Delectable details

A walk along the beach presents a feast of fleeting features: shorebirds, deer tracks, driftwood, breaking waves, bores, minnows, dabble pits, the list goes on. But, this document only treats features involving sand or ice; and even then, some relevant topics are treated elsewhere—longshore drift, wet sand, beach cusps. Yet, herein are many delightful delectables to espy while walking the shoreline. Of course, if your trek takes you along Nelson’s waterfront path, then you will likely miss much of what is discussed, as you are rather high above the water; so, seek a place where you can walk along the edge of the wet sand.

Dew on the beach  In the early morning, dew moistens the ridges between the footprints of the previous day. The valleys (of the footprints) are warmer than the ridges because they receive some (infrared) radiation from the valley walls while the ridges only receive radiation from the much colder sky. Wet sand is darker than dry sand because the refractive index of water and sand are nearly the same. This allows some light to penetrate deeper than when the sand is dry, so more light is absorbed and less is reflected.

Frost on the beach  As with the picture of dew on the beach (above), this early-morning shot shows frost confined to the ridges. The explanation is the same: the ridges are colder and so condensation takes place there preferentially. But, as the frost forms (white) crystals rather than wetting the sand does not wet the sand, the ridges now appear lighter than the valleys (even apart from the shadows).

Swash marks  Following the breaking of a wave, a thin (about a half-centimeter) sheet of water flows up the beach. Called an uprush, or a swash, the forward edge acts as a plow to push sand and other debris ahead of it. Usually, the swash flows over the finer sand but pushes the somewhat larger grains. When the swash loses its momentum, some of the water sinks into the sand, and some flows back down the beach (the backrush). A thin line of sand (and sometimes organic material) is left at the maximum reach of the swash: the swash mark.

Swash marks leave a record of succeeding waves, the record lasts only until another big swash wipes out all previous marks with a new mark higher on the beach. Subsequent swashes build new patterns.

Sand ripples  Waves don’t just move sand at the water’s edge, in shallow water, waves also move sand along the bottom. The back and forth motion of the water sculpts the bottom into a series of sand ripples.

These ripples in the sand should not be confused with the ripples which form on the surface of the water. The latter are much smaller and have a very different origin.

Frozen swash marks  Not seen as often as regular swash marks (above) are those made of ice. Typically, to see a pattern such as this, the overnight temperature must reach about -10°C and the lake must be falling. Alas, not as many folk are out walking the beach on such occasions. Now, the water from successive swashes doesn’t retreat but, having washed up on the beach, freezes.

Fozen ripples  When the temperature drops, ice forms first in the shallows. If those shallows also have sand ripples, an interesting pattern of ice may emerge.

Dabble pits  I used to ponder the strange indentations often seen in the shallows along the shore. What physical process could produce these, I wondered? None, as it turns out. Dabbling ducks and geese root around in the silt for things to eat; the dabble pits are the remnants of their foraging.

Dabbling  A Mallard drake and hen dabble in the shallows with their tails pointed up. Their beaks sweep out little pits in the silt and sand as they look for food.

Dabble pits are sometimes still evident in the sand even after the Lake goes down.

Shore ice  When the temperature drops well below 0°C, ice forms first right along the shore where the water is shallow. There seems to be two reason for this:
1) katabatic winds (cold air) draining down the mountain slope encounters the water first at the edge, but are somewhat warmer farther out as a result of flowing over the water;
2) mixing between surface and warmer, deeper water farther out in the lake impedes surface freezing.

Of course, on the interesting question of why the shore ice displays the striking patterns it does, alas, I don’t have a clue.

Ice flowers (also called Frost flowers) These can form under the same circumstances as steam fog, but, of course, the temperature must be below 0C for these. The frost can grow because humidity in a thin layer near the surface is well above 100%. These are seen quite rarely along the water’s edge.