The New Brunswick Subcommittee on River Ice was established under the Flood Forecasting Technical Committee within the Canada-New Brunswick Flood Damage Reduction Program (FDRP). The following agencies are represented on the Subcommittee: the New Brunswick Department of Environment (NB ENV), the New Brunswick Electric Power Commission (NBEPC), the University of New Brunswick (UNB), and Environment Canada as represented by the National Water Research Institute (NWRI), the National Hydrology Research Institute (NHRI), the Atmospheric Environment Service (AES), the Water Resources Branch (WRB) and the Water Planning and Management Branch (WPM).

The Subcommittee plays a leading role in identifying ice related problems and in coordinating activities directed at their solution. This Manual was produced to inform the public of general river ice processes, specific characteristics of the ice regime in New Brunswick rivers, ice monitoring techniques and predictive and control methodologies.

Ice jam at a railway bridge.

ACKNOWLEDGEMENTS

This Manual could not have been produced without the close cooperation of the member agencies of the New Brunswick Subcommittee on River Ice. Special recognition is given to the following :

Editors:

N.E. Elhadi, NB ENV

J.G. Lockhart, FDRP

Authors:

S. Beltaos, NWRI

B.C. Burrell, NB ENV

K.S. Davar, UNB

J. Dublin, AES

S. Ismail, NBEPC

R.J. Lane, WRB

T.D. Prowse, NHRI

Reviewers:

J.E. Anderson, NB ENV

T.M. Humes, WPM

Bridge collapsing under ice forces.

1.0 INTRODUCTION

River ice related problems are common in New Brunswick throughout the winter and spring seasons; from early winter when frazil and anchor ice are generated, through the formation and growth of ice covers, to the eventual breakup and jamming in the spring. In many areas of the Province, ice jams frequently cause flooding, destruction of bridges and extensive socio-economic damages.

Residents of New Brunswick need hardly be reminded of recent ice related events which were of near catastrophic proportions. In 1970, ice runs and jams destroyed 32 bridges and resulted in about $ 14 million in damages, in 1987 dollars. In 1976, ice jams on the Saint John River resulted in severe flooding and damages at Perth-Andover and Woodstock that totalled $ 3.6 million. Most recently, in 1987, ice jamming at and below Perth-Andover resulted in about $30 million in damages. The undocumented and intangible damages would substantially increase these amounts.

Recently, considerable advances have been made in understanding and predicting ice problems and in minimizing the associated socio-economic damages. It is appropriate that this progress in scientific knowledge be made available to the concerned public as their understanding and cooperation is crucial in effectively managing ice related problems in rivers. The prime purpose of this Manual is to present the basics of river ice processes, as well as to identify ice problems and mitigative techniques.

The scope of this Manual is limited to the presentation of a simple and concise review of current knowledge of the subject. In Section 2, a description of the various ice processes is presented. A summary of climatic conditions and a description of the ice regime in New Brunswick are presented in Section 3. In Section 4, various ice monitoring techniques as well as the essential data to be collected through ground-based monitoring programs are given. Predictive methods and quantitative applications are summarized in Section 5; while methods of ice control are described in Section 6. Many of the scientific and engineering analyses involve complex theoretical and computational procedures which cannot be described in a publication of this nature. The interested reader is referred to the references cited herein for further information.

2.0 RIVER ICE PROCESSES

2.1 Freeze-up

2.1.1 Border Ice
2.1.2 Frazil Ice
2.1.3 Thermal Ice Cover

2.2 Breakup

2.2.1 Winter Breakup
2.2.2 Spring Breakup
2.3 Ice Jams

Most rivers in New Brunswick are ice-covered during much of the winter season. River ice processes can normally be classified as either freeze-up or breakup. The characteristics of these processes are dependent upon the weather and flow conditions prevalent during the freeze-up and breakup periods. The predominant river ice processes and characteristics observed in New Brunswick rivers are presented in the following sections.

