Abstract:
Surface size analyses of Twenty and Sixteen Mile
Creeks, the Grand and Genesee Rivers and Cazenovia
Creek show three distinct types of bed-surface
sediment: 1) a "continuous" armor coat which has a
mean size of -6.5 phi and coarser, 2) a
"discontinuous" armor coat which has a mean size of
approximately -6.0 phi and 3) a bed with no armor coat
which has a mean surface size of -5.0 phi and finer.
The continuous armor coat completely covers and
protects the subsurface from the flow. The
discontinuous armor coat is composed of
intermittently-spaced surface clasts, which provide
the subsurface with only limited protection from the
flow. The bed with no armor coat allows complete
exposure of the subsurface to the flow.
The subsurface beneath the continuous armor coats
of Twenty and Sixteen Mile Creeks is possibly modified
by a "vertical winnowing" process when the armor coat
is p«natrat«d. This process results in a welld
«v«loped inversely graded sediment sequence.vertical winnowing is reduced beneath the
discontinuous armor coats of the Grand and Genesee
Rivers. The reduction of vertical winnowing results
in a more poorly-developed inverse grading than that
found in Twenty and sixteen Mile Creeks. The
streambed of Cazenovia Creek normally is not armored
resulting in a homogeneous subsurface which shows no
modification by vertical winnowing. This streambed
forms during waning or moderate flows, suggesting it
does not represent the maximum competence of the
stream.
Each population of grains in the subsurface layers
of Twenty and sixteen Mile Creeks has been modified by
vertical winnowing and does not represent a mode of
transport. Each population in the subsurface layers
beneath a discontinuous armor coat may partially
reflect a transport mode. These layers are still
inversely graded suggesting that each population is
affected to some degree by vertical winnowing. The
populations for sediment beneath a surface which is
not armored are probably indicative of transport modes
because such sediment has not been modified by
vertical winnowing.
Bed photographs taken in each of the five streams
before and after the 1982-83 snow-melt show that the
probability of movement for the surface clasts is a function of grain size. The greatest probability of
of clast movement and scour depth of this study were
recorded on Cazenovia Creek in areas where no armor
coat is present. The scour depth in the armored beds
of Twenty and Sixteen Mile Creeks is related to the
probability of movement for a given mean surface
size.
