WINTER PROCESSES IN AN ESTUARINE ENVIRONMENT

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 22 International Association of Hydraulic Engineering and Research WINTER PROCESSES IN AN ESTUARINE ENVIRONMENT Brian Morse 1, Danielle Messier 2, Edward Stander 3 and Tung-Thanh Quach 2 ABSTRACT Three years of winter process monitoring along a 5.8 km long meso-tidal estuary is presented. The Portneuf Estuary, located on the north shore of the St. Lawrence River in Quebec, Canada, is bounded upstream by a hydroelectric dam, and empties into the St. Lawrence at Forrestville, Quebec. While river spring flows and tidally induced flows may reach several hundred m 3 /s, winter water entering the estuary is generally on the order of 2 m 3 /s. Ice hinges and pressure ridges characterize the estuary. Ice thickness varies laterally and is at a minimum in the centre of the channel where snow-ice overlays S1 (and locally S2) ice. Despite the saline environment, virtually all the ice is brine free. Daily ice formation and/or melt appears to be dependent on the temperature of the incoming St. Lawrence waters, the distance from the Portneuf estuary mouth, and tidal range (neap or spring). In turn, the presence of the ice cover attenuates the tidal range and the water flow rates. Velocity profiles are quite uniform over the depth except during the time salt water influx. INTRODUCTION The present study was initiated by Laval University and Hydro-Quebec in 1999 at the bequest of the Canadian Committee on River Ice Processes and the Environment (Morse et al., 1999). A review by Morse et al. (21) concluded that ice behaviour in estuaries depends primarily on tidal range. Ice processes in weakly tidal rivers (tidal range 3 to 6 cm) were found to behave similarly to non-tidal rivers except for some particularities such as the formation of an ice bridge at the mouth which may have many meters of frazil accumulation; a fresh water plume in the upper water column of the outer estuary and a reduced salt wedge intrusion due to the modification of the tidal range. Ice processes in macro-tidal estuaries (having typically a 11 m tidal range) are very significant. Studies by Desplanque and Bray (1986) demonstrated that the geometry of these estuaries was dramatically reduced in winter due to the formation of massive ice walls and that the observed ice processes were indicative of both river and sea ice environments. There is an on-going interest in these estuaries since they are subject to flooding by ice jams and to extreme sediment transport processes. 1 Université Laval, Sainte-Foy, Québec, Canada, Brian.Morse@gci.ulaval.ca 2 Hydro Québec, Montréal, Québec, Canada 3 State University of New York, Cobbleskill, NY

2 The subject of this paper is the Portneuf meso-tidal estuary (48:39:42 N, 69:1:1 W) located in Quebec, Canada (Figure 1). Its estuary portion is 5.8 km long, has a main channel 2 m wide and is subject to semi-diurnal tides having a neap/spring cycle of 1.9 m to 3.8 m (Savard, 1998). At the upstream end, there is a run-of-the-river hydro dam and the Portneuf Estuary empties into the St. Lawrence Estuary on its north shore near Forrestville. Spring runoff typically varies between 2 and 7 m 3 /s. During winter months, the river flow is only about 15 2 m 3 /s while tidally induced flows at the mouth are typically 3 4 m 3 /s. Being a meso-tidal estuary, the ice processes are similar to river-like processes but are strongly influenced by tidal dynamics. While this study is still underway, we have thus far collected three winters of field data. During the first year's program in 2, the emphasis was on acquiring traditional ecological knowledge (Clément, 2) and performing preliminary field observations (Morse et al., 21). In 21, we instrumented the estuary with salinity and temperature probes to which we added water current measurements in 22. Portneuf dam N St. Lawrence Estuary Portneuf mouth Figure 1 : Portneuf estuary, Quebec, Canada ICE PROCESS IN THE PORTNEUF ESTUARY The most striking winter feature of the estuary is a semi-continuous ice pressure ridge 1 to 3 m high that runs along the estuary. While this ridge usually parallels the banks of the estuary, it occasionally switches from one bank to the other at river bends. It is normally situated at the demarcation between the main channel and the tidal flats. Within the pressure ridge, one can observe a number of ice push events of varying thickness (4 to 15 cm) originating from the tidal insertion of freezing water in the tidal

