RE: Some thoughts on recurrence intervals
Alfred E. Holland, Jr. (aholland@unm.edu)
Wed, 1 Oct 1997 12:58:01 -0700
Yes, Virginia [and Ben], water can flow on snow.
We had a fairly exciting snow and rain episode in the Sierra Nevada
last winter that may offer some observed evidence to this discussion of the
ensuing cocktail (rain on the rocks), run off, delta formation, and "can
water flow on snow" question.
Engineers may not recognize them all, but every Eskimo knows a
bidarka-load of names for different kinds of snow. They also know that
temperatures vary through the vertical axis of a pack. Folks that ski in
the back country know fewer of those names but several of snow's
properties. They're keenly aware of stratigraphic structures in snowpack.
When the tropical rains commenced to fall on about six feet of snow pack,
the recent (that is, as yet unconsolidated) stuff, about four feet at 6,000
feet, disappeared in a matter of about four hours. The river crested a
couple days later. The recent discussions here ask why the delay.
While scouting for down timber for the firewood stash for one of the
Sierra Club's back country huts last Sunday, my brother and I stumbled on
to several depositions of debris and silt that had settled out of what we
suppose for lack of other material residue to have been ice dam-impounded
pools. It seems to me that an ice dam works by stopping water from flowing
straight down the fall like. That might be considered a "negative value"
instance of water flowing on a snowbank?
These evidences of delay on the way to the sinks in Nevada and the
Pacific Ocean stand in marked contrast to the feather-form, flowing
water-etched surface of the snow we saw during and immediately after the
deluge. This surface was most visible on north slopes and increasingly
subtle on those with greater exposure to the south. The surface of the
waning and remaining snowfields took on a corrugated form (like Strongbarn
roofing in cross section, but three to five feet wave length and eight to
fourteen inch amplitude) with the roughly parallel troughs seeking the fall
line. Each of the resulting ridges was similarly etched with tributary
corrugations that joined the major troughs at an angle that varied with the
slope, steeper slopes showed more acute angles. This novel demonstration of
headward erosion remains intriguing. My hunch is that the rain came faster
than it could filter through the increasingly dense, saturated snow the way
a stream of water flows off rather than through an already saturated sponge
or the way a thunderstorm races down the Little Colorado rather than
soaking into the Colorado Plateau. Trying to ski on the compound
corrugations led to several attempts to penetrate the crust with body parts
sharp and blunt ranging from bottom to skull. The crust won every contest
until I tried chipping at it with a steel shovel. The crust was to packed
snow as quartzite is to sandstone: neither tight fit nor cemented amalgam,
that stuff had grown together. The crust had a pair of skins, one facing
the sun and a second facing what I'm guessing was the cold in the depth of
the pack. Hypothesis at this stage is that the residual cold caused the
crust to form from the inside and then the dense crust eroded in a
combination of melting (for you engineers and your thermal calculations)
and abrasion (for those of us who get by on the gritty). The water flowing
on the surface of the snow pack requires no hypothesis. It just plain did
so. It acted like all the water I've ever seen in channels, no matter the
substrate. That the crust had formed from water trapped in the spaces
between the snow and ice crystals also accounts for some of the delay in
the creeks' crestings.
Although this Tennessean was far from "the marge of Lake Labarge" at
the time, that ranks as the strangest flowing-water phenomenon I've
witnessed.
See you on the water,
Al
Alfred E. Holland, Jr. Department of History
7321 Santa Juanita Avenue University of New Mexico
Orangevale, California Albuquerque, New Mexico
95662-2834 87131-1181
(916) 988-1300 aholland@unm.edu
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