FLOOD05589

• County: Statewide
• Community: Statewide
• Stream Name: All
• Basin: Statewide
• Title: Ephemeral Streams Hydraulic Factors and Their Relation to the Drainage Net
• Date: 1/1/1956
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

30

PHYSIOGRAPHIC AKD HYDRAULIC STUDIES OF RIVERS

of formation of diseontinuou.5 and continuous arroyos
is different, If the mechanics of the two are appreciably
different, the choice of control measures presumably
would be affected,
The salient characteristic of a discontinuous gully is
the relatively small gradient, or slope, of its bed, It is
this flat slope-less steep than the floor of the original
ungullied alluvial valley-that makes the gully dis-
continuous, for the bed profile must at some dO\,,~nstream
point intersect the profile of the original valley floor.
At that point the gully depth has diminished to zero,
and the flow of the gully spreads out over the valley
floor and at least part of the load is deposited in a low
fan.
Therefore. it is important to find an explanation for
the low gradient. Any discussion of such gullies which
does not provide an explanation of the mechanics by
which this flat slope is developed would miss the domi-
nant feature. Our approach to this aspect of the prob-
lem is essentially inductive, for there is a paucity of
adequate data on essential elements in the hydraulic
relations, However, as will be shown later, field charac-
teristics appear to support the conclusion reached in-
ductively,
Consider the wide, grassy floor of an alluvial valley
in the semiarid West before the recent epicycle of ero-
sion. Though the summer rains are of short duration
and intensity, the flash flows spread widely over the
valley, and channel storage consequently keeps the peak
discharge at any point to a small value per unit of
drainage area. The grassy floor offers resistance to the
flow of water and sediment contributed to the vallcy
floor from adjoining slopes, This resistance keeps the
velocity low and thus the sediment concentration is
small. The profile and cross section of such an ungullied
valley floor are shown as diagram 1 of figurc 26.
. Assume that local weakening of vegetation allows an
initial furrow, sCfil'plet, or small basin to form by erosion.
The cause of this lowered resistance to erosion could be
grazing or trampling by stock, fire, or an exceptional
local storm. The regional erosion in the vVest, which
began about 1880, is considered to be a result of a com-
bination of overgrazing and meteorologic shift,
Subsequent storms cause the head of this initial
erosion feature to progress up-vallcy, and the debris
excavated splays out at the downstream toc in the form
of a low fan, As soon as a short channel is formed,
terminating in a vertical head-cut, the concentration
of water in the f1umelike trench reduces channel
storage. Consequently, from a storm of given size, the
peak discharge passing through the channel is greater
than would have been experienced on the ungullied
valley floor. This increased peak discharge is accom-
panied by grcater velocity and cutting power, and the

initial gully advances so rapidly during storlllS that
growth of vegetation in the intervals between storms
cannot heal it.
Water pours over the lip and develops a plunge pool
at the toe. The original sod, even when weakened and
incomplete as a protective cover, keeps the lip of the
head-cut relatively stronger than the underlying
alluvium and the latter is cut away by the turbulence
in the plunge pool, leaving the root-bound lip over-
hanging the plunge pool. Undercutting by plunge-pool
action during storm flow is greatly aided by-and
perhaps is even less important than-slumping of the
moistened headwall after the storm flow ends. This
shun ping is promoted by piping holes (see fig. 5) which
develop on a diagonal line between the lower part of
the vertical headwall and the valley floor many feet
upstream from the head-cut. We have often observed
the upper cntrance to these piping holes on the swale
floor 50 feet from the head-cut. Piping permits water
to penctrate deeply into the material into which the
head-cut is eating, and this moisture aids the process
of sapping and slumping at the base of the headwall.
The plunge pool is always seen to be dug deeper
than the level of the floor of the discontinuous gully
just downstream from the plunge pool. This clearly
means that the floor of the discontinuous gully is
composed of a layer of newly deposited material which
overlies the undisturbed alluvium. This deposit is
laid in immediately below the plunge pool, and deposi-
tion proceeds upstream as fast as the plunge pool does.
These details of gully growth are important to an
understanding of the mechanics, for they show that
when a plunge pool is present, and it usually is, the
flat slope of the discontinuous gully is a grade oj deposi-
tion. Furthermore, these details point up a significant
difference between the action of flood flow in the
channel of a normal river and that of a developing
gully. The plunge-pool action tends to deepen the
channel faster than to widen it. In thc early stages
of gully development, then, the channel is relatively
narrow and deep.
At any time, the gully floor is built at such a slope
that the material coming into the reach will be trans-
ported through the reach under the particular condition
of roughness. At the early stage of gully development
no\v under consideration; the channel has eonsidcrable
depth but a restricted width. At the same time it is
forming a deposit just downstream from the plunge
pool and thus is currently forming its bed slope under
conditions in which slope can be adjusted with relative
rapidity, as compared to width. Under conditions
which prevail in ordinary rivers the reverse is true;
width adjusts rapidly during floods, but because of the
large amounts of material involved in appreciably