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```shown in Fig. 6.14b. An example of a
measured storm surge on Lake Erie is presented in Fig. 6.15. A maximum water
level difference in excess of 3 m existed between Bar Point near the West (upwind)
shore and Port Colborne near the downwind shore, 300 km to the East. This makes
136 Introduction to Coastal Engineering and Management
the average water slope due to storm surge (dS/dx) about 1 x l o 5 . If we assume the
whole lake to have an average depth of 25 m, then according to Eq. 6.2 the wind
speed needed to generate this storm surge is about 27 m/s (or 55 knots), which is a
severe storm for the area. The storm surge in Fig. 6.15 is therefore quite large for
Lake Erie.
0 4 8 12 16 20 24 28 32 36 40
Time (hrs.)
Figure 6.15 Measured Storm Surge on Lake Erie, Dec 1-3, 1985
(after Moulton and Cuthbert, 1987)
6.4 Barometric Surge
Since strong winds are the result of large pressure fluctuations, a barometric surge
will accompany storm surge. Suppose there is a difference in barometric pressure
Ap between the sea and the shore in Fig. 6.14a, or between the upwind and
downwind shore in Fig. 6.14b, then an additional water level rise will be generated
Chapter 6 - Rdes and Water Levels 137
where p is the density of water. Equation 6.5 results in a water level rise of about
0.1 m for each kPa of pressure difference. A major depression can easily generate a
pressure difference of 5 Wa, resulting in a potential barometric surge of 0.5 m.
6.5 Seiche
When the wind that formed a storm surge stops blowing, the water level will begin
to oscillate back and forth (seiche). The oscillations will continue for some time
because fiiction forces are quite small. The wave length of the fundamental mode of
the oscillation (a standing wave) for a closed basin (Fig. 6.16) is twice the effective
basin length (Be). In general, the wave length is 2BJ( 1 +nh) for the nh harmonic. For
an open ended basin (open coast), the flmdamental wave length is 4 times the
effective length of the shelf (Be) over which the storm surge was initially set up. In
general, for the nh harmonic it is 4BJ( 1 +2nh).
Figure 6.16 Seiche Wave Lengths
The period of oscillation (T=L/C) for a closed basin may be calculated as
T m = 2%
(1 + n h )@
138
and for an open ended basin,
Introduction to Coastal Engineering and Management
For the Lake Erie example in Fig. 6.15, the fundamental period of oscillation along
Lake Erie (with an average depth of 25 m and an effective length of about 300 km)
would be about 10.6 hours. The fkdamental period for the seiche across the lake
(about 55 km) is about 2.0 hours. The currents needed to displace the large volumes
of water can be considerable. For Fig. 6.15, currents would be as high as 0.25 m / s
in the lake itself. The oscillations may cause severe currents and water level
changes in bays and rivers that connect to such a seiching water body. The currents
can break ships and pleasure craft from their moorings. In the case of the Napanee
River, which enters Lake Ontario (a tideless sea), the “tides” resulting from seiche
on the lake were present so often that they were counted on by the sailing vessels to
negotiate the river.
~~ ~ ~
Example 6.2 Water Level Fluctuation at Venice
An interesting example of the combination of short term water level fluctuations
may be found in Fig. 6.17, where the water level in Venice during the first few days
of the 1992 international Coastal Engineering Conference is shown. Figure 6.17
shows the astronomical tide predicted fiom the tidal constituents. Superimposed on
this we see an oscillation of about 23 to 25 hour period, which caused very high
water levels (flooding San Marco Square in the centre of the city by 0.4 m).
We will now calculate what we might expect at Venice using the above equations
and recognizing that we will make many simplifying assumptions. Integrating Eq.
6.2 for a wind speed of 25 knots ( 1 3 m/s) over the Adriatic Sea gives a total storm
surge of 0.58 m. Inside the lagoon in which Venice is situated, there is a further
storm surge of 0.05 m. The water level rise due to barometric pressure is calculated
with Eq. 6.3 as 0.07 m. The total water level rise was therefore 0.58 + 0.05 + 0.07 =
0.70 m. This is close to the difference between the actual water level and the
predicted astronomic tide in Fig. 6.17 for the two highest water level peaks.
The storm surge would not have created so much difficulty, if its peak had not
coincided with the high water from the astronomic tides. This surge coincided with
several high waters in a row, since the surge period was equal to about twice the
Chapter 6 - Tides and Water LRvels 139
basic tidal period. Let us see if we can calculate the surge period. The calculation
depends very much on the assumed average depth of water over the portion of the
Adriatic Sea involved in the oscillation. A reasonable estimate of average depth is
150 m and the effective length is 800 km. For these characteristics the period for the
hndamental mode according to Eq. 6.7 is 23 hours.
Figure 6.17 High Water at Venice
From these relatively crude computations, it is clear that the 'Aqua Aka' (high water)
at Venice is a combination of storm surge, barometric surge and seiche. The simple
equations permit a basic understanding of the complex problem. More elegant
solutions are needed to solve actual design problems. Flooding problems in Venice
are being studied with sophisticated numerical models, using the two-dimensional,
depth-averaged equations of motion and continuity. To give better results than the
above approximations, such models require extensive field measurements for
calibration.
In passing, note that the barometric pressure was lowest when the first high water
level occurred. Thus it appears that barometric pressure drop gave rise to high
winds that caused a storm surge, which was enhanced by the barometric surge also
resulting from the pressure drop.
140 Introduction to Coastal Engineering and Management
6.6 Seasonal Fluctuations
Seasonal water level fluctuations do not occur along the open ocean, but they do
occur on lakes and in the upper reaches of estuaries. Extreme fluctuations occur in
power and water supply reservoirs. Normally, seasonal fluctuations are taken into
account in design as a matter of course and hence they are not of much concern,
even along the Great Lakes. Figure 6.18 shows examples for Lakes Michigan-
Huron and Ontario. The seasonal fluctuations are about 0.5 and 1 m respectively.
1 7 8 . 0
ff 177 .5
9 1 7 7 . 0
0 - E 176.5
f 1 7 6 . 0
5 175 .5
175 .0
7
1
1 9 0 0
7 6 0
0
3 7 5 . 0
E
p 7 5 . 5
r
I -
4 7 4 . 5 8 7 4 . 0
73 5
1 9 0 0
Figure 6.18
L a k a M Ic h Ipan-H u r o e
1920
I
1940 1960 1 9 8 0 z o o 0
Lake Ontar lo
1 9 2 0 1 9 4 0 1 9 6 0 1 9 8 0 2 0 0 0
Tlm e
Monthly Water Levels on Lakes Michigan-Huron and Ontario 1
(after Monthly Water Level Bulletin, Environment Canada)
Chapter 6 - Tides and Water Levels 141
6.7 Long-Term Water Level Changes
Water level fluctuations that are the response to long term climatic change such as
extended wet and dry periods are a cause for concern. The danger of such longer
term water level fluctuations is that everyoneforgets in a few years how high (or
low) the water can actually be. Therefore, a few years after high water, development
begins once again to encroach on the shore, exposing new properties to extreme
stress during the next high water. Similarly, once a low water has not occurred for a
few years, docks and marinas will be built in areas of shallower water, so that when
the next low water occurs, the water is too shallow for these facilities to be useful.
One recurring theme resulting from such l o n g - t e ~ ~uctuations is that gove~ment
funds are requested to help out the “unfortunate property owners”. Fortunately there
is a trend toward```