Stormwater Management

Mosman Council is responsible for the stormwater system which drains Mosman‘s 8.5km². This system collects stormwater from public and private property and conveys it through kilometres of pipes to stormwater outlets in Middle Harbour or Port Jackson. Council has implemented initiatives to reduce flooding including:

• adopting stringent design standards to prevent flooding through private property;
• developing a Stormwater Asset Management Plan (SWAMP), which prioritises upgrade works to its stormwater assets, and directing significant funding towards SWAMP projects; and
• adopting a Policy for On Site Detention which ensures that peak flows of stormwater from private property do not increase when development occurs.

Property owners are responsible for ensuring that any stormwater drainage systems from their property are maintained in good working order to prevent flooding and damage to their own property or any downstream properties.

Both surface and sub-surface run-off occur naturally downhill between properties, and Mosman is especially prone to heavy ground water flows as the bedrock of Sydney Sandstone is rarely more than 1 to 1.5m deep, and ground water travels through the subsoil above this rock layer.

If an unusually high groundwater flow is being experienced in dry periods residents should have a licensed plumber ensure that their water service, sewer, stormwater and sprinkler systems are in good working order. If a problem is suspected from a neighbouring property then residents should talk to their neighbour and investigate the source of any overland or sub surface flow in a cooperative way. Council only has powers to act when there is a defect with existing systems.

In Mosman, stormwater is ultimately discharged into the waters of Sydney Harbour. Any stormwater pollutants in Mosman‘s catchment such as litter, sediment, organic matter, weeds, nutrients, oils, grease and pesticides can impact on the health of our harbour waterways and beaches, and have a negative effect on water quality and aquatic biodiversity.

Heavy rain can also cause the sewerage system to overflow because illegal connections and cracks in old pipes allow excess water into the system. When the sewerage system overflows, it can drain back into the stormwater system, and from there into the harbour.

Mosman Council is concerned with the quality of stormwater that flows from the Municipality into Sydney Harbour. Council has been improving stormwater quality by:

• Installing Stormwater Quality Improvement Devices (SQIDs) on stormwater outlets throughout the Municipality. Currently 75% of Mosman is treated by SQIDs. To date these devices have captured a total of 1443 tonnes of pollutants that would otherwise have washed into our beaches and bays.
• Undertaking environmental engineering works and bushland management solutions to reduce erosion and sedimentation of creek systems, and to rehabilitate creeks and bushland areas.
• Running education programs to raise community awareness and encourage people to identify and modify behaviours that cause stormwater pollution.

Water quality describes the health of water, any chemical or microbiological contaminants it may contain, and the consequences for the ecology and health of aquatic systems and human health.

The Office of Environment and Heritage conducts water quality monitoring of beaches within the Mosman municipality, including Chinamans, Edwards and Balmoral Beach, Clifton Gardens and Little Sirius Cove. Refer to Beachwatch Program for daily Harbourwatch water quality results.

As all of Mosman forms the catchment for our harbour waterways, we all have a role to play in preventing stormwater pollution:

• Put litter in bins. Litter can kill seabirds and marine mammals if they swallow it, and plastic waste can strangulate and smother them. Litter is also unsightly.
• Use a plastic bag to pick up your dog‘s droppings and put them in the bin. Animal droppings can increase the level of bacteria such as faecal coliforms and make our water unsafe for swimming. Excess nutrients encourage the growth of weeds, which can smother native species.
• Bin your cigarette butts. Butts take several years to breakdown, and they introduce toxins into waterways. Carry a small container such as an old film canister to store your butts until you can dispose of them properly.
• Wash your vehicle over grass or on gravel, so that the waste water can be absorbed by the ground. Use a bucket and minimal detergent, and empty the soapy water down the sink or toilet. Or use a commercial car wash that recycles its water.
• Care for your car to prevent it leaking oil and lubricants onto the road. These hydrocarbons can be toxic to marine life when washed into the stormwater system.
• Rake garden and lawn clippings and use as compost or mulch. Never hose garden sweepings into the stormwater system. Organic matter uses up oxygen as it decomposes in waterways, suffocating aquatic plants and animals.
• Allow unused paint to dry out, then dispose of residue in the bin. Do not clean brushes or rollers into gutters or drains.
• Only put rain down the drain. It is illegal to place any other material into the stormwater system. Fines can be issued for offences.

Council’s consultant Manly Hydraulics Lab (MHL) recently installed a rainfall gauge at Mosman Bowling Club. Residents can view a webpage which displays rainfall data collected over the previous seven days.