Contributions to IHP-V by Canadian experts

-

UNESCO

INTERNATIONAL HYDROLOGICAL PROGRAMME @

Contributions to IHP-V by Canadian experts

Canadian National Committee for the

In terna tional Hydrological Programme (/HP)

IHP-V 1 Technical Documents in Hydrology 1 No. 33 UNESCO, Paris, 2000

SC-2OOO/WS/18

The designations employed and the presentation of material throughout the publication do not imply the expression of any opinion whatsoever on the part of UNESCO concerning the legal

status of any country, territory, city or of its authorities, or concerning the delimitation of its frontiers or boundaries.

Introduction

In 1998 the Canadian National Committee for IHP was formed, with following objectives in mind:

(a) To advance Hydrology and Water Sciences on an international level;

(b) To initiate research into an investigation of problems which require international cooperation;

(c) To determine the trends in research on the most urgent problems and to increase communication within the Canadian hydrology and water sciences community;

(d) To strengthen Canada’s involvement in UNESCO’s International Hydrological Programme (IHP) by:

i. Developing Canadian linkages to present IHP projects, and

ii. Providing a focal point for Canadian input to the design of future IHP programs;

(e) To stimulate international awareness and understanding of Canada’s hydrologic and water sciences; and

(f) To act as a hydrological and water sciences advisor to the Canadian Commission for UNESCO and participate in meetings of its Sub-Commission for Natural Sciences.

Membership of the Committee is made up of one person designated by the Canadian Geophysical Union, Hydrology Section (Denis Gratton, Univeriste du Quebec a Trois-Rivieres) and one person designated by the Canadian Water Resources Association (Russel Boals, Environment Canada), and three members at large drawn one each from governments (Fred J. Wrona, National Water Research Institute), universities (Slobodan P. Simonovic, University of Manitoba) and industry (John S. Gladwell, Hydro Tech International).

The Fifth phase of the IHP was in progress when the Canadian National Committee came into existence. Therefore, one of.our primary goals was to determine how to provide contributions to IHP V. An intensive survey of work in progress related to the themes of IHP V was conducted and publication of this book was decided to be the best way to share the results of the on-going Canadian work with the rest of the IHP community. We have tried to document the work based on those efforts that we believe have relevance to international activities. However, this book is far from being a comprehensive review of Canadian hydrology. Selection of papers for the publication in this volume has been based on their scientific merits and relevance to themes of IHP V.

Contributions by Whitfield & Cannon, Beltaos & Prowse, Jacobs & Banfiled and Belore et al. are directly related to the IHP V Theme 1 - Global hydrological and geochemical processes especially topic 1.1 - Application of methods of hydrological analysis using regional data sets. Papers by Schertzer & Lam is of direct relevance to the topic 1.3 - Hydrological interpretation of global change predictions of Theme 1. Three contributions are provided to the Theme 2 Ecohydrological processes in the surfkial environment. Issues related to the topic 2.2 - Sedimentation processes in reservoirs and

deltas are addressed in the contribution by Pettucrew & Droppo. Work by Stolte is addressing issues under the topic 2.3 - Interactions between river systems, flood plains and wetlands. Contribution to the topic 2.4 - Comprehensive assessment of the surficial eco-hydrological processes is provided by Levesque & de Boer.

Many people have contributed to the publication of this monograph. First I would like to thank all the authors, then the members of the Canadian National Committee for IHP and the Canadian Commission for UNESCO (in particular, Mrs. Gisele Trubey).

The hardest part of the work in preparing volumes like this, is one of the chief editor. We are very grateful for expertise and commitment of Dr. John S. Gladwell (Hydro Tech International) whose leadership was essential for the success of this publication. Help of the people from the UNESCO IHP Secretariat in Paris was instrumental in bringing this book into your hands.

As a final note, I would like to invite the readers to get in touch with us.

Canadian expertise in hydrology and water resources is vast and we hope that this publication will help in building new networks, improving communication between the professionals around the world and transferring some of the Canadian knowledge to other regions.

Slobodan P. Simonovic Chairperson

Canadian National Committee for IHP University of Manitoba

Winnipeg, MB R3T 2N2 CANADA

s~@&tcha.ca [March 20001

Contents

Page Introduction . . . , . . . i Recent Variations in Climate and Hydrology of British Columbia and Yukon,

by Paul H. Whitfield and Alex J. Cannon . . . I Climate Impacts on Extreme Ice Jam Events in Canadian Rivers,

by Spyros Beltaos and Terry Prowse . . . 22 Aspects of the Hydroclimatology of Newfoundland under a Varying Climate,

by John D. Jacobs and Colin E. Banfield . . . 47 Surficial Fine-Grained Laminae as Indicators of Water Quality,

by Lucie M.J. LCvesque and Dirk H. de Boer . . . 63 The Hydrology of Salinization, Wetlands and Reclaimed Areas,

by W.J. Stolte . . . 72 Application of GIS for Regional Hydrological Analysis,

by Harold S. Belore, Ross D. Zhou and Dr. Lloyd Logan . . . 94 The Morphology and Settling Characteristics of Fine-Grained Sediment from a

Selection of Canadian Rivers

byEllenL.PetticrewandIanG.Droppo . . . 111 Modeling of Climate-Change on Large Lakes and Basins with Consideration of

Effects in Related Sectors

by William M. Schertzer and David C.L. Lam . . . 127

. . .

111

Recent Variations in Climate and Hydrology of British Columbia and Yukon

Paul H. Whitfield and Alex J. Cannon’

Abstract

Climatic and hydrologic variations between the decades 1976-1985 and 1986-95 are examined at climate stations and hydrometric stations from British Columbia and Yukon. The variations in climate are distributed across a broad spatial area. Temperatures were generally warmer in the most recent decade, with many stations showing significant increases during spring and fall time periods. No significant decreases in temperature were found. Significant increases in temperature were more frequent in the south than in the northern portions of the region. Significant changes in precipitation were also more prevalent in the south, and particularly in the summer. In coastal areas, there were significant decreases in precipitation during the dry season, and significant increases during the wet season. In the BC interior, significant precipitation decreases occurred during the fall, with significant increases during the winter and spring. In the north there were few changes in precipitation. The hydrologic responses to these variations in climate are classified into seven groups based upon the type of hydrology and the pattern shifts between the two decades. These seven classes appear to be spatially linked to the distribution of ecozones. The recent variations illustrate the leverage effect of small variations in climate, particularly temperature, on different hydrologic systems.

