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Sydney

                                                                                                   

14.2 Surface water flow out of the region

                             

Supporting information   


Outflow from the Sydney region is the streamflow reaching the sea. The estimated volume for the line item is 1,196,052 ML.

This item includes streamflows resulting from:

  • rule-based environmental flows, other unspecified releases and spills from most downstream reservoirs, and weirs to rivers
  • natural run-off from the catchment of rivers downstream of reservoirs and weirs, and unregulated 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 14.7.9.7. 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 2009–10 (ML)

Rule-based environmental flow releases from storage reservoirs

79,932

Unspecified releases from storage reservoirs

14,746

Spills from most downstream reservoirs and weirs

161,559

Natural run-off from the catchment of rivers downstream of reservoirs and weirs, and unregulated streams leading to the sea

942,301

Treated water releases from wastewater treatment plants

56,556

Less downstream diversions

(59,042)

Outflow to the sea

1,196,052

 

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 2009–10 (ML)

The Sydney Catchment Authority 

Woronora

3,628

Lake Burragorang (Warragamba Dam)

12,154

Prospect

Not applicable*

Lake Yarrunga (Tallowa Dam)

55,727

Blue Mountains**

Not applicable*

Pheasants Nest Weir

7,835

Broughton Pass Weir

588

Total

79,932


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

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

In addition to the volumes shown in the above table, a further 2,450 ML of environmental flow was released from Woronora Reservoir (a high-flow release from the environmental flow bank). This volume is not included here, as it is included in Line item 14.7.9.7. Total environmental flow released during 2009–10 was therefore 82,382 ML.
The above table shows net environmental flow reaching the sea because the released flows were not diverted to downstream reservoirs/weirs and not used for other purposes. Details of released environmental flows including volumes reached at downstream reservoirs/weirs are available in the note 'Water for environmental benefit'. 

Unspecified releases and spills from storage reservoirs

Storage/weir

Other releases during 2009–10 (ML)*

Spills during 2009–10 (ML)

Total during 2009–10 (ML)

Lake Burragorang (Warragamba Dam)

1,147

0

1147

Lake Yarrunga (Tallowa Dam)

13,129**

152,400

165,529

Blue Mountains

0

366

366

Pheasants Nest Weir

0

1,399

1,399

Broughton Pass Weir

470

2,644

3,114

Danjera**

0

4,750

4,750

Total

14,746

161,559

176,305

* Other 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 reservoir release of 736 ML is not accounted in the table because the flow was received at Lake Yarrunga (Tallowa Dam).

** When 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 run-off from the catchment of rivers downstream of reservoirs and weirs, and unregulated streams leading to the sea

Gauged flows are not available to cover all river reaches downstream of reservoirs and weirs, and unregulated 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 run-off was therefore estimated from available gauged flows for the rivers listed in the following table, and estimated run-off for other streams.

Total natural run-off is 942,301 ML for 2009–10, made up of 458,080 ML of gauged flows and 484,221 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.*

Flow volume for 2009–10 (ML)

Comments

Macdonald River at St Albans

212228

4,839

 

Colo River at Upper Colo

212290

191,977

 

Gross River at Burralow

212291

129,759

 

South Creek at Richmond Road

212297

20,537

Excludes treated wastewater released from St Marys Water Recycling Plant, which was accounted for separately

Woronora River at Needles North Engadine

213211

10,607

Excludes 6,078 ML environmental flow release, which was accounted for separately

Shoalhaven River at Grassy Gully No. 2

215216

100,361

Excludes (from the gauged flow):

  • 14,057 ML diverted by the Sydney Catchment Authority to Shoalhaven City Council pumping at Burrier
  • 55,727 ML environmental flow release, 152,400 ML dam spilling and 13,129 ML of other releases from Lake Yarrunga.

Excluded items were accounted for separately.

Total gauged flow

458,080

 

* The Sydney Catchment Authority manages the gauging stations.

