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Tides and their seminal impact on the geology, geography, history, and socio-economics of the Bay of Fundy, eastern Canada.

7. Ebb and Flow in Estuaries

7.1. INTRODUCTION

Because tides are variable in strength, the tidal prism and tidal volume in an estuary are also variable as is the wet cross-sectional area, which is that part of the total cross-sectional area filled with water. As discussed in section 6.3, it is through this wet area that the water which fills the tidal prism must flow. The upper parts of an estuary may become virtually empty during Low Water. Water levels in the estuary are only able to match the rising tide if enough water flows through the available cross-sectional area to fill the tidal prism to the level reached by the tide (Lauff 1967; Carter 1998). In many cases this is physically impossible as the maximum current speed possible in a cross-sectional area is limited, being proportional to the square root of the water depth (Kjerfve 1988). Rectangular cross-sectional areas are able to discharge much more water than either parabolic or triangular cross-sectional areas because the larger cross-sections allow higher current velocities.

In channels that empty, or nearly empty, during Low Water, the first water to return after the turn of the tide is very much restricted moving into the estuary. This is so even discounting friction, because the water depth and cross-sectional area are very small or zero. However the following water finds a partly filled channel and can thus move faster and at a greater discharge rate. As long as the discharge is inadequate to fill the tidal prism at the same rate of rise of water surface as the outside water, the result is that the seaward gradient of the incoming water will initially have a strong upward curvature. This curvature may become so strong that a bore will form at the bottom of the incoming wave.

What happens when, at a certain location in the estuary, the water reaches its highest level? Certainly the more upstream parts of the estuary remain unfilled with water. Seaward of this point the water level is already falling. Thus water continues moving landward on its own inertia when the high tide level is reached at that point. As a result, the water surface has already dropped before the direction of flow reverses. Thus the time of peak elevation and zero velocity are not coincident. When the water ceases its advance into the estuary it has already a considerable downward slope seaward. Soon a sizable seaward current develops, occupying a smaller cross-sectional area than when it moved into the estuary on the rising tide. The falling water level may at first follow the outside tide rather well because of the large cross-sectional areas, but progressively the reduced depth restricts water outflow. Consequently the seaward gradient of the water surface during ebb flow now has a downward curvature.

In terms of sediment transport this means that while the incoming tide is at high levels and gradually reducing in speed, its sediment carrying capacity is reduced (Amos and Tee 1989; Amos 1995a). Much of the released sediment is spread over the full height of the channel banks, and during very high tides, over the salt marshes also (Postma 1967). However the outgoing tide is concentrated toward the bottom of the channel, having at times higher speeds than the incoming tide. This situation promotes erosion through undercutting of the channel banks. Excess material high on the banks will slide down, resulting in unstable soil conditions. In testimony, the banks of tidal creeks and rivers in the Bay of Fundy region are characteristically bare of any vegetation except perhaps for some being transported downslope by undercutting (Scott 1980). Only where currents are slight is vegetation able to take root. Overall the intertidal flats of estuaries may vary seasonally, storing clays and silts during summers and removing them during heavy freshwater run-offs.

7.2. RESHAPING THE TIDAL WAVE THROUGH TIME

7.2.1. The Shubenacadie estuary

When the tidal wave enters a shallow estuary its sinusoidal symmetry becomes distorted. The rising limb of the curve becomes compacted within a shorter period, whereas the period of the falling tidal wave increases. The manner in which a tidal wave may be reshaped as it moves into an estuary is well illustrated by conditions in the Shubenacadie and Cornwallis river estuaries in Nova Scotia (for locations see Figs 3 and 43). At the mouth of the Shubenacadie estuary the rising tide is retarded by the shallow waters of Cobequid Bay (see Fig. 36). However, the centreline ([CL.sub.1]), being the locus halfway between rising and falling tide levels at Black Rock at the mouth of the river, is almost straight and vertical in the uppermost part of the marigram. This means that there is little distortion in the symmetrical shape of the local tide wave. The near-vertical rise of the tide at the bottom of the incoming tide wave (K) indicates that a bore may possibly develop.

[FIGURES 36, 43 OMITTED]

The curvature (asymmetry) of the centre lines at locations further upstream indicates that the tidal wave is becoming increasingly distorted with much steeper rises of the tide, and slower rates of drop. Meanwhile, the thalweg (the median line of the channel) of the estuary is rising at some distance beneath points K, L, M, N, and O (which indicate the water level in the river before the tide wave arrives), thus increasingly limiting the inflow of tidal water. The High Water levels rise for about the first 8.3 km (A to B in Fig. 36) before gradually dropping (B to E). This situation is reflected by the asymmetry of the marigrams, as indicated by the increasing curvatures of their centrelines ([CL.sub.1] to [CL.sub.5]) with distance from the river mouth. Eventually, the water levels in the river will fall back to K, L, M, N and O, as the water exits the estuary prior to the next incoming tide.

7.2.2. The Cornwallis River estuary

Figure 37 shows the reshaping of the tidal curve in the uppermost 4 m, near the peak of the tide, for a tidal wave in the Cornwallis River, N.S. The asymmetry of the marigrams, and the shape of the centrelines ([CL.sub.1], etc.,) show that the further the wave goes into the estuary (Site #1 to Site #5), the slower the rate of rise of tide near the peak (A, B, etc., on Fig. 37) and the faster the drop after the peak has passed. Initially, the curvature of the centrelines is reversed near the top of the wave (eg., A), but further upstream (eg., D) this reversal disappears.

[FIGURE 37 OMITTED]

Figure 38 shows the momentary gradients of the water surfaces at different times during the passage of the top of the Cornwallis River tidal wave. While the wave is moving upstream on its own inertia through this section (between 12:15 and 13:30 hours), the gradients of the water surface seaward of the wave top are negative and rather uniform. The moment its inertia is spent, the water starts moving backward toward the sea. The difference is that now, on the outflow, the gradients are not only reversed but are steeper because water levels have dropped considerably, forcing flow through small cross-sections at higher velocities. Note too that the water's surface is curved downward, concentrating the flow of outgoing water in the lower section of the channel.

[FIGURE 38 OMITTED]

The difference between tidal rivers and tidal creeks is that tidal rivers discharge fresh water from upland watersheds. In the upper reaches of the estuary, freshwater flow maintains a drainage channel, the size of which is related to the river discharges. In contrast, a tidal creek is formed by tidal water that moves onto a tidal marsh during Higher High Waters, discharging following the turn of the tide. The only fresh water moving in such creeks is rain water collected on the salt marsh itself. In effect, creek formation requires a certain area of marsh because the erosion process demands a minimum flow volume from the marsh in order to initiate channel cutting through vegetated soil. For this reason there always exists a margin of land not cut by tidal creeks on the seaward side of dykes built on reclaimed salt marshes. This feature is readily visible on aerial photographs of out-to-sea marshes.

