ICE-INDUCED DAMAGE TO SHORELINE PROPERTIES ON ROUND AND GOLDEN LAKES

Transcription

1 ICE-INDUCED DAMAGE TO SHORELINE PROPERTIES ON ROUND AND GOLDEN LAKES G. Comfort 1 and A. Liddiard 1 1 BMT Fleet Technology Ltd., Kanata, Canada ABSTRACT Ice-induced damage has occurred in 3 of the past 11 winters at Round and Golden Lakes, which are located about 200 km from Ottawa, Ontario. Thermally-induced ice push has been responsible for damage to the shoreline and to properties near shore. Water levels on these lakes are regulated by a hydro-electric power producer located downstream. The problem was investigated. First, the root causes of the events were identified. Criteria were developed to define whether an ice push event would, or would not, occur. Then, methods for mitigating the problem were assessed. Recommendations were developed regarding the preferred hydraulic operating regime to avoid ice-related problems. KEY WORDS: Ice push; Shoreline damage; Ice load modeling; Water management INTRODUCTION AND OBJECTIVES Ice push events have occurred during three of the past eleven winters (i.e., , and ) on Round and Golden Lakes, which are located about 200 km northwest of Ottawa, Canada. These events have caused significant damage to the shoreline and to some riparian properties (Figure 1). Lake levels are controlled by Renfrew Power Generation (RPG), which is a hydropower producer downstream of the lakes, and regulated within the Bonnechere River Water Management Plan (WMP) (MNR, 2004) under the governance of the Ontario Ministry of Natural Resources (MNR). This paper summarizes a study (Comfort and Liddiard, 2005) that was undertaken to: (a) assess the root causes of the ice damage events, and the effects of the contributing factors; (b) evaluate the most appropriate hydraulic operating regimes for Round and Golden Lakes. BACKGROUND Ice push onto a shoreline can result from two different mechanisms: (a) expansion due to ice temperature increases (termed thermal ice push here), and; (b) wind-driven ice movements. Kovacs and Sodhi (1988) described wind-generated ice push that has occurred along shorelines in the Arctic. These two mechanisms are mutually exclusive for practical purposes. Because the available anecdotal evidence from the MNR and RPG indicated that that ice push events on Round and -21-

2 Golden Lakes tend to occur after an ice cover has been fully established on Round and Golden Lakes, the work was focussed on thermal ice push. Figure 1 Round Lake 2005 Golden Lake 2004 Ice Related Damage on Round and Golden Lakes (photos by RPG) Thermal ice push has been observed at many locations besides Round and Golden Lakes. Paul (2003) provided a description of the ice push phenomenon, as well as a website with photos of many recent ice push events at lakes in Minnesota ( Gatto et al (2001) presented field measurements made at Lake Bomoseen, Vermont, to investigate the ice push phenomenon. Zumberge and Wilson (1953) (cited by Gatto et al, 2001) investigated ice push that occurred at Wamplers Lake, Michigan. Pessl (1969) (cited by Gatto et al, 2001) attributed the formation of a rampart 2-4 ft high and 4-10 feet wide on Gardner Lake, Connecticut to thermal ice expansion. Sommerville and Burns (1968) described reservoir damage near Winnipeg, Manitoba that was caused by thermal ice expansion. PROJECT SCOPE AND APPROACH Analyses were done for the years to , as follows: (a) an environmental model was run for each winter, and; (b) the ice-shoreline bond length was calculated. Environmental Analyses These were done using available environmental data and an environmental model that had been developed based on an 11-year investigation of ice loads on hydro-electric dams (Comfort et al, 2003a, b, c; Singh et al, 1997, 1998). First, the model was verified against ice thickness data from 1956 to 1971 at Golden Lake (Allen, 1977). Second, the model was used to evaluate environmental conditions for to The output of the model was used to identify thermal ice load events, which on these lakes would likely lead to shoreline damage. The timing of these predicted events was then compared to the occurrence of known shoreline damage events. Investigation of those predicted events that did not correspond to actual shoreline damage events led to the analysis of the ice-shoreline bond lengths. -22-

