Blog, coast, coastal flooding, tides

After the supermoon, comes the supertide

The Conversation

This article was originally published on The Conversation. Read the original article.

Ivan Haigh, University of Southampton and Kevin Horsburgh, National Oceanography Centre

The city of Plymouth, on England’s south coast, normally has fairly moderate tides. However this week it will have a 6m “supertide” – the highest tide in 18 years. This comes just days after the celebrated “supermoon”.

In fact, many locations along the UK, US and Australian coasts will experience their highest tides for tens of years around September 29 or 30. Coastal roads in Miami, for instance, have already been closed in anticipation of exceptional tides.

These high tides may bring water levels uncomfortably close to the tops of harbour walks and flood defences, emphasising the threat of rising sea levels. In the UK they are unlikely to be a major problem on their own unless they coincide with storms (a strong storm surge has a greater impact than even the most exotic of tides). However in other areas, like in parts of America and the Pacific, no storms are necessary: these high tides on their own can lead to nuisance flooding.

Why do we expect such extreme tides?

Tides are controlled by changes in the position and alignment of the moon and sun relative to Earth. Every fortnight – at new moon or full moon – the Earth, sun and moon are in an approximately straight line as seen from space and the additional gravitational pull of the sun causes stronger tides, known as spring tides.

The Bay of Fundy on Canada’s Atlantic coast has the world’s highest tides.

Yet each month one set of spring tides is higher than the other. This is because tidal forces are strengthened when the moon is at “perigee” and its elliptical orbit takes it closest to Earth. Tide-generating forces are also enhanced when the moon is directly overhead at the equator, part of a cycle lasting 27.2 days – a so-called “draconic month”.

Elliptical orbits of: (A) the moon around the Earth; and (B) the Earth around the sun.
Author provided

Tides can differ over the course of a year, as the Earth moves from its closest (perihelion) to furthest (aphelion) point from the sun and back. More important is the variation in the sun’s position north or south of the equator, which causes the seasons. The tide-generating forces are greatest at the equinoxes in March and September when the sun is directly overhead at the equator. Spring tides are always higher at these times of year.

A perfect tide?

Over periods longer than a year, very large spring tides occur when all the astronomical factors we have mentioned earlier coincide.

Two longer-term motions of the moon’s orbit around the Earth are important. These motions (astronomers call them precessions) are the reason we are seeing unusually large spring tides this year.

The first precession is known as the cycle of lunar perigee, and influences tides about every four to five years. The elliptical orbit of the moon around the Earth slowly moves in relation to the sun, completing a full circuit every 8.8 years. This means at either the March or September equinox approximately every 4.5 years the moon is both at its closest point to the Earth, and is also overhead at the equator.

The second precession is known as the lunar nodal cycle and is due to a very slow change in the moon’s orbit. Imagine the Earth’s orbit around the sun took place on an enormous sheet of glass – what astronomers call the ecliptic plane. The moon’s orbit cuts this surface at an angle of approximately 5 degrees. Over 18.6 years the moon’s orbit slowly rotates around so it cuts through the ecliptic plane in a different place.

One effect of this is to change how far above or below the equator the moon can reach in its orbit. In 2015 the moon is at the point where it deviates the least from the equator. This slightly increases the chances of the moon being directly overhead at the equator at any given point, and thus coinciding with the other factors that contribute to extreme tidal forces.

A lot of things have to fall in place at once to generate record-breaking tides and this year the cycle of lunar perigee and the lunar nodal cycle nearly perfectly coincide, resulting in some of the highest spring tides for decades.


The authors help run the SurgeWatch website and would welcome any photos of high tides during this period.

Ivan Haigh, Lecturer in Coastal Oceanography, University of Southampton and Kevin Horsburgh, Head of Marine Physics and Ocean Climate, National Oceanography Centre

Blog, coast, coastal flooding, science, sea level, storm surge, tides

Fieldwork at the Steart managed realignment scheme, Somerset, September 2014

My new PhD student Clementine Chirol has recently been undertaking field work at the Steart peninsula Managed Realignment Scheme.