1 INTRODUCTION

Recent variations in climate may provide insight into potential hydrologic impacts of persistent changes in climate. Burn (1994) examined the impact of climatic change on timing of the spring runoff. He found that there are a greater number of rivers showing earlier spring runoff than expected by chance alone. Leith & Whitfield (1998), Whitfield & Taylor (1998), and Whitfield (1999) assessed the hydrologic response to recent climate variations in British Columbia and Yukon. Leith & Whitfield (1998) focused on changes in timing of hydrologic events such as the onset of spring snowmelt in a warmer climate. Whitfield & Taylor (1998) applied the same methodology to coastal areas of British Columbia. Whitfield (1999) found that temperatures had generally increased between two decades (1976-85 and 1986-1995).

Whitfield (1999) then applied the methodology to 275 rivers in British Columbia and the Yukon and found that hydrologic patterns respond to variations in climate in similar ways in the different ecoregions. Whitfield (1999) found six distinct patterns of response that were linked to the hydrologic pattern and to time shifts based upon a visual classification system.

For example, in coastal BC the dry season is starting earlier, and lasting longer resulting in lower streamflows in most coastal streams; in southern BC the onset of snowmelt is advanced in a warmer climate and the summer recession is extended in duration resulting in reduced fall streamflows. Other areas of British Columbia and Yukon respond in other manners

’ Environment Canada; 700 - 1200 West 73’d Avenue; Vancouver, B.C. V6P 6H9 CANADA

linked to the climate variations, the hydrologic pattern, and the controls on hydrology within that ecoregion. Whitfield and Cannon (in press) extended that work in two ways. They introduced a method to cluster similar stations on a statistical basis rather than a visual interpretation, and they extended the analysis to the phenology of hydrologic events to all of Canada. In the present work we present a reanalysis of the natural streamflow stations in British Columbia and Yukon using the unconstrained clustering procedure of Whitfield &

Cannon (in press) to replace the visual classification system used by Whitfield (1999).

Water is not uniformly distributed across the study area and this uneven distribution of water on the landscape results in the current distribution of ecoregions. British Columbia and Yukon is one of the more ecologically diverse areas of Canada. When climate shifts, many hydrologic features may be affected. Frequently, concern is expressed over the impacts on the total amount of runoff, and more often on extreme events such as floods. Small shifts in the availability of water during the annual cycle of seasons also have important ecological consequence. Silver-town et al. (1999) shows the linkage between plant habitat and the availability of water. Large changes may alter the current distribution of vegetation and aquatic habitats and result in ecological shifts. Similarly, hydrologic shifts also affect other aspects of the importance of water to society and to economics - irrigation, hydroelectric or water supplies.

In the present work we extend the analysis to 244 hydrologic stations across British Columbia and Yukon. The objective of this work is to demonstrate (1) the recent changes in temperature and precipitation patterns in BC and Yukon, (2) the concurrent changes in hydrological patterns, and (3) that the changes are linked to existing ecological regions.

2 METHODS

2.1 Data Sources

Monthly maximum temperature, monthly minimum temperature, monthly total precipitation, and daily mean streamflow records were obtained from climatic and hydrometric stations in Canada for the decades 1976-1985 and 1986-1995. Temperature and precipitation data selected for analysis were taken from the homogeneous series prepared by the Climate Data and Interpretation group of Environment Canada and described by Vincent (1998), Vincent &

Gullett (1999), and Mekis & Hogg (1999). Both sets of data have been adjusted for measurement inhomogeneities. Missing values in the temperature series have been estimated using records from neighbouring stations. As only maximum and minimum temperatures were available, a pseudo-mean temperature series was calculated based on averages of the maximum and minimum temperatures for each month. In all, 46 temperature and 83 precipitation stations were included in the current study. Hydrometric records selected for analysis were taken from stations in watersheds with natural flows and where the record length was greater than 20 years. In total, 244 stations with natural flows were considered.

All hydrometric data were obtained from the Hydat 96-1.04 CD-ROM provided by the Climate and Water Products Division of Environment Canada.

The analysis was performed on hydrologic data that me the criteria used by Leith &

Whitfield (1998): [l] the stream or river was considered to have natural streamflow (i.e., dams and diversions were either not present or affected only a small percentage of the watershed, [2] there were no substantial periods of missing data (stations which are operated only seasonally, or where entire years of observations were missing were eliminated).

2.2 Statistical Analysis

Whitfield & Cannon (in press) detected significant differences in temperature, precipitation, and streamflow between decades for stations across Canada following a procedure similar to that described by Whitfield (1999), Leith & Whitfield (1998) and Whitfield & Taylor (1998).

For temperature and precipitation records significance tests were performed on the monthly data. To allow for better detection of phase shifts in the hydrographs, 5-day averages of the streamflow records were calculated. Periods with more than one missing value were removed from further analysis, and decades in which more than 4 years of data were missing were not considered for testing. Significance levels of differences in monthly temperatures, monthly precipitation, and 5-day average streamflows between 1976-1985 and 1986-1995 were determined using the two-tailed Mann-Whitney U test, a non-parametric rank-based test that is robust against non-normality in distributions of the two test samples. For each monthly or 5-day period at each station, the level of signilicancep of the test was recorded, as was the direction of the observed change in the decadal median value.

Whitfield & Cannon (in press) identified regions with similar decadal variations in temperature, precipitation, and streamflow clustered using a batch k-means algorithm (Hartigan, 1975). For temperature and precipitation at each station, monthly values of (l-p) were multiplied by the sign of the observed monthly changes (+l if an increase in the median was observed between 1976-1985 and 1985-1995, -1 if a decrease was observed, and zero if no change was recorded). The series of monthly values for all stations were then standardized and entered into the clustering algorithm. Numbers of clusters to retain were determined using the Davies-Bouldin separation measure (Davies & Bouldin, 1979) and visual inspection of cluster locations and cluster boxplots. Six clusters were calculated for both temperature and precipitation data. Cluster centres were initialized using the method suggested by Katsavounidis et al. (1994). The same methodology was used for the five-day discharges, and on the shift in 5-day discharges. Clustering of hydrology stations based upon stream flow pattern resulted in four clusters, while unconstrained clustering on the shifts between decades results in six clusters. The intersection of these two clusters results in a classification system with 24 alternatives. Rather than perform the analysis using a subset of the data we used the clusters of temperature, precipitation and hydrometric stations, developed for the entire Canadian landscape by Whitfield & Cannon (in press).

3 RESULTS

3.1 Temperature

The basic input was the monthly temperatures for each of two decades. The probability of a shift in temperature between that month for the two decades for each of the 46 stations were used in the clustering. The unconstrained clustering resulted in six clusters for all of Canada, each with its own pattern of variation between the two decades. Only four of these patterns were observed in BC. A summary and description of the clustering of the shift in temperature between the two decades are shown in Table 1. The pattern of shift in temperature for each of the four clusters is illustrated for one station from the center of the cluster in Figure 1, these being the most typical example of each group. Figure 2 provides the monthly probability plots for the four clustered groups. In this plot significance of change is shown by values at the extremes of 1 or -1. The spatial distribution of these climate stations, grouped by

the pattern in shifts in temperature based upon the unconstrained clustering, is shown in Figure 3.