Tidal channels reach a quasi-equilibrium condition ranging between certain limits. In spring, river discharges erode silt deposited during the summer and early fall when the river flows are greatly reduced; consequently, river bottoms may accrete or erode by as much as two metres (see Fig. 39). Before dyke construction the tidal marshes served as sinks for excess sediment in the system. However, the construction of dykes, causeways and tidal dams in estuaries further upsets the quasi-equilibrium state of tidal channels.

[FIGURE 39 OMITTED]

Uplands, or freshwater bogs in the case of extensive salt marshes, form the landward limit of the high marsh. The latter situation can be explained by the fact that most of the sedimentation that occurs when sediment-charged tidal water floods the marsh, happens soon after it enters the marsh, forming a natural levee. Thus a pond is formed between the tidal estuary and uplands. With submergence of the landmass, the levee becomes higher, and the pond, which receives thinner layers of fine-grained material, becomes deeper and extended horizontally. At a late stage of development the far reaches of these ponds receive hardly any salt water, thus allowing freshwater vegetation a chance to develop.

7.3. THE REVERSING FALLS--A UNIQUE ESTUARINE FEATURE

The Reversing Falls at the mouth of the Saint John River, N.B. is a perfect example of what happens when an estuary of limited cross-sectional area must serve a tidal prism with a large surface area. It was on Jean Baptiste day, 24 June, 1604, that the first Europeans in the region, Pierre de Gua, the Sieur de Monts, a Huguenot merchant, and Samuel de Champlain, Royal Geographer, discovered the mouth of one of the largest rivers on the eastern seaboard of North America. These pioneers and others who came later described its mouth in their logbooks and reports. According to the Jesuit missionary Pierre Baird (1611), "The entrance to this river is very narrow and dangerous, for a ship has to pass between two rocks, where the current is tossed from one side to the other, flashing between them as an arrow. Upstream from the rocks is a frightful and horrible precipice, and if you do not pass it at the proper moment, and when the water is smoothly heaped up, of a hundred thousand barques not an atom would escape, but men and goods would all be lost." (Raymond 1910). Doubtless Baird was well aware of the Indian legend concerning the hazardous entrance to Saint John Harbour. Here the Indian cultural hero Glooscap is said to have created the Reversing Falls when he destroyed a large beaver dam built across the river's mouth. (The remains of Glooscap's beaver dam support a popular restaurant!) The natural feature responsible for the legend lies at the downstream end of a 4 km long, tortuous river section. At Indiantown, water surface levels can vary between 0.3 m above Geodetic Survey of Canada Datum (GD) during periods of small river discharges, to 5.2 m above GD during extremely large runoffs. Two kilometres downstream of Indiantown the river suddenly narrows to 215 m, and a sill occurs about 4.5 m below GD (see Fig. 40). Downstream of this sill the channel deepens to 45 m below GD and steep rocks conspire to form a gorge 106 m wide, with the bottom 40 m below High Tide level and a cross-sectional area of 2125 [m.sup.2]. During Low Tide the river at the reversing Falls is only 78 m wide and the cross-sectional area reduced to a mere 1400 [m.sup.2] (Fig. 41).

[FIGURES 40-41 OMITTED]

The average freshwater discharge of the Saint John River is approximately 1100 [ms.sup.-1]. In spring, the melting snowpack in the watershed, combined with precipitation, can generate freshets of more than 10 000 [m.sup.3]/s. The cross-sections over the sill and in the gorge are far too small to allow these storm discharges to pass at normal levels, which are little above MSL. Therefore at times the water has to rise over 5 m, even during low water, before such freshets can fully discharge into the sea. Even then, tides restrict the outflow twice a day during High Water. The average range of tide in Saint John Harbour is 6.7 m, reaching an elevation of 3.4 m + (above) GD. Large tides have a range of 9.1 m and reach 5 m + GD.

Upstream of the falls a system of drowned valleys (Grand Bay, Kennebecasis Bay, Long Reach and Belleisle Bay) provides drainage channels for the river and some of its tributaries. They have a combined surface area of 200 [km.sup.2] at the present sea level, and 250 [km.sup.2] at flood levels about 5 m above GD. Due to sea-level rise this system has gradually been filling over the past several thousand years. The tide has a range of about 0.5 m, but in the section of river upstream from Evansdale it is reduced to 0.3 m, and half that again 10 km further upstream. Upstream of the Falls the tidal prism is approximately 0.09 [km.sup.3], and can be filled within 6.2 hours by an average flow of 4000 [m.sup.3][s.sup.-1] with an average velocity of 5.5 [ms.sup.-1] through the area of 730 [km.sup.2] over the sill, requiring a drop of 1.5 m (Hansen 1970). When the river is low and with only a small gradient in the water surface, backwater effects due to the tides are observed near Fredericton, New Brunswick's capital, 125 km upstream of the falls.

In Saint John Harbour the tide ranges from -2.0 m to +2.0 m with respect to MSL during small tides and from -4.5 m to +4.5 m during large tides. Upstream of the falls the range is at most between 0.0 m and 0.6 m above MSL. This means that at High Water in the Harbour, the water will flow through the gorge and over the sill in a landward direction, dropping 1.75 m to 4.2 m depending on the strength of the tide. But at Low Water in the Harbour, the water will flow seaward dropping over a short distance 2.3 m to 4.8 m. Tidal water, augmented by the river discharge, gains a considerable velocity when moving over the sill where the available wet cross-sectional area is smallest. Rapids result in this section as the water moves turbulently through the gorge into the Harbour.

Navigation under these conditions clearly poses problems. Since tidal current and river flow are always present, there is no appreciable slack water in the Reversing Falls section. The ideal time to navigate the section is as close as possible to the time of slack water as indicated in the tide tables. Slack water at the end of the inward current is 2.4 hours after the time of High Water predicted for Saint John. The tide has then dropped sufficiently to be close to the same level as it is in the river upstream of the falls. Another slack time occurs 3.8 hours after Low Water, when the tide has reached the river level. Although slack water lasts less than 10 minutes, the section can be navigated for half an hour before and after this slack. Variations in meteorological conditions can alter the time of slack water by up to about an hour. At times it is impossible for ships to move upstream. An example is during spring runoff when river levels upstream of the falls are so high that they are not even reached at High Water levels of the tide. Under these conditions a steady seaward current will persist throughout the whole tidal cycle.

Flooding of the several lakes upstream on the Saint John River has troubled settlers on the fertile freshwater delta since 1694. In April 1987 heavy flooding associated with an ice jam caused water levels to rise over 5 m. The basic problem was recognized as early as 1693 by Lamothe Cadillac who, while conceding that the Saint John is the most beautiful, most navigable, and the most favoured river of Acadia, was all for blowing up the rock on which the Reversing Falls restaurant is situated. Indeed, the idea has been revived on more than one occasion since. Fortunately there has been strong opposition. People perceive that such a project would change the Saint John River into a tidal river with unsightly mud flats and alternating currents of turbid water. The remedy for protecting the low-lying lands would be worse than the problem because the annual inundation with fresh water would be replaced with inundations of salt or brackish water at every set of high tides. Moreover the Harbour would be ruined because tidal currents would become much stronger than they are at present, increasing the navigational hazards. For these good reasons the Falls remain a unique estuarine feature of the Fundy landscape.

