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Accueil du site Doctorat États-Unis 1972 THE DYNAMIC STRUCTURE OF EPHEMERAL STREAMS

**Titre :** THE DYNAMIC STRUCTURE OF EPHEMERAL STREAMS

**Auteur : ** Renard, Kenneth G.

**Université de soutenance : ** University of Arizona

**Grade : ** Doctor of Philosophy (PhD) 1972

**Résumé **

The hydraulic features of ephemeral streams are dynamic in
response to the variable streamflow available for sediment transport.
For a given water and sediment discharge, the hydraulic stream features
of width, depth, velocity, slope, and roughness result from mutual
adjustments. Although the problem of quantifying the stream behavior
can be approximated using laboratory and field developed hydraulic relations,
variations in the temporal and spatial stream roughness and
sediment sizes available for transport add an additional element of uncertainty
because of the difficulties of measuring these factors in the
prototype. The dynamic stream behavior is discussed qualitatively with
consideration of the profile shape, bed slope, channel width, flow
depth, sediment discharge and channel controls.
Transmission losses (that water infiltrating to the alluvium)
in ephemeral streams result in a reduced volume of water to transport
sediment. The losses tend to be offset by additional runoff from
tributaries when the runoff-producing thunderstorms, which are of
limited areal extent, are confined to the lower portion of the drainage
basin.
There are two tendencies apparent in an ephemeral stream ; to
be concave down because of loss of discharge by infiltration through
the normally dry channel alluvium, and to be concave up because there
is more flow downstream than upstream due to tributary inflow. These
two tendencies seem to be in balance on Walnut Gulch resulting in a
remarkably constant slope for the main channel from one end to the
other.
The principal channel reach on the Walnut Gulch Experimental
Watershed was modeled incorporating a geomorphic parameter approach
based on the Horton-Strahler stream order numbering concept for tributary
intersections. The intersections by stream order are described
by a geometric probablity function with the probability and the number
of intersections related to the rate of drainage area increase per
length of stream channel. The channel slope and width and the watershed
drainage area for the tributaries were found to be related to
stream order.
The log-normal probability distribution was used to model the
particle size distribution of the alluvial beds. Although minor deviations
from the theoretical distributions were sometimes observed at the
extremes, the distributions can be specified using the mean and standard
deviation. These values were then used in the Laursen relation to describe
the percentages in various sizes. Prediction equations developed
using multiple linear regression were used to predict the mean and standard
deviations from the channel width in the tributaries. For the main
channel, however, the regression equations indicated an increase in the
mean grain size at the downstream end of the channel which did not agree
with the sampling data at the ends of the reach.
To quantify the inputs and output of water and sediment, a
stochastic model of runoff based on the Diskin-Lane model was used with
the Manning open channel flow relation and the Laursen sediment transport
relation. The composite model was then tested against Walnut Gulch
data for the 36,200 foot channel reach between Flume 6 and Flume 1 and
was found to produce synthetic sediment volumes and peak discharges
which agree quite well with the prototype data. The importance of the
bed material size distribution was demonstrated using several sizes
at the upstream end of the channel reach. For example, reducing the
mean of the logarithmic transformations of the bed sediment from 1.85
to 1.00 mm changed the sediment balance for the reach from about
7 acre-feet per year of erosion to about the same amount of deposition.
This rate represents about one-half of a foot over the channel area
for a 10 year period. Because it would involve the combined effects of
bank scour and bed scour or deposition, the depth change might be
expected to be less. In reality, however, the bed composition probably
reacts and changes in response to the flow sequences and the availability
of sediment from tributaries. Because of complex conditions across
a typical channel section, changes in bed form and hydraulic roughness
temporally and spatially are important when predicting the channel
behavior

** Mots clés : ** Bed load — Measurement. ; Sediment transport — Mathematical models. ; Ephemeral streams — Mathematical models.

Page publiée le 27 avril 2016, mise à jour le 23 mars 2018