Wilson Inlet 6
 |
 |
 |
Managing the bar and the Inlet
Natural variability
As we have seen a number of variables have been identified as fundamental drivers of the water circulation within the Inlet, water exchange between the Inlet and the ocean and sand movement into and out of the Inlet. There are significant seasonal and interannual variations in the magnitude and effect of these variables that result in very different rates of water and sand movement from year to year.Those variables and some idea of the magnitude of their fluctuations are:
Rainfall/river flow and evaporation: The volumes of rainfall and river flow are major drivers of the water level and salinity in the Inlet and the flushing of the Inlet water out to sea. They have a major impact on the scour of the bar, the dimensions of the channel through the bar and the duration of opening and hence also influence the volume of marine exchange. In the past 50 years the annual Denmark rainfall ranged from 795 mm to 1580 mm; a range of 785 mm, with an average of 1030 mm.
| The rainiest year had almost twice the rainfall of the driest year. |
In the past 25 years the annual Denmark River flow ranged from 5 GL to 81 GL; a range of 76 GL, with an average of 32 GL.
| In other words the highest flow year had about 16 times as much river flow as the lowest flow year. |
In the past 25 year the annual Albany evaporation ranged from about 1200 mm to about 1600 mm; a range of 400 mm, with an average of about 1400 mm.
| In other words the driest year had 30% more evaporation than the wettest year. |
Ocean storminess (swell, sea-breeze waves and storm waves): The ocean storminess is a major driver of sand transport onto, along, and off of Ocean Beach and therefore is a major driver of bar closure and bar channel dimensions, and influences the infill of the delta. Because of its effects on the bar and delta channel dimensions the ocean storminess also effects the amount of marine exchange and therefore stratification. In the past 25 or so years the annual south west WA ocean storminess, based on an arbitrary scale ranged from 0.06 to 4.6; a range of about 4.5, with an average of 1.0.
| In other words the stormiest year was about 75 times stormier than the calmest year. |
Ocean mean sea level and tides: (astronomic tides and most significantly barometric effects.) The ocean mean sea level and tidal effects are major drivers of the scour of the bar channel and more importantly are major drivers of the marine exchange via the pumping of water into and out of the Inlet. Consequently they are also important influences on the salinity of the Inlet and the extent of stratification. In the past six years the annual Albany mean sea levels ranged from 8 cm below AHD to 10 cm above AHD; a range of 18 cm, with an average of 1 cm above AHD.
| In other words the mean sea level was 18 cm higher in the highest year than in the lowest; this volume represents about 10% of the Inlet volume, and roughly a 20% difference between years in scouring capacity when the bar is opened. |
In the past six years for the spring period the average Albany barometric pressure ranged from 1014 hPa to 1020 hPa with between 4 to 8 low pressure systems passing through, while the sum of air pressure changes varied from 550 hPa to 850 hPa over the spring period.
| In other words there was at least 1.5 times more barometric pumping of water into and out of the Inlet in some years compared to others. |
Wind (direction, speed and persistence): The wind is the major driver of vertical mixing in the Inlet and therefore has an impact on the extent of stratification. An analysis of Albany wind data gives some broad indication of variability. The annual average of the magnitude of the Albany half hourly mean wind speed data for the past six years ranges from 3.4 knots to 5.1 knots; a range of 1.7 knots.
| In other words the windiest year was 1.5 times windier than the least windy year. |
