Albedo
able compared to estimates made by others from
force measurements on piers at a downstream
The absorption of solar radiation by an ice cover
bridge.
is a function of the reflectivity of its surface. Dif-
ferent surfaces (e.g., water, snow, ice) reflect solar
radiation to different degrees. The ratio of the so-
Thermal effects on ice jams
Once an ice cover or ice accumulation has
lar radiation reflected by a given surface to the
formed, it will thicken largely because of heat
incoming solar radiation is termed the albedo of
transfer processes, which Ashton (1986) identified
the surface. A high albedo indicates high
as shortwave (solar) radiation, longwave radia-
reflectivity (and low absorption), while a low al-
tion, evaporation or condensation, convection, and
bedo indicates low reflectivity (and a high degree
precipitation. Ice cover decay is also a function of
of absorption). The albedo depends on both the
heat transfer processes, but it is more complicated
condition of the surface (i.e., rough, smooth) and
because melting at both the top and bottom sur-
the sun angle. According to Williams and Gold
face of the ice, and melting within the ice cover
(1963), the albedo of new snow is about 0.8 to 0.9,
matrix itself are involved. Calculating a detailed
compared to an albedo of 0.4 to 0.6 for melting
heat budget that describes the decay process can
snow, and 0.05 to 0.15 for water. Gray and Male
be quite complex, because it depends on latitude,
(1981) report albedos of 0.75 to 0.95 for fresh snow,
time of year, time of day, hours of sunlight, cloud
0.3 to 0.4 for sea ice, and 0.05 to 0.30 for water.
cover, wind speed, air temperature, water tem-
Kondratyev (1954), as presented in Gray and Male
perature, humidity, ice type, and other variables.
(1981), reports an inverse relationship between sun
Solar radiation plays an important role in the
angle and albedo. Measured albedo increased from
heat budget, particularly in the spring when the
0.86 to 0.95 for compact, dry, clean snow as the
hours of daylight are increasing, because melting
sun angle decreased from 30.3 to 25.1 degrees.
within the ice cover is largely caused by in-
They reported smaller increases for wetter and
creased absorption of solar radiation. This inter-
more porous snow. Measurements of albedo for
nal melting process reduces the internal strength
brash ice reported in Prowse and Marsh (1989)
of the ice cover and its resistance to failure at low
ranged from 0.04 to 0.15, with a mean value of 0.08.
stress levels.
Candled thermally grown ice had an albedo of
0.39, while granular thermally grown ice had an
Heat flux
albedo of 0.55. During an ice dusting operation
Huokuna (1988) reported on the modeling of
on the Platte River, Nebraska, the albedos of
ice cover growth and decay on a 37.6-km reach of
dusted and non-dusted areas were measured be-
the Oulujoki River, Finland. The heat flux between
tween 1 March and 7 March 1979 (USAED Omaha
the water and air includes contributions from
1979). The average albedo of the non-dusted ice
short- and longwave radiation, convection, and
decreased during the measurement period from
about 0.65 on 1 March to 0.40 on 5 March, prob-
between the water and the channel bed. The heat
ably because of the formation of pools of water on
flux between the water and the ice cover is a func-
the ice surface from rainfall, snowmelt, and ice
tion of flow turbulence and water temperature.
melting. The albedo of the dusted ice was about
The model assumed the thermal conductivity of
0.2 to 0.25.
the (snow-covered) ice cover to be 1.8 W/mK,
compared to 2.24 W/mK for pure ice. Calculated
Photosynthetically active radiation (PAR)
ice thickness resulting from thermal decay at two
The presence of an ice cover is thought to pre-
cross sections compared favorably to observed ice
vent or minimize photosynthesis through the ab-
thickness. Careful measurements of the decay of
sorption or reflection of solar radiation in the form
the pre-breakup ice cover and the 1983 jam at the
of visible light. The wavelengths from approxi-
confluence of the Liard and Mackenzie Rivers
mately 380 to 760 nm are used by photosynthetic
(Northwest Territories) provided information used
organisms located throughout the water column
in determining the magnitude of the major heat
(Brock et al. 1984, Atlas and Bartha 1998), with the
fluxes contributing to ice decay (Prowse 1988, 1990;
range between 400 and 700 nm known as photo-
Prowse and Marsh 1989). Between 6 May and 9
synthetically active radiation (PAR). Bolsenga et
May 1983, approximately 3.24 107 m3 of ice
al. (1991, 1996) reported that transmittance of PAR
melted.
through clear freshwater ice ranged from less than
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