The “Venice Effect”

On my recent excursion to Port Stephens, I drove along Marsh Road between Salt Ash and Bobs Farm. This road runs parallel to Tilligerry Creek and is bordered on the north side by a healthy stand of mangroves. The road serves as a dyke to prevent salt water from entering reclaimed farm land on the opposite side. Over the years I have observed the penetration of mangrove seedlings and now tall shrubs onto land that has been drained and farmed. Many farms are now abandoned. The question I ask myself when I visit this area is whether I am witnessing a slow rise in sea level with more frequent high tide inundation along the roadside, or simply a disintegration of the drainage system allowing salt waters to re-enter reclaimed land.

I became aware of the issue of frequency of high tide inundation in estuaries in reading the work of Jim Titus in Maryland Law Review (1998). Jim produced this work as part of his graduate studies while employed by the US EPA. He was one of the first to articulate the legal and policy aspects of what was being observed in and around Chesapeake Bay. He visited Marsh Road with me in 2002 and saw similarities with that in eastern USA. In 2008 I visited Venice and experienced a minor flood event in St Mark’s Square on a very calm day. On making inquiries I learnt that such was not uncommon, in fact similar events were occurring around 250 days a year compared to less than 100 per year 50 years or so before. What constituted an event was not clear although the well documented acqua alta resulting in board walks being placed across the square are well known. So what is happening? Has there been an increase in the frequency of tidal inundation in Venice, if so what is the cause?

It is not difficult to find information on flooding of Venice. There are many causes including winter storms, subsiding land, wind direction, sea level rise and human intervention in the management of the waterways. A combination of these phenomena in 1966 saw the waters rise to record heights when waters rose nearly 2m higher than the more usual flood levels. But the figure that did interest me was the record of what could be called normal or simply nuisance flooding which is defined as only covering the pavement. In comparing 1972-74 with 1962-64 there was an increase in 80% of normal flooding. That was what I was hearing on my visit. Again it was not clear as to cause but the increase in frequency has become more than a nuisance; one shop keeper is reported to have recently said she can expect 100 days a year of some form of flooding whereas 10 years ago when she moved in the frequency was 30 to 40 days per year.

A recent paper in Earth’s Future (2017) by Moftakhari et al., on nuisance flooding by the tides discussed the cumulative effects of tidal inundation occurring at increased frequency in several US cities. For instance, in Washington D.C. the number of hours of nuisance flooding per year has grown from 19 between 1930 and 1970 to 94 over the last two decades. By 2050 they are projecting the increased frequency to be as much as 700 hours per year impacting adversely on infrastructure, monuments and businesses. The authors refer to the inundation as clear-sky flooding and attribute the change in frequency to sea level rise associated with climate change. They have created a cumulative hazards index to pinpoint locations that are at risk in the long-term and will require intervention to prevent serious adverse impacts beyond mere “nuisance”.

I have used the term Venice Effect on occasions to highlight the issue of increased frequency of high tide inundation. The king tide photo project initiated by Phil Watson of OEH in NSW is one attempt to document the local changes in tidal elevation over time. We recognise that such changes at any one place may not be easily determined. However, it is important that local records are kept. I do this at Parsley Bay in Sydney Harbour on occasions when I know tides will be at or above the predicted, and casual observations at places like Marsh Road may help. In addition, there are those studying mangroves and salt marshes in a systematic way who are in a position to pinpoint the direction of change and ecological impacts. I would encourage our coastal ambassadors of the future to be involved in taking observations from fixed positions to record what is happening in their area.

Words by Prof Bruce Thom. Please respect Bruce Thom’s thoughts and reference where appropriately: (c) ACS, 2017, posted 7th March 2017, for correspondence about this blog post please email admin@australiancoastalsociety.org

#72