2.1 FREEZE-UP

Air temperature and flow velocity are the two most important factors affecting ice processes during the freeze-up period. Heat exchange at the open water surface is the primary process by which the river water temperature drops in the fall. Heat transfer depends upon: solar or short-wave radiation, long-wave radiation, evaporation or condensation, convection, and precipitation. Corresponding heat loss rates (rates of heat transferred per unit area) can be calculated according to hydrothermal and meteorologic principles (Michel, 1971). Minor heat exchanges may also occur at the stream bed due to: groundwater flow, heat stored in bottom sediments, geothermal heat and flow friction. These means of heat exchange are generally negligible but may become significant when an ice cover is present.

For simplicity, the heat loss rate is often calculated as the product of the difference between air and water temperatures and a factor reflecting local and meteorological conditions. For example, this factor was found to range from 20 to 60 watts/m^2oC for the St. Lawrence River (Shen et al, 1984; Prowse, 1987). By knowing the rate of heat loss and stream hydraulics, it is possible to predict the water temperature. Ice formation is imminent when the water temperature drops to near 0^oC.

A description of the freeze-up processes for the most common types of ice observed in New Brunswick rivers are described below.

2.1.1 Border Ice

The first ice to appear on a river usually forms along the banks where the velocity is low, Plate 1. Border ice grows vertically and also laterally toward mid-stream. Lateral growth can take place even with the main water temperatures slightly above freezing, depending on the flow velocity and meteorologic conditions, Fig. 1 (Devik, 1964). Presently, the rate of lateral growth can best be predicted by using empirical relationships, developed by Newbury, 1968 and Matousek, 1984.

PLATE 1. BORDER ICE FORMATION.

2.1.2 Frazil Ice

Frazil ice is very common in New Brunswick rivers at freeze-up as well as through the winter in ice-free turbulent reaches. It is made up of tiny ice particles which result from the slight supercooling of water (to approximately -0.05^oC). The amount of frazil ice is proportional to the extent of the open water area and the rate of heat loss.

In supercooled water, frazil particles adhere to each other leading to the formation of frazil "clusters" or "flocs" which are buoyant and rise to the surface. This property of particle adherence normally results in one of the following ice accumulation forms, Fig.2:

Fig. 2. River Ice Formation Processes

Ice Pans: As the frazil clusters grow in size, they form ice pans, Plate 2. These ice pans may stop at a channel constriction or may come to rest against an ice cover at river sections where the velocity is less than 0.6 m/s or where the Froude number is less than 0.08. The Froude number is defined as (V/(gH)^0.5); where V is the mean velocity, g is the acceleration of gravity and H is the mean depth. As the water surface continues to lose some of its heat to the atmosphere, these ice pans freeze together to form a continuous ice cover.
Hanging Dams: The accumulation of frazil ice against an ice cover progresses in an upstream direction when the velocity at the upstream edge of the cover is low. At high velocities,the frazil is transported downstream under the ice cover where it adheres to the undersurface in a low velocity area. As the supply of frazil continues, the ice accumulation under the cover grows in size forming a "hanging dam". A hanging dam can cause extensive blockage of the flow area resulting in increased upstream water levels and potential flooding.

PLATE 2. FRAZIL ICE ACCUMULATION (ICE PANS)

Anchor Ice: In turbulent reaches of wide, shallow rivers, frazil particles may adhere to the river bed and accumulate to form "anchor" ice. Anchor ice generally forms in the evening when the rate of heat loss is the greatest and supercooling occurs. Anchor ice can cause a significant blockage of the flow cross-section resulting in local flooding. Little is known about anchor ice formation, growth and detachment.

