ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

PEDRO BLANCO SEGUNDO, JOSE R. HERNANDEZ SANTANA,
ANTONIO MAGAZ GARCIA & JORGE L. DIAZ DIAZ
Basic principles for the classification
of the recent exogenous processes of the Cuban relief
Instituto de Geografia Tropical,
Ministerio de Ciencia, Tecnologla y Medio Ambiente, Cuba
New classification criteria of the recent exogenous dinamics
of the Cuban territory relief sustained on the expeditionary
geomorphologic research for the elaboration of the
«Recent Exogenous Processes» map of the Nuevo Atlas
Nacional de Cuba (Nanc) are presented. The carried out
classification has four hierarchic levels: 1) Zonal type of the
process, which is the fluvial one for Cuba; 2) Category of
the process, which according to its direction can be denudation
accumulation and weathering; 3) Mechanism of the
exogenous modeling, that is connected to the genesis of
the process (fundamental type or level of classification)
which can be erosive, denudative, karstic, etc, and 4) Intensity
of the process, given by the degree which the earth
surface is transformed. The general geologic and geomorphologic
characteristics of the relief, are the basis for
the determination of the category of the process and the
mechanism of the exogenous modeling. The degree of intensity
was qualitatively determined by the condition of
the plant cover, land use of soil, angle of the slope, genetic
type and the annnal mean precipitation. The present classification
synthesizes the main criteria and ideas of the exogenous
dinamic researchers in Cuba.
JOHN BOARDMAN 1 & DAVID FAVIS-MoRTLOCK 2
Frequency and magnitude in soil erosion-some effects of
spatial scale
1 University of Oxford, School of Geography and Environmental
Change Unit, Mansfield Road, Oxford OXl 3TB, UK
2 University of Oxford, Environmental Change Unit,
Mansfield road, Oxford OXl 3TB, UK
Soil erosion by water results from both rainfall and runoff.
Because of the important role of the land's surface, frequency-magnitude
distributions of erosion events differ
markedly from those for rainfall and runoff. This difference
is due in part to temporal changes in vegetation cover
and soil properties. It is also related to variation in the relative
importance of erosional processes with rainfall amount
and intensity. Examples are the increase in the contribution
of splash with increased rainfall intensity, and the shift
in the balance between rill and gully erosion with changed
rainfall intensity (Poesen & alii, 1996). Also the importance
of these processes does not necessarily vary smoothly
across the whole range of rainfall magnitudes: there appear
to be broad thresholds separating the domains in which
each dominates. Frequency-magnitude analysis of rainfall
or runoff is therefore a poor predictor of erosion.
Knowledge of frequency-magnitude relationships for erosion
events is hindered by a lack of long-term studies incorporating
high magnitude events (Favis-Mortlock &
Boardman, this volume). Even for shorter periods of observation,
knowledge of the frequency-magnitude distributions
of erosion events at the field and catchment scale is
limited, since the majority of existing time series of erosion
rates are derived from small plots. In particular, we lack
knowledge of the effects of spatial scale on frequency-magnitude
distributions. Moving from hillslope plots to small
catchments, increased heterogeneity of vegetation cover
may become important. For example, while it is clear that
on the agricultural UK South Downs most erosion occurs
as the result of a few storms, these are only effective when
and where there is low crop cover. While storms of >1000
year return period have little erosive effect in summer,
storms of 25 year return period may give rise to erosion rates
>200 m'ha' on individual fields with little crop cover in
the autumn period.
A second effect of increased scale is increased opportunity
for storage. Work done by erosional runoff in the agriculturallandscape
is difficult to quantify because of the possibility
of long-term sediment storage. Trimble (1983) shows
that eroded sediment may be eventually released from storage
sites on slopes and flood plains. A central issue here is
therefore the degree of connectivity between landscape
elements; this varies through time and in relation to the
magnitude of runoff events. Experimental plots fail to address
these scale issues.
This study presents an analysis of erosional frequency-magnitude
relationships for sites in the South Downs and
other localities at a range of scales.
JAN C. BOELHOUWERS
Periglacial slope deposits in the hex river mountains,
Western Cape, South Africa
Department of Earth Sciences, University of the Western Cape,
Priv. Bag Xl7, Bellville 7535, South Africa
Coarse debris deposits are of widespread occurrence in the
Western Cape mountains of South Africa. Although no
hard evidence exists, the origin of the material, which occurs
down to sea level, has generally been related to Late
Quaternary phases of cryoclastic weathering. The processes
of deposition of the slope materials and the environmental
conditions under which they took place remain
equally debatable. While various types of slope processes
have been suggested for the lower slope deposits, block-
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