3 The Ice-Shoreline Bond The ice-shoreline bond is a key factor affecting whether or not an ice push event occurs. A strong ice-shoreline bond will prevent ice displacements since the thermal ice forces will be reacted into the lakebed. Conversely, a weak ice-shoreline bond will allow thermally-induced ice sheet displacements to occur, driving the ice sheet up the beach slope. In the absence of any lake level changes, the length of the ice-shoreline bond will steadily increase over the winter as the ice thickness increases. This increases the force necessary to break the bond, which makes ice push events increasingly unlikely as the winter progresses. The ice-shoreline bond is also affected by lake level changes. A drop in lake level will increase the strength of the ice-shoreline bond by increasing the contact length (Figure 2). This can greatly increase the force required to break the ice free from the shoreline, thus reducing the risk of ice push events. Conversely, an increase in lake level will weaken or even break, the ice-shoreline bond. A large increase in lake level will break the ice free immediately (Figure 3). A small increase will crack the ice but not immediately break it free from the shoreline (Figure 4). In this latter case, surface flooding is expected which will weaken the ice-shoreline bond. Therefore, although different mechanisms accompany large and small lake level increases, the net effect on the ice-shoreline bond will be the same as it will be weakened and destroyed. Figure 2 Effect of a Drop in Lake Level Figure 3 Effect of a Large Increase in Lake Level -23-

4 Figure 4 Effect of a Small Increase in Lake Level The length of the ice-shoreline bond was calculated for each week of each of the eleven winters from freeze-up (Dec 15) to mid-winter (Feb. 16). The calculations for Round and Golden Lakes were done for beach slopes of and respectively, as these are the beach slopes at locations where ice push events have occurred in the past. Ice-shoreline bond lengths were computed taking into account changes in both ice thickness and lake levels. RESULTS Factors Causing an Ice Push Event Ice push events result from: (a) ice temperature increases, in combination with (b) a poor bond between the ice and shoreline which allows the ice sheet to move upslope (Figure 5). Each of these depends on several contributing factors. Ice temperature increases can result from: (a) air temperature increases; (b) a lack of snow cover on the ice, which allows ice temperatures to fluctuate in concert with air temperature variations; (c) a snowfall that adds insulation to the ice, warming it from the bottom up; the effect is most pronounced when there was little to no snow cover on the ice initially; and (d) a rainfall, which adds heat and also removes snow cover. Poor bonding between the ice and the shoreline is likely to result from one or more of: (a) lake level rises which act to weaken or destroy the ice-shoreline bond (b) thin ice, or ice covered with slush, which warms the ice and tends to bring the ice temperature to 0 C throughout the full ice thickness. Winter-by-Winter Assessment The ice-shoreline bond length required to prevent an ice push event increases with the magnitude of the ice temperature rise. This fact was used to determine the minimum bond length required to prevent significant ice push events, which, based on the available data, would result from ice surface temperature rises of more than 5C. These analyses indicate that an ice-shoreline bond length of more than about 15 m and 30 m will prevent significant ice push events on Round and Golden Lakes, respectively. Longer bond lengths are required for Golden Lake due to its shallower beach slope. -24-

5 The interdependence of ice temperature changes and good bonding at the shoreline is illustrated by the results for the winter. Of the eleven winters studied in this project, this winter saw the largest predicted ice temperature increase of 17C, however, ice push did not occur. The absence of an event can probably be attributed to: (a) a steady drop in lake level over the early winter, and; (b) cold air temperatures during the early winter. These factors would have combined to produce a strong ice-shoreline bond which prevented ice push. No Large Air Temperature Change? Snowfall or Rainfall Occurs? Little Snow on the Ice? No Slush on the Ice? Yes Contributes to the Lack of an Ice Temperature Change Contributes to a Large Ice Temperature Change Large Level Change? o Dropped Contributes to Good Ice-Shoreline Bond o Raised Contributes to Poor Ice-Shoreline Bond Ice Thickness: o Large Contributes to Good Ice-Shoreline Bond o Small Contributes to Poor Ice-Shoreline Bond No Large Ice Temperature Change Occurs? Yes Ice Well-Bonded to the Shoreline When The Ice Temperature Change Occurs? No Yes No Ice Push Figure 5 Ice Push Event Occurs Schematic of Factors Causing an Ice Push Event No Ice Push Event Tables 1 and 2 summarize why ice push events did and did not occur in these respective winters. Table 1 Winters with Ice Push: Summary of Major Contributing Factors Winter Ice Temperature Increases 1 Ice-Shoreline Bond Large 1 increase: 11.2 C at ice surface Bond was short (31 & 55 m) 2 when ice Contributing factors: (a) increase in air temperature increase occurred temperature; (b) little snow on the ice Contributing factors: thin ice Low 1 increase: 3.4 C at ice surface Contributing factors: rainfalls warmed the ice sheet and produced surface slush Medium 1 increase: 8.2 C at ice surface Contributing factors: snowfall accompanied by an increase in air temperature Bond was very short (10 & 20 m) 2 when ice temperature increase occurred Contributing factors: lake levels rose; thin ice Bond was short (40 & 10 m) 2 when ice temperature increase occurred Contributing factors: lake levels rose; thin ice -25-

