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Sewers Studied

Looking at infrastructure needs

 

Click here for larger map of above aerial photo

Is Dilution the Solution?

To get around just dumping raw sewage in harbour – city built a deep outfall (“I”) which ‘traps’ the plume well below surface, unlike the other 8 outfalls (above)

Present system of Outfalls

The yellow circles are sanitary outfalls the white ones are ‘combined’ sewage and run-off

Outfall I is deep and the plume rises to only about 40 meters

 

Outfall

Diameter mm

Length M

Depth of Discharge Below LLW—M

A

600

60

18

B

450

71

6

C

1200

58

15

F

200

50

15

G

375

37

11

H

1200

58

22

I

750

316

64

J

300

120

22

K

300

41

22

L

450

103

8

 

Sewage means both ‘nutrients’ but also bacteria (“coliforms” are measured) – here red = highest counts--- computer files included in CD Rom so can follow dispersion over a day and at different depths--- Model ‘detected’ an eddy – top right (white arrow)—at Venn Passage that has effect of pushing sewage water onto far side of harbour on flood tides

Outfall I sends out a plume that is mainly below 40 meters --- though I suspect a weakness of this work is that we have little real information on how quickly waters at that depth moves out of harbour – and if any gets ‘trapped’

 

Ugly chemicals accumulate in sediments

While the above images are computer models that have not been verified – the graph left is from actual testing of harbour sediments for PAHs etc .
Bad news – outfall I has large hit of material --- coming out of the sewer line and not dispersing very far away.

Real Data and the Model

Thermoclines

This study has lots of information that is useful to anyone interested in how the harbour system works

 

Here is the summer and winter water profile – in summer there is a freshwater layer on top – then some transition water – then deeper colder saline water below

In winter the water column loses its stratification and the winds mix the water column well down. But the deeper more saline water persists and thermocline not lost – just deeper

 

Created by LG Wednesday, May 28, 2003