Sydney
17.2 River outflow from the region

Supporting Information

Outflow from the Sydney region is the streamflow reaching the sea. The estimated volume for the line item is 3,039,529 ML.

This item includes streamflows resulting from:

  • rule-based environmental flows, other unspecified releases and spills from most downstream storages, and weirs to rivers
  • natural runoff from the catchment of rivers downstream of storages and weirs, and streams leading to the sea (less downstream diversions)
  • treated water releases from wastewater treatment plants.

Banked environmental flow releases have been separately accounted for in line item 17.16. The following table provides a summary of outflow from the Sydney region to the sea.


Outflow from the Sydney region to the sea – summary

Description

Volume during 2010–11 (ML)

Rule-based environmental flow releases from storages

293,094

Unspecified releases from storages

19,950

Spills from most downstream storages and weirs

576,421

Natural runoff from the catchment of rivers downstream of storages and weirs, and unregulated streams leading to the sea

2,142,348

Treated water releases from wastewater treatment plants

74,554

Less downstream diversions

(66,838)

Outflow to the sea

3,039,529

 

Rule-based environmental flow releases from storage reservoirs – flow reaching the sea

Authority responsible for environmental flow releases

Storage reservoir/weir from which the release was made

Environmental flow release during 2010–11 (ML)

Sydney Catchment Authority

 

Woronora

7,126

Lake Burragorang (Warragamba Dam)

3,197

Prospect

Not applicable1

Lake Yarrunga (Tallowa Dam)

219,245

Blue Mountains2

Not applicable1

Pheasants Nest Weir

38,877

Broughton Pass Weir

24,649

Total

293,094

1Environmental flows have not been specified from these storage reservoirs.

2Blue Mountains storage reservoirs include Medlow Reservoir, Greaves Creek Reservoir, Upper Cascade Reservoir, Middle Cascade Reservoir and Lower Cascade Reservoir.


The previous table shows net environmental flow reaching the sea because the released flows were not diverted to downstream storages/weirs and not used for other purposes. Details of released environmental flows including volumes reached at downstream storages/weirs are available in Water for environmental benefit under Water access.


Unspecified releases and spills from storages

Storage/weir

Other releases during 2010–11 (ML)1

Spills during 2010–11 (ML)

Total during 2010–11 (ML)

Lake Burragorang (Warragamba Dam)

2,907

0

2,907

Lake Yarrunga (Tallowa Dam)

16,4192

538,645

555,064

Blue Mountains

224

180

404

Pheasants Nest Weir

0

6,462

6,462

Broughton Pass Weir

0

7,601

7,601

Danjera2

0

23,533

23,533

Total

19,950

576,421

595,971

1Other releases were made to facilitate scheduled deliveries (e.g. to meet minimum discharge required to operate a gate — also known as 'conveyance water'). Fitzroy Falls storage release of 736 ML is not accounted in the table because the flow was received at Lake Yarrunga (Tallowa Dam).

2When the spill rate is above the required environmental release no additional release is required. However, for operational reasons, the release from the outlet is maintained through the period of spill.


Natural runoff from the catchment downstream of reservoirs and weirs, and unregulated streams leading to the sea

Gauged flows are not available to cover all river reaches downstream of storages and weirs, and streams leading to the sea. Many of the gauges are tidal and only river height data are available for some of them. Gauged flow data are not available for:

  • the Hawkesbury River and the lower Nepean River (except the tributaries shown in the following table)
  • northern Sydney rivers
  • southern Sydney rivers, except for the Woronora River (at Gauge No. 213211)
  • the Illawarra rivers
  • downstream river reaches below the gauges shown in the following table.

Natural runoff was therefore estimated from available gauged flows for the rivers listed in the following table, and estimated runoff for other streams.

Total natural runoff is 2,142,348 ML for 2010–11, made up of 740,134 ML of gauged flows and 1,402,214 ML of flows below these gauges, as well as in ungauged and tidal streams. This is detailed in the following two tables.