6 Table 2 Winters Without Ice Push: Summary of Major Contributing Factors Winter Ice Temperature Increases 1 Ice-Shoreline Bond Large 1 increase: 17 C 3 at ice surface Bond was long enough (55 & 153 m) 2 when Contributing factors: rainfalls accompanied ice temperature increase occurred by air temperature rise; little snow on the ice Contributing Factors: lake levels dropped; ice thickness increased rapidly in early Medium 1 Increase: 7.1 C at Ice Surface Factors Contributing to Temp Increase: rainfalls accompanied by an increase in air temperature, and; little snow on the ice at the time Medium 1 Increase: 7.6 C at Ice Surface Factors Contributing to Temp Increase: increase in air temperature, and; little snow on the ice at the time No Temperature Increases Contributing factors: heavy early snowfall; rainfalls causing slush on the ice Medium 1 increases (2 events): 8.8 & 7.7 C at ice surface Contributing factors: rainfalls accompanied by an increase in air temperature, and; little snow on the ice at the time No Temperature Increases Contributing factor: heavy early snowfalls season Bond was long enough (25 & 50 m) 2 when ice temperature increase occurred Contributing Factors: lake levels dropped; ice thickness increased rapidly in earlyseason Bond was long enough (21 & 51 m) 2 when ice temperature increase occurred Contributing Factors: lake levels were dropped slowly; lake levels were low, and; ice thickness increased rapidly in earlyseason Not relevant this winter as there were no temperature increases No Temperature Increases Not relevant this winter as there were no Contributing factor: warm winter temperature increases Low 1 Increase: 3.4 C at Ice Surface Bond was long enough (26 & 55 m) 2 when Contributing factors: small increase in air ice temperature increase occurred temperature; little snow on the ice at the Contributing factors: lake levels dropped time steadily Notes to Tables 1 and 2: 1 The ice temperature increases were ranked as small, medium and large, which corresponded to increases of less than 5C ; 5C to 10C, and more than 10C, respectively. 2 These are the bond lengths calculated for Round & Golden Lakes, respectively. 3 This was the largest ice temperature rise predicted for all winters analysed. SHORELINE MAINTENANCE Ice push events produce soil berms at the ice edge, which have been observed at Round and Golden Lakes (Figure 6), as well as in other locations. A site visit to Round and Golden Lakes showed that these berms had been cleared away at many locations, leaving a smooth beach profile. Soil berms are a natural response to ice push events, and by removing them, the beach is left vulnerable to future ice push events (e.g., Paul, 2003; Minnesota DNR, 2005). Paul (2003) noted that ice-induced natural barriers will build up over time, and become fortified with items such as roots, trees and rocks (if they are present on the beach). This will act to produce a natural barrier that both persists over time (in the presence of erosive forces), Bond was long enough (58 & 75, and 117 & 155 m) 2 when ice temperature increase occurred Contributing Factors: lake levels dropped steadily for 2-3 weeks prior to the events, and; ice thickness increased rapidly in earlyseason Not relevant this winter as there were no temperature increases -26-