7.4. TIDAL POWER

Tidal energy, exploited in Europe over 900 years ago, is highly predictable and is harnessed in much the same way as hydropower; all that is needed is a barrage across a suitable estuary that allows the bay behind it to alternately empty and fill with the tides. The greatest amount of energy available from falling water, at least in theory, is obtained by dropping the largest amount possible over the greatest vertical distance (Daborn 1977).

The construction and operation of a tidal power plant is however much more complex than a hydro plant on a river. At a given site, the amount of energy available depends on the range of the tides and the area of the enclosed bay. Since the head can never exceed the tidal range, unless water is pumped from the sea into the reservoir using energy generated elsewhere, the capacity of the plant can only be increased by moving more water through the turbines. This water must move by tidal action, within the period of 6 hours or less, from the sea into a reservoir when the sea levels exceed the reservoir levels. This water must then be temporarily stored in the reservoir until the tide drops enough to create a head that will drive the turbines. The larger the reservoir, the more power the plant can extract from the water. In principle it would be best to build the barrage at a point in the estuary close to the sea. Most estuaries flare and deepen toward to sea, requiring the barrage to be more voluminous and therefore more expensive and challenging to build. No matter what the location the control of tidal waters through the channel, particularly during the late stages of construction of the barrage, is extremely difficult. High tides during this construction period force large amounts of water through the closure gap. Nevertheless, in order to make a tidal power plant energy effective, an estuary is needed where large tides prevail. To illustrate (Bray et al. 1982; Gordon 1984; Gordon and Dadswell 1984), large sections of estuaries in the Bay of Fundy fall dry at Low Water. When the tide rises, the surface area of the water gradually increases. In most cases the water surface area for a trapezoid is approximated as follows:

(40) Sy = (M + N x y) x [10.sup.6] [m.sup.2]

where M = surface area in [km.sup.2] , at MWL, N = increase in surface area in [km.sup.2]/m, y = the height above MWL in metres. The volume R of a tidal section between levels a and b (see Fig. 42) is approximately:

R = [M + N(a + b)/2] x (a - b),

whence

(41) R = [M x (a - b) + N x ([a.sup.2] - [b.sup.2]) / 2 ] x [10.sup.6] [m.sup.3]

or where b is negative,

R = M (a + b) + N ([a.sup.2] - [b.sup.2]) / 2 x [10.sup.6] [m.sup.3]

[FIGURE 42 OMITTED]

The tidal prism P, with a total amplitude of A metres then becomes:

(42) P = 2A x M x [10.sup.6] [m.sup.3]

In the upper reaches of the Bay, the volumes of R and P can be large. Examples are given in Table 16 together with the values of potential energy Ep, during MHW.

[FIGURE 42 OMITTED]

The energy in a body of tidal water can be compared to the energy in the pendulum of a grandfather clock. The weight, or bob of this instrument, with mass m, is suspended a distance L from a rigid support. If the system is at rest, the force of gravitation will keep the bob at its lowest position, exactly vertical, below the support. However when the bob is drawn aside a distance x, it has to be lifted the vertical distance y. The relationship between y, x, and L can be expressed as:

(43) y = L[1 - [(1 -[(x/L).sup.2]).sup.0.5]] = L (0.5 [(x/L).sup.2)] + much smaller terms.

The potential energy [E.sub.p] is equal to 0.5 m x g x [x.sup.2]/L, and the kinetic energy [E.sub.k] can be given as 0.5 m [(dx/dt).sup.2], when x is small compared with L. The total energy is then the sum of [E.sub.p] and [E.sub.k], and can be expressed as:

(44) [E.sub.t] = 0.5 m (g x [x.sup.2]/L + [(dx/dt).sup.2])

indicating a simple harmonic motion. When the bob, with a mass of 1 kg, and suspended 1 m below its support, is moved aside horizontally 0.44 m from its equilibrium position, it must be lifted 0.1 m, thus being supplied with an energy of 0.1 x g Joules. When released it will be accelerated, reaching its maximum velocity of 1.4 [ms.sup.-1] when passing its equilibrium position. This velocity enables the bob to be lifted 0.1 m at the other side of the equilibrium position. In the clock, the energy lost due to friction with the support and the surrounding air, is replenished by a dropping weight via the escapement mechanism. When the motion of the bob is interrupted at the equilibrium point, the system is released of its total energy. In order to start the bob moving again, the same energy must be resupplied.

A body of tidal water can be likened to a pendulum lying on its side. At Mean Sea Level, its equilibrium position, the strongest currents occur and thus the maximum amount of kinetic energy (Godin 1990). The highest potential energy occurs when the surface is close to its High Water or Low Water positions. In order to reach its Low Water position, the water must be evacuated toward the sea by means of its current velocity. The water then has to move uphill, decelerate, and eventually come to a halt, thus limiting the volume that can be evacuated near the head of the estuary. Because this movement causes eddies and friction, the oscillating tidal movement can only be maintained when fresh supplies of energy are introduced into the system by the oceanic tides. Nor is all of the energy dissipated during each tide, otherwise the outgoing water would not have the energy to move uphill during the latter half of the ebb cycle and a noticeable imbalance between the High Water and Low Water amplitudes would result. There should also be a relatively large dissipation of energy otherwise the tide would not respond as fast as it does to the changing gravitational influences of the Moon and Sun.

The energy [E.sub.1], of a layer of water with velocity v, density D, a surface area [S.sub.y] [km.sup.2], [d.sub.y] metres thick, at elevation y, with the capacity of dropping a vertical distance h, and subjected to an atmospheric pressure p kPa, can be described as:

(45) [E.sub.1] = (D x g x h + [p.sub.1] + 0.5 D x [v.sub.1.sup.2]) x [S.sub.y] x [d.sub.y] x [10.sup.9] Joules

When the water is moving from a deep reservoir through the turbines, the original velocity [v.sub.1], of the water in the reservoir, is negligible in relation to the current velocity [v.sub.2] that it has when leaving the turbines after the h metre drop. The atmospheric pressure [p.sub.1], affecting the water in the reservoir, will be on average around 101 kPa, with possible deviations of 4 kPa. However the pressure [p.sub.2], in the turbines can be much lower because of the Venturi-shaped passage. The drop in pressure can account for the measured discharge coefficients of such orifices, which can considerably exceed unity. Generally velocities in such passages can not be higher than [(2g x h).sup.0.5], but certain shapes allow underpressures and higher velocities.

