The question is sometimes asked is “how deep is the water in the canals?”.   There is not a single simple answer as it depends on time and location.

In discussing depth, it is first necessary to understand various datums from which the base elevation of the canals may be measured from.  For most Civil Engineering projects in Australia constructed since the mid 1970’s elevations are referenced to Australian Height Datum (AHD).  This datum was established as part of the preparation for conversion to the metric system in Australia.  Prior to the establishment of AHD, local datums were used.  For Perth this was Low Water Mark Fremantle.  In a rural town it may be some local reference point like the foundation stone of the post office. Over the period of 1966 to 1968 tide records for 30 locations were analysed and the results tied together with high precision surveys. AHD was the mean sea level for 1966 to 1968.  The nearest port to Busselton included in the establishment of AHD was Bunbury.  Thus 0.0mAHD in Busselton is approximately equal to mean sea level (MSL) in Bunbury nearly 60 years ago.  MSL in Port Geographe is now about 0.13m higher than AHD.  This is not all sea level rise but is a combination of many factors such as the density of underlying bedrock, the existence of a broad shallow bay and the Leeuwin Current bring warm, less dense water down from the North. Depending on the location, the canals in Port Geographe had a design elevation for the base of -2.8, -3.0 or -4.0 relative to AHD.   Broadly speaking the elevation of -4mAHD applies to the Marina area and the others to canals.  The water is shallower within about 6m to 10m of the canal walls where there is rock rip rap to prevent undermining of canal walls by propeller wash.

AHD is great for civil design and construction but not so good for navigation of boats.  For boating, sailors are more interested in how much water they have at low tide.  For Port Geographe, water depths are linked to Chart Datum (CD).  The tide gauge for Port Geographe is located near The Deck.  You can get up to date tide levels relative to CD by googling “real time tide data” and going to the dot.wa.gov site and clicking on Busselton.  For Port Geographe, a Chart Datum value of 0.0m CD corresponds to 0.68m AHD or 0.81m below MSL.  If a canal was excavated to -3.0m AHD (and there has been no change) and the tide gauge reads 0.4m above CD, then the water depth is (3.0 -0.68) + 0.40 = 2.72m.

If you get your tide information from Willy Weather or a similar source, then this will be different to that from the dot.wa.gov site. Willy Weather uses a slightly different datum again and only gives predicted tides.  The dot.wa.gov site gives real time tides that can be influenced by factors such as barometric pressure and wind direction.

The depth at construction is not the only factor affecting water depth. Over time there is an accumulation of material that settles in the base of the canals reducing the available depth for boats.  This is most obvious in the entrance and North end of the Grand Canal, where seagrass, shed and dislodged from Geographe Bay, in early winter is carried into Port Geographe by a rising tide and then settles in the low energy protected waters.  There is also a West to East movement of sand in Geographe Bay and some of this is carried into the entrance and settles.  Sand can also for a sandbar in the Bay, near the Entrance.  It is the Entrance and North end of the Grand Canal where you can observe the dredge working.

Other contributions to reduced navigable depth in the canals and marina come from sand and dust carried by offshore winds and from the occasional bit of outdoor furniture and other items blown into the canals in storms.

Comprehensive depth soundings were carried out by Department of Transport WA for the whole of the canal system.  Apart from the Entrance and Grand Canal, other places with a significant reduction in depth are Pearler Canal (the one behind what used to be Sensations Café) and Trysail Canal.  In these two canals the accumulation of seagrass is so great that it could impede yachts with deep keels.  There has also been a noticeable loss of depth in Mizzen Canal and Fathom Canal.  This is of lesser concern as being South of the footbridge, these canals are not used by yachts with deep keels.  If you become aware of other significant shallow spots, please mark these on a plan as accurately as you can and forward the plan to info@portgeographe.com.au.

If you thought that datums for depth measurement was a bit complicated, the sorting out who is responsible for maintaining canal depths is even more of a challenge. With past developers going into administration and agreements/deeds of agreement when the breakwaters were realigned, things get a bit messy. The following is an Engineer’s interpretation.  A Lawyer might come up with a different interpretation.

Overarching responsibility for maintenance of canals (outside the privately owned mooring envelopes on residential Lots) sits with the City of Busselton.  When the breakwaters were realigned about 15 years ago, The WA Department of Transport became their “agent” and took on responsibility for canal and entrance dredging in the Port Geographe Coastal Management Area.  The City hands over the Specified Area Rate (SAR) funds to the DoT as a contribution to dredging costs.  Within the Marina Lease, maintenance of adequate depth sits with the Marina leaseholder.  For the balance of the canals, (except for the mooring envelopes within canal Lots) responsibility sits with the City of Busselton.

Who do you contact if you are having trouble moving your boat due to accumulated seagrass in the canals?  In the first instance you should contact the City of Busselton.  Please also let Port Geographe Landowners Association know so we can lobby on behalf of affected Landowners and Residents.