7 and also protects against ice push. It is believed that the shoreline management practices contributed to the ice push events. Soil Berms May, 1995 Smooth Beach August, 2005 Figure 6 Beach Profiles on Round Lake (photos by RPG and BMT) RECOMMENDATIONS REGARDING THE HYDRAULIC OPERATING REGIME The following principles should be used in the management of the lake levels over the winter: (a) a steady drop in lake level after freeze-up should be targeted: (b) the rate of drop should be selected to establish a strong (and long) ice-shoreline bond as early as possible; (c) the lake level should be in the mid-range of the limits set out in the WMP at freeze-up. If the lake level is near the upper part of the range in the WMP at freeze-up, the lake level may exceed the limits should rainfalls occur (as was the case in ). Also, a high lake level brings the ice edge closer to properties making them more vulnerable to ice push events. Recommendations were developed based on: (a) the above principles; (b) the goal of achieving ice contact lengths of at least 15 and 30 m for Round and Golden lakes, respectively, early in the winter, and; (c) the goal of not exceeding the lower lake level limits set out in the WMP (MNR, 2004). A drawdown rate of 4cm/week was recommended for both lakes. It was also recommended that high lake levels be avoided, and that the lake level elevations at freeze-up be no more than m and m for Round and Golden Lakes, respectively. These target elevations are both about 0.2 m below the upper lake level limits set out in the WMP (MNR, 2004). CONCLUSIONS No single parameter was identified as the root cause of an ice push event. A combination of factors, or unfavorable occurrences, is necessary to bring about an ice push event. Three general contributors to an ice push event were identified: (a) environmental conditions; (b) the hydraulic operating regime, and; (c) the shoreline management practices. Obviously, one cannot control the environmental parameters; however, the operational and shoreline-management parameters are under some control by human intervention. Thus, the occurrence of an ice push event is neither completely random, nor is it completely deterministic. In brief, it is a combination of both. -27-

8 ACKNOWLEDGEMENTS The project was funded by the Ontario Ministry of Natural Resources (MNR) and Renfrew Power Generation (RPG). The MNR and RPG are both acknowledged for the information that they provided, and their efforts to expedite the project. Canadian Electricity Association Technology Incorporated is thanked for allowing the models developed in an 11-year investigation of ice loads on dams to be used in this project. REFERENCES Allen, W.T Freeze-Up, Break-Up And Ice Thickness In Canada, Fisheries and Environment Canada, Atmospheric Environment, Report No. CLI Comfort, G., Gong, Y. and Liddiard, A., 2003a, Static Ice Loads on Dams, proc. ICOLD (International Conference On Large Dams), Montreal. Comfort, G., Gong, Y., Singh, S., Abdelnour, R. and Liddiard, A., 2003b, Static Ice Loads On Hydro-Electric Structures: Summary Report; Ice Load Design Guide, and; Ice Load Prediction Computer Program, CEATI report T Comfort, G., Gong, Y., Singh, S., and Abdelnour, R. 2003c, Static Ice Loads on Dams, Canadian Journal of Civil Engineering, River Ice Special Issue. Comfort, G., and Liddiard, A., 2005, Investigation of Ice-Related Damages to Private Properties on Round and Golden Lakes, BMT FTL report 5951 submitted to MNR and RPG. Gatto, L., Ferrick, M., and Calkins, D., 2001, Ice-Push Damage on Lake Bomoseen, Vermont, CRREL report LR Kovacs, A., and Sodhi, S., 1988, Onshore Ice Pile-Up and Ride-Up: Observations and Theoretical Assessment. In Arctic Coastal Processes and Slope Protection design (edited by A.T. Chen and C.B. Leidersdorf), ASCE Technical Council on Cold Regions Engineering Monograph. Minnesota DNR, 2005, Shoreline Alterations: Ice Ridges, available at (website for Minnesota Dep t of Natural Resources) MNR, 2004, Bonnechere River Water Management Plan, published by the Ontario Ministry of Natural Resources, September, 2004 Amended March Paul, G., 2003, Ice Power, available at (website for Minnesota Dep t of Natural Resources). Pessl, F., 1969, Formation of a Modern Ice-Push Ridge by Thermal Expansion of Lake Ice in Southeastern Connecticut, CRREL Research Report 259. Singh, S., and Comfort, G., 1997, Static Ice Loads on Hydro-Electric Structures: Environmental and Probabilistic Analyses of Thermal Loads, CEA EG Singh, S., and Comfort, G.,1998, Expected Thermal Ice Load in Reservoirs, IAHR Conference, Potsdam, NY. Sommerville, R., and Burns, G., 1966, Damage to a Winnipeg Reservoir Due to Ice, proc. Conference held by the National Research Council in Ottawa on Ice Pressures Against Structures, NRC Technical Memorandum no. 92. Zumberge, J.H., and Wilson, J.T., 1953, Quantitative Studies on Thermal Expansion and Contraction of Lake ice, Journal of Glaciology, 61 (4):