IMPACTS OF WIDENING THE RIP

Packo's prediction for the slack water times at Port Phillip Heads.

Moderator: packo

DAE avatar
baza
Posts: 3
Joined: Sat, 09 May 2020 4:59 pm

IMPACTS OF WIDENING THE RIP

Postby baza » Mon, 11 May 2020 2:51 pm

I did send you (I think I did it correctly but if not here goes again ) a request last week for help to see if you could point me in the direction to any body or hydrologist who might be able to assist me in working out the impact that widening the rip at port Phillip heads in the early 1900's and then again recently in 2004 in the channel dredging episode.

I am trying to estimate the impact those 2 events had on the average high water mark across the whole of port Phillip Bay.



DAE avatar
packo
Forum Moderator
Posts: 40
Joined: Thu, 22 Mar 2018 8:34 am

Re: IMPACTS OF WIDENING THE RIP

Postby packo » Tue, 12 May 2020 4:32 pm

Hello baza. You wrote:
(I think I did it correctly but if not here goes again ) . . .

Not sure what happen in that first try - but the system says you have only made the one post, and there have been no PM's (personal messages) to me. Never mind all that as I now have received your question - and it is a bit of a controversial one!

I'm afraid I can't help with any resources relating to the early 1900's "widening" (it is really only "deepening"). I know they dynamited quite a few limestone pinnacles that were considered a shipping danger. However I assume this was fairly small scale in the scheme of things because the major dangers of Corsair & Lonsdale rocks seem to have been left pretty much intact. It is doubtful if these early episodes had any measurable effects on the tides.

The most recent "Channel Deepening Project" or "CDP" removed rock and sand on a far more "industrial scale". Although the design work started around 2004, the project was opposed by many groups with the result the dredging at the Rip didn't occur until 2008. The main targets at the Rip were removing around 3m to 5m from parts of Rip Bank and Nepean Bank along the main shipping leads. This gave a minimum depth of around 17m over a width of about 250m.

They also nibbled off a few bits & pieces offshore from Queenscliff and Portsea to achieve a minimum depth of 16m in that region. A major aspect of the project was sand dredging of some South Channel areas between Rye and McCrae to achieve a minimum depth there of 15m. There was also some significant sand/mud dredging at the north end of the Bay but I don't have any details on that.

Back to the major point of your post:
I am trying to estimate the impact those 2 events had on the average high water mark across the whole of port Phillip Bay.

The answer here is basically "not much". Although a "Greenie" at heart, at the time I was frustrated because way too much time was spent battling this project on its "increased tides" aspect, rather than the far more important long term issues of buried toxic waste, increased swell penetration, and potentially increased storm surges.

The effect of the CDP on the tides has been proved to be close to the anticipated result: high tides a little less than 1cm higher and low tides a little less than 1cm lower. The increase in tidal range is around 1%.

Common sense said this would be the likely effect of increasing the cross-sectional area of the Rip by about 1%. The image below is a bit fuzzy, but shows my checking of how much the Rip dredging would change its cross-sectional area.

Image

Some blue wedges folk banged on about "increased tides" claiming a huge loss of foreshore area due to CDP without putting it into the proper perspective. Everyday weather effects can easily raise (or lower) the high tide levels by twenty times this amount, and up to 70 times during severe storms.

So the "increased tides" issue wasn't a big deal in my book despite much public fuss and confusion at the time. I did hear one very elderly lady getting rather upset saying "I hear the tides will be 100% higher!" (rather than 1% higher).

****** Other Effects ******

As a long time boater in the Heads area I would claim that swell penetration into the Bay on the flood tide has increased significantly. In the pre-internet days we used to judge outside swell height by the clearly visible rise and fall in Bay waters while heading westward past the Quarantine Station. Nowadays you can begin to feel a big outside swell on the flood tide while passing Pt King near Sorrento (about 5km further inside the Bay.)

As far as higher storm surges goes I think there is evidence for this as quite a number of 50+ year old beach boxes from Rye to Mt Martha have been undermined and collapsed since the CDP. Some low lying areas on the Bellarine Peninsula also seem to have experienced more frequent and higher inundation levels. Some will claim it is climate change rather than CDP.