After the water has dropped the distance h, the energy changes in character. The potential energy Ep, represented by the factor D x g x h, is transformed into other energy forms, such as kinetic energy [E.sub.k], or heat energy [E.sub.h]; it can also be extracted as electric energy [E.sub.e]. Consequently the energy distribution [E.sub.2] after the drop h, becomes:

(46) [E.sub.2] = [E.sub.1] = ([p.sub.2] + 0.5 D x [v.sub.2.sup.2] x [S.sub.y] x [d.sub.y] x [10.sup.9] Joules

Thus, the potential for electrical extraction will be:

(47) [E.sub.e] = (D x g x h + [p.sub.1] - [p.sub.2] - D x ([v.sub.2.sup.2] - [v.sub.1.sup.2] /2) x [S.sub.y] x [d.sub.y] x [10.sup.9] Joules

Neglecting energy gain due to pressure differences, and assuming negligible initial velocity [v.sub.1], the energy that can be extracted annually (706 tides) with a single-effect operation can be set at:

(48) [E.sub.e] = 1971.3 (h - [v.sub.2.sup.2]/(2g)) x [S.sub.y] x [d.sub.y] MWh / year

Note that no energy extraction is possible when [v.sub.2] = [(2g x h).sup.0.5]. To extract the largest amount of electric power, the value of [v.sub.2] must be kept as small as possible. If the combined working cross-sectional area of all turbines is X [m.sup.2], and extraction occurs during t seconds, the velocity, [v.sub.2] will be:

(49) [v.sub.2] = [S.sub.y] x [d.sub.y] x [10.sup.6]/(X x t) m/sec

The velocity will decrease as the values of X and t increase. Because it is not possible to extract all of the energy during the relatively short interval of slack Low Water, sufficiently large discharge channels must be available. Obviously, it is not practicable to employ the total tidal range as an energy head.

Another interesting and important consideration concerns the value of N x y in (41) for [S.sub.y]. When N x y is large relative to M, the potential for extraction of energy on the ebb flow becomes much more attractive than during the incoming flood flow because of the larger surface areas at higher levels of the reservoir. Double-effect extraction is less attractive (Larsen and Topinka 1984; Charlier 1982) because, although more time is available for extraction, the available head will be smaller, and very large sluiceways are needed in order to fill and void the reservoir of large volumes of water during shorter intervals.

Promoters of tidal power usually use the following simplified equations for calculating the potential energy extraction. Here it is assumed that the base level to which the water in a reservoir can fall is at elevation z, and that the total energy [E.sub.e] can be derived by the integration of the following equation:

(50) [dE.sub.e] = K x (M + N x y) x (y - z) x [d.sub.y] Joules

where K = D x g x [10.sup.9] = 10.05 x [10.sup.9]. When y drops from level a to level b, the value of [E.sub.e] becomes:

(51) [E.sub.e] = K x (M - N x z)([a.sup.2] - [b.sup.2])/2 - M x z x (a - b) + N x ([a.sup.3] - [b.sup.3])/3 Joules

If the range of the reservoir is Y and a = Y/2, and b = z = -Y/2, then (51) becomes:

(52) [E.sub.e] = K x (M x [Y.sup.2]/2 + N x [Y.sup.3]/12)Joules

For a single-effect operation during 706 tides, the annual output can be estimated at:

(53) [E.sub.e] = 3.548 x [10.sup.12] (M x [Y.sup.2] + N x [Y.sup.3]/6) Joules/year = 0.9856 x [10.sup.6] (M x [Y.sup.2] + N x [Y.sup.3]/6) kWh/year

Promoters may replace Y by the tidal range, and use twice the value, indicating a double-effect operation in which the reservoir is emptied simultaneously at Low Water, and filled again instantaneously at High Water. This yields a highly inflated value of the potential power extraction.

7.5. PROSPECTS FOR FUNDY TIDAL POWER

Greenberg (1987) has calculated the mean potential energy of the Minas Basin at 1.15 x [10.sup.14] J per tide. For a smaller area (850 [km.sup.2], east of Cape Split), Godin (1990) estimated energy output at 2.657 x [10.sup.14] J. The theoretical potential energy for this area, calculated using (52), and Y as the local amplitude of the mean tide, is 1.8 x [10.sup.14] J per tide. However, Charlier (1982) claims that a total annual energy of 169.5 billion kWh can be generated (this is equivalent to about 8.65 x [10.sup.14] J per tide) with a double-effect unit.

A single-effect power station lets the water flow through the turbines in one direction only, generally from the basin into the sea, thus utilizing the greater basin storage at the higher levels. Since the amplitude of the tide can vary between 60 and 140%, its mean potential value can be even higher. As the output is proportional to the square of the amplitude, the importance of a large tidal range becomes obvious (see Table 16). Tidal range is even more crucial in areas where the water surface area increases significantly. In practice of course all this energy cannot be tapped. This would require the reservoir to be filled to high tide level, and then almost instantaneously released when the tide is out.

Experts on tidal power development estimate that only about 25 to 30% of theoretical capacity can be realized. For example, the French tidal power station on the Rance River has a basin with a surface area of 22 [km.sup.2] and a mean tidal amplitude of 4.25 m. The theoretical annual energy output should thus be 1567 GWh. However with an annual output of 544 GWh, the efficiency is close to 35% thanks to refined computer software, which optimizes plant operation, including pumping and double-effect power generation. A similar system is operated by the Russians in Kislaya Bay, near Murmansk. It has a 1.1 [km.sup.2] basin and a theoretical annual output of 6775 MWh of which only 2300 MWh is realized. The Russian and French tidal power stations operate with reservoirs used as holding ponds to create large heads, and turbines to extract the energy.

Alternate Fundy tidal power proposals call for placing paddle wheels or egg-beater-type propellers in the flowing water, without confining the water to either reservoir or restricted channel. One possible location is the Minas Channel, where current speeds can reach 7 to 8 knots. A current speed of 8 knots can be generated by a head of 0.86 m. Theoretically, by building a dam in the Minas Channel, a head of 10 m can be created. This is 11.5 times the head that the paddle wheel can be subjected to under the fastest currents. Nevertheless, in 1916 the president of Acadia University, together with two members of its engineering department and the head of its business academy, formed the Cape Split Development Company. During summer, a survey was made of the topographical and hydrographical conditions between Cape Split and Squaw Cap Rock (just offshore from the Cape). The maximum tidal current was clocked at 11 mph (4.92-5.66 m/sec). A model was constructed of a tidal power contraption consisting of pairs of endless chains linked to concave vanes and led over sprocket wheels that would drive pumps. The idea was that the pumps would continually top up two 250 000 [m.sup.3] storage tanks erected on the 100 m high Cape Split. Water would generate electric power utilizing machinery built in four open sluices and housed in the 120 m-wide gap between Cape Split and Squaw Cap Rock. Although shares were quickly issued, the company dissolved in 1929.

There were a few early success stories relating to tidal power worldwide. Records show that tide mills existed on both sides of the English Channel as far back as the 12th Century. Similar mills were also in operation along the coastline of eastern North America shortly after the first settlers arrived. Some of the mills served several purposes. In 1634, a cove and marshland north of Boston were dyked off. A three metre-wide sluice admitted tide water, which eventually powered two grist mills, a saw mill, and a chocolate mill. Most of these plants were abandoned when more convenient sources of energy became available.