Figure 1. Patterns of temperature shift between the two decades for selected BC and Yukon climate stations.

Table 1. Temperature grouping based upon unconstrained clustering of temperature shifts between the decades 1976-85 and 1986-95 using the classification system of Whitfield and Cannon (in press) for Canada, where n is the number of stations in that group.

Cluster In PJhIQpsxrties

1 0 Lower winter temperatures in January, February, and March.

2 34 Higher temperatures during all months except February

3 2 Shifting temperatures during winter - up in December, January, March.

4 4 Higher temperatures in spring and summer, down in Jan. and Nov.

5 6 Increases in temperature in January, June and July, decreases in Feb.

6 0 Increases in temnerature in March, decreases in May

4

Cluster 2 (N=34)

Cluster 4 (N=4)

Cluster 3 (N=2)

Cluster 5 (N=6)

Jl”‘r.;

a- -I -

Figure 2. Box plots of probability of monthly temperature shifts for the four clusters observed in BC. Significance increases are indicated by bars toward the top of the graph (+l.O) and significant decreases towards the bottom of the graph (-1 .O).

3.2 Precipitation

The basic input was the monthly precipitation for each of two decades. The probability of a shift in precipitation between that month for the two decades for each of the 83 stations were used in the clustering using the same procedure as for temperature. For Canada, the unconstrained clustering resulted in six clusters, each with its own pattern of variation between the two decades. Five of these patterns were observed in BC. A summary and description of the clustering of the shift in precipitation between the two decades are shown in Table 2. The pattern of shift in precipitation for each of the five clusters is illustrated for one station from the center of the cluster in Figure 4, these being the most typical example of each group. Figure 5 provides the monthly probability plots for the four clustered groups. The spatial distribution of these climate stations, grouped by the pattern in shifts in precipitation, based upon the unconstrained clustering, is shown in Figure 3.

Temperature Group 0 2

l

3

@ 4

@ 5 Eco Zone

rnB oreal Cordillera Boreal Plain

ml Montaine Cordiller /j@@@j Pacific Maritime 111 Southern Arctic

~ Taiga Cordillera (1 Taiga Plain

Figure 3. Distribution of temperature shift patterns between the decades for Yukon

BC and

r YPln

Figure 4. Patterns of precipitation shifts between the two decades for selected BC and Yukon climate stations.

Cluster 2 (N=9) Cluster 3 (N=35)

- - Y

- Y -

Cluster 4 (N=2)

Cluster 6 (N=4)

- -

= -

Cluster 5 (N=33)

Figure 5. Box plots of probability of monthly precipitation shifts for the four clusters observed in BC. Significance increases are indicated by bars toward the top of the graph (+l.O) and significant decreases towards the bottom of the graph (-1.0).

8

Precipitation Group

q

2

n

3

5 4

q 5

n

6

Eco Zone

&@j Boreal Cordillera CAB oreal Plain

Montaine Cordillera Pacific Maritime Southern Arctic vl A,.:.>>:.>:.~:.:.:

Cordillera r] Taiga Plain

:. . . . :.:._

,‘::I: . 1. .I:.:

s!iL

;j;: ;::j:_

::!I: .:.. :, .,.:. _:. .I::.:.

:::::.

,I. .:. .:’ ::.

:.:.::::::. ::::.

I;&:1;1::,: .:.:.”

:

Figure 6. Distribution of precipitation shift patterns between the decades for BC and Yukon.

Table 2. Precipitation groupings based upon unconstrained clustering of precipitation shifts between the decades 1976-85 and 1986-95 using the dlassification system of Whitfield and Cannon (in press) for all of Canada, where n is the number of stations in that group.

Group LI Properties

1 0 Slight decreases through most the year, increases in Aug.

2 9

3 35

Increases through most of year. (except Aug.)

Increases in the winter and decreases in Sept. and Oct.

4 2 Increases during summer and late fall, down in SeptDec.

5 33 Increases during Apr. and Nov., decreases in May, Aug., and Sept.

6 4 Increases in late fall, decreases in Dec., spring and summer

3.3 Hydrology

Whitfield & Cannon (in press) clustered 650 hydrology stations in two ways. First they performed an unconstrained clustering of the stations based upon hydrologic pattern based upon five-day [pentad] average discharges. These four hydrologic types represent the general nature of natural streams across Canada, however, only three of these occur in BC and Yukon. The streamflow of Canadian rivers is dominated by snowmelt runoff (Types b and c), while coastal rivers in BC exhibit the rainfall driven pattern shown in Type d. Second, Whitfield & Cannon (in press) clustered the probability of shifts between the two decades, and found that there were six clusters. We found that fourteen of these groups were observed in BC and Yukon. The generalized hydrograph for each group is shown in Figure 7. The probability of an interdecadal shift for each of these groups is shown in Figure 8. These groups are not randomly distributed across the British Columbia and Yukon landscape.

When each of the stations is plotted on an ecoregion map of the study area several patterns emerge (Figure 9). In Figure 9, the symbol shape denoted the type of hydrology, with a circle type being b, a square type c and a star type d. The numeric portion of the classification indicates the observed pattern shift. Examining figure 10 will show the association of Type d with the coast of BC. Colour is used to denote the pattern shift between the two decades. The first thing the reader will note is that there are distinct clustering of certain colours in specific regions.

Table 3. Summary of the classification of hydrologic records on the basis of hydrograph type and pattern of shift, using the classification system proposed by Whitfield & Cannon (in press) for Canadian Rivers.

10

2 3

1 6

-

a

OII.W.3,

-kn,“.s,

f ” .‘.,..p., .r ----

I . I j\ ~l*li”.ii,,,“ij~ ,,, .,“...::‘:::::::&,,, .I

b

-*,m ^-1,*1,

.-

C

-

d

Figure 7. Hydrologic pattern for the 14 clusters observed in BC and Yukon.

I 2 3 4 5 6

a

b -

C

- --w-l

d

Figure 8. Shift in hydrology between pentads in two decades for the 14 clusters observed in BC and Yukon.

12

Figure 9. Distribution of hydrologic shift types for natural flow stations in BC and Yukon.