Sea level rise itself is currently running at about 2.4cm per decade, so that in itself is unlikely to be the cause. More frequent and or more intense storms might be a possibility. It seems the only way to differentiate between climate change and the CDP is to look at the time-scale of any increase of storm surges. We might expect any increase to be gradual over time if climate change is the cause, but more suddenly after 2008 if it is a CDP effect.

The CDP planning process had experts claiming any increase in storm surge height would be minimal and only detectable by "sensitive instruments". This claim is based on the fact that although the Heads are narrow, they do not offer any protection from storm surges entering from Bass Strait. This is because the time scale of storm surges is several times longer than that of the normal tides. This means the surges aren't "filtered" by the Heads and will come through at almost 100%, regardless of any small changes made by dredging. Note the faster changing tides are reduced by around 40% in height when they come through the Heads.

This is a perfectly valid argument that I do accept, even though I have the "feel" of higher surge levels at my local beaches. (More erosion, more frequent "zero freeboard" at the local boat ramp jetty, completely "drowned beaches" more often, etc.) Maybe there is some other effect in operation such as the combination of swell penetration and storm surge that delivers higher water levels to places well beyond the reach of the swell alone. The quote below is from an old "Dive-Oz" post (2016) of mine on this topic.
The "trigger level" for serious beach erosion in my area seems to be when the Hovell Pile tide gauge off Rosebud reaches a reading of 1.5m or more. This gauge has been in operation for 25 years. In the 17 years before the CDP, the 1.5m level was breached three times: 1991, 2003 and 2004. On average, this was once every 5.5 years.

In the 8 years since the CDP, the 1.5m level has been breached eleven times: 2008, 2009(twice), 2011(twice), 2013, 2014(twice), 2015, and 2016(twice - so far). On average this is about once every 9 months. The strong el Nino in the winter-spring of 2015 depressed sea levels by around 10-15cm, and in doing so just saved 2015 from being another "double banger" year.

If we correct these results for the fact that the sea level itself has risen by about 0.05m during the 25 years, we get two more events in 1994, and one more in 2009 that might be regarded as "today's 1.5m equivalent". This would bring the "pre-dredge" average down to once every 3.5 years and the "post-dredge" average down to once every 8 months. By this crude measure, the significant erosion frequency seems about five times more often now.

This higher frequency does not allow the beaches to recover before the next onslaught. Several quiet years are needed before wind blown sand accumulating at the back of the beach can be colonised and stabilised by grasses.

Unfortunately I can't update that with any authority as data beyond 2015 is no longer updated. However from memory the pattern continues with the 1.5m level being exceeded at least a half-dozen times over the period from 2016 to now.
Update 17/10/2020: Well the 2015+ data did eventually get updated. The years for 1.5m+ level occurrences at Hovell Pile are: 2016 (twice), 2017, 2018 (3 times), 2019 (twice).

The reason the Bay and bayside infrastructure is more susceptible to storm tides compared to areas along the ocean coast is because a big storm surge of +80cm on the ocean coast is less than half of the typical tidal range, whereas inside the Bay that same +80cm surge can double the normal tide range.

It should be noted that the Hovell Pile values mentioned earlier are all max "storm tide" heights, ie. storm tide = observed tide = predicted tide + storm surge. A proper analysis would need to separate out just the storm surge component and do a frequency analysis of surge heights both pre and post CDP, before any proper conclusions might be revealed. Unfortunately this was never done, largely because the OEM (the then Office of Environmental Monitor) spent much of its efforts comprehensively studying the "increased tides" beat-up, rather than the potentially more serious effects. (Thanks Greenies!)

So bazza while the answer to your query is "hardly any effect on the normal daily tides", you might be aware that the proposal for a new "Bay West" container port includes a much more significant dredging program. I think they are talking about 20m minimum depths and doubling the width of the Great Ship Channel at the Heads. That might be a whole different and unwelcome ball game!

cheers,
packo



DAE avatar
baza
Posts: 3
Joined: Sat, 09 May 2020 4:59 pm

Re: IMPACTS OF WIDENING THE RIP

Postby baza » Tue, 12 May 2020 9:52 pm

Thanks for the reply

I've done a bit of research over the years and basically, in a nutshell, the answer re the "blasting" of the rip is as follows, and its a bit more than you had imagined I think.

