ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

MARK E. RUSE & MR. PEART
The spatial variability of fallout Caesium-137
in Hong Kong
Department of Geography and Geology, Hui Oi Chow Building,
University of Hong Kong, Pokfulam Road, Hong Kong
An understanding of hillslope sediment movement is important
both to theoretical and to applied geomorphology.
The standard approach to its study includes long-term monitoring
of sediment yield and erosion rates from drainage
basins or controlled erosion-plots. Another technique is to
use the fallout radionuclide Caesium-137 C 37 Cs) as a tracer
of sediment movement. This provides useful data aggregated
for a period of around 40 years without the need for time-consuming
monitoring.
The use of 137CS in this way depends substantially on a valid
assessment of an input reference value. This issue has
gained particular importance as recent work suggests the
spatial distribution of 137Cs in the landscape is more variable
than previously assumed. Consequently there is a need
for research into the range of 137Cs values both at input sites
and between different sites. Better still would be an improved
understanding of the controls on 137Cs behaviour in
the landscape and of the influence of differing environmental
characteristics.
To these ends the present research has investigated 137Cs
tracer input sites in the monsoonal tropics. Hong Kong is a
good location to test the variability of 137Cs in the landscape
because of the large range of mean rainfall totals (ca.
1500 to 3000 mm per annum) within a small area (ca. 1000
krn'). As rainfall controls 137Cs fallout from the atmosphere,
137Cs input totals are expected to reflect this pattern. The
paucity of 137Cs studies in the tropics coupled with tropical
air circulation patterns cast some doubt on the possibility
of using 137Cs as a tracer in such areas. Preliminary assessments
show that the method is applicable in the Hong
Kong environment.
Ten input sites within the Territory have been chosen to
reflect different rainfall totals. Qualification of the uneroded
status of the input sites was made by assessing a profile
of 137Cs totals in 2 em increments, the form of the profile
reflecting the movement of 137Cs in the soil. Results from
some other locations confirmed the difficulty of selecting
input sites that have experienced no significant sediment
movement in the past four decades.
Some basic soil parameters of the profile samples were also
evaluated. This permitted a correlation assessment of controIs
on 137Cs behaviour. This includes the ability of the soil
to fix 137Cs and subsequently the ability of natural processes
to move it.
Another major component of the research was to assess
the variability of 137Cs totals across the input sites. Ten core
samples have been taken at each input site to characterise
the variability of 137Cs within a small area. This permitted
an evaluation of sampling methodology, such as the number
of samples needed to make an accurate assessment of
input totals. Such data are essential if the technique is to
be useful to environmental managers.
ANDREW 1. RUSSELL1, OSKAR KNUDSEN 2,
JUDITH K. MAIZELS 3 & PHILIP M. MARREN 1
Controls on the geomorphic impact of the
November 1996 jokulhlaup, Skei8ararsandur, Iceland
1 Department of Earth Sciences, Keele University, Keele,
Staffordshire, ST5 5BG, UK
2 Iceland Meteorological Office, BustaGavegi 9,
IS-150 Reykjavik, Iceland
3 Department of Geology and Geophysics, Grant Institute,
University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW
This paper examines the spectacular geomorphic and sedimentary
impact of the November 1996 jokulhlaup on the
proximal zone of the Skeidarrirsandur, Iceland. Detailed
comparison of pre- and post-jokulhlaup river morphology
and sedimentology has allowed an assessment of the main
controls on the geomorphic effectiveness, of this event in a
variety of different ice-marginal and proglacial settings.
A volcanic eruption beneath the Vatnajokull ice cap began
on September 29, 1996. Over the next month some 3.2
km' of meltwater travelled subglacially into the Grimsvotn
caldera, raising the subglacial lake to a critical level. The
jokulhlaup began on the most easterly outlet river, the
Skei8ara, at 0800h on November 5, and reached a reported
peak discharge of 45000 m's" within 12 hours. The jokulhlaup
also burst from numerous other locations, including
single conduit outlets and crevasses up to 2 km in length.
Most outlets were in new locations, although some existing
outlets were occupied and enlarged. On exiting the glacier,
jokulhlaup flows coalesced into four other main outlets,
each",successively occupied from east to west: namely,
Skei8ara, Saeluhusavatn, Gigjukvisl and Sula.
Specific flood powers are estimated to have reached 8000­
16000 Wm- 2 at the moraine constriction on the Gigjukvisl,
decreasing to 400-900 Wm- 2 within 2 km downstream.
Flood flows on the Gigjukvisl initially included large debris
lobes that surged from the glacier margin at velocities
up to 6 ms", However, the main flood was dominated by
watery, turbulent runoff. At flood peak, 60-100 x 10 6m3 of
water was temporarily stored in a backwater lake upstream
of the Gigjukvisl moraine constriction. Vast numbers of ice
blocks, many over 10m in diameter, were transported by
floating, rolling, and sliding, depending on their sediment
content and density, many reaching the offshore zone, 24
km downstream. Huge volumes of sediment were transported
by the jokulhlaup resulting in all extensive sediment
plume offshore of the Skei8ara, although only relatively
small sizes were transported, especially in the Gigjukvisl
channel.
Grounded ice blocks, especially those forming large clusters
on the highest bar surfaces, acted as major sources of
resistance to the flows, equivalent in effect to those imposed
by boulders of similar dimensions. The backwater zone
played a crucial role in (i) delaying and attenuating the
flood hydrograph, and thereby reducing peak flow competence;
(ii) acting as a sediment trap, thereby releasing mo-
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