A modest proposal for Fundy tidal power was advanced by K.E. Whitman in 1944 for a double-basin scheme using the Maccan River estuary as a head-water pond and the Hebert River estuary as the tail-water basin. Hicks (1965) identified seven possible sites between Cape Split and the mouth of the Bay of Fundy, and between Long Island, N.S., and Cutler, Maine. That same year it was seriously proposed to build a barrage through the 5 km-wide Minas Channel. The channel is 100 m deep at low water, and a suggested dam at this site would have a volume of over 75 million cubic metres! At that date, only the Fort Peck dam in the U.S.A. was more voluminous. To handle the volume of material needed in construction would require exclusive use for two years of loading facilities equivalent to all those available at the world's largest shipping centre, Rotterdam.

There have also been proposals based on the difference in tidal range on opposite sides of the Chignecto Isthmus. Tides in Cumberland Basin have an average range of 10 m, while 23 km away, in the Northumberland Strait near Baie Verte the average range is less than 5 m. Many people have had the mistaken idea that the Low Waters of these tides are at the same level as the Fundy tides, tides on both sides of the isthmus being measured from Chart Datum. The fact is that each tidal station has its own Chart Datum set at some particular elevation below Mean Sea Level. (Chart Datums are set at elevations so low that the tide at any given location will seldom if ever fall below it). Thus, and contrary to some proposals that have actually been advanced (see for example, Pogany 1958), the perennial economic problems of the Maritime Provinces can not be solved by simply digging a canal through the isthmus and installing a tidal power plant on it.

A more thoroughly researched report was prepared in 1945 for the Government of Canada (H.G. Acres and Co. 1946) concerning tidal power development in the Petitcodiac and Memramcook River estuaries. However this report, like that prepared by the Atlantic Tidal Power Programming Board (1969) for the Federal and Provincial governments, concluded that development could not be justified under the prevailing economic circumstances. A different conclusion was reached in the Bay of Fundy Tidal Power Review Board (1977). Out of 37 possible sites, the three most promising were in the Minas Basin, Cumberland Basin, and Shepody Bay. According to the report (which really focused on the results of market research), the largest, in the Minas Basin, would be capable of generating about 5000 MW; it was given preference over runner-up Cumberland Basin.

Many proposals have been advanced to try and tap the power of the Fundy tides (for details see Desplanque and Mossman 1998a). To date, however, only a modest experimental plant exists in the Bay of Fundy region. Installed in 1984 and located in the Annapolis River estuary, this small unit (20 MW) uses a "Straflo" combined turbine-generator to extract energy on an ebb tide, using a tidal range of 4.5 to 10.0 m. It may ultimately be the pilot for a vastly more ambitious undertaking to harness the great tidal ranges of the Minas Basin. In any event the construction of barrages in tidal waters is a formidable assignment. At La Rance, the French built the power units (total 250 MW) for the plant under dry conditions by placing cofferdams part way across the Rance estuary (Table 17). The placement of the cofferdams at La Rance succeeded, but not without a few tense moments. The relatively small power unit (800 kW) of the Russian plant was built into a floating caisson, 36 m long and 15.35 m high, which was then towed to the site and placed on a prepared bed. To smooth this bed the Russians sent some divers to the bottom equipped with hand rakes. It is extremely doubtful that such a procedure could be used in Fundy waters. A submerged tide gauge held down by pieces of railway track was placed in the upper portion of Cumberland Basin on 21 May, 1978, in 12 m of water. The current carried it away, and it was never relocated despite a thorough search.

Closing off parts of estuaries in the upper reaches of the Bay of Fundy would be a complicated undertaking, comparable but more difficult than the massive works carried out for the Deltaworks in The Netherlands. The latter are carried out through the cooperation of government personnel working together with contractors and a labour force experienced in this sort of work for many decades. The Deltaworks actually began with the construction of the enclosure dam and the first polder of the Zuiderzeeworks in 1927. Before that, the best scientific minds (among them H.A. Lorentz, Einstein's mentor) studied the implications of the closure of the Zuiderzee. The practical knowledge gathered during operations on both the Zuiderzee-and Deltaworks have established that undertakings of such magnitude require a sound logistical base in order to guarantee success. Planning a tidal power facility is not just a matter of placing a line on often outdated maps, and launching a public relations campaign. It involves plans that incorporate basic facilities such as sheltering harbours, and requires a sound infrastructure for constructing the facilities. Within the rigorous timetable ordained by the unforgiving tides, many new techniques would need to be developed and perfected.

"Fundy Tidal Power--Update '82" (Baker 1982) stated, among other conclusions, that: "Significant reductions in overall cost of a tidal power plant can be achieved by shortening the construction period and that should be one of the objectives of definitive design" (Baker 1982). In order to reach this objective it would be imperative that labour peace is guaranteed during the construction period. The "Update" also calls for the construction of 50 sluiceways and 64 turbine caissons, each 59 m long, 39 m wide and 46.25 m high, to be towed and placed on mattresses. This latter manoeuvre is an extremely delicate operation to be carried out during the periods of slack tide with split second precision, and in very close proximity of previously placed caissons. Any error of judgement will result in damaging collisions or time-consuming strandings of caissons. The construction and transportation of these 16-storey-high structures would require the ultimate of technical know-how. From the human perspective, these structures would be enormous, but in fact they are as delicate as oversized aquariums. They will have to be placed on platforms that will support the caissons evenly; otherwise internal stresses can play havoc with the structures and their contained expensive turbines or sluice gates. The successful construction and emplacement of such platforms would be a colossal engineering feat.
Table 16. Characteristic water surfaces, tidal prisms,
potential energy, etc., of various sections of the Bay
of Fundy

 M N Mean A P
 [km.sup.2] [km.sup.2]/m m [km.sup.3]

Cobequid Bay 210.6 14.6 6 2.53
Southern Bight 157.3 9.4 6 1.89
Minas Basin 608 8.8 5.5 6.69
Total Minas Basin 975.9 32.8 5.8 11.11

Cumberland Basin 78.3 8.0 5 0.78
Shepody Bay 115.2 8.4 5 1.15
Chignecto Bay 280.5 2.2 4.5 2.52
Total Chignecto 474.0 18.6 4.8 4.45

Inner Bay 4653.8 6.4 4 37.23
Subtotal 6103.7 57.8 4.4 52.79

Outer Bay 6976.1 10.3 2.5 34.88
Total Bay 13079.8 68.1 2.9 87.67

 R+MSL R-MSL Ep
 [km.sup.3] [km.sup.3] 1014J

Cobequid Bay 1.53 1 0.487
Southern Bight 1.11 0.77 0.353
Minas Basin 3.48 3.21 0.973
Total Minas Basin 6.12 4.98 1.813

