DIFF~ISION AND DEPOSITION OF FLOATING POLLLITANTS
377
same big eddies which move floating particles horizontally into confluences,
also govern the dispersal of marked fluid. From our point of view the main
conclusion is that for floating particles we may use the classical diffusion
equation in two dimensions, for phenomena ofa time scale much larger than
lo3 sec, and using an estimated diffusivity of the order of lo3 cm2 sec- I .
111. BEACH DEPOSITION OF AZosa pseudoharengus
While the horizontal diffusion in a field of windrows of a batch of any kind
of floating objects is likely to proceed in an identical manner according to
the above arguments, differences are certain to arise at the location of
greatest practical interest, namely, the beaches where floating objects are
washed up. Globules of oil are likely to be retained on beaches according
to different laws than larger flotsam. To render the discussion more
concrete, wc shall here focus our attention to the beach deposition of AIosa
pseudoharrngus in the Great Lakes.
I have made a few observations over the years 1967-1973 on the deposition of alewives on a sandy beach fronting our cottage on Lake Huron. One
finds larger quantities washed up at random times in May and June, but
never later in the season. Thus the large scale die-off of these fish is definitely
tied to the "spring" thermal regime of the Great Lakes, when a warm band
of water (t0"C or so) at the shores rings a cold (- 4°C) central mass. The
width of the warm band is of the order of 5 km (see, e.g., Csanady, 1972). On
the Victoria day long weekend (Monday nearest 24th May) one may often
observe cottagers collecting and incinerating quantities of alewives on their
beaches.
On this year's Victoria day (May 21, 1973) the following could be observed. The water temperature near shore was between 10' and 12'C. but it
must have been much less one or two kilometres off shore, where on the
preceding day the water surface was mirror-smooth, in contrast to the
rougher surface (covered by capillary waves) closer to shore. Such differences in surface appearance are caused by the stability of the air over cold
water, m y frequently be seen in the Great Lakes, and have been reported in
other lakes by Ragotskic (1962). On the 20th May, one could see the full
mirror image of boats travelling beyond the sharp boundary of the nearshore rough band of water.
About noon on the 21st a school of alewives, many dead, but some just
dying, floated slowly toward the shore. There was a light onshore breeze. by
which the lake surface was very slightly roughened (wave amplitude
estimated at 5 cm). When within 100 m or so of the shore, the density of the
alewives floating belly up was I per 10 square meters or so, in some places 1
per rn2. As they arrived at the shore. the alewives concentrated within a band
377
same big eddies which move floating particles horizontally into confluences,
also govern the dispersal of marked fluid. From our point of view the main
conclusion is that for floating particles we may use the classical diffusion
equation in two dimensions, for phenomena ofa time scale much larger than
lo3 sec, and using an estimated diffusivity of the order of lo3 cm2 sec- I .
111. BEACH DEPOSITION OF AZosa pseudoharengus
While the horizontal diffusion in a field of windrows of a batch of any kind
of floating objects is likely to proceed in an identical manner according to
the above arguments, differences are certain to arise at the location of
greatest practical interest, namely, the beaches where floating objects are
washed up. Globules of oil are likely to be retained on beaches according
to different laws than larger flotsam. To render the discussion more
concrete, wc shall here focus our attention to the beach deposition of AIosa
pseudoharrngus in the Great Lakes.
I have made a few observations over the years 1967-1973 on the deposition of alewives on a sandy beach fronting our cottage on Lake Huron. One
finds larger quantities washed up at random times in May and June, but
never later in the season. Thus the large scale die-off of these fish is definitely
tied to the "spring" thermal regime of the Great Lakes, when a warm band
of water (t0"C or so) at the shores rings a cold (- 4°C) central mass. The
width of the warm band is of the order of 5 km (see, e.g., Csanady, 1972). On
the Victoria day long weekend (Monday nearest 24th May) one may often
observe cottagers collecting and incinerating quantities of alewives on their
beaches.
On this year's Victoria day (May 21, 1973) the following could be observed. The water temperature near shore was between 10' and 12'C. but it
must have been much less one or two kilometres off shore, where on the
preceding day the water surface was mirror-smooth, in contrast to the
rougher surface (covered by capillary waves) closer to shore. Such differences in surface appearance are caused by the stability of the air over cold
water, m y frequently be seen in the Great Lakes, and have been reported in
other lakes by Ragotskic (1962). On the 20th May, one could see the full
mirror image of boats travelling beyond the sharp boundary of the nearshore rough band of water.
About noon on the 21st a school of alewives, many dead, but some just
dying, floated slowly toward the shore. There was a light onshore breeze. by
which the lake surface was very slightly roughened (wave amplitude
estimated at 5 cm). When within 100 m or so of the shore, the density of the
alewives floating belly up was I per 10 square meters or so, in some places 1
per rn2. As they arrived at the shore. the alewives concentrated within a band