The records indicate that the blasting in the early 1900’s removed approximately 3 million CBM of rock and the more recent CDP effort removed 500,000 cbm (of rock ) and 14.5 million CBM of sand

this means that in round numbers 3.5 million cbm of permanent rock bottom has been removed since the early 1900's .

I am interested in this because I live on the beachfront and my story in relation to that is described in a proposal I was going to send to the various heads of departments who are involved in the management of the Bay, but what I am trying to determine is if the removal of 3.5 million CBM of material could cause a 50ft (horizontal) change to the high watermark in Port Phllip bay over that 70 year period.

here is a link to where I started to explain my position and my interest in this matter..its an interesting story if nothing else, but in all of the hoo-ha that went on when the CDP exercise was in the news , I never saw any comment about the fact that the previous "blasting" episode at the rip in the early 1900's supposedly had this sort of impact on the bay that the council held up to me in the mid 70's

So I was hopeful of trying to find someone who might be able to help me corroborate it as a possibility now that we know how much material was taken out altogether since Mr Batman decided to visit these shores.

https://www.dropbox.com/s/9bqyjouho7nxp ... l.doc?dl=0

here is a copy of the report from a local paper at the time and added to that an extract of the report of the amount of material removed in CPD

https://www.dropbox.com/s/iotd3fna5pe2i ... .docx?dl=0

Interested to hear your reaction to this.



DAE avatar
packo
Forum Moderator
Posts: 40
Joined: Thu, 22 Mar 2018 8:34 am

Re: IMPACTS OF WIDENING THE RIP

Postby packo » Tue, 19 May 2020 12:54 pm

Hello baza,

Apologies for the delay in replying but I used the recent good weather with a couple of days doing measurements off Portsea and in the Rip. Its all done from a Kayak so is covid-19 "lockdown legal" (I think?). Plenty of "essential exercise" in that, but it leaves me a little worn out.

You are right, the extent of the "1900s" Rip blasting was way more than I thought. I found both your drop-box articles a very interesting read. I have also had frustrations with "Councils being Councils". Good that your battle seems to have resulted in a slightly satisfactory outcome.

In regard to the Rip blasting, the amount of explosives used was a surprise. As a diver I really feel for the poor old marine life resident on Rip Bank. It would have been total carnage, and it carried on for so many years. Not to mention that it seems back then, they were allowed to "dispose" of some of the rocks and rubble by allowing it to fall down into the deep entrance canyon (over the sponge beds and sessile animals living there). Although some of that happened in 2008, there were measures to try and prevent that sort of extra damage.

As a young kid, my family used to holiday at Rye in the late 1950s and early 1960s. I do remember the Ports & Harbours steamer "The Rip", mentioned in your second article, used to regularly tie up off the end of the Rye pier. We knew from an uncle, a long time Rye Fisherman, that she was involved in blasting operations at Port Phillip Heads.

I'm not sure whether she was too big to actually tie alongside the pier, or it was the presence of explosives onboard that made her anchor some distance out, but with a couple of stern lines back to the pier. I think there were warning signs "danger keep clear" but the stern was close enough to the pier to talk to the off duty crew who occasionally passed their time on the rear deck.

We used to snorkel out and swim around her. I don't recall ever being told off by the crew. In fact my brothers and I used to dare each other to do a breath hold swim right under the hull from one side to the other. I do remember when we popped up on the other side and surprised pier onlookers, we then used to gasp heavily and generally carry on about our daring feat. Probably just playing to the audience, but the truth is there wasn't much clearance under the keel and it was a very dark and gloomy place at the half way point. Also not the best place to be if the explosives magazine did happen to blow up!

Back to your original enquiry about horizontal shifts in the high water mark (HWM). There are several points I would make.

a) The 3 million m3 rock removal you mention for the "1900s" blasting period was I believe for the original plan. Because of cost and navigational improvements this was scaled back to around 75% of that figure. Nevertheless it is still say four times the size of the 2008 CDP rock removal. I don't think many people are are aware of that comparison. Thank you for bringing it to our attention.