Cumberland Basin 0.49 0.29 0.132
Shepody Bay 0.68 0.47 0.18
Chignecto Bay 1.28 1.24 0.292
Total Chignecto 2.45 2 0.604

Inner Bay 18.67 18.56 3.755
Subtotal 27.24 25.54 6.172

Outer Bay 17.47 17.4 2.196
Total Bay 44.71 42.94 8.368

Notes. M = surface area in [km.sup.2] at MSL; N = increase in surface
area in [km.sup.2]; A = amplitude of the tidal prism; R = volume of a
tidal section between levels a and b; MSL = mean sea level; R + MSL is
the volume between MSL and tide level + A; R - MSL is the volume between
MSL and tide level - A. (R + MSL) + (R - MSL) = P. [E.sub.p] = potential
energy

Table 17. Summary of tidal power generating
capabilities of various sites

Petitcodiac and Memramcook--1945

Tidal Range 6.4 to 15.8 m
High Basin, Petitcodiac 31 [km.sup.2]
Low Basin, Memramcook 5.6 [km.sup.2]
Channel width 2650 m

30 generators at 9 MW each; 150 MW continuous
power is typical

Passamaquoddy--1963.

Tidal Range 4 to 7.9 m
High Basin, Passamaquoddy 262 [km.sup.2]
Low Basin, Cobscook 106 [km.sup.2]

100 generators at 10 MW each

La Rance--1967

Tidal Range 5.3 to 13.5 m
Basin Area 22 [km.sup.2] at high tide
Channel width 750 m

24 generators at 10 MW each; double effect with
pumping at extremes

Cumberland Basin--1977

Tidal Range 8 to 13 m
Basin Area 119 [km.sup.2] at mean level
Channel width 2560 m

37 generators at 31 MW each; single effect, falling
tide

Annapolis Royal--1983
Tidal Range 4.5 to 10 m

One generator at 20 MW; single effect, falling tide


8. Ice Phenomena in a Bay of Fundy Estuary

8.1. WINTER CONDITIONS: A SHORT CASE HISTORY

The transition from fall to winter in the Bay of Fundy is a time of stark contrasts. Huge amounts of ice can be formed in a few days of heavy frost. As we shall see, this can rapidly bring about dramatic change in the character of tidal estuaries in the Bay of Fundy (Jennings et al. 1993). On Wednesday, 10 December, 1980, the Sun was out, the wind was light, and the temperature hovered about freezing on the marshes bordering Cumberland basin (Fig. 43). No snow was on the ground and not a speck of ice in the estuary. A survey of ice conditions in the area, planned for the coming winter, was about to begin. The plan called for surveys on foot, on skis, and by helicopter throughout the period that ice would be present in the Bay. That day scientists from the Bedford Institute of Oceanography would be inspecting sites that the senior author had volunteered to visit during the winter.

By the following morning, as the temperature fell, the weather had changed significantly. On Saturday, 13 December, it was so cold and windy that it was dangerous to be out on the marsh 'alone. However on Sunday, 14 December, the Sun appeared again and conditions moderated. Five centimetres of snow covered the ground. In the morning, as the tide ebbed past Lusby marsh (LM on Fig. 43), one third of the ebb channel was covered with ice moving in the outside bend of the basin. Already formed were most types of ice common here in winter.

The drifting ice was mainly slush, or frazil ice, formed in open water areas termed "ice factories" (Knight and Dalrymple 1976), and made up of an unconsolidated mixture of needle-like ice crystals and sediment-laden water. There was also a good representation of pan ice, and cake ice. Pan ice, also present, is formed from accumulations of slush ice, frozen together in flat slabs up to 15 cm thick. Cake ice is considerably thicker, and probably forms as pan ice is jostled in fast-moving, ice-packed water. On its perimeter, cake ice picks up slush ice, creating an elevated ridge, or levee. The resulting basin-shaped central part can at least temporarily, hold silt-laden water. However the water usually seeps through the porous ridge leaving the silt behind. Collisions with other ice cakes evidently round the undersides of the cakes. Floating by too were a few scattered blocks of composite ice protruding 0.5 m and higher above the surrounding ice. On 14 December the only type of ice not yet in evidence was floe ice, normally formed where salinity and tidal energy are greatly reduced. Floe ice consists of frozen assemblages of all other types of floating ice, 50 to 100 m in diameter, that move restlessly with the tides, up and down the estuaries.

Following Desplanque and Bray (1986), the foreshore is here divided into three subzones (see also Fig. 44). The upper subzone is the vegetated high marsh that, in the upper reaches of the Bay of Fundy, is approximately 1.2 m below the highest levels that tides normally reach. Dominated by river-related processes, this subzone is not often covered with tidal water. During some winters, tides do not reach this level, in which case the high marsh escapes ice deposition (Dionne 1989). However, frozen crust, another major type of ice also present on 14 December, usually forms on the surface of the intertidal sediment. This "shorefast ice" results from the combined action of downward-freezing pore water, upward accretion of precipitation, run-off and, depending on the season, sea water (see also Desplanque and Mossman 1998b).

[FIGURE 44 OMITTED]

The landward limit of the high marsh is formed by dykes, uplands, or in the case of extensive salt marshes, freshwater bogs. On the seaward side a vertical scarp I m or more high separates high marsh from the middle subzone. In sheltered sections of the shoreline the upper portion of the middle subzone may be covered during the summer by Spartina alterniflora (low marsh) and algae. Where rock is present seaweed may cling to it. Slippery mud, underlain by semi-consolidated material of similar grain size, covers the lower sections. The middle subzone has a relatively gentle gradient (approximately a 1 m drop in about 50 m), and its lower limit is also a scarp (Fig. 44) although not as high as the upper scarp. The lower subzone slopes toward the edge of the ebb channel, forming the thalweg (the median line of the channel) in Cumberland Basin.

On 14 December, 1980, the lower half of the middle zone was completely covered with cake and pan ice. Several centimetres thickness of silt had accumulated on some of the cake ice. On the upper half, a scattering of ice indicated the height of the tide during the preceding few days. The foot of the upper scarp had not been reached by the ice. In some places the lower subzone was covered with a crust of glazed ice beneath which water flowed toward the lower edge of the zone; in others, the ground was bare and unfrozen.

Several ice blocks stranded in the middle zone were constructed of 4 or 5 layers of ice of differing structure. Most layers were parallel, but some were oriented up to 45[degrees] relative to the others. The blocks, from 1 to 1.5 m high and 3 m square, contained much silt. However their buoyancy was very high because of the numerous air pockets. These sizeable blocks must have formed during the preceding three days because, as noted, ice was not present on 10 December.

During this three day period, the tides had not been large by Bay of Fundy standards; they ranged from a predicted 7.8 m + CD at Saint John on 10 December, to 7.3 m + CD on 14 and 15 December. However from this day on the tides became stronger, reaching a predicted height of 8.5 m + CD on 22 December. This semi-monthly inequality of 1.2 m at Saint John translates into an inequality of 2.0 m or more in Cumberland basin.