Hydrographs and polar plots of examples from each of the fourteen response groups are given in Figures 1 O-l 5. In Figure 10 the pattern of a slightly earlier spring and lower flows (Groups 1 b and lc) during the remainder of the year are shown. In Figure 11, Groups 2b and 2c, the higher year round flows are shown. In Figure 12, Groups 3b and 3c the earlier summer freshet and lower post-freshet flows are shown. In Figure 13, Groups 4b, 4c, and 4d shows the pattern for coastal BC of higher winter flows and lower summer flows. Figure 14 shows the hydrologic response of Group 5b, 5c, and 5d. Groups 5b and 5c have only one member in BC and Yukon and cluster with similar stations elsewhere in Canada. Visually, Whitfield (1999) clustered these with other groups. Group 5d is the dominant response type for coastal BC and shows the higher flows that were observe at the onset and into the wet season. Finally, Figure 15 show the response types 6b and 6c. These are the dominant types with 43% of all station being this type. The plots illustrate the earlier spring freshet and

Figure 10. Example Rivers showing shifts type 1 for hydrology type b [left]

and c [right].

Figure 11. Example Rivers showing shifts type 2 for hydrology type b [left]

and c [right].

14

Figure 13. Example Rivers showing shifts type 4 for hydrology type b [upper left], c [upper right], and d [lower].

4 DISCUSSION

Whitfield & Cannon (in press) describe variations in climate between the two decades indicates a general warming in western Canada. These shifts in temperatures between the decades are not uniform nor random across the study area. Within BC and Yukon four patterns of temperature shift all showing a form of temperature increase between the two decades (Figures 1 and 2). These four clusters have a definite spatial pattern (Figure 3).

Dominating the pattern is cluster 2, which has warmer temperatures in every month except February occurs throughout most of British Columbia [34 stations]. Northeastern BC contains 2 stations from cluster 3, which exhibit warmer winter temperatures in the more recent decade. The northern areas of the Yukon are occupied by 4 stations from cluster 4 which has a cooler early winter and warmer late winter and summer. The southern Yukon shows warmer temperatures in January and June/July [cluster 5 - 6 stations]. These results indicate that most of the study area was warmer in the recent decade, and that the change is persistent and widespread.

Figure 14. Example Rivers showing shifts type 5 for hydrology type b [upper left], c [upper right], and d [lower].

t

Figure 15. Example Rivers showing shifts type 6 for hydrology type b [left] and c [right].

Precipitation also varied between the two decades. Whitfield & Cannon (in press) also clustered Canadian precipitation stations into six clusters. Five of these clusters are represent

16

in BC and Yukon. Cluster 2 showed increases throughout the entire year, which has groupings in North western BC [9 stations]. Cluster 3 showed winter increases and fall decreases and the members of this cluster are found in coastal BC, and south central BC [35 stations]. Two stations in cluster 4 showed increased summer and fall precipitation. Much of the BC interior showed increases in spring precipitation and decreases in summer [clusters 5 - 331. TWO stations from cluster 6 showed increases in precipitation in fall and decreases in winter, spring and summer. These pattern shifts are not as clearly separated spatially as are the clusters obtained for temperature shifts, but on a smaller scale indicate shifts in precipitation regimes between the decades that would influence hydrology.

Regional changes in climate may be inferred from the spatial analysis of temperature and precipitation records. One of the difficulties of the analysis presented here is that the records for temperature, precipitation, and hydrology are not collected from coincident locations. Hence we infer a spatial temperature pattern from one set of locations, a spatial precipitation pattern from a second set of locations, and the hydrologic response from yet a third set of locations. In an ideal world, there would be data available from a more conjoined set of observations.

Temperature has been implicated as important in shifts in the timing of hydrologic properties (Westmacott & Bums, 1977; Leith & Whitfield, 1998; Whitfield & Taylor, 1998;

Whitfield & Cannon, in press). Westmacott & Bum (1997) showed that temperature changes over the last 100 years affected both the magnitude and timing of hydrologic events in the Churchill-Nelson River system. Westmacott & Bum (1997) found that the magnitude of hydrologic events decreased over time while snowmelt events occurred earlier. With recent variations in climate the hydrology responds. The first changes observed are not changes in volumes, but changes in timing. This has been shown by Leith & Whitfield (1998) Whitfield

& Taylor (1998), and Whitfield (1999). Whitfield & Cannon (in press) report more hydrologic shifts in areas where temperature has increased than for precipitation shifts.

Intuitively, shifts in precipitation patterns will also affect hydrology. This relationship should be most clear in systems where precipitation is not stored as snow as was shown by Whitfield & Taylor (1998). However, when there is a transference of water from one season to another as is typical of the snowmelt systems which dominate Canada the impact of variations in precipitation on runoff is less easily assessed.

Whitfield and Cannon (in press) observed that Canada has four basic types of hydrologic patterns. These four patterns represent the range from snowmelt to rainfall driven systems. Only three of these types are observed in BC and Yukon. Type b streams exhibit a broad summer freshet with higher flows lasting for several months. These watersheds are broadly distributed in western Canada where there is significant variation in elevation and aspect. The variations in elevation and aspect effectively spread out the snowmelt process over a broad time period and results in the type of discharge peak we observe. Type c streams are also snowmelt systems, but these streams have a distinctively sharp peak during freshet.

The peak of discharge is relatively short lived and results from snow melting in watersheds which do not have much vertical elevation or mountainous terrain which can shadow accumulated snow. These streams occur in the interior plateaus of British Columbia. The final pattern, Type d, are rainfall driven systems. These systems generate runoff during the rainy season of the year, and any snow accumulations are of short duration. Streams of this type occur in the coastal areas of British Columbia.

Using an unconstrained clustering of the shifts in hydrology between the two decades Whitfield & Cannon (in press) found that all 650 natural streams in Canada could be placed into six clusters. These six shift types (Figure 8) are found in different areas of Canada, and all six patterns are observed in the study area (Table 3). Cluster 1 showed decreases in

streamflow throughout the entire year, these 9 streams occurring in the Northern Yukon and in the lee of the coastal mountains in Southern BC. This pattern is typical of the boreal forest (Whitfield & Cannon, in press) Shift type 2 -showed increases in streamflow during all portions of the year and were found in the Taiga ecoregions and in the mountainous areas of the west [42 stations]. The third cluster showed increases in winter and spring flows and decreases in fall, these streams are located in the mountainous regions of BC [69 stations].

The streams in the fourth cluster, of which only 6 occur in the study area, have flows which are lower in the late summer and higher in the early winter and spring (Figure 8). This type of stream is generally found in the Maritimes and southern Quebec (Whitfield & Cannon, in press). The fifth group has lower flows during late summer and fall. These 13 streams are dominantly type d streams occurring in coastal BC. This group of rivers is described by Whitfield & Taylor (1998). The sixth cluster has higher spring flows and lower summer and fall flows, and dominates the streams in the mountainous areas of BC. These 105 stations exhibit the pattern described by Leith & Whitfield (1998).

