Water sensitive urban design

What is water sensitive urban design?

Urban development impacts the natural water cycle by creating impervious surfaces that affect the quantity and quality of stormwater. This in turn generates increased pollution and erosion. In Merri-bek, stormwater runoff is discharged to Port Phillip Bay via Merri Creek, Edgars Creek and Moonee Ponds Creek. Stormwater runoff and pollutants are detrimental to these creeks, the Bay and the ocean.

Water sensitive urban design mitigates these impacts while reducing water bills and creating greener urban areas. 

Benefits of water sensitive urban design

Water sensitive urban design also provides many social, economic and environmental benefits including:

•  Minimising impact on receiving waters

•  Reducing potable water use

•  Recharging local groundwater through the infiltration of stormwater

•  Creating greener urban environments with high visual amenity, and

•  Passive cooling through increased vegetation cover.

Best practice

The Victorian Urban Stormwater Best Practice Environmental Management Guidelines (Victorian Stormwater Committee, 1999) mentioned in the VPP's define best practice stormwater pollutant removal as:

• 80% reduction in the typical urban load of total suspended solids

• 45% reduction in the typical urban load of total phosphorous

• 45% reduction in the typical urban load of total nitrogen

• 70% retention of typical urban load of litter.

In addition, the VPP require flow from the site to be:

• Designed to ensure that flows downstream of the site are restricted to pre-development levels unless increased flows are approved by the relevant drainage authority and there are no detrimental downstream impacts.

Moreland City Council is located in the middle reaches of Merri Creek and Moonee Ponds Creek, with downstream flows impacting the lower reaches of the Yarra River. Meeting the stormwater quality objectives is important to maintain the health of the downstream Yarra River, Bay and Ocean. Meeting the stormwater flow objectives is important to ensure the health and stability of the Merri and Moonee Ponds creeks.

To find out more visit the Melbourne Water website.

Demonstrating meeting best practice

Meeting the VPP requirements can currently be demonstrated in two ways:

• Submitting a MUSIC model demonstrating a treatment train that achieves the above targets.

OR:

• Submitting a STORM report achieving a score of 100% or above.

Option 1: STORM Calculator

The STORM Calculator is a user friendly, free online tool developed by Melbourne Water.  It is designed to be suitable for applicants without any formal training on using the calculator or designing stormwater treatment systems.

STORM Calculator inputs include the total development area and all impervious areas (including impervious areas where no treatment will be provided for stormwater runoff). The calculator enables users to select from a range of WSUD treatment types.

An overall STORM score of at least 100% is required to demonstrate that best practice Stormwater Management has been achieved.

Option 2: MUSIC

The Model for Urban Stormwater Improvement Conceptualisation (MUSIC) is a modelling tool that uses historic rainfall data to estimate catchment runoff and predict the performance of WSUD infrastructure. It enables a significantly higher degree of modelling complexity and flexibility compared to the STORM calculator. 

The MUSIC model should only be used by those with appropriate expertise.  MUSIC models used to prepare WSUD Responses for the City of Merri-bek must be developed in accordance with Melbourne Water MUSIC Guidelines.

MUSIC users must have a software licence and a minimum level of training and competency to develop a MUSIC model. MUSIC training is provided by eWater.  MUSIC is generally used by professionals with stormwater treatment expertise.  MUSIC is generally the most suitable assessment tool for complex and/or large developments (e.g. large multi-lot subdivisions) and any proposal that involves stormwater harvesting. 

Water sensitive urban design treatments

A range of water sensitive urban design treatments can be used to demonstrate best practice stormwater management. 

Below are principles for Merri-bek's preferred stormwater management within townhouse developments: 

• Maximise roof drainage to rainwater tanks and plumb this water into the townhouse toilets for flushing. Merri-bek will not accept the use of charged pipes underneath dwelling slab components, including garages. 
• Where draining the entire roof to a rainwater tank is not realistic, a split catchment should be provided, such as: roof draining to a second rainwater harvesting tank or to an above-ground planter box raingarden.
• Driveways to be treated by permeable paving. Merri-bek will not accept driveways draining to in-ground raingardens adjacent to neighbouring properties or proposed building, buffer strips, swales, or proprietary stormwater management treatment systems.

Moreland has prepared example townhouse WSUD response. Please see the set of plans (in particular page 3) and accompanying STORM report within the sample SDA response. 

Moreland has also prepared standard WSUD treatments in this pdf. 

How to develop a response

A water sensitive urban design (WSUD) response must clearly demonstrate how stormwater runoff will managed in accordance with the VPP Stormwater Management objectives.

Step 1: When should you develop a WSUD response?

WSUD responses should be developed as early as possible in the development process to allow WSUD to be fully integrated with the site design, such as roof shape and the site levels. This will result in a better WSUD response and easier assessment process.

Step 2: Determine catchment area and discharge point(s)

Identify and measure the area of all outdoor “hard” surfaces on your site plan.  Hard surfaces include roofs, balconies, verandas, pergolas, concreted and paved areas.  (Note that permeable paving does not count as a hard surface with respect to generating stormwater runoff). 

Depending upon the type of roof construction, sections of the roofed areas may drain to different points of the development, and therefore may need to be separated into sub-roof areas. Sub-roof areas may be combined where the roof runoff will be diverted to a common WSUD treatment, i.e. rainwater tank.

The legal point of discharge for the property should also be identified. Discharges from WSUD treatments will need to be conveyed to this point.

Step 3: Choosing WSUD infrastructure

Select which WSUD treatment will be used to treat runoff from each hard surface using the above hierarchy, such as a rainwater tank or a raingarden. Take into account any constraints on available space and site levels relative to the legal point of discharge.

Step 4: Size rainwater tank and/or treatment system

Size your rainwater tanks using the assistance of STORM and MUSIC.