It is however difficult to "guesstimate" how the average tide height would have increased without details of exactly where it was removed from. (The diagrams referred to in the article didn't appear in the download I got.) The best guess might still be some number less than a 5cm increase for the two projects combined.

Depending on the beach slope (typically from 1 in 10 to 1 in 50) this would move the high tide mark by at most a meter or two IF THE BEACH REMAINED PRECISELY THE SAME OVER THAT 100 YEARS.

b) The reality is that beaches and shorelines are essentially "dynamic in nature". In cases where the beach profile appears to be stable over some decades, the reality is that the sand deposition and removal rates due to wind, waves, & weather have both remained as fairly equal numbers over long periods of time. If these "to" and "from" transport rates along a beach are even slightly different there will either be beach erosion (net loss) or beach accretion (net gain).

c) As I understand, your predicament is(was) that the rear boundary was defined in the original title as being 100 feet back from the HWM. The 100 feet setback from the HWM being the width of the public reserve between you and the water. The flaw in this type of title is that while you might have expected it to mean the HWM at that time, discovering that in practice it really means "whatever the HWM is today".

From my perspective the major reason you may have lost 50 feet of your property is not because a slightly higher tide level has "flooded" half of the original public reserve width (and so pushed your rear boundary back), but most likely that it is beach erosion that has removed some of the public reserve sand and so allowed the "old high tide level" to advance say 45 feet closer to your original back boundary, plus say an extra metre or two of advance due to the say 5cm increase in the high tide level due to the combined dredging projects.

I know that is probably not the opinion you wanted to hear. By the same token I think it is prickish of the Council or land titles office to be trimming back your block when it appears to me that it is really the public reserve land that has eroded and not yours. Perhaps in such circumstances it would be more logical for the landholder and the public to share any loss. (Can't imagine the difficulties of having a time varying "rear boundary" though.)

From aerial photos I see the problem seems to have been resolved and that it was not so much a fight over "a backyard" in the normal sense, but all about what must be a glorious view over the beach, pier and Bay. The extraordinary coincidence about your local MP seems to have played a major part in him tackling your Councils earlier position.

d) Your story highlights the tricky nature of "beachside land ownership" in the present times and into the future where most scientists expect rising sea levels to really get going. Is your local government area Kingston? There is a group called "ABM" (Association of Bayside Municipalities) who fund and run the "Port Phillip Bay - Managing Better Now" program. They have engaged consultants to provide background info and possible future directions. So far there have been five reports released. You might find Nos, 2,3,& 4 could be useful. (No. 5 might be too scary.)

e) You might not want to hear this but in this battle with Mother Nature there are sometimes winners as well as losers. Among the losers are some areas around Portsea and Sorrento. Midway between these is Lindsay Fox's beachside property - a stunning winner over the last decade. It has been astonishing to see (as I regularly pass by on the water) the enormous beach growth there. It is well over 50m now, and all well elevated and stabilised with a good grass cover.

Extra land for free! He went to various high courts and got some special parliamentary legislation to make sure his legal rear boundary grew seaward as the HWM retreated. Neighbours might not be so pleased though as some have private jetties, fully equipped with covered boat hoist, swimming change rooms, etc - all now "stranded assets" totally surrounded by high and dry sand.

cheers, and thanks for your unusual yarn
packo

PS. Re your comment about partially "shallowing" the Rip again and sending the bigger ships around to Westernport. That wouldn't help much because it is not the regular (or slightly raised) tides that is the problem but waves and storm surge that are doing the damage. Only some form of lock system can prevent high surges getting through.

I did recently read some group was seriously proposing "Resealing the Nepean Bar". This translates into building up a barrage across "The Great Sands" from say St Leonards to Rosebud. There would be locks for smaller scale ships to proceed to either Melbourne or Geelong. In addition a deep-water port would be established just north of "Snapper Deep" in the south channel (opposite Sorrento).

This scheme would expose parts of the Mornington and Bellarine peninsulas to significantly higher tides, but would protect the "Greater Melbourne" shoreline.