Whitfield & Cannon (in press) demonstrate the linkage between hydrologic pattern and recent shift pattern across Canadian Ecosystems. The subset of stations we considered within the present study is summarized in Table 4.

Table 4. Correspondence between ecozones and hydrologic shift types for BC and Yukon hydrometric stations. The letter indicates the hydrologic pattern, while the number indicates the type of shift observed.

Total 1 b lc 2b 2c 3b 3c 4b 4c 4d 5b SC 5d 6b 6c Boreal Cordillera

Boreal Plain Montaine Cordillera Pacific Maritime Southern Arctic Taiga Cordillera Taiga Plain

Total Stations 244

7 2 4 1

3 2 1

3 6 39 3 47 22 2 1 3 1 1 11 39 66

Percent of Total 7 2 16 1 19 9 1 0 1 0 0 5 16 27

The ecosystems of Canada are inextricably linked to the hydrology of their watersheds. Each Canadian ecozone has a specific relationship between the landscape, the timing and form of precipitation, the seasonal variation in temperature that moderates the form and controls the summer melt of accumulated snow, and the flora and fauna which typify that region. When the climate varies within an ecozone the hydrology of the streams respond.

Changes in temperature have a pronounced impact on water resources. Dvorak et al.

(1997) considered the hydrologic impact of GCM scenarios using three hydrological models.

Their results show a shift forward in time of spring runoff coupled to lower late summer runoff. Brubaker & Rango (1996) indicate that in mountainous regions a warmer climate is expected to shift snowmelt earlier into the winter and spring decreasing summer runoff. This is the dominant response of streams in the mountainous areas of BC and Yukon. Changes in precipitation that may accompany temperature changes may modify that signal. The shifts in temperature patterns observed can be expected to affect streamflows as predicted for mountainous areas by Brubacker & Rango (1996) and as evidenced for the Canadian prairie by Westmacott & Bums (1997).

18

Bum (1994) examined 84 natural rivers in west-central Canada and found spring runoff to be occurring earlier. Leith & Whitfield (1998) demonstrated the shift in onset of spring runoff to be earlier, and the shift to be wide-spread in south central British Columbia.

Whitfield (1999) examined more that 200 natural streams in British Columbia and found a number of shifts in hydrologic patterns that occurred in a warmer decade. Whitfield &

Taylor examined rainfall driven streams in coastal BC and found that in a warmer decade the onset of summer flow regime was commencing earlier and lasting longer into the fall. In each of these temperature was attributed the primary climatic shift between the two decades.

The climate variations between the two decades are not consistent across the study area. This coupled to the hydrologic diversity of the BC and Yukon results in a complex series of changes. We reworked the BC and Yukon subset of the results presented for Canada by Whitfield & Cannon (in press) and found temperature and precipitation changes between the two decades in four and five of the six groups respectively. These changes in climate are relatively small, and many of the variations are not statistically significant. While the climate variation observed was rather small, the hydrologic response to these changes is impressive. The hydrological changes that were observed indicate a response to these variations in climate. Two things stand out in these responses. First, different hydrologic types respond to the climate variations in different manners. Second, there is a clear linkage between the type of hydrology, the response to climate variation, and the ecozone of the stream. The response characteristics vary across the region, but fall into 6 distinctive groups.

Only a few stations of response type 1 and 4 were found in BC and Yukon. Statistically, the shift at these stations are similar to stations located elsewhere in Canada. Whitfield (1999) examined these stations visually and identified them as members of groups, or distinctive sub-groups. The hydrologic change between decades is dominated by four responses type 2 with higher flows throughout the entire year [42 stations], type 3 with earlier spring and lower summer and fall flows [69 stations], type 5 with higher winter flows and lower summer flows which occurred in 11 stations on the BC coast, and type 6 that has the same pattern as cluster 3 but with an earlier spring. While clusters 3 and 6 are statistically distinct, the principal difference is that type 3 has freshet in summer (Figure 12) while type 6 has freshet in spring (Figure 1 S).These groups reflect the basic nature of the hydrological system in these areas and how the water on the landscape responds to the shifts in temperature and precipitation between the two decades we examined. The spatial mapping of these response classes (Figure 10) indicates that the distribution of responses is not random. Each hydrologic response class occurs in a specific geographic and ecological area.

From an ecological perspective, the landscapes of western Canada had longer and warmer summers during the period 1986-95 than in the previous ten years. This results in a longer growing season, and likely increases in total evapotranspiration. Summer rainfalls have increased through most of the BC interior and may be the result of increased convectivity.

5 CONCLUSIONS

The climate of BC and Yukon varied between the two decades 1976-85 and 1986-95. Most marked is the increase in temperatures in the period 1986-95. The Yukon had warmer Januarys and Julys. Precipitation patterns also shifted between the decades. The dominant patterns of precipitation shift were winter increases and spring and fall decreases.

The streams in the study area responded to the observed climate variations. Where conditions were warmer the onset of hydrologic spring occurred earlier in the more recent decade than in the previous period. We observed three types of hydrology, and six patterns of

shift in streamflow patterns. The first type of hydrologic shift are year round decreases in streamflow. The second type of stream shows year round increases in discharge. These streams are located in northwestern Canada. In this region of Canada streams have higher runoff throughout the entire year were increased as the result of a combination of the warmer and wetter conditions. The third pattern is higher winter flows with an earlier summer freshet and lower flows during the summer months. These streams are located in northern BC and in the Rocky Mountains. Here warmer temperatures throughout the year and increased early winter precipitation. The fourth pattern is one of lower late winter flows, an earlier spring and lower streamflows after the peak in freshet. The fifth pattern is one of higher winter flows with a delayed spring peak and generally lower summer flows. These streams are located in coastal British Columbia, and reflect the warmer winter temperatures and higher winter precipitation. The sixth pattern is earlier onset to spring freshet, followed by lower flows through the late summer and fall of the year. These streams are located in south-central BC and have been previously described by Leith & Whitfield (1998). In all parts of the study area, small variations in temperature and precipitation result in significant shifts in hydrologic patterns.

6 ACKNOWLEDGEMENTS

The author would like to thank all those who contributed to the preparation of this work. Eric Taylor provided useful comments and discussions during the preparation of this work. Norm Wade prepared the maps and provided GIS support.

7 REFERENCES

Brubaker, K.L., and A. Rango, 1996. Response of snowmelt hydrology to climate change.

Water, Air, and Soil Pollution 90,335-343.

Bum, D.H, 1994. Hydrologic effects of climatic change in west-central Canada. Journal of Hydrology 160:53-70.

