Key features

The water sensitive urban design (WSUD) development includes a unique stormwater treatment train that includes rainwater tanks, protection of old river red gums in multiple purpose rain gardens, bio-retention swales and restored waterways. The project has successfully operated for over eight years.

Use of linear rain gardens for amenity, protection of river red gums, flood protection and management of stormwater runoff and quality is a unique feature of this project.

Overview

Dr. Peter Coombes was commissioned in 2005 to develop a conceptual design of the former Groves Estate at Mernda in Victoria that includes water sensitive urban design (WSUD) and integrated water cycle management (IWCM). Stage 1 of the Plenty Valley development near Mernda includes approximately 150 dwellings and occupies a land area of 9.37 Ha. The urban development, now called Renaissance Rise, has successfully operated over the last 8 years.

A feature of the stormwater treatment train for this project was the retention of the old river red gums in rain gardens, multiple purpose rain gardens and restoration of ephemeral waterways. The multiple purpose rain gardens provided amenity, flood protection, and management of stormwater runoff and quality.

Impacts of the proposed development on stormwater runoff quality and quantity were analysed using the Systems Framework created by Dr Coombes during his research activities at the University of Newcastle. This process combined the PURRS water balance model, WUFS rainfall runoff design model and the MUSIC water quality model were used to analyse the performance a various water sensitive urban design (WSUD) and integrated water cycle management (IWCM) scenarios. Local climate and demographic data was also used in the analysis.

It was established that the use of traditional drainage methods in the proposed development will result in significant increases in peak stormwater discharges, annual stormwater runoff volumes and annual pollutant loads. This would result in decreases in receiving water quality and increased flood risks in the downstream environment. A substantial amount of downstream infrastructure would be required to mitigate these impacts including gross pollutant traps (GPT), detention basins, floodways and wetlands. This infrastructure will occupy a large land area (at least 0.5 Ha) and require considerable annual maintenance.

A full WSUD solution that includes rainwater tanks, bio-retention swales, small sediment basins, rain gardens and contour banks will reduce peak stormwater discharges by 67% to 89%, annual stormwater runoff volumes by 78%, annual loads of TSS by 98%, annual loads of TP by 90% and annual loads of TN by 82%. In addition, the combination of water efficient appliances (3A shower roses, 6/3 flush toilets and tap regulators) and 3 kL rainwater tanks used to supply toilet and outdoor uses will decrease annual mains water demand by 10.9 ML, peak day water demands by 20% and instantaneous water demands by 52%.

The use of rainwater tanks and water efficient appliances will reduce impacts on regional water security whilst decreasing the requirement for water pumps stations, water storage reservoirs, water mains and downstream stormwater management infrastructure. The requirement for downstream stormwater management infrastructure is eliminated and the need for water distribution infrastructure is reduced whilst stormwater quality and quality discharges from the sub-catchment are similar to the predevelopment discharges.

The WSUD solution will produce stormwater quality that is significantly in excess of the ‘design standard’ requirement for 80%, 45% and 45% reduction in suspended solids, phosphorus and nitrogen respectively. This will have the impact of reducing the impacts on stormwater quality and flow regimes on downstream catchments. In addition the full WSUD solution will comply with standard minor/major stormwater design safety requirements.

Land scaped buffer and swale at the interface of the site with Bridge Inn Road

The Site

The development, with an area of 9.37 Ha, is located between Bridge Inn Road and an existing open drain as shown in the Figure below. The topography of the site is generally low lying with a small escarpment that is parallel to Bridge Inn Road.

Schematic of the development site

Design of the Stormwater Management System

The stormwater management system for the development employs a water sensitive urban design (WSUD) treatment train philosophy and is shown in the Figure below.

 

Schematic of the WSUD layout for the development

The street drainage system consists of road pavements with one-way cross falls that discharge stormwater runoff to a bio-retention swale system (see following Figures) or to a traditional pipe drainage system.  The bio-retention swale system will consist of a shallow grass swale over a gravel trench (0.5 m wide and 0.6 m deep) that contains a 100 mm diameter agricultural pipe (see following Figures). The dimensions of the trench and diameter of the agricultural pipes are varied at some locations in the development.

The ephemeral stream corridor downstream from the subdivided land has been restored and includes six shallow dry basins in the restored stream corridor. Note that the ephemeral stream corridor was previously an earth lined open channel. The stormwater management system also includes rain gardens and rainwater tanks.

Rainfall falling on 50% of each domestic roof area is directed to rainwater tanks. Overflow from the rainwater tanks and rainfall runoff from the balance of the domestic roof areas will be directed to the street drainage system. The road cross section employed to manage stormwater runoff from the access road is shown in the Figure below.

