1. The Problem

In recent times there have been several dramatic increases of flooding in the Upper Parramatta River Catchment. Up until 10 years ago many houses, shops and factories thought to be safe from flooding have been repeatedly under water.

In 1990 a study found that 2316 residential properties were subject to over-ground flooding in a 100-year storm, whilst 631 homes and 356 businesses would be flooded over-floor.

Since then works undertaken by the Upper Parramatta River Trust and the four councils have reduced the number of flood liable residential properties by 31% and the number of flood liable businesses by 65%.

Flooding is a natural periodic event in the highly variable Australian climate. As early as the 1880’s colonists in the Upper Parramatta River Catchment experienced flood devastation of property and crops. In the early 1800’s the upper Parramatta River and its tributaries had only to drain the natural precipitation. Runoff was slowed due to the presence of natural vegetation whilst infiltration was high due to the presence of permeable exposed soils and very few impermeable, man-made surfaces.

However with increased urbanisation the impact on our land and waterways has increased. Only about 30% of the natural land in the catchment is now available to soak up the rainwater and most of the trees which once intercepted and stored water have also gone from the system. As development of the catchment has expanded to the current 70% urbanised area, the natural tendency of local creeks to flood has been exacerbated.

The Upper Parramatta River Catchment now receives water from two sources – precipitation and piped water from catchments such as Warragamba Dam. Most of the imported water is removed via the sewerage system but water used for washing cars, hosing factory floors, watering gardens and that escaping from broken or overloaded sewerage pipes does not. This excess water has to be channelled out of the catchment via the stormwater drains and local waterways, adding to the load from precipitation.

2. Why it is a Problem

The mean monthly flow at stream gauging stations in the Parramatta River, in normal rainfall regimes, varies between 0.976 and 3.621 m³, (see Hydrology Overview). Peak Discharges for flood waters in the Parramatta River show readings hundreds of times higher than this.

Peak Discharges for Flood Waters in the Parramatta River

The rise and fall of flood waters in the Parramatta River can be shown in a hydrograph, (the amount of water passing a point, its discharge, over time). Development upstream in the catchment such as at Baulkham Hills and West Pennant Hills have had devastating effects downstream. An increasing quantity of water entering the creeks increases the likelihood of floods particularly in the Parramatta CBD which is the lowest point in the catchment.

In the late 1980’s through to 1991 floodwaters caused considerable damage along Darling Mills Creek and the Upper Parramatta River. Hardest hit areas have been Northmead, North Rocks and North Parramatta. Between 1986 and 1991 some properties were flooded repeatedly. In 1889 a flood greater than these invaded the low lying areas of Parramatta township. However in 1889 the upstream catchment was mainly farms and orchards. With the catchment almost fully urbanised the same rainfall today would result in much higher flood levels. Prior to flood mitigation works in a major flood, such as the 100 year flood, a third of Parramatta CBD would be inundated. Urbanisation has resulted in accelerated runoff as: ·           roughly 70% of the catchment is urbanised and increasingly sealed by roads, carparks, paths, factories, housing, hospitals, schools etc. ·           gutters, drains and canals deliver stormwater very quickly to main channels which then overflow ·           siltation due to urban runoff from housing sites and cleared land raises river beds leaving less depth for water ·           rubbish and weeds collecting in creeks cause blockage of channels so reducing their efficiency in discharging water. Results of floods can be measured in terms of social, economic and environmental costs. These include: ·           more floods, more often ·           damage to property ·           distress, fear and panic experienced by flood victims ·           loss of valuable topsoil due to erosion ·           erosion of river banks due to increased water flow and loss of riparian vegetation ·           accumulation  of rubbish in creeks and stormwater drains.

3.    Management Solutions

In 1990 over 2,300 residential and 350 business properties were identified as being subject to over-floor flooding in a 100-year event. By 1999 31% less homes and 65% less businesses were deemed to be similarly affected.

There is a range of flood mitigation (control) options:-

·      Structural – those, such as retarding basins or detention systems, which attempt to reduce the problems of floods for existing developments. These temporarily store water during storm runoff and lessen the flow rates and water levels downstream. They are designed to mitigate downstream flooding.

·       Non-structural- those suited to avoiding future flood, ie they seek to restrict the course of development so that the problems are not created e.g., leaving flood prone areas as open space.

Current best-practice in creek management favours allowing natural processes to work where appropriate. However intervention of some sort is often required when channel instability threatens infrastructure such as roads, pathways or private property. Past and present local examples include the eroding creek banks at lower Greystanes Creek near Toongabbie Bowling Club and at lower Darling Mills Creek near Speers Road, North Rocks. Runoff must be reduced, discharge into streams controlled or flood prone areas left as open space. In the Upper Parramatta River Catchment the following solutions have been suggested or implemented:-

·         Retarding basins have been built in such places as Toongabbie, Greystanes, Prospect and west Baulkham Hills.

·         Flood retarding basin at Loyalty Road, on Darling Mills Creek has reduced the level of flooding in Parramatta by at least one metre.

·         Grassed floodways developed e.g. through residential properties in Toongabbie.

·         Most new developments must incorporate stormwater detention ponds or tanks in their design.

·         Native gardens and reduced paved areas promoted

·         Stream banks can be regenerated , revegetated or raised

·         Houses built on flood plains demolished and the floodway used as open space

·         Gross pollutant taps and trash racks installed in creeks and drains

·         Houses raised above the likely flood water level, e.g. Hopkins Street, North Wentworthville, Budgeree Road, Toongabbie.

·         Voluntary acquisition of flood prone houses by the council e.g. in Hood Street, Toongabbie and Darcy Rd, Wentworthville.

List of Detention Basins in the Upper Parramatta River Catchment (From Stormwater Management Plan 1999)

Constructed Wetlands in the Upper Parramatta River Catchment.

4.What can you do?

Because human activity has increased localised flooding, people are responsible for providing solutions both through government intervention and individual efforts. Individual efforts include such things as:

·         Reduce the amount of water leaving your property, e.g. more grass less concrete, to encourage infiltration

·         Get the litter out of the drains. On Clean Up Day in 1991 about 500 tonnes of rubbish and 30 cars were shifted out of creeks and drains.

·         Install a rainwater tank to reduce mains water consumption.

·         Plant a native vegetation garden rather than exotics as these require less watering

·         Become involved in the local Landcare or Bush Regeneration group and help revegetate creek corridors.

5. Sites where this is a problem

A series of catchment wide floods in the 1980’s resulted in several million dollars of damage to local commercial and residential property. Hardest hit areas have been Northmead, North Rocks and North Parramatta as a result of flooding in the Darling Mills Creek. Since that time the Upper Parramatta River Catchment Trust and councils have implemented a long term strategy to reduce the potential risk and impacts of flooding. Several large projects on Toongabbie Creek (McCoy Park and Sierra Place retarding basins) and Darling Mills Creek (Loyalty Road basin) have significantly reduced flood risk across the catchment.

6. Further Information & Links

Upper Parramatta River Catchment Trust Website www.uprct.nsw.gov.au