ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

TIM B. ABBE & D'AVID R. MONTGOMERY
Patterns, processes and geomorphological consequences
of wood debris accumulations in channel networks
Department of Geological Sciences, University of Washington, Seattle,
WA, 98195, U.S.A.
In unmanaged forested landscapes wood debris can comprise
a significant portion of the sediment budget and form
the principle physical element defining the morphology of
channels and alluvial valleys throughout a wide range of sizes.
In small channels, wood debris can account for a significant
portion of the elevation loss, whereas in larger channels
such debris can form the dominate roughness element
and force flow constriction, pool formation, and textural
variance of the bed surface sediment. Accumulations of
wood debris exhibit distinctive patterns that reflect their
formative processes in different parts of a drainage basin.
Field surveys in the 724 km 2
Queets River watershed on
the west slope of the Olympic Mountains in Northwest
Washington reveal basin-wide patterns in distinctive structural
types of log jams that arise from differences in the
mechanism of log recruitment, hydraulic geometry, the
physical characteristics of logs, and the position.of logs relative
to one-another and the channel (jam structure). Classification
of logs comprising wood debris accumulations
into three stability categories (key, racked, and loose members)
defines unique domains on a dimensionless plot of
log diameter and length relative to the depth and width of
the bankfull channel. Although log length is an important
factor, it's influence on log stability becomes less significant
as channel size increases. Ourresults show that log
diameter and geometry are important factors governing the
stability of in-stream wood debris, especially in large channels.
The magnitude of local wood debris inputs also influences
jam formation and becomes more important with
increasing channel size. The presence of roorwads or multiple
stems was also found to be an importantcontrol on
log jam stability. Ten fundamental wood debris jam types
can be distinguished based on the orientation" of key,
racked, and loose debris relative to the channel axis. The
position of a jam within the channel, jam width to length
ratio, channel planform geometry, topographic and bed
texture variations, and spatial patterns in the age structure
of trees growing on and around a jam also serve to delineate
different jam types. The stability and structural integrity
of a jam depends on the presence and arrangement ofkey
and racked members. Specific jam types fall intothreebasic
jam categories based on whether these members underwent
transport once introduced into the channel. In-situ
jams consist of logs lying approximately at the point
they entered the channel. Transport jams form from logs
that were routed downstream before coming to rest and
combination jams consist of both elements. Jam types
exhibit a systematic spatial distribution as a function of
drainage area and channel gradient. In-situ jam frequency
per unit channel length tends to decrease with increasing
drainage area, whereas the frequency of combination and
transport jams tends to increase to a maximum at progressively
larger catchment areas before decreasing with
further increases in channel size. Although some jam types
have little geomorphic impact, others form stable in-stream
structures influencing alluvial morphology at both sub-reach
and reach length scales. Stable log jams directly influence
channel anabranching, planform geometry, floodplain
topography, and establishment of long-term riparian refugia.
Because the formation of stable jams depends on the
size and supply of wood debris introduced to the channel,
these structures and their effects are integrally linked to riparian
forest management. Systematic patterns and physical
attributes of log accumulations and their relationship to
channel morphology and gradient throughout a large drainage
basin provide a basic model for patterns and processes
of wood debris accumulations in forested environments.
This template of natural wood debris structures offers
a foundation for managing channel debris and developing
stable in-stream log structures for habitat restoration
and engineering applications.
ESSAM ABDEL-MoTAAL
Neotectonicandmorphotectonic implications,
the Nile Delta Basin, Egypt
Department of Geology, Faculty of Science, Al-Azhar University,
Nasr City, Cairo, Egypt
The present work is concerned with the study of the influence
of Neogene and Quaternary tectonics (Neotectonics)
on the configuration of the Nile Delta basin. Nature ,',
and distribution of the Late Miocene thick clastic sediments
reveal the occurrence of three main morphotectonic
units (fault - scarp, mid ,- delta canyon and alluvial fans) in
the Late Miocene Delta basin. Morphotectonic map and
diagram showing the configuration of the basin, which experienced
neotectonic activity along the Hinge Zone faults
during this time, have been suggested.
Comparison of the isopach maps of the Delta deposits,
from Pliocene to Holocene, gives evidences of more neotectonic
subsidences in the eastern reaches of the Delta basin.
Distribution of the ancient Delta branches seems to
have been significantly influenced by such subsidences.
Changes in the configuration of the Mediterranean coastline'
along the Nile Delta, may reflect neotectonic oscillatory
movement during the Holocene time. Neotectonic activities
of the Delta seem to be due to rejuvenations along the
WNW Hinge Zone and NEPleusium faults.
Available seismic records of the Nile Delta suggest that the
area has experienced a number of shocks which, though
few in numbers, are indicators of neotectonic activity. It is
concluded that the Nile Delta site seems to be affected by
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