3 hinges. Where the channel passes close to the bank, one observes the horizontal layering of discrete ice sheets possibly formed by ice-thrust events. Tidal hinges and pressure ridges make navigating on the ice surface extremely hazardous. Most of the time there are some areas where water floods the ice. As such, the estuary is never used for recreational snow mobiling apart from occasional winter ice fishing. The shoreward edges of the tidal flats are rarely covered by water. Instead, falling snow accumulates as the ground freezes. Subsequent flooding of this region during spring tides leads to the inundation of these snow banks and the development of massive, rough ice floes. Breakup usually occurs during spring thaw, but its timing is solely dependent on the arrival of the spring tides. During the rising portion of the neap/spring cycle, the tide gets progressively larger and water fluctuations intrude further and further into the estuary both longitudinally and laterally. During each intrusion, high water breaks up the ice along the shores and flats. Ice pieces about 3 m in diameter then get ripped out and transported down the estuary during the ebb flow. Normally, the whole estuary is cleaned out within a few days. Major ice jams are extremely rare because the ice is pulled out by the tides rather than pushed out by a spring runoff event. However, when the broken ice pieces are not fully removed by the ebb tide, they fall into the main channel and create a series of porous weirs that the water cascades through. During such periods, the flow in the estuary is singularly unique. Ice formation in the main channel follows a well behaved pattern. In 2, the vast majority of ice core samples were identical in basic structure. The cores primarily consisted of a thick layer of transparent, competent S1 ice overlain by 1 to 3 centimeters of frozen snow-slush (T1 ice). Only three cores displayed the columnar pencils indicative of S2 ice, and of these, only one contained pockets of brine. S1 ice is present at all locations. In the upper reaches, this layer extends to the base of the ice sheet. The S1 crystals are generally quite large (1 3 cm), and bubble free, although some cores contain well defined horizontal layers of bubbles. S1 ice indicates that the initial ice cover formed during quiescent fresh water flow conditions. The S1 layer abruptly terminates at depth in two cores taken near the mouth by a thin frazil nucleation layer. This layer is very thin (perhaps 2 3 crystals thick), and led to the nucleation of columnar S2 ice towards the bottom of the sheet. S2 ice also developed in a third core closer to the estuary mouth, though in this case the transition from S1 to S2 ice is associated with an increase in thin brine pockets. In 21, eight cores were collected of the full ice thickness, most of which were taken near the estuary mouth, as ice upriver was rarely (if ever) affected by the saline plume. These cores were identical to those collected in 2. The base of the ice sheet was particularly interesting in that there was much evidence of thermal erosion. Ice immediately upstream of the Forestville bridge displayed beautifully polished thermal grooves oriented parallel to the channel. Thermal effects were also in evidence about some protruding temperature probes installed on February 9. These probes nearly broke through the ice sheet as a result of under-ice melting.

4 TIDAL DYNAMICS Ice growth and recession in the 5.8 km long estuary demonstrated an obvious link to thermal processes driven by tidal dynamics. Our one dimensional numerical simulations demonstrated that the tide intruded typically 4 km during low winter flow conditions under spring tides but would only intrude 1 km when the flow increased to 1 m 3 /s and neap tide conditions. Figure 2 illustrates the tidal wedge intrusion near the bridge (2.7 km upstream of the mouth). At high tide (2 p.m.) the tidal wedge approached the dam (at 5.6 km upstream of the mouth). It had flattened out considerably and was overlain by a fresh water layer (S <.5 o / oo ) about 2 m thick. Half way through ebb tide (at 4: p.m.), we sampled profiles along the whole estuary. All the way from the head to the mouth, salt content increased with depth. In the upper 2 m, the water was fresh, while in the bottom 2 m, salinity was in excess of 25 ppt. Later on, at ebb tide (7 p.m.), except for some deep holes, the whole estuary was fresh water. 5 4 Notes: 1. Profiles were taken every 5 minutes from 9:46 to 11:55 2. Salinty is expressed in ppt Level of Ice Local water depth (m) 3 2 Water level Level of river bed 1: 1:3 11: 11:3 Local time, 21st February, 21 Figure 2: Salt wedge intrusion near bridge (2.7 km from mouth) These data show that the ice sheet near the mouth is strongly affected by the incoming saline wedge while ice further inland only comes in contact with St. Lawrence River waters during the spring tides. As the plume migrates upstream, it flattens out with the result that fresh water entering the estuary from upstream appears to simply ride over the intrusion with a minimal amount of mixing. Salt water always freezes at temperatures below that required for fresh water ( C). The relationship between salinity and freezing temperature is defined by the solid line in figures 3 and 4. If the temperature of the water column falls below the point of ice formation, it is supercooled and is liable to form ice nuclei within the column. If the temperature of the water column lies above this line, it will melt any ice it comes in contact with. Equilibrium is achieved when the ambient water temperature falls directly on the line.

5 Figure 3 shows the relationship between plume salinity and temperature at the base of the water column over an 11 day period from February 9 th to February 2 th. The monitoring station is upstream of the bridge (2.7 km from the mouth). As all of the data lies above the solid line, we can argue that the plume was generally too warm to form ice, and would initially melt the ice sheet were it to come in contact with it. 5 Salinity (ppt) Points above the line indicate "warm" water Observations : Sonde YSI Température de fusion = f(salinité) ,2 -,4 -,6 -,8 Temperature (degrees C) -1 Points below the line indicate supercooled water Figure 3: Water temperature and salinity near the channel bottom -1,2-1,4-1,6 In contrast, the water column near the ice/water interface was often cooled to the point that ice formed, and was frequently supercooled below the temperature of ice formation (figure 4). This suggests that the water column was cooled at times by the overlying ice sheet Salinity (ppt) Points above the line indicate "warm" water Observations : Sonde YSI Température de fusion = f(salinité) 15 1 Points below the line indicate supercooled water 5 -,2 -,4 -,6 -,8-1 -1,2-1,4-1,6 Temperature (degrees C) Figure 4: Water temperature and salinity near the ice surface