Dvorak, V., Hladny, J. and L. Kasparek, 1997. Climate change hydrology and water resources impact and adaptation for selected river basins in the Czech Republic.

Climate Change 36,93-106.

Idso, S. B., and A.J. Brazel, 1984. Rising atmospheric carbon dioxide may increase streamflow. Nature 312,51-53.

Leith, R.M, 199 1. Patterns in snowcourse and annual mean flow data in British Columbia and the Yukon. In Using Hydrometric Data to Detect and Monitor Climatic Change, G.W. Kite and K.D.Harvey [eds] Proceedings of NHRI Workshop No 8. p 225-23 1.

Kite, G.W, 1991. Analyzing hydrometeorological time series to detect climate change. In Using Hydrometric Data to Detect and Monitor Climatic Change, G.W. Kite and K.D.Harvey [eds] Proceedings of NHRI Workshop No 8. p 13 1-142.

Leith, R.M., and P.H. Whitfield, 1998. Evidence of Climate Change Effects on the Hydrology of Streams In South-Central B.C. Canadian Water Resources Journal 23, 219-230.

Loaiciga, H.A., J.B. Valdez, R. Vogel, J. Garvey, and H. Schwarz, 1996. Global warming and the hydrological cycle. Journal of HydroZogy 174, 83-127.

Marengo, J.A., 1995. Variations and change in South American Streamflow. Climate Change

31,99-l

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Michel, F.A and R.O. van Everdingen, 1994. Changes in hydrogeologic regimes in permafrost regions due to climate change. Permafrost and Periglacial Processes 5:191-195.

Silvertown, J., M.E. Dodd, D.J.G. Gowing, and J.O. Mountford, 1999. Hydrologically defined niches reveal a basis for species richness in plant communities. Nature 400:61-63.

Taylor, B, 1997. The Climates of British Columbia and Yukon. In Responding to Global Climate Change in British Columbia and Yukon, by E. Taylor and B. Taylor. 16pp.

Westmacott, J.R., and D.H. Bum, 1997. Climate change effects on the hydrologic regime within the Churchill-Nelson River Basin. Journal of Hydrology 202:263-279.

Whitfield, P.H, 1999. Linked Hydrologic and Climate Variations in British Columbia and Yukon. To appear in Environmental Monitoring and Assessment.

Whitfield, P.H. and A. Cannon, 1999. Recent Variations in Climate and Hydrology in Canadian Ecosystems. Submitted to Canadian Water Resource Journal.

Whitfield, P.H., and M.J.R. Clark, 1997. Driving Forces. Water Quality International March/April 1997, 20-2 1.

Whitfield, P.H. and E. Taylor, 1998. Apparent Recent Changes in Hydrology and Climate of Coastal British Columbia. In: Y. Alila, Mountains to Sea: Human Interactions with the HydroZogic Cycle. Proceedings of the 5 1” Canadian Water Resources Conference.

pg 22-29.

Xu, C-Y., and S. Halldin, 1997 . The effects of climate change on river flow and snow cover in the NOPEX area simulated by a simple water balance model. Nordic Hydrology 28,273- 282

Climate Impacts on Extreme Ice Jam Events in Canadian Rivers

Spyros Beltaos’ and Terry Prowse2

Abstract

River ice jams can produce extreme flood events with major social, economic and ecological impacts throughout Canada. River ice processes in general, and ice breakup and jamming in particular, are governed by the flow hydrograph, the thickness and strength of the winter ice cover, and the stream morphology. These factors are directly or indirectly influenced by climate, and particularly by temperature and precipitation. Consequently, there is concern over the potential impacts of changing climatic conditions on the severity of ice jamming in Canadian rivers, and, thence, on stream ecology and local economy. Relevant work has to date focused on simple measures of climatic effects such as the timing of freeze-up and breakup. Changes over the past one hundred years are consistent with concomitant changes in air temperature, though additional factors are known to be at work. However, very little work has been done on how climate change may influence the frequency and severity of ice jams: it shows a number of trends that may be of concern in certain parts of Canada, such as increased incidence of mid- winter breakup events, and increased freshet flows. Using current understanding of river ice processes, various scenarios can be examined and associated impacts identified in a general manner. Detailed impacts at specified locations may include a variety of changes, both detrimental and beneficial, but are difficult to quantify due to limitations in atmospheric, hydrologic and river ice modelling capabilities. Research needs are discussed and pertinent river ice studies identified.

1 INTRODUCTION

River ice is present in nearly every Canadian river, for a period that ranges from days to many months. Whether moving or stationary, it interacts with the river flow in various ways, resulting in multiple impacts on the economy and ecosystem, and often posing a major threat to riverside communities. Extreme events resulting from ice jamming are responsible for a large part of such impacts. They include flooding, damage to property and infrastructure, interference with navigation, and inhibition of hydropower generation. Ice jams attain thicknesses of several metres and have highly irregular undersides. To pass the river flow, the water level has to rise drastically in order to accommodate both the large additional resistance created by this new boundary and the keel of the jam, itself being a large part of the thickness.

This can be seen from the following equation, which describes gravity flow under a floating ice cover in terms of the open-water depth for the same discharge (e.g., see Beltaos, 1982):

’ National Water Research Institute, Burlington, Ontario

2 National Water Research Institute, Saskatoon, Saskatchewan

22

~~[l+(~)qi+o*92~ (1) in which Y is water depth, while the suffixes “cov” and “op” denote ice-covered and open-water conditions; h = ice cover thickness; and n,, n,, denote Manning roughness coefficients for the bottom of the cover and the riverbed, respectively. The first term on the RHS of Equation 1 expresses the ratio of the under-ice depth of flow to Y, while the second term is the ratio of the submerged thickness of the cover to Y,. It is not uncommon for ice jams to have Manning coefficients two-to-three times as much as those of typical river beds, and a thickness comparable to, or even greater than, Y,. Substitution in Equation1 indicates that the presence of a jam can easily raise the water level so high as to attain 2.5 to 3 times the open-water depth required to pass the same flow. The latter depends primarily on discharge as well as channel slope and width.

Serious flooding is thus often the outcome of ice jamming, even where the discharge is modest relative to that of open-water floods. Unlike the latter, which can usually be anticipated days or even weeks in advance, ice-jam flooding is sudden and allows little time in which to plan and implement mitigation measures or evacuation of local residents. The release of an ice jam is also a matter of concern because a large volume of water is able to come out of storage at once, creating a surge. This phenomenon is qualitatively similar to a dam-break, though not as pronounced. Ice-jam surges are characterized by very rapid stage increases downstream and very high flow velocities (5 m/s is not uncommon), capable of substantial bank erosion and bed scour. Major damage can therefore occur within minutes of the arrival of a surge.

