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EFFECT OF THE TIDES ON RIVERS.
121
like a dike, which prevented the entrance of the tide into the bed of the
river. The encounter of the mascaret and the fluvial current have again
raised this bank of sand at a little distance. On striking against this
new obstacle, the tidal wave rises up to surmount it. Different hydraulic
works, undertaken in the beds of the Garonne and the Dordogne above the
Bec-d'Ambez, have also often modified the phenomena of the mascaret there.
The sudden appearance of the tide in estuaries raise^§the fluvial waters
very rapidly from the level of low to that of high water. At Tancarville,
which is the precise spot where the Seine discharges itself into the bay,
and where the tide exceeds a mean amplitude of about thirteen feet, the
entire rising of the waters is accomplished in two hours, while the fall of
the liquid mass, driven back by the tide, occupies about ten hours. The
river having to discharge during the period of ebb not only that which
the flow had brought to it, but also the fresh waters from higher up,
must follow its normal course toward the sea during a space of time
longer than that in which it is driven back by the rising tide. For each
point of the river-bed the duration of the flow is generally the shorter
the farther that point is from the sea: the force of the tide is gradually
exhausted, and toward the end of its course it only momentarily retards
the speed of the fluvial current.
£
«3> 3
$ 5i
----;
-Jfigi
s&ter
'--
/
/"
,
■''
/
•5.6*
■*$&'
_._._- "
:ey^
15y^
liv*3?
Fig. 46.—Tides of the Garonne.
The amplitude of the tides diminishes, likewise, in proportion to their
progress up the stream in rivers. The mass of fresh water flowing incessantly within the channel prevents" the low tide from sinking, as it does
on the sea-shore; and as to the high tide, its shorter duration does not
allow it to rise to a much higher level than that which it attains on the
strands and cliffs by the ocean. Thus, in the Garonne, the difference between the ebb and flow diminishes gradually above the Bec-d'Ambez;
and near Castets, at about ninety-five miles from the sea, it is finally reduced to zero. In certain places, it is true, particular circumstances may
cause apparent exceptions to this general law; a promontory rising before the tidal wave like that of Tancarville, in the bay of the Seine, bars
the way to the marine waters, and gives them, in consequence, a greater
relative height above low water. But in spite of these abrupt projections, the mean amplitude of the tide diminishes from the lower to the
upper course, and finally it becomes imperceptible.
| Title | The ocean, atmosphere, and life |
| Creator | Reclus, Elisée |
| Publisher | Harper |
| Place of Publication | New York |
| Date | 1873 |
| Language | eng |
| Type | Books/Pamphlets |
| Title | 00000136 |
| Type | Books/Pamphlets |
| Transcript | EFFECT OF THE TIDES ON RIVERS. 121 like a dike, which prevented the entrance of the tide into the bed of the river. The encounter of the mascaret and the fluvial current have again raised this bank of sand at a little distance. On striking against this new obstacle, the tidal wave rises up to surmount it. Different hydraulic works, undertaken in the beds of the Garonne and the Dordogne above the Bec-d'Ambez, have also often modified the phenomena of the mascaret there. The sudden appearance of the tide in estuaries raise^§the fluvial waters very rapidly from the level of low to that of high water. At Tancarville, which is the precise spot where the Seine discharges itself into the bay, and where the tide exceeds a mean amplitude of about thirteen feet, the entire rising of the waters is accomplished in two hours, while the fall of the liquid mass, driven back by the tide, occupies about ten hours. The river having to discharge during the period of ebb not only that which the flow had brought to it, but also the fresh waters from higher up, must follow its normal course toward the sea during a space of time longer than that in which it is driven back by the rising tide. For each point of the river-bed the duration of the flow is generally the shorter the farther that point is from the sea: the force of the tide is gradually exhausted, and toward the end of its course it only momentarily retards the speed of the fluvial current. £ «3> 3 $ 5i ----; -Jfigi s&ter '-- / /" , ■'' / •5.6* ■*$&' _._._- " :ey^ 15y^ liv*3? Fig. 46.—Tides of the Garonne. The amplitude of the tides diminishes, likewise, in proportion to their progress up the stream in rivers. The mass of fresh water flowing incessantly within the channel prevents" the low tide from sinking, as it does on the sea-shore; and as to the high tide, its shorter duration does not allow it to rise to a much higher level than that which it attains on the strands and cliffs by the ocean. Thus, in the Garonne, the difference between the ebb and flow diminishes gradually above the Bec-d'Ambez; and near Castets, at about ninety-five miles from the sea, it is finally reduced to zero. In certain places, it is true, particular circumstances may cause apparent exceptions to this general law; a promontory rising before the tidal wave like that of Tancarville, in the bay of the Seine, bars the way to the marine waters, and gives them, in consequence, a greater relative height above low water. But in spite of these abrupt projections, the mean amplitude of the tide diminishes from the lower to the upper course, and finally it becomes imperceptible. |
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