New Ulm review. (New Ulm, Brown County, Minn.) 1892-1961, 1916-07-26
PECULIARITIES OF STREAMS IN
FLOOD. !: I

FLOOD. a: a,
Interesting features in the behavior Of
streams during floods are explained by a
scientist of the United States Geological
Survey in recent official letter. It has
long been known that swift streams are
higher at the center than near the banks,
that driftwood NOV es to the banks dur
ing the rise and crest of food and re-
turns to the center as the waters fall, and
that foam is generally abundant on a
rising stream and absent from one that is
falling, but owing to the imperfect de-
velopment of the science of NEUROLOGY as
applied to streams and to the special
obscurity of the laws relating to the
energies involved in stream flow the
causes of these phenomena are not
generally known. ~at
It can be shown, however, that the in
tricate and bewildering play of energy in
streams, like all other natural phe-
nomena, is fundamentally systematic.
Different causes create distinct sets of
forces, which act at the same time but
independently in the mass of stream.
A single particle in motion is commonly
affected at any instant by forces belong
ing to several different sets, some local
and some general, and the forces of first
one and then another set may dominate
its movement. The composite move
ment of groups of particles constitutes
currents, and the composite of currents
constitutes stream flow. To understand
these phenomena it is necessary to study
the origin of the different sets of forces
and to see how they affect the currents
and how the currents affect the shape of
stream. .....
The elevation of the center of stream
above it's margins during flood is gener-
ally the result of the roughness of it's bed.
Bowlders, sand waves, and rifles, and
even sand grains, turn parts of the cur
rent that strike against them outward
from the banks and upward from the
bottom of the channel. The particles
directly DETECTED by the obstacles may
move but short distance in that direc-
ton, but their energy is transmitted to
other particles and by them to still others,
setting up system of forces that act
toward the upper middle part of the
stream. All forces acting in.. a liquid
cause motion until friction completely
wastes the energy involved. Therefore
these forces cause movements in the
streams mass that converge toward the
upper central part of the cross section.
This causes the center of the stream to
rise above the margins until hydraulic
head balances the DETECTED forces. The
height to which the center of stream
may rise above it's margins depends upon
the size, shape, and roughness of the chan
mel and the velocity of the current. It
is greater in relatively deep and narrow
streams and in rough than in smooth
channels. Artificial troughs lined with
paraffin, planed wood, rough lumber,
sand, and gravel are increasingly effective
in the order named in producing the phe-
nomenon, and natural streams that have
their channels in coarse material show
greater effect than those whose beds are
in fine material. The phenomenon is so
pronounced in the rapids of Niagara
River that it is indicated on the topo-
graphic maps published by the Survey.
The peculiar behavior of driftwood dur
ing floods is due to the fact that the deep
central part of a stream, because of it's
greater freedom of flow, tends both to rise
and to fall slightly in advance of the
margins. The swelling of the central
part of a stream during a rise gives a
shoreward movement to the surface
currents which carries the drift toward
the banks. During the falling stages the
center of the stream is slightly depressed
and the surface currents converge toward
the center, carrying the drift with them.
Eddies also play an important part in this
phenomenon, for water is added to the
eddy mainly at the surface and is drawn
off some distance below the surface, where
it is dragged away by the swifter part of
the main current. This gives a surface
slope toward the eddy and away from the
center OF the stream. The eddy begins
to give up its drift only when the depres-
sion of the center of the stream due to the
falling stage overcomes the shoreward
slope due to the eddy.
The development of foam along the
margins of rising streams is due to the es-
cape of air and gas from the soil after it
has been covered with water. Observa-
ton along the edge of stream during a
rise will show quantities of air bubbling
to the surface and forming foam. Foam
ing is especially notable where the water
rises over ground covered by vegetation,
but it occurs also even on bare, loose sand
! and gravel bars.