ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

white sand valley fills and terraces, where numerous exposures
are found in both formations. The multi-convex hills
are associated with expected profile and catenary changes
and as valley floors are approached the saprolite exhibits
colours associated with reducing conditions, and appears
as a white, kaolinitic clay. Where water issues from macropores
in this material there is an immediate oxidation of
FeO which is in striking contrast to the water flowing
within and from exposures in the contiguous stream sediments.
It is concluded that the sediments are not derived
from the lower slopes of valleys, but obtain their materials
from the erosion of upper slopes and zones in the weathering
profiles. These can be shown to be leached of clay and
Fe, and this can lead to the shedding of sandy sediment by
surface wash under conditions of exposure to intense
rains. Further leaching of Fe will occur within the hydromorphic
environments of the valley sediments. A geomorphic
interpretation of the white sands involves a combination
of weathering under Equatorial conditions in which
lessivage is more or less continuous, alternating with
Quaternary seasonal climates, now understood as having
widely affected the inner tropics during at least the Last
Glacial Maximum and possibly during other ice advances.
Thus neither continuous pedognenesis, nor a simple catenary
model is adequate to explain these formations which
appear to reflect the oscillatory nature of tropical climates,
even in the Equatorial zone. A further complication, however,
concerns the age relations of the thick saprolite mantle.
In Kalimantan this has been called the «old Sundaland
regolith» and ascribed to a Tertiary evolution. While this is
not refuted, it is thought that a model of continuous
downwearing of the multi-convex landscape may be a relevant
precursor to the evolution of the white sands during
Quaternary climate changes.
The model developed from this study is compared with
other published models of pedogenesis and morphogenesis
in perhumid tropical environments.
COLIN R. THORNE 1 & STEPHEN E. DARBy 2
Magnitude and frequency analysis of large alluvial rivers
1Department of Geography, University of Nottingham, Nottingham,
NG77RD, UK
2 Usda-Ars National Sedimentation Laboratory, p.o. box 1157,
Oxford, Miss. 38655, USA
The magnitude-frequency analysis first proposed by W 01man
and Miller (1960) is applied to a range of large alluvial
rivers situated in contrasting physiographic and climatic
regions. Study rivers include the Brahmaputra River (SE
Asia), Upper Missouri and Lower Mississippi Rivers
(North America) and the Parana and Paraguay Rivers
(South America).
Analyses were based on data from established stream.gauging
stations with at least 30 years of historical record. Sediment
transport data were obtained from routine measurements
made at gauging stations during the period of record.
Return periods and durations for important flows were
calculated using the standard hydrological techniques.
Water surface profiles and stages associated with the range
of flows transporting most sediment were plotted on to
longitudinal profiles and channel cross-sections to investigate
whether there were significant associations between
the effective range of flows transporting most sediment
and major forms and features of channel morphology.
The results of the magnitude-frequency analyses for each
river were applied in engineering-geomorphic studies and
proved to be useful in identifying process-form linkages
and in predicting the sensitivity of morphological response
to river regulation or river training works that would impose
changes in the boundary conditions, flow regime or sediment
supply.
In each case the analyses revealed a clearly defined range
of flow discharges that was responsible for performing the
great majority of geomorphological work on the channel
through sediment transport. Within that range of effective
flows it was possible to define a dominant discharge equating
to the single flow doing most sediment transport. Return
periods and durations of formative flows were found
to be consistently within the range of 1 to 2 years described
by Wolman & Miller (1960), suggesting that over medium
to long timespans it is in fact flows of moderate magnitude
and frequency that are responsible for driving
morphological adjustments through sediment transport in
large rivers.
Correlation between the dominant discharge, the range of
effective flows and specific geomorphic surfaces revealed
that the concept that dominant discharge corresponds to
bankfull discharge may be a special case. More generally it
was found that the range of effective flows was bounded
by stages corresponding to «barfull» and bankfull flows at
the lower and upper limits respectively. This finding is independent
of channel planform, which included both single-thread
meandering and multi-thread braided patterns.
Specific applications to engineering-geomorphic analyses
included the identification of node-island patterns in the
planform of the braided Brahmaputra River, establishment
of the likely morphological impacts of proposed changes
to the regulated regime of the Upper Missouri River, development
of geomorphological guidelines for the design of
river training works on the Lower Mississippi River, and
mapping of the line of the active river corridor on the Parana
and Paraguay Rivers for flood zoning and flood plain
management.
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