 

Natural streamflow in the river reaches downstream of the gauges (above table), and in ungauged and tidal streams

Description

Volume for 2009–10 (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)

484,221

Based on the Bureau of Meteorology calculation of Line item 13.4.1 Landscape water drain to connected surface water
 

Treated water releases from wastewater treatment plants

Description

Volume for 2009–10 (ML)

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

51,834

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

1,980

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

2,742

Total

56,556

 

Deductions from natural river flows for downstream diversions 

Description

Volume for 2009–10 (ML)

Comments

Riparian right releases

14,446

Based on total estimated riparian use provided by the NSW Office of Water (3,650 ML released from Lake Burragorang was deducted from the total use)

Stock and domestic and other lumped access license 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—greater metropolitan region

Sydney Water Corporation diversion from the Hawkesbury River for North Richmond

6,065

Based on the value provided by Sydney Water Corporation

 

Total

59,042

 

 

Quantification approach   


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 reservoirs 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 for rule-based environmental flow releases

 

Data source

Daily Return System (DRS) database.

 

Data provider

The Sydney Catchment Authority.

 

Method

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

 

Uncertainty

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%.

 

Approximations, assumptions, caveats/limitations

Nil.

 

Quantification approach for unspecified releases and spills from reservoirs and weirs to downstream rivers

 

Data source

Agency Data source
The Bureau of Meteorology

Spilling volume from Danjera Reservoir for 2009–10 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).
The Sydney Catchment Authority Sydney Catchment Authority Hydstra system and Daily Return System (DRS) databases

 

 

Data provider

Agency Data provider
The Bureau of Meteorology Spilling volume from Danjera Reservoir
The Sydney Catchment Authority Spilling volumes and other releases from the storage reservoirs and weirs managed by the authority

 

 

Method

Agency Method
The Bureau of Meteorology Reservoir mass balance calculations using measured storage volumes, estimated precipitation and evaporation, and modelled run-off
The Sydney Catchment Authority Spill volumes derived from water levels and rating tables for spillways

 


 

Uncertainty

Agency Uncertainty
The 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 ungraded. 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%.
The 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%.

 

 

Approximations, assumptions, caveats/limitations

Agency Approximations, assumptions, caveats/limitations
The Bureau of Meteorology

Based on data provided by Shoalhaven City Council, water was not supplied from Danjera Reservoir during 2009–10. Reservoir losses other than spills have been assumed as negligible. For other approximations and assumptions, see the following line items:

  • 13.1 for precipitation 
  • 13.4.1 for inflow 
  • 14.1 for evaporation.
The Sydney Catchment Authority Nil.

 

Quantification approach for natural streamflows in gauged streams

 

 

Data source

Stream gauge data from the Hydstra system.

 

Data provider

The 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 Sydney Catchment Authority’s (SCA) 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.

 

Uncertainty

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.

 

Approximations, assumptions, caveats/limitations

Nil.

 

Quantification approach for natural streamflows in ungauged streams

 

Data source

The Bureau of Meteorology modelled the landscape water balance to estimate inflows using 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.

 

Data provider

The Bureau of Meteorology (the Bureau)

 

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 rivers
  • 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 for run-off to Line item 13.4.1 River reaches not within SCA operational area and the reservoirs).

 

Uncertainty

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

 

Approximations, assumptions, caveats/limitations

See in Line item 13.4.1 River reaches not within SCA operational area and the reservoirs.

 

Quantification approach for treated water releases from wastewater treatment plants

See Line item 13.7.1 Urban wastewater discharge to connected surface water and Line item 13.7.2 Urban recycled water discharge to connected surface water.

 

Quantification approach for downstream diversions from flow leading to sea

See the following:

  • Line item 14.5.1 Surface water diversion – riparian right for diversion associated with riparian right
  • Line item 14.6.5.4 Diversion of unregulated flows – urban holder for diversions by Sydney Water Corporation from the Hawkesbury River
  • Line item 14.6.5.5 Surface diversion of unregulated flows – individual holder for stock and domestic diversions
  • Line item 14.6.5.6 Surface diversion of unregulated flows – other lumped holders for diversions associated with other access licences.