Expert Details


Ever wondered what role the ocean plays in controlling the climate? Ask Giselle

Name: Giselle Firme
Title: Marine Conservation Officer
Organisation: Nature Conservation Council of NSW

Description:

Giselle Firme is a biological oceanographer and an expert on iron fertilization. Her interests lie in the role of the ocean in regulating climate, and educating the public on the importance of conserving the incredible rich life of the sea that we happen to depend on to breathe. She has recently presented the talk Iron Fertilization: Ecological and Policy Implications, at the 2008 Global Conference on Oceans, Coasts and Islands, in Vietnam.

Giselle at the Eat Local. Save Planet : Sustainable Seafoodies dinner hosted by food writer John Newton on September 26


 
Giselle at Manly Ocean Day

Questions and Answers

Question asked by: Dr Tim Thornton
Date: 12 June 2008

Qn: Has there been any definitive studies on the effects of boat wash/wake on river bank erosion?? Does a typical tinnie on the plane cause more erosion to the river banks than when it is travelling at 4-6knots ??

Ans: Dear Tim, The Belligen Environment Centre has been trying to implement a river wide speed limit due to that very reason. They have submitted to the Belligen Shire Council their reasoning for protecting their waterways supported by scientific papers, which I make available here. The Tasmanian government has also issued a pamplhet with information on the issue. I hope that is helpful.

Wake up Slow Down - Wave Wake Brochure
Bellingen Environment Centre: Submission to Bellinger/Kalang Rivers Estuary Management Plan.

Comments from other experts:
David Rissik:

Dear Tim, Following up on the comments from Giselle below, Will Glamore from the University of NSW did some work on wake erosion and management a few years ago and wrote a few conference papers and perhaps other papers as well. I have provided one reference that may be useful but there are others. I suggest that you follow up directly with the Water Research laboratory at UNSW.

Glamore, William C; Hudson, Rohan and Cox, Ronald J. Measurement and Analysis of Boat Wake Waves: Management Implications [online]. In: Coasts and Ports: Coastal Living - Living Coast; Australasian Conference 2005; Proceedings; pages: 67-72. Townsend, Murray Robert (Editor); Walker, David (Editor). Barton, A.C.T.: Institution of Engineers, Australia, 2005.

Guest Expert :
David Edwards

Lecturer in Geomorphology
School of Biological, Earth and
Environmental Sciences
University of New South Wales

Ans: I have worked on several projects on bank erosion on the Hawkesbury River over the past few years. All are still in the process of being written up/ finalised but I can make a few comments now. As always with these things though its a hard question to answer in 25 words or less.

In a nutshell: a boat travelling at speed on the plane causes a much smaller wake and therefore less bank erosion compared to a boat travelling at lower speeds and displacing much more water.


However, it is much more complicated than that - of course.

The wake height is dependent on something known as the depth based Froude Number. We usually use the Froude number in fluid hydraulics to define the ration of inertial forces to gravity forces in a moving body of water. In the case of boat wakes we calculate a value as:

F = Vs / (gd)0.5 ( Vs divided by square root of g*d)

where: F is the Froude number,
Vs is the vessel speed
g is the acceleration due to gravity
and d is the water depth.

So essentially we see that slower boats in shallower water have a smaller Froude number. We call this sub critical. The critical value occurs where F = 1 and above this the boat is travelling a lot faster and/or in deeper water at super critical speeds.

Now we get different types of wakes at different Froude numbers. At sub critical values we get generally short crested waves that are a mixture of divergent and transverse waves. Diverging waves propagate out from the stern of the vessel whereas transverse waves form a pattern in an opposite direction to the sailing line. Where these two waves types intersect causes the largest resultant waves that may be several times larger than the single waves alone. This effect is greatest as the boat approaches critical speed (F= 1) or where the boat appears to be riding its own wake.

This critical speed may occur in depth of 1 - 2 metres for a boat travelling at 6 - 8 knots, but for a boat travelling at 10 - 12 knots it will not occur until 4 metres of depth.

Above F = 1 the waves can no longer keep up with the speed of the vessel and trail behind. These are long crested transverse waves with the longest and fastest waves being on the outside
of the group and travelling at a higher angle with respect to the sailing line. For these waves the specific wave parameters will be determined by a range of factors including the vessel characteristics.

Basically then wave height is generally highest when F ~1 .

The problem is knowing what happens to those waves as they approach the shoreline. The general principles of wave mechanics in the open ocean do not usually apply for wake waves.

We know of course that the waves shoal in shallow water and that they tend to become smaller as the travel away from the boat but these concepts are compounded by larger waves overtaking smaller waves and the interaction of divergent and transverse waves. Not to mention the variable riverbank morphology, the role of tidal pulses or wind sheer and also the variable water depth and boat speed as the boat travels along the river.

So where does that leave us?/

EFFECTIVE boat regulation requires NO WASH zones not speed limits. My observations have shown that a large ski boat travelling down the middle of a river at 20 knots issues a resultant river bank wave the same as a wind wave but a small overloaded tinny close to shore can create a wave greater than 50 cm breaking on the bank!!

Of course that leads us on the nature of the bank material and the susceptibility to erosion etc. But that's a whole essay in itself.

The frightening aspect of my research is that many popular boat activities involve sub critical velocities including wake boarding and towing kids around on inflatable tubes etc. Plus they often involve looping around in one part of the river where reflected waves from opposite banks start to become a problem as well. In addition we have concentrations of activity: at one site on the Hawkesbury over summer we observed nearly 300 boat passes in a two hour period!!

Hope this helps answer the question. I have lots of references etc I would be glad to forward on if you need it.

 

Qn: How do oceans control climate?

Ans: Not only do ocean currents distribute heat from the equator to the poles, the ocean is responsible for producing over 50% of the oxygen we breathe, in turn taking up most of the carbon dioxide in the atmosphere, a greenhouse gas. Tiny plants, called phytoplankton, are responsible for taking up carbon dioxide and releasing oxygen- much in the same way that trees and land plants do. They form the bottom of the food chain in the ocean, and are extremely important in maintaining ecosystem balance and the role that the ocean has in making Earth a livable planet for us. Recently it has been found that by adding iron to selected areas of the ocean, the phytoplankton would "bloom" and potentially carbon could be stored in the deep sea becoming a practical solution for mitigating climate change, but to date there has not been scientific evidence to support this fact, and there are ecological consequences to consider.