2.1.3 Thermal Ice Cover

In rivers with velocities less than 0.6 m/s and water temperatures below the freezing point, ice crystals form on the surface and rapidly link together to create a thin ice sheet. Once a thin ice sheet has formed, it begins to grow downward by freezing at the ice-water interface. Heat loss is retarded by the ice cover itself and by snow cover that may be present. Where freezing takes place in a frazil accumulation, the rate of thickening of the solid ice layer is inversely proportional to the porosity of the accumulation. A simple, semi-empirical formula is often used to calculate the solid ice thickness, h_i, in cm as:

h_i = a_i (D_F)^0.5(1)

in which D_F = accumulated degree-days of freezing (^oC-days) and a_i = empirical coefficient. Values of a_i have been found to be as follows: a_i = 2.7 for a windy lake without snow; 1.7 - 2.4 for an average lake with snow; 1.4 - 1.7 for an average river with snow; and 0.7 - 1.4 for a sheltered small river with rapid flow.

In central New Brunswick, the thickness of thermal ice covers typically ranges between 38 and 80 cm; depending on the severity of the winter season.

2.2 BREAKUP

Warm weather normally melts the snow pack and weakens the ice cover. In addition, longitudinal and transverse cracks, Plate 3, further reduce the strength of the cover. The rate of the snowmelt and rainfall and the subsequent runoff are the major factors affecting the breakup process. A high rate of snowmelt and rainfall, and consequently a rapid increase in river flows, normally results in an early breakup of a relativly strong ice cover. This form of breakup may occur in mid-winter or early spring and generally causes the worst flooding. On the other hand, slow snowmelt causes a gradual increase of the river flow and a gradual decay of the ice cover resulting in significantly lower peak water levels.

PLATE 3. LONGITUDINAL HINGE CRACKS.

2.2.1 Winter Breakup

Unseasonally warm weather is frequently experienced in central and southern New Brunswick during the late January - early February period. It is occasionally accompanied by a moderate amount of rain resulting in a high rate of snowmelt and consequently high river flows. As a result, increased downstream forces are exerted on the ice cover causing localized breakup and, in some cases, ice jams and associated flooding. When the normal mid-winter freezing temperatures return, the ice jam may freeze and consolidate in place. In many cases this condition results in more severe ice jamming and flooding the following spring.

2.2.2 Spring Breakup

In late March - early April, a general warming trend prevails resulting in the melting of the snow pack and the gradual thawing of the ice cover. This process is sometimes aggravated by rainfall and results in a decrease in ice cover strength and increased streamflows and water levels. These phenomena cause an increase in the forces exerted on the ice cover and also result in the formation of hinge cracks parallel to both banks which further reduce the resistance capabilities of the ice cover. At some point, as the downstream forces exerted on the cover increase while the resistance forces decrease, the ice cover begins its movement resulting in the breakup of the ice sheet into pieces progressing in a downstream direction. The progress of the breakup depends on flow conditions, the geometry of the river channel and the weather conditions immediately preceeding and during the breakup period.

As the broken ice continues its travel downstream, its momentum contributes further to the breakup of the ice sheet until it lodges at a point of greater resistance; such as at a channel constriction, at the edge of a strong ice cover, or a combination of both. The duration of the ice lodgement at a particular location has a significant impact on the progression of the breakup,the upstream water levels and the extent of flooding.

2.3 ICE JAMS

Ice jams are the most dramatic events caused by the breakup and rapid accumulation of fragmented river ice. Ice jams often cause sudden massive increases in the water level resulting in severe flood damages, in some cases exceeding those associated with extreme open water flood events. In New Brunswick, approximately seventy percent of recorded flood damages are caused by ice related floods.

There are locations which are more susceptible to ice jam formation than others. These include the confluence of two rivers, channel constrictions, sharp bends, islands, bridge piers, shallow river reaches, the edge of a solid ice cover, and at sudden changes in the slope of the water surface. Often ice jams are caused by a combination of two or more of these factors.

The severity of an ice jam event is generally influenced by: river flow, volume and strength of river ice, length of the breakup period, rate of heat transfer, snow depth, and precipitation. Of these, river flow is the single most important determinant of ice jam severity.

TABLE OF CONTENTS-----CHAPTER 3 - RIVER ICE REGIME

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New Brunswick River Ice Manual

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