The weather remained cold. On 21 December it was unsafe to visit the site, but on 23 December we observed the effect of the increasing strength of the tides on the accumulation of shorefast ice forming along the shoreline in the zone between neap and spring High Waters. The middle subzone of the foreshore was now filled with a thick layer of chocolate-coloured ice to 0.3 m below the level of the high marsh. It was impossible to reach the lower subzone because ice-covered crevasses between the cakes made the going extremely hazardous. Gradually the rising tide submerged the lower subzone and all types of floating ice covered the basin. Pan ice penetrated marsh creeks, becoming stranded on their banks.

Flat pan ice is commonly deposited on earlier-formed pan ice ferried in on weaker tides to freeze onto the banks. On higher parts of the foreshore stranded ice is left longer exposed to freezing air temperatures, and is more likely to become anchored to the banks (Sweet 1967). Commonly, ice may become bonded so strongly to a clay or silt substrate that it will not refloat when covered by a higher tide. In contrast, the connection between ice stranded on gravel or loose rock is rather more fragile. Sheet ice formed in the lower subzone of the foreshore moves up and down with the tides because the substrate cannot freeze during the shorter exposure to super-cooled air. This vertical movement of the sheet ice creates a bellows-like action between the ice and soft mud, promoting vigorous erosion.

8.2. THE PHENOMENON OF ICE WALLS

In the upper reaches of tidal estuaries, sea water can bring in ice that gradually builds up high vertical walls. Forced into the ever-narrowing sections of the estuary by a sort of ratchet movement, the ice is unable to exit. Initially, ice walls are rather porous and irregularly-shaped accumulations of ice. However, passing tides charged with floating ice cakes smooth off the rough edges, filling the pores and leaving a film of silt-laden water to freeze into a smooth surface layer of ice. Before long the trapezoidal cross-sections of tidal creeks, with side slopes of approximately 1:3.5, are transformed into rectangular channels of much smaller cross-sectional area (Gordon and Desplanque 1981). This in turn reduces the tidal prism and eventually also the flow rate of tide in the estuary (see Fig. 45).

[FIGURE 45 OMITTED]

During winter, ice walls 5 m high can build up in a week or less. Typically, ice forms a levee that is higher beside open water than toward the bank of a tidal creek (see Figs. 46, 47, and 48). These figures illustrate that impressive ice walls can form even in years of moderate tides. The foot of an ice wall tends to be slightly lower than the level reached by the lowest High Waters occurring during the frost period. Most of the ice becomes stranded shortly after High Water. Slack water intervals in tidal creeks are brief, to the point of non-existence. They occur after the water level has dropped following High Water. At High Water the water continues to move into the estuary on its own inertia. This process delays filling of the upper parts of the estuary with tidal water. Thus at High Water the ice is still moving in, to become stranded shortly afterward. Ice that is lifted onto the top of the banks or onto previously stranded ice will likewise be left high and dry when the ebb sets in. Before the next High Water the chances are good that this ice will become solidly frozen to the underlying base and will not be refloated. If it does manage to break loose, this will occur on the rising tide and stranding will occur at some point further into the estuary. The process sounds mundane perhaps, but in actual fact it is a most remarkable exercise in ice block gymnastics. The performance seems surreal because the delayed release of ice blocks from the substrate beneath the rising tidal waters causes them to be released by buoyancy, as if possessed of life.

[FIGURE 46-48 OMITTED]

8.3. ASTRONOMICAL CYCLES AND ICE BUILD-UP

Ice build-up will be heavier during some winters than in others because of tidal conditions (see Fig. 49, Table 18). Peaks of perigean tides and spring tides coincide in cycles of 206 days. Two of these cycles last 412 days with the result that from year to year the especially strong tides occur 47 days later than in the previous year (412 - 365). During the first half of the 206-day cycle, the difference in height between neap tide and spring tide is decreasing, or below average, allowing a lesser ice wall build-up. Conversely, during the second half of the cycle, tides will gradually rise to higher levels during the week before the perigean tides. One of the key ingredients in heavy ice build-up is thus the timing of the greatest difference between neap tide levels and spring tide levels. This occurs one or two months before the perigean and spring tides combine to form the strongest tide of the cycle. Thus, when the latter half of the 206-day cycle occurs during the frost period between December and the end of March, one can expect the greatest build-up of ice walls. The formation of high ice walls in the Bay of Fundy was first recorded by Henry Y. Hind on or near 25 April, 1875 (Hind 1875). One hundred days earlier, the spring tide will have been at its minimum. Thus, the latter half of the 206-day cycle fell during the first period allowing ice to accumulate on the strand a month early, with accumulation probably peaking toward the end of April.

[FIGURE 49 OMITTED]

Naturally, the ice walls on either side of an estuary converge inland. Near the upper end of the estuary they will almost touch, leaving a strongly reduced channel in which freshwater runoff takes over the main role in channel-shaping. The drastic reduction in cross-sectional area can lead to flooding during a sudden winter thaw or spring break-up. The channel may then become choked with freshwater runoff, and/or ice. Certain rivers, such as the Salmon River near Truro, N.S., seem more prone than others to such flooding.

8.4. HAZARDS VERSUS BENEFITS OF ICE WALLS

The development of ice walls can contribute to disastrous problems in tidal waters. In 1965 a fertilizer plant was built at Dorchester Cape, N.B., near the confluence of the Memramcook and Petitcodiac River estuaries (see Fig. 43). In order to handle the bulk material to and from the plant, a wharf was constructed nearby. Because of the large local tides, a $2.5 million floating dock was constructed in the form of a 25 m x 90 m x 7 m concrete caisson connected to a wharf and concrete platform on shore by means of a bridge and connecting arms. At this location (see also Fig. 3) the average range of the tide is over 11 m, increased during large tides to more than 15 m. Since trucks had to move over the bridge the slope needed to be maintained within certain limits. Hence the distance between platform and caisson was substantial. Also, the tidal channel was dredged, allowing the caisson and ships tied to it to move freely up and down with the tide.

The facility, named the Port of Moncton, was officially opened on 24 November. Winter followed the departure of the first ship. An ice wall developed along the shoreline, and worse, between the platform and the caisson beneath the caisson arms. When this wall became sufficiently high and strong, it blocked the free movement of the connecting arm and bridge. The bridge lifted out of the water, the connectors buckled and broke, and the caisson drifted away, stranding on a nearby silt flat. The entire operation was subsequently shut down. Ironically, predicted and observed tides for November, 1965 were low, so that ice conditions might have been a lot worse!

What are the ecological implications of ice in the upper reaches of the Bay? At one time it was believed that ice moving from the high marsh during winter was the main agent responsible for transporting organic matter from the marsh into the Bay. However during most winters, ice is unable to move onto the high marsh. Thus the organic detritus of high marsh vegetation survives until spring only to disappear beneath flesh vegetation in the same manner as non-cut grasses on upland meadows, presumably by microbial decomposition.