Bio-retention swale at the centre of dual carriageways

Bio-retention swale at the centre of the road

The road pavements in the access road reserves slope towards a bio-retention swale system situated at the centre of dual carriageway roads. Stormwater runoff from the pavement infiltrates through the base of the grass swale into the gravel trench below. Excess stormwater flows along the swale for ultimate collection in drainage pits that discharge stormwater into the agricultural pipe within the gravel trench. The diameter of the agricultural pipe was normally 100 mm and up to 250 mm in places to minimise excess surface flows in the swale. The trench contains gravel with a nominal diameter of 20 mm to 30 mm that is surrounded by geofabric. Surface flows in the swale are directed to inlet pits that discharge excess stormwater into the gravel trench. Within the gravel trench stormwater infiltrates to the surrounding soil and flows downstream via the agricultural pipe.

The stormwater management train includes the use of bio-retention swale and contour systems placed at the interface between the development and the drainage reserve. A schematic of the swale/contour solution is shown in the Figure below.

Details of GPT, diffuser trench, swale and contour bank arrangements

Interface between the drainage system with the restored waterway at the diffuser outlet

This swale contour system includes bio-retention areas and gravel diffusers that remove energy and pollutants from stormwater runoff. The impacts of stormwater runoff on the receiving environment and the performance of the linear swale/contour/bio-retention system was further improved by the addition of mandatory rainwater tanks that supply outdoor, toilet and laundry water uses.

An important design element of the WSUD strategy for this project is the use of small contributing stormwater catchments to each perimeter treatment system that minimises and equalises the stormwater flows that require management at each outlet. This approach is the opposite of current stormwater practice that endeavours to minimise stormwater outlets with subsequent maximum stormwater flows and impacts on receiving waters.

In addition, the use of rainwater tanks and linear measures at the perimeter of the development minimised the requirement for wetlands and detention basins freeing up valuable land for development and public use whilst eliminating the considerable maintenance burden of constructed wetlands. Employment of the small contributing catchment strategy also minimises stormwater treatment, maintenance and replacement costs. A traditional inter-allotment drainage system consisting of pipes and pits is proposed to drain stormwater from roofs on allotments that slope away from streets to the nearest street drainage system.

The unique stormwater treatment train for this project included the retention of the old river red gums in rain gardens, multiple purpose rain gardens and restoration of ephemeral waterways. The multiple purpose rain gardens provided amenity, flood protection, and management of stormwater runoff and quality.

A multiple purpose rain garden built around a river red gum tree

Performance of the development

Stormwater runoff from development was analysed using a hydrological model for design storm events with average recurrence intervals (ARI) from 1 to 100 years and durations ranging from 10 to 360 minutes. The performance of the WSUD solution is compared to the performance of traditional pipe drainage, rainwater tanks and bio-retention strategies are shown in the Table below.

Performance of stormwater management strategies

The use of traditional pipe drainage in Stage 1 will result in considerable increases in peak stormwater discharges in comparison to stormwater peak discharges from the sub-catchment in the predevelopment state. Addition of the 3 kL rainwater tanks to traditional pipe drainage solution will reduce stormwater peak discharges by 12% to 32%.

In contrast the WSUD solution including rain gardens, bio-retention swales and rainwater tanks will reduce stormwater peak discharges by 67% to 89%. In addition, the WSUD solution produces similar peak stormwater discharges to the sub-catchment in the predevelopment state.

The results for urban stormwater quality for the various stormwater management solutions are summarised in the following Table.

Summary of water quality results

The traditional drainage solution will increase annual stormwater runoff volumes by 358%, annual loads of total suspended solids (TSS) by 1,378%, annual loads of total phosphorus (TP) by 687% and annual loads of total nitrogen (TN) by 420%. Urbanisation of the sub-catchment and the use of a traditional drainage system will result in dramatic increases in stormwater runoff volumes and associated pollutant loads.

Addition of 3 kL rainwater tanks, used to supply toilet and outdoor water uses, to the traditional drainage solution will reduce the annual stormwater runoff volumes by 16%, annual loads of TSS by 26%, annual loads of TP by 12% and annual loads of TN by 10%. The use of the rainwater tanks will provide significant improvements in urban stormwater quality as well as considerable reductions in mains water demand.

Adoption of the full water sensitive urban design (WSUD) solution that includes bio-retention swales, rainwater tanks, small sediment basins, rain gardens, contour banks and restored waterways will reduce the annual stormwater runoff volumes by 78%, annual loads of TSS by 98%, annual loads of TP by 90% and annual loads of TN by 82%. In addition, the WSUD solution reduced the annual stormwater runoff volumes and pollutants loads discharging from the sub-catchment in comparison to the site in the predevelopment state, and provides significant water conservation.

Dual carriageway road within the project with central swale