The total tangible annual cost of ice jams to the Canadian economy has been estimated as $CAN 60 million in 1990 dollars (Gerard & Davar, 1995), comparable to the 100 million estimated for the United States at about the same time (Carlson et al., 1989). A much greater amount is attributed to missed hydro-electricity production opportunities because of inadequate understanding of river ice processes in general (Raban, 1995). In New Brunswick, where detailed damage records are available, it has been found that ice jams cause a third of all flood events but are more destructive than open-water floods because they account for two-thirds of all flood damages (Humes & Dublin, 1988).

Equally important are the many ecological impacts of river ice, which arise from the intimate relationship between ice processes and river-me ecosystems. Extreme ice jam events are again major causes of ecological impacts that can be both beneficial and detrimental. For instance, ice-jam flooding provides essential replenishment to the multitude of lakes and ponds characteristic of the northern Canadian deltas, which are havens for wildlife, especially waterfowl and aquatic animals. (Peace-Athabasca Delta Project, 1973; Marsh & Hey, 1989).

On the other hand, flooding caused by ice jams and the surges produced by their release can result in severe fish mortality and loss of spawning grounds. Because ice is ubiquitous on Canadian rivers, many aquatic species have adapted their life cycles so as to take advantage of, or cope with hazards posed by, various phases of the annual ice regime.

In response to the increasing recognition of the economic and ecological significance of river ice, there has also been a rising need to forecast how it will react to changes resulting from global warming (e.g., Fitzharris et al., 1995). To this end, the links between climate and river ice have been poorly explored and most evaluations to date have focussed on simple measures, thought to be directly related to, and principally determined by, changes in temperature. These may include, for example, changes in freeze-up and breakup dates,

duration of the ice season, and ice cover thickness. Evaluation of more complex occurrences, such as the magnitude and frequency of ice-jam floods,’ depends on numerous factors driven by climate but interacting through a variety of geophysical processes such as ice mechanics, micrometeorology, hydrology and hydraulics.

The main objective of this paper is to review the status of research on the climatic aspects of river ice jams and associated extreme events, with a focus on Canadian rivers and research activities. Following a brief outline of agencies and institutions where such research is carried out, the current understanding of breakup and jamming phenomena is reviewed and climate-related factors are summarized. With this background, the available evidence of changing river ice conditions over the past one hundred years or so is reviewed and changes that may result from a doubling of carbon dioxide in the atmosphere are discussed. Probable socio-economic and ecological impacts in Canada are examined, and research needs outlined.

2 RIVER ICE RESEARCH IN CANADA

Despite its significance to the Canadian environment and economy, river ice science is still in its infancy, having only begun during this century and achieved a relatively slow advance. In part, this can be attributed to the complexity of river ice phenomena, which encompass such diverse areas of physical science as hydrology and hydraulics, thermodynamics and structural mechanics, strength of materials and rubble processes. At the same time, it has to be admitted that only. a limited amount of resources have been dedicated to concerted research on the subject. In Canada, private consultants carry out occasional research and development work in conjunction with studies of problems for which no obvious or standard solutions are available. A very small number of Universities offer courses on river ice subjects or conduct related research; predictably, they are located in provinces that are prone to river ice impacts (Quebec, Manitoba, Alberta) though not in all such provinces. It is mainly in provincial and federal government departments where short- and long-term research programs are pursued, in support of initiatives undertaken to address major public issues, e.g. climate change impacts and adaptation, sustenance of aquatic ecosystems, and protection of communities from flood damage. Such work is carried out at various institutes and locations across the country, including Environment Canada’s National Water Research Institute (Burlington, Saskatoon), Atmospheric Environment Service (Halifax, Ottawa); New Brunswick Departments of the Environment and Transportation (Fredericton); and Indian and Northern Affairs Canada (Yellowknife, Whitehorse).

Apart from the funding supplied by various levels of government in support of departmental and academic research, hydroelectric utilities also provide substantial amounts.

The latter usually target specific engineering and ecological problems, and often lead to acquisition of original field data and fundamental theoretical findings. The degree of hydro- industry involvement in river ice research and engineering depends on how much river ice interferes with power generation facilities and operations. In the latter half of the 20” century, this impetus has stimulated work in several major rivers of Canada, including, for example, the Saint John, Saint Lawrence, La Grande, Niagara, Nelson, Churchill, and the Peace.

3 ICE BREAKUP PROCESSES AND ICE JAMS 3.1 General

24

Ice jams occur during both the freeze-up and the breakup periods. Because of the much higher flows that usually prevail during breakup, freeze-up jams have a lesser potential for damages to structures and habitat than do breakup ones, even though they are known to pose various other problems, especially with respect to hydropower generation. Herein we shall concentrate on breakup jamming, which is the main cause of extreme ice-related events.

The breakup of river ice is triggered by mild weather and encompasses a variety of processes associated with thermal deterioration, initial fracture, movement, fragmentation, transport, jamming, and final clearance of the ice. Though several or all of these processes may be occurring simultaneously within a given reach, it is convenient to visualize the breakup period as a succession of distinct phases such as pre-breakup, onset, drive, wash. During the pre-breakup phase, the ice cover becomes more susceptible to fracture and movement via thermally induced reductions in thickness and strength. At the same time, the warming weather brings about increased flow discharges, due to snowmelt or rainfall or both. The rising water levels fracture the ice cover and reduce its attachment to the riverbanks while the increased flow velocities cause it to move and break down into relatively small blocks. This is the onset of breakup, and is followed by the drive, that is, the transport of ice blocks and slabs by the current. The onset is governed by many factors, including channel morphology, which is highly variable along the river. It is thus typical to find reaches where breakup has started alternating with reaches where the winter ice cover has not yet moved.

Invariably, this situation leads to jamming because ice blocks moving down the river in one reach encounter stationary ice cover and begin to pile up behind it, initiating a jam. Ice jams can stay in place for a few minutes or for many days; they can be a few hundred metres or many kilometres long. As already indicated, ice jams can cause very high water levels, well above the equivalent-discharge, open-water flow stages (Figure 1). The flow-rating curve, applicable to open

9-

a-

l 0reakup o Freezeup

method of Beltaos

83e

01 I I I I I I I I I I I /

0 100 200 300 400 500 600 700

800

Figure 1. Peak ice-influenced water levels recorded at the Water Survey of

Canada hydrometric station on the Restigouche River near Rafting Ground Brook, N.B. (t = computed thickness of ice jam at the indicated discharge).