Decaying high marsh vegetation is commonly deposited along the dykes in thick accumulations (Nova Scotia Department of Agriculture and Marketing 1987). Hydraulic conditions on the marsh are such that they inhibit most if not all seaward movement of organic debris; flow is too gradual at these shallow depths to accomplish transport.

On the low marsh, exposed vegetation is crushed and frozen into blocks of ice. Subsequently these blocks may be lifted by the spring tide, pulling the encased vegetation from the marsh. Removed to higher elevations they will remain stranded until the ice thaws, leaving the vegetation behind. In tidal rivers, ice walls begin to collapse by the end of March and most ice disappears except for shorefast ice that may persist until late April (see Figs. 50, 51). Stranding of large blocks of composite ice on the mud flats, acting in concert with the bellows action of sheet ice hinged to the shoreline, may cause the complete reworking of the soil under the sheet ice during the late winter and early spring. In late spring some tidal flats resemble plowed fields with great scars left by chunks of moving ice. Any form of life in this mudflat ecosystem survives only under a great deal of stress (Gordon et al. 1985). Macrofaunal diversity is very low. In addition to organic secretions of diatoms, most of the productive biomass derives from three species (Hicklin et al. 1980): a bivalve (Macoma balthica), a polychaete (Heteromastus filiformis) and an amphipod (Corophium volutator). The amphipod, so important to migrating bird life in the Bay, is widely distributed over the mudflats. Its survival in winter is doubtless predicated by its ability to burrow well beneath the zone of scour and erosion. Since ice scouring is most intense on the outer portions of mudflats, the role of shorefast ice may afford an important protection to these species in the inner several hundred metres of mudflats at the mouths of estuaries. Corophium volutator may also occur on exposed tidal flats with no ice.

[FIGURES 50-51 OMITTED]

With approaching spring, flows of freshwater and tidal water progressively undercut the basal portion of ice walls and ice blocks. In places the ice is sculpted into dark-coloured, ephemeral toadstool-shaped formations charged with concentrations of estuarine mud as high as 18g per kg (i.e., 18 wt %) mud. By early April the thinner ice will have disappeared, but ice walls may persist until month's end (Fig. 52). At their base, huge accumulations of silt occur in sections sheltered from strong currents. It is remarkable that the ice fields are in the same locations where silt built up after construction of the Petitcodiac River causeway in 1968. Causeway construction promotes silt accumulation, with concomitant depth reduction of tidal rivers, and dire results for much aquatic life (Daborn and Dadswell 1988). With hindsight it can be seen that the river and its ice accumulations act like an enormous hydraulic sluice. Thus, whereas ice walls and associated phenomena may prove hazardous to man-made constructions they serve very important natural purposes. In the case of ice walls, their smoothness resists further narrowing of the channel, increases the net ebb and river flushing currents, maintains river depth, increases the surface slope seaward, forces the salinity intrusion seaward, and obstructs massive influx of ice from the estuary mouth.

[FIGURE 52 OMITTED]

8.5. ICE-RELATED PROBLEMS IN ESTUARIES OF THE BAY OF FUNDY

There are many lessons to be learned from a consideration of winter conditions in a tidal regime as complex as that of the Bay of Fundy. Intertidal ice is an extremely active agent in diverse environmental processes, both physical and biological, in this macrotidal region.

From a strictly engineering perspective, the sequence of tides, temperatures, and wind velocities must be carefully evaluated in estuaries in northern regions subjected to a large tidal range. Floating structures attached to an estuary bank are at risk due to build-up of ice walls. So too, are bridges, where ice build-up increases the size of bridge piers, resulting in reduced cross-sectional area at the crossing (Figs. 53, 54). Construction designed to control flooding of marshlands must consider the downstream channel changes from trapezoidal to rectangular as ice factories swing into production. Shorefast ice can greatly inhibit the workings of one-way drainage devices, whether mechanical (flap gates) or electrical (gate slots), on hydraulic structures that are installed without due regard to winter conditions.

[FIGURES 53-54 OMITTED]

Concerning megaprojects like tidal power production, it is quite certain that a tidal power project would result in major changes in the dynamics and distribution of ice in an estuary. Reduction of tidal energy would promote water column stratification, which would extend the ice season. Sheet ice could expand near tidal rivers and by excluding extreme tides, the development of shorefast ice would be retarded. Not least, accumulation of drift ice at the barrage would need to be considered at the early design stage.

February is usually the time of heaviest ice build-up. Ice jams (dams) are most likely to occur in the upper part of an estuary during a period of low temperatures and spring tides. Conditions in the Bay of Fundy will be greatly exacerbated by strong, prevailing southerly to southeasterly winds that might coincide with a rapid thaw and heavy rains. Doubtless the most important factor affecting intertidal ice conditions in the Bay of Fundy is the unusually high tides. This results in several conditions not commonly encountered in other regions. Thick ice walls may be unique to Fundy estuaries due to the pronounced variation in the elevation of High Water during the spring/neap cycle, and the prolific High Water stranding of drift ice. Also unique, as we have seen, is the substantial variation in elevation of extreme (extraordinary) tides over longer periods than the spring/neap cycle. This factor affects the extent to which shorefast ice develops, and the degree to which the high marsh areas are influenced by sea ice. Hind (187.5, p. 193) elegantly described the behaviour of ice in the lower zone of a Fundy estuary: "The appearance of an estuary in the Bay of Fundy at any time in midwinter presents some singular and striking phenomena, which may contribute to our knowledge of the manner in which different agents have assisted in excavating this extraordinary bay, and are now engaged in extending its domains in some directions and reducing it in others." At the time, his concern was with the potential impact of ice on the Bale Verte canal, proposed to link Cumberland Basin to the Northumberland Strait. Although this project was never completed, various large scale construction projects and a multitude of smaller scale coastal management and development schemes will continue to merit quality time applied to the task of understanding the dynamics and environmental effects of intertidal ice.
Table 18. Summary characteristics of major constituents
of tidal cycles in upper sections of the Bay of Fundy

Cycle Period Approx.
 tidal range

1. Diurnal cycle due to 0.517 days
 relation of Moon to Earth (12 hr 25 min) 11.0 m

2. Spring/neap cycle 14.77 days 13.5 m

3. Perigee/apogee 27.55 days 14.5 m
 206 day cycle due to

4. spring/neap, and 206.0 days 15.5 m
 perigee/apogee cycles

5. Saros cycle (last peaked in
 1994-1995) 18.03 years 16.0 m


CON DESPLANQUE (1) AND DAVID J. MOSSMAN (2) *

(1.) 27 Harding Avenue, Amherst, NS B4H 2A8

(2.) Department of Geography, Mount Allison University, 144 Main St., Sackville, NB E4L 1A7

* Corresponding author
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Title Annotation:Part 3: Chapter 7-Chapter 8
Author:Desplanque, Con; Mossman, David J.
Publication:Atlantic Geology
Geographic Code:1CANA
Date:Mar 1, 2004
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