Interestingly this proposal would dramatically reduce the tidal currents through the Heads down to around just a tenth of what they are today and make the port safer for shipping. It is a very "left field" proposal but the proponents say that they believe we are near a tipping point where unstoppable collapses in various ice sheets will give 2m to 5m sea level rises one or two hundred years out from now. Without such dramatic and desperate intervention they claim many coastal cities would be "stuffed" if those scenarios do play out.

I watched the ABC's "4-corners" program last night that was a shocking story about politics and Australia's abject failure to address climate change risks over the last 20 years. Parallels too in the way some counties are ignoring the science around how to best deal with the Covid-19 pandemic.



DAE avatar
packo
Forum Moderator
Posts: 40
Joined: Thu, 22 Mar 2018 8:34 am

Re: IMPACTS OF WIDENING THE RIP

Postby packo » Wed, 03 Jun 2020 11:59 pm

Packo again, with an afterthought post:

I've had some second thoughts about my last reply to baza. Perhaps it was the way baza's original post was framed, but I unintentionally slipped into the "its all about the Heads" mindset that I have previously criticised others for, including the port authorities and those behind the official slack water predictions.

The fact is that in terms of controlling water flow into or out of the Bay, the shallow Great Sands area is as important as the narrow entrance at the Heads. The CSIRO confirmed this during their big Bay study in the 1990s. It is also fairly obvious from the tide curves which show that at max Heads inflow, the height drop across the Great Sands is roughly the same as the height drop through the Heads.

I momentarily overlooked this when I saw baza's point that compared to the 2008 CDP dredging at the Heads, the prior long periods of blasting during the early to mid 1900s actually removed around four times as much rock at that location.

Reeling a little in surprise, I then responded that perhaps the increase in tidal amplitude caused by the earlier deepening might have been for times more than the approximately 1cm increase caused by the more recent channel deepening project (CDP). I now am pulling back a little from that position and suggesting a lower "guesstimate" of around just a 2.5cm high tide increase due to both projects.

What I overlooked was the CDP was mainly about South Channel dredging (14.5 million m3) compared to 0.5 million m3 of dredging at the Heads. So to compare the sizes of the two dredging projects based only on the Heads component could be misleading.

It is likely that any 1900s dredging in the South Channel was much smaller in size than the CDP because it mainly concentrated on a small section called "the cut". Most other areas already had sufficient natural depth for the planned drafts back then.

I note that in the OEM "before and after" tidal analysis reports following the CDP shows that while the increase is similar in absolute terms (0.5 to 0.8cm) for all Bay areas, in percentage terms the increase is higher north of The Great Sands than south of it. This supports the view that part of the increase in tidal range is attributable to the dredging of the South Channel rather than it being "all about the Heads".

So the final points are that the increase in high tide due to:
a) ALL dredging programs is fairly small with a "Packo guesstimate" at around 2.5cm.
b) sea level rise from 1920 to 2020, is approximately 15cm.
c) severe storm surges, is around 70cm.

cheers,
packo



DAE avatar
baza
Posts: 3
Joined: Sat, 09 May 2020 4:59 pm

Re: IMPACTS OF WIDENING THE RIP

Postby baza » Sun, 07 Jun 2020 9:26 pm

Thanks Packo ,

Appreciate the time you have spent on my enquiry.

Based on your estimate, the figure I was advised of a 50 ft impact on the high water position on the shoreline of Port Phillip bay (or at least in the city of Kingston) due to the early 1900's operation might be incorrect, although I am not sure how much 25 mm or so overall increase in bay water level would have on the shallow shoreline.

When I thought about it a bit more I guess it doesn't make much difference to the level of the Bay as a whole how deep they blast the channel at the heads , that would allow more local water to flow in and out between the high and low tides and or storm surges and that seems to have been evident by the more recent beach erosion down the southern end of the bay, but natural equilibrium eventually takes over as far as the whole bay is concerned I guess, and that can only really be effected by an overall increase of sea level in the ocean over time..or subsidence in the bay whichever comes first.

Nice chatting with you and keep safe.




Who is online

Users browsing this forum: No registered users and 0 guests