Gauged flows

River

Gauge No.1

Flow volume for 2010–11 (ML)

Comments

Macdonald River at St Albans

212228

100,910

 

Colo River at Upper Colo

212290

352,476

 

Gross River at Burralow

212291

89,804

 

South Creek at Richmond Road

212297

7,876

Excludes 3,662 ML of treated wastewater released from St Marys Water Recycling Plant, this has been accounted for separately

Woronora River at Needles North Engadine

213211

24,150

Excludes 7,126 ML environmental flow release, which was accounted for separately

Shoalhaven River at Grassy Gully No. 2

215216

164,918

Excludes (from the gauged flow):

  • 16,419 ML diverted by the Sydney Catchment Authority to Shoalhaven City Council pumping at Burrier
  • 19,245 ML environmental flow release and  538,645 ML dam spilling from Lake Yarrunga.

Excluded items were accounted for separately

Total gauged flow

740,134

 

 1The Sydney Catchment Authority manages the gauging stations.


Natural streamflow in the river reaches in ungauged and tidal streams

Description

Volume for 2010–11 (ML)

Comment

Natural streamflow in the river reaches downstream of the gauges, and in ungauged and tidal streams (before any addition and diversions at further downstream reaches)

1,402,214

Based on the Bureau of Meteorology calculation of line item 9.4 Runoff to surface water

 

Treated water releases from wastewater treatment plants

Description

Volume for 2010–11 (ML)

Wastewater releases to the rivers from the area managed by Sydney Water Corporation

55,648

Recycled water releases to Boundary Creek (a tributary of the Nepean River) by Sydney Water Corporation

15,989

Wastewater releases to the Shoalhaven River below Gauge No. 215216, from the area managed by Shoalhaven City Council

2,917

Total

74,554

 

Deductions from natural river flows for downstream diversions

Description

Volume for 2010–11 (ML)

Comments

Riparian right releases

22,718

Based on total estimated riparian use provided by the NSW Office of Water (9,837 ML released from Lake Burragorang was deducted from the total use)

Stock and domestic, and other lumped access licence releases in the respective river reaches

38,531

As estimated by the NSW Office of Water based on entitlement information available in Draft Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources (2012a)

Sydney Water Corporation diversion from the Hawkesbury River for North Richmond

5,589

Based on the value provided by Sydney Water Corporation

 

Total

66,838

 

 

Quantification approaches are provided separately for the following inputs used to calculate outflow to the sea:

  • rule-based environmental flow releases
  • unspecified releases and spills from storages and weirs to downstream rivers
  • natural streamflows in gauged streams
  • natural streamflows in ungauged streams
  • treated water releases from wastewater treatment plants
  • downstream diversions from flow leading to sea.

Quantification Approach

Downstream diversions from flow leading to sea

Data Source

Line item 17.6 and 17.11.

Provided by

Various.

Method

Assumptions, Limitations, Caveats and Approximations

Uncertainty Information

Natural streamflows in gauged streams

Data Source

Stream gauge data from the Hydstra system.

Provided by

Sydney Catchment Authority.

Method

River levels are gauged and converted to a flow rate using a rating table. Rating tables are updated by manual stream gauging.

Some of the gauging sites have been linked to the SCA's radio telemetry system so that gauging data are readily available. For the other sites, data are downloaded to SCA's Hydstra system in regular intervals from logged data at the site.

Assumptions, Limitations, Caveats and Approximations

Nil.

Uncertainty Information

The volume is based on measured data. Estimated uncertainty based on measurement accuracy and professional judgment on rating tables is +/– 10% to 50%, depending on the site characteristics and flow rate.

Quantification approach for natural streamflows in ungauged streams

Data Source

The Bureau modelled the landscape water balance to estimate inflows using the following data sources: the Bureau: climate data and geographic information system (GIS layers), Commonwealth Scientific and Industrial Research Organisation (CSIRO): raster spatial data, WaterDyn and AWRA-L model parameters, and radiation data; NSW Office of Water: GIS data on waterbody and river channel sections; SCA: GIS data on its operation area; Geoscience Australia: built-up areas and other GIS layers.