Steart

The Steart peninsula is the largest managed realignment scheme undertaken in the UK, with 400 hectares of new habitats created to compensate for the losses related to coastal squeeze. To that end, flood defences are moved further inland and previously reclaimed farmlands are opened up to tidal inundation. As saltmarshes naturally attenuate wave and tide energy, the new flood defences will be more durable.

The project was designed and modelled by CH2M Hill on behalf of the Environment Agency; the construction phase was carried out by Team Van Oord. The completed site is now managed by the Wildfowl and Wetlands Trust (Tim McGrath).

The flood defences were breached on 1st September 2014 by Team Van Oord. With the increasing spring tide, the site’s channel was first flooded to full bank on the 7th. The highest spring tide was reached on 10th September (Hinkley Point: 7.05 m).

steart peninsula breach

The aim of Clementine’s PhD is to monitor the morphological evolution of the entry channel and creek network over several years as the site transitions to a more natural shape. Results from this project should help improve the design of future realignment schemes.

This early fieldwork campaign undertaken around the time of the first inundation focused on the creek system: in fact, due to the rapid erosion and turbulent flow, no deployment could be made in the breach area. We had two objectives: firstly, to perform a baseline survey of one of the creeks’ morphology and sedimentology that would help quantify all future changes. Secondly, to observe the early effects of the tide on the morphology and sediment strength at the Steart managed realignment site.

TGPS survey o this end a GPS survey of one of the creeks’ outline was realised, as well as several cross-sections along the length of the creek. Stakes in the ground were used to evaluate the accumulation or erosion of sediment at the banks. Sediment samples and syringe cores were taken to assess the bulk density and the organic matter concentration. The cohesive strength of the sediment was measured every day along the creek with a CSM (Cohesive Strength Meter) during the time of the fieldwork. The successive inundations of the site were monitored by two Gopro cameras covering the studied creek and the entry channel.

This fieldwork was a great opportunity to witness the realisation of an ambitious realignment project. It will also provide a valuable baseline to monitor the evolution of this area.

Australia, Blog, coast, coastal flooding, extreme events, flooding, Journal paper, science, sea level, storm surge, tides

New paper, just published: Australian Sea Levels – Trends, Regional Variability and Influencing Factors

While there has been significant progress in describing and understanding global-mean sea-level rise, the regional departures from this global-mean rise are more poorly described and understood. In this new paper, which you can view here, we present a comprehensive analysis of Australian sea-level data from the 1880s to the present, including an assessment of satellite-altimeter data since 1993.

We find that After the influence of El Niño Southern Oscillation is removed and allowing for the impact of Glacial Isostatic Adjustment and atmospheric pressure effects, Australian mean sea-level trends are close to global-mean trends from 1966 to 2010, including an increase in the rate of rise in the early 1990s. Given that past changes in Australian sea level are similar to global-mean changes over the last 45 years, it is likely that future changes over the 21st century will be consistent with global changes.

 

Blog, science, tides

Time and tide bell

The “Time and Tide Bell” by Marcus Vergette is very cool. From his website,

“This project is to make a permanent installation of the Time and Tide Bell at the high tide mark at a number of diverse sites around the country, from urban centres to open stretches of coastline. The rise of the water at high tide moves the clapper to strike the bell. Played by the movement of the waves, the bell creates a varying, gentle, musical pattern. As the effect of global warming increases, the periods of bell strikes will become more and more frequent, and as the bell becomes submerged in the rising water the pitch will vary.”

Blog, tides

Counting the Cogs – The Mystery Machine

I have always had a fascination with tide prediction machines.

Rescuing Historical UK Sea Level Data

Today I attended a meeting of the Sea Level and Ocean Climate group at the National Oceanography Centre, Liverpool, where Professor Philip Woodworth gave a short talk about tide prediction machines.

Prof Woodworth started with a brief history of tide predicting machines. The concept was demonstrated by Sir William Thomson (later Lord Kelvin) but Prof Woodworth argued that Edward Roberts should also be considered for the title of ‘Father of tide predicting machines’. Roberts was a mechanical engineer who built many of the machines put into use. Only around 25 machines were ever built and 20 of them were made in Britain. The very first machine is now in the Science Museum in London.

One of the very interesting aspects of the talk was a bit of detective work that Prof Woodworth and his colleagues had been involved in. I blogged recently about a film of a tide predicting…

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