From Beltaos and Burrell, 1992; data provided by R. Lane, WSC, Fredericton, NB.

water conditions is also shown for comparison. The difference between an observed ice- influenced stage and the open-water stage for the same discharge is called the backwater. The

“equilibrium-jam” curve is seen to provide an upper bound to the data points, consistent with theoretical concepts (Beltaos, 1995a; see also later discussion).

The sheet-ice-cover curve represents the stage that prevails shortly before breakup is initiated, when the ice cover is still intact but its underside has already been roughened by heat transfer from the water. Both the stage and backwater due to an ice jam are seen in Figure 1 to increase with discharge so that the severity of breakup is compounded by the fact that the spring runoff is often associated with the annual peak flow.

When a jam lets go, a large amount of water comes out of storage in short time, producing a surge. The water level drops very quickly upstream of the jam, but rises rapidly downstream; at the same time, water speeds increase to extreme values. Intact ice cover may be broken up and carried by the surge or, if still very competent, it may stay in place and initiate another jam. In this manner, more and more ice is broken up and carried down the river, until the final jam releases. This is the start of the wash or final clearance of ice.

3.2 Types of breakup events

Depending on hydro-meteorological conditions, the severity of a breakup event can vary between two extremes, those of the thermal or overmature breakup and the premature breakup.

The former type occurs when mild weather is accompanied by low runoff, due to gradual slow melt and lack of rain. The ice cover deteriorates in place and eventually disintegrates under the limited forces applied by the modest current. Ice jamming is minimal, if any, and water levels remain low. Typically, a thermal event would be represented in Figure 1 by a data point that plots near the sheet-ice curve. Premature breakup on the other hand, is associated with rapid runoff, usually due to a combination of rapid melt and heavy rain. The hydrodynamic forces are sufficient to lift and break segments of the ice cover before significant thermal deterioration can occur. Ice jams are now the most persistent because they are held in place by sheet ice that retains its strength and thickness. This is aggravated by the high river flows caused by the intense runoff, rendering premature events the most severe in terms of flooding and damages.

Usually, a breakup event falls somewhere between these two extremes, and involves a combination of thermal effects and mechanical fracture of the ice. Herein, the term mechanical breakup will be used to denote all non-thermal events because they are at least partly governed by the mechanical properties of the ice cover.

In the colder continental parts of Canada such as the Prairies or Territories, we are most familiar with a single event, the spring breakup, triggered by snowmelt. In more temperate regions, however, such as parts of Atlantic Canada, Quebec, Ontario and British Columbia, events called winter thaws are common. Usually occurring in January and February, they consist of a few days of mild weather and typically come with significant rainfall. River flows may rise very rapidly and sufficiently to trigger breakup on many local rivers. This is the winter breakup which can be more severe than a spring event, not only because of its premature nature:

26

dealing with the aftermath of flooding is hampered by the cold weather that resumes in a few days; while many jams do not release but Ii-eeze in place, posing an additional threat during subsequent runoff events.

3.3 Onset of breakup

Defining the onset of the breakup event at any particular location along a river as the time when the winter ice cover is set in sustained motion (e.g., Beltaos, 1995b), a number of onset criteria have been formulated in the past few decades. Most are completely empirical, relying on various combinations of water level, ice thickness, freeze-up conditions, and air temperature indices such as degree-days of thaw. A common empirical criterion is:

HB - HF = kh, -F(S)

in which H, = water surface elevation at which the ice cover starts to move; H, = water surface elevation at which the ice cover formed during the preceding freeze-up event = freeze-up level;

h, = ice cover thickness prior to the start of melt; F = a site-specific function of S, the latter being an index of thermal effects on the ice cover, often taken as the cumulative heat flux to the ice or simply the accumulated degree-days of thaw; and k = site-specific coefficient, so far known to vary between 2 and 10. Note that this type of criterion does not apply to thermal breakup events, characterized by in situ disintegration of the ice cover and insignificant ice breaking or jamming.

Equation 2 and others like it do not explicitly account for hydrodynamic or morphological effects; hence, they can only be applied to the particular river site at which they have been calibrated, i.e. they are site-specific. Application to another site on the same river, or to a different river, can only be made if adequate local data are available. This limitation can, in principle, be overcome with criteria that are based on a physical-process hypothesis. A number of these have been proposed in the literature and were recently reviewed and evaluated using field data from six different river sites (Beltaos, 1997a, 1999). The following equation, based on the simple requirement that ice plates formed by transverse cracking are set in motion when there is adequate water surface width, was found to adequately describe all six data sets:

W, - 4 P(m-0.50)0,

h, =

(3)

in which W, = water surface width at the stage at which the breakup is initiated; Wi = width of ice cover = river width at the freeze-up stage minus side strips caused by hinge cracking prior to breakup; h, o = ice cover thickness and flexural strength, while the suffix o denotes initial values, just before thermal deterioration begins; m = radius of channel curvature divided by channel width; ‘c = downslope force per unit area applied on the ice cover due to its own weight and flow shear; and 13 = dimensionless coefficient between 0.3 and 1.5. The ratio oh/o,h, quantifies the loss of ice “competence” due to thermal deterioration during the pre-breakup period. This process involves reductions in both ice thickness via top and bottom melt, and in strength, due to penetrating solar radiation and preferential melting at crystal boundaries (Ashton, 1985; Prowse et al., 1990). It is difficult to predict such effects, however, owing to complexities introduced by the snow cover and its changing reflective/absorptive properties as

melt progresses (Prowse & Marsh, 1989). Consequently, the competence ratio has been expressed as an empirical function of accumulated degree-days of thaw, DD, above a base of -5°C (Bilello, 1980).

3.4 Evolution of breakup

Noting that channel width depends on water level and, thence, on discharge, we see that Equation 3 includes the effects of such factors as antecedent weather and hydrologic conditions (Wi, h,,, DD), channel morphology and slope (m, Wi, W,, z), and hydrodynamic forces applied on the ice cover (z). For simplicity, let us first consider a premature breakup, so that thermal deterioration is not a factor. The breakup initiating discharge, Qin, corresponds to the stage at which the water surface width is equal to W,. At a given river site, where m is fixed and the stage - discharge - z relationship is defined by local bathymetry and slope, Qi,, depends on the pre-breakup (or winter maximum) ice thickness, and on the water level. during the preceding tieeze-up, H,. At the same time, the variable channel slope, bathymetry and planform cause Qi, to change irregularly along a river reach, as depicted in Figure 2. The highest Q;, values are often associated with sharp bends where the radius of curvature is relatively small.

breakup initiated . intact ice cover

Downstream Distance

Figure 2. Schematic illustration of the conditions for breakup initiation, ice-jam formation and ice clearance, using the concept of a variable Qio, as indicated by Equation 2.

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