6 It may be further argued that the longer a saline water body lies in contact with the ice sheet, the greater this cooldown effect will be. Figure 5 shows that the cold salt water stays along the bottom of the channel during a much longer period than it does near the ice surface. We found that the time of contact during a tidal cycle was directly proportional to the tidal range and was proportional to the distance of the location from the mouth. 5 salinité près de la surface 3 salinité près du fond Salinity (ppt) Température de l'eau près de la surface Niveau d'eau (m) à la marina Water level (m) and temperature (degrees C) Feb 18: 19 Feb 2: 19 Feb 22: 2 Feb : 2 Feb 2: 2 Feb 4: 2 Feb 6: 21 Figure 5: Time history of water temperature and salinity during one tidal cycle EFFECT OF ICE COVER ON HYDRODYNAMICS Just as the tidal range influences ice dynamics, so too does the presence of the ice cover influence water currents. One-dimensional numerical modelling demonstrates that the presence of the ice cover increases the water level elevations by about 1 cm at high tide and 5 cm at low tide. Maximum ebb and flood tide water discharge are reduced by about 1 % throughout the estuary. In 22, we collected water current profiles using an ADCP installed at the ice surface. Figure 6 displays the velocities as a function of time and depth. Note that the top 3 cm are the ice sheet and then there is a 2 cm window under the ice sheet that is blind to the ADCP. In addition, velocities near the bed cannot be detected by the instruments. Whereas under open water conditions, we would expect a logarithmic type of profile, figure 6 demonstrates that in the presence of an ice cover, the velocity is more or less constant as a in those depths observed by the ADCP. It is only during the rising flood tide that there is a significant variability of velocity as a funciton of depth. We note the increase in the lower half of the water column during the indtrusion of the salt-wedge on the rising tide. DISCUSSION There is an interplay between thermodynamics and hydrodynamics in the estuary. The development of ice under estuarine conditions appears to be a balancing act between thermal erosion and ice growth. The saline plume arriving from the St. Lawrence is generally too warm to nucleate ice, and comes in contact with ice that is at or near C (i.e. in equilibrium with fresh water). This condition leads to the rapid melting and cooling of the ice at the water interface. However, given the proper conditions, the ice

7 will equilibrate with the higher salinity (and lower temperature), and a new layer of ice will form at the base. Here residence time is probably important, since changes in ice temperature require time to overcome the thermal inertia of the sheet and water. m Ice bottom Channel bottom m From noon March 2 to 5:4 am March 3, 22 Figure 6: Water current profiles during a large tide at the bridge As the tide falls, the arriving fresh water plume may well come in contact with an ice sheet which is colder than its fresh-water freezing point. This leads to supercooling of the water column and the rapid growth of ice at the interface. With continued ice formation, supercooling ceases - at least until the next saline plume retreats. Ice thickening, then, appears to depend on the relationship between tide, temperature, and salinity: a) High tidal differences lead to long residence times of salt water near the ice/water interface, which in turn favors equilibrium temperatures and ice formation after the initial melt period. b) Higher salinities lead to an increased likelihood of thermal erosion at the interface, and an increased probability of supercooling during the falling tide. From the above, it appears that saline ice will rarely form in estuarine environments, an observation supported by core analysis. In order for saline ice to form, one requires a combination of low ambient (air) temperatures (with no insulating snow layer) and high resident plume times (spring tides). Even so, these same conditions favor supercooling, which will likely lead to the growth of fresh water ice between the saline layers. While field observations suggest that the saline plume was more apt to melt than thicken the overlying ice sheet during our study period, there is some evidence that local ice

8 growth did occur during the falling tide. REFERENCES Clément. D. Le Savoir Écologique Local Relatif aux Conditions de glace de L'estuaire de la Portneuf. Rapport présenté à Hydro-Québec, Hydraulique et Environnement (2) 71p plus annexes. Desplanque, C. and Bray, D.I. Winter ice regime in the tidal estuaries of the northeastern portion of the Bay of Fundy, New Brunswick. Can. J. Civ. Eng. 13: (1986). Morse, B., Messier, D., Stander, E. and Quach-Thanh, T. Le savoir écologique local de la dynamique des glaces dans l'estuaire Portneuf. In 11th Workshop on River Ice "River Ice Processes Within A Changing Environment, May 14 16, 21, Ottawa, Ontario. Canada (21). Morse, B., Burrell, B., St. Hilaire, A., Bergeron, N., Messier, D. and Quach-Thanh, T. River ice processes in tidal rivers: Research needs. In Proceedings of the 1th Workshop on River Ice "River Ice Management with a Changing Climate: Dealing with Extreme Events". June 8-11, Winnipeg. Manitoba. Canada (1999) Savard, J.-P. Avis Scientifique sur la Dynamique Sédimentaire dans L'estuaire de la Rivière Portneuf. Rapport d'étude réalisé pour Hydro-Québec par InteRives Ltée (1998) 5p.

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