied music for a conceptual stormwater management plan. For the conceptual planning they considered the architectural plan for the site and stormwater discharge locations.

“On the conceptual music model we have to show how to collect stormwater discharge and think of appropriate treatment trains. We investigated where to place devices in order to integrate them into the architectural plan,” says Steve Williams, Senior Civil Engineers within the Property Team at Hyder Consulting.

The Property Team also put together an Engineering Service Report detailing utilities connections, road access, water and stormwater connections of the site and the impact on existing developments around the site.

The Hilton Hotel and Residences development is built right to the boundaries and space is limited.

“We knew we had to apply small devices: bioretention planter boxes, and incorporate them into the landscaping.” says Frost. “music allows us to size stormwater devices and decide where to put them. The sizes have to treat the stormwater to meet pollutant guidelines by the Council. We are familiar with the devices”.

“The targets set the framework for what we are trying to achieve. With music we were able to meet targets during the conceptual design process and size the devices to comply with pollutant guidelines by Council,” Williams says. “music helps getting the balance right for optimised treatment devices and targets”.

Frost says “music is a conceptual model tool. We can experiment and test what works and what doesn’t”.

An important aspect of the treatment trains is the reuse of rainwater. Frost says “We look at the amount of reuse that is likely to occur. It is important to capture runoff and reuse the rainwater for several recycle options, dual reticulation and irrigation requirements. music lets us investigate options and articulate how much reuse can be expected”.

The Stormwater Management Plan of the two towers resulted in three bioretention systems per tower in landscape areas. Devices in the form of planter boxes collect stormwater which drains into gross pollutant traps. The gross pollutant traps goup stream into a large concrete water tank in the basement of each building. Rainwater from roofs is collected in the same water tank in the basement level. Williams says “There can be a large runoff from roof and podium areas. Stormwater and rainwater overflow runs into the Council’s stormwater system”.

“We have to balance the size of the bioretention area with the size of the rainwater tanks. In urban developments, space is scarce and rainwater tanks are often placed in the basement of buildings. Basement space is tight and can impose constraints on other usage and limit the numbers of car parks for instance,” Williams says “In the concept design, the music model provides flexibility on other usage that has to fit around the device, and allows for a lot of adaptability down the track”.

The water balance modelling functionality in music helps with sizing rainwater tanks and ties in with the Urban Developer software from eWater. Frost finds developers ecologically sensitive, saying they try to reuse stormwater and rainwater as much as possible. “music enables us to calculate how much stormwater can be captured, and developers get a good idea of stormwater reuse as a reliable source” says Frost.

“Hyder Consulting’s main focus is to optimise the size of treatment systems to meet pollutant targets with the least amount of bioretenetion systems”.

Useful Lessons

During the conceptual music modelling Frost learnt that it is important to calculate the impervious area on site as accurately as possible. “Slight differences can produce relatively large changes in the effectiveness of the treatment train”.

Another important learning Frost points out is to source Bureau of Meteorology data (i.e rainfall data) that is as specific to the site as possible. He says “It is possible to have generic data that is specific to the city the development is in, however there may be data that is sourced from rainfall stations that may be more applicable. The ‘music Modelling Guidelines for South East Queensland’ as published by Water by Design is a very good way to tailor models to suit the location that the site in question is located in such a rainfall data to use, recommended rainfall-runoff parameters and pollutant export parameters”.

Frost also says “A lesson specific to the Hilton Hotel and Residences project, and other similar high rise projects, is to use architectural plans to get a detailed conceptual model without getting so specific that it will provide constraints and ‘shoe-box’ detailed designers later on in the design process”.