Provided by

The Bureau of Meteorology

Method

The Bureau calculated natural streamflow for the following ungauged and tidal rivers:

  • the Hawkesbury River and the lower Nepean River, except catchment areas upstream of

-        Warragamba River at Lake Burragorang (Warragamba Dam) (Gauge No. 212243)

-        Pheasants Nest Weir (Gauge No. 212203)

-        Broughtons Pass Weir (Gauge No. 212233)

-        Macdonald River at St Albans (Gauge No. 212228)

-        Colo River at Upper Colo (Gauge No. 212290)

-        Gross River at Burralow (Gauge No. 212291)

-        South Creek at Richmond Road (Gauge No. 212297)

  • northern Sydney rivers
  • southern Sydney rivers, except the catchment area upstream of the Woronora River at Needles North Engadine (Gauge No. 213211)
  • the Illawarra river
  • the Shoalhaven River, except the catchment area upstream of Grassy Gully No. 2 (Gauge No. 215216).

Natural river flow in the river reaches is based on landscape water draining into the rivers modelled by the Bureau (see the method described in line item 9.4 Runoff to surface water).

Assumptions, Limitations, Caveats and Approximations

See line item 9.4 Runoff to surface water - rivers below the reservoirs.

 

Uncertainty Information

The volume is based on modelled and estimated data. Uncertainty is ungraded.

Rule-based environmental flow releases

Data Source

Daily Return System (DRS) database.

Provided by

SCA.

Method

Known release rates from flow meters, orifice plates, valves with rating tables or measured outlets were used to derive the volume.

Assumptions, Limitations, Caveats and Approximations

Nil.

Uncertainty Information

The volume is based on measured data. Estimated uncertainty based on meter accuracy, and professional judgment on rating tables and site configuration issues is +/– 10%.

Treated water releases from wastewater treatment plants

Data Source

See Line item 9.9.

Provided by

Sydney Water Corporation and Shoalhaven City Council.

Method

Assumptions, Limitations, Caveats and Approximations

Uncertainty Information

Unspecified releases and spills from reservoirs and weirs to downstream rivers

Data Source

Bureau of Meteorology: spilling volume from Danjera Reservoir for 2010-10 was estimated using the following input data; estimated precipitation and evaporation, and modelled inflow to Danjera Reservoir using Bureau of Meteorology data, storage volumes of the reservoir provided by Shoalhaven City Council (source: level monitoring at the reservoirs with hydrometric sensors linking to council's radio telemetry system); diversions and other losses for the reservoir provided by Shoalhaven City Council (source: Shoalhaven City Council Telemetry System and daily data registers in respect of the operations of water treatment plants); SCA; SCA Hydstra system and Daily Return System (DRS) databases.

Provided by

Bureau of Meteorology: spilling volume from Danjera Reservoir; SCA: Spilling volumes and other releases from the storage reservoirs and weirs managed by SCA.

Method


Data source

Agency

Method

Bureau of Meteorology

Storage mass balance calculations using measured storage volumes, estimated precipitation and evaporation, and modelled runoff

Sydney Catchment Authority

Spill volumes derived from water levels and rating tables for spillways

Assumptions, Limitations, Caveats and Approximations

Breakdown table by agency

Agency

Approximations, assumptions, caveats/limitations

Bureau of Meteorology

Based on data provided by Shoalhaven City Council, water was not supplied from Danjera Reservoir during 2010–11. Storage losses other than spills have been assumed as negligible. For other approximations and assumptions, see the following line items:

Sydney Catchment Authority

Nil

Uncertainty Information

Uncertainty by agency

Agency

Uncertainty

Bureau of Meteorology

The volume was estimated from measured and modelled data. Estimated uncertainties for some of the inputs used for the calculation of spilling volume from Danjera Reservoir are not quantified. These inputs are precipitation, evaporation and modelled run-off. Therefore, uncertainty for the spilling volume from Danjera Reservoir is ungraded. Estimated uncertainty for storage volume, which was the remaining input for the calculation, is +/– 5%

Sydney Catchment Authority

The volume is based on measured data. Estimated uncertainty based on meter accuracy, and professional judgment on rating tables, site configuration issues and local issues at the time of spilling at each site is +/– 5% to 10%

Comparative year

The following table compares the value at 30 June 2010 reported in this account with the corresponding value reported in the 2010 Account.

 

Comparative information from the 2010 Account for line item 17.2

2011 Account line item

2010 Account line item

Value reported in the 2010 Account (ML)

Value reported in the 2011 Account (ML)

Additional information

17.2 River outflow from the region

14.2 Streamflow to downstream entity

1,196,052

1,196,052

Change in line item name and number