Swimming Conditions

Swimming conditions in this website are predicted from the likelihood of bacterial contamination at typical recreational sites based on correlations with salinity levels as a proxy for rainfall and pollutants runoff. Swimming sites could also be affected by harmful algae blooms which would not be informed through this website. For further information on the presence of harmful algae blooms visit Hornsby Water Quality or call Water NSW algae hotline 1800 999 457.

  Pollution is likely  Swim at own risk   Pollution is unlikely



Location Salinity
(average last 24 hours, parts per 1000)
Latest Data Status % 'Pollution unlikely'
in last 12 months
Bar Island 30.8 23 Jun 06:15 96
Bennetts Bay 29.4 23 Jun 06:15 96
Brooklyn Baths 33.9 23 Jun 06:00 100
Calabash Bay 28 23 Jun 05:45 86
Courangra Point 25.8 23 Jun 06:15 99
Crosslands Reserve 15.9 23 Jun 06:15 17
Dangar Island 34.1 23 Jun 06:00 100
Deadhorse Bay 33.2 23 Jun 06:00 100
Gunyah Point 34.3 23 Jun 06:00 100
Laughtondale 13.5 23 Jun 06:30 99
Milsons Passage 32.3 23 Jun 06:15 100
Sandbrook Inlet 31.8 23 Jun 06:15 100
Twin Beaches 29.9 23 Jun 06:15 100


Overview of Swimming Conditions Calculations

Swimming conditions are predicted from the likelihood of bacterial contamination based on recorded salinity levels as a proxy for rainfall and pollutants runoff. The following steps are undertaken in the calculation of swimming conditions:

  1. Salinity data is collected every 15 min in the lower estuary using six real-time monitoring stations. Recorded data is download at 7 am each morning at all monitoring stations.
  2. At locations without recorded data, salinity is estimated using correlated values from nearby recorded data.
  3. The latest salinity is calculated at each location as the average of the last 24 hours.
  4. The latest salinity is compared to different thresholds depending on the location to determine a given swimming condition. Thresholds for each location are shown in the table below.
LocationThresholds
Crosslands ReservePollution is likely = Salinity is less than 20 ppt
Swim at your own risk = Salinity between 20 and 22 ppt
Pollution is unlikely = Salinity is more than 22 ppt
Courangra Point & LaughtondaleThresholds are based on the historical 30th and 10th salinity percentiles
All other locationsPollution is likely = Salinity is less than 21 ppt
Swim at your own risk = Salinity between 21 and 24 ppt
Pollution is unlikely = Salinity is more than 24 ppt

The above thresholds have been calculated based on Hornsby Shire Council's water quality monitoring program and the 2018 National Health and Medical Research Council Guidelines for Managing Risks in Recreational Water. These thresholds take into consideration the NHMRC categories based on the 95th percentile for Enterococci and the salinity values being monitored from Council’s on-site water quality monitoring station. These thresholds will change as more bacteria data comes available in particular after wet weather sampling. The occurrence of freshwater in the estuary following rainfall increases the likelihood of pollution being present. Obvious signs of stormwater pollution are water discoloration as well as debris in the water and tide line. When pollution is likely, swimmers may be at increased risk of contracting illness and swimming at these times should be avoided.


Disclaimer

This system utilises recorded data captured by Manly Hydraulics Laboratory (MHL) on behalf of Hornsby Shire Council. Recorded data is download at 7 am each morning at all monitoring locations. Displayed salinity values are calculated as the average of salinity measurements for the latest 24 hours of data. At locations without recorded data, salinity is estimated using correlated values from nearby recorded data.

IMPORTANT: This information has been recovered directly from automatic recording equipment and has not been quality controlled by MHL. The State of NSW (of which MHL is an agency) does not warrant the quality, accuracy, currency, reliability or correctness of this information and accepts no liability for any loss, injury (including death), or damage which any person relying on this information may suffer.