(Quebec) “I wonder if the size and speed of the boats are responsible for the erosion of the St. Lawrence shoreline. The waves hitting the shores year round are much larger than it was 30 or 40 years, “writes Maryse Aubin.
It is true that maritime traffic has greatly increased since 30 or 40 years, both in number and in the size of buildings. But, in general, “I do not see big alarm about the erosion caused by boat waves,” said Jannette immediately Frandsen, coastal engineering researcher at the Centre Eau, Terre et Environnement INRS, which just works with a huge “wave generator” brand new to recreate the conditions of waves and erosion.
“What is obvious at first, it is the large amplitude waves, the ones that come with storms, she continues. The moderate waves like those boats will also erode the beaches, they do their share of damage, but as soon as a storm occurs, it clears everything and it will change the geometry of the track quickly and significantly. ”
Of course, this all depends (not bad) from where it is located. On some lakes, like small lakes or narrow and tranquil rivers, there may be almost no wave – at least, not big. In such cases, if boaters begin to circulate regularly with boats powerful enough engine, their waves will damage the banks fail.
But this is not the situation that prevails in the St. Lawrence, said Ms. Frandsen. Downstream of Quebec and throughout the Gulf, it is mainly the waves created by the wind and particularly the storms that eat away shores. “Upstream of Quebec and moving to Montreal, I do not think the wind produces waves problematic,” she adds. In this sector, it is mainly the tides and currents that cause erosion, says the researcher.
According to a Coast Guard document, in the broadest sectors of the river, boat wakes is responsible for only 2 to 5% of the energy that erodes the banks, but in the most grand channels like that is seen between some islands, up to
Montreal – it can reach 95 to 98%.
Interestingly, the physics of waves and their interaction with the banks is not yet understood very finely. In its Quebec facilities, Mrs Frandsen generates waves that can measure 1.5 meters high, sends them in a 120-meter channel on 5 wide and deep, and then measuring the pressure forces on the other side channel. “And what we see is that these pressures then seem random. It is literally unable to predict how much the measured pressure on the back wall will be strong nor where in the wall will be the strongest. ”
Story to follow, so …
COAST GUARD. Shoreline erosion by wave action, nd goo.gl/iJeJHh
“According to our calendar, the autumn equinox in 2014 was the 23septembre. That said said equinox day length as equal to that of the night. So we had fun note the length of the day during the week of the equinox, but we noticed that it is rather September 25 as the day lasted nearly 12 hours battery. On 23 September, official day of the equinox, the time was 12:08. What is the explanation? “Asked Clement Castonguay, Quebec.
From a purely etymological point of view, Mr. Castonguay is right there in that word aequus and Latin nox, “equal” and “night”. But the etymology and definition of the equinox are two entirely different things.
Let us imagine, to fully understand it, an invisible thread linking the center of the Earth at the center of the sun. As our planet is slightly tilted on its orbit, this thread would pass by a point on the surface of the Earth, located at different latitudes by season. In the summertime, he “come out” in the northern hemisphere may reach its highest latitude (that is when the solstice) and then start to go down to the south and finally pass in the Southern Hemisphere – to stay until spring.
The equinox is simply when this imaginary crosses over the equator. This is not a “day” per se, mind you – that’s even an infinitely brief moment, that does not even time. This year, that moment occurred last Wednesday at dawn, around 3:21.
If it is called “equinox” after all, is that it’s about this time of the year that the day and night are approximately equal durations. But it’s never the same day of the equinox that these periods are the most equal, for two reasons. First, because it does not use the same points of the Sun to define the equinox (the center of the Sun) as for its sunrises and sunsets, which are now the margin of the Sun is shown / hidden in horizon. And then, because the gases from the atmosphere slightly deflect sunlight, showing the Sun a little higher in the sky than it really is.
These two phenomena have the same effect, lengthen the “artificially” a few minutes day length. At this time of year, it plays against the trend towards shorter days, and that is that the day lasts for the time night comes two days behind
the equinox. And spring, it’s the opposite: it plays in the same direction as the trend toward longer days, and it precedes the day of 12 hours battery compared to the equinox.