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| Titel |
Understanding sediment sources in a peri-urban Mediterranean catchment using
geochemical tracers |
| VerfasserIn |
Carla Ferreira, Rory Walsh, Ryunosuke Kikuchi, Will Blake |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122174
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| Publikation (Nr.) |
 EGU/EGU2016-1134.pdf |
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| Zusammenfassung |
| One of the main physical environmental impacts of urbanization is an increase in
suspended sediment concentrations and loads, particularly in the constructional phase.
Impacts in peri-urban catchments characterized by a mosaic of urban and non-urban
landscape elements with varying roles in acting as sources and sinks of overland
flow and slope wash have received little attention, particularly in Mediterranean
environments.
The present study uses a sediment ’fingerprinting’ approach to determine the
main sediment sources in the peri-urban Ribeira dos Covões catchment (6.2km2) in
Portugal and how they change during storm events following contrasting antecedent
weather. The catchment, rural until 1972, underwent discontinuous urbanization in
1973-1993, followed by an urban consolidation phase. Currently, its land-use is
a complex mosaic of woodland (56%), urban (40%) and agricultural (4%) land
parcels. Distinct urban patterns include some well-defined urban residential centres,
but also areas of discontinuous urban sprawl. Since 2010, a major road was built
and an enterprise park has been under construction, covering 1% and 5% of the
catchment, respectively. The catchment has a Mediterranean climate. The geology
comprises sandstone (56%), limestone (41%) and alluvial deposits (3%). Soils
are generally deep (>3.0m), but shallow (<0.4m) on steeper limestone terrain.
The catchment has an average slope of 9˚ , but includes steep slopes of up to 46˚
.
The sediment fingerprinting methodology involved characterizing the chemical properties
of sediments from individual upstream sub-catchments and comparing these to the properties
of downstream transported fluvial material. Three fine bed-sediment sampling surveys
were carried out after (i) a long dry period (21/09/2012), (ii) a winter storm of
relatively high rainfall intensity (23.2mm day−1) (19/02/2015), and (iii) after several
storms in Spring (22/04/2015). All samples were oven-dried (at 38˚ C) and sieved
to obtain different particle size fractions (0.125-2.000mm, 0.063-0.125mm and
<0.063mm). Seventeen stream sites were sampled plus a sample of sediment from a road
surface immediately it entered the stream network. The elemental composition (40
elements) of each size fraction was assessed using a Niton X-ray fluorescence elemental
analyser.
Results show that rock type has a profound influence on the geochemical properties of
bed-sediments. Catchment outlet sediment collected after the summer and a storm of
high rainfall intensity following dry weather displayed geochemical properties
closer to those of sediment from sandstone sub-catchments, and in particularly
sediment from the enterprise park under construction. After the storm that followed
very wet weather, however, limestone areas became of much greater significance
as sediment sources, probably because of the high soil saturation. At limestone
stream sites receiving runoff from the newly constructed road, fine bed-sediment
geochemistry was found to be similar to that of road sediment, indicating a high
contribution of this source. These results are supported by spatio-temporal differences in
streamflow and suspended sediment concentrations at instrumented monitoring
stations.
It is concluded that this methodology represents a potentially useful tool to enable river
managers to detect and assess sediment sources in urbanized and partly urbanized
catchments, and to supporting them in designing and implementing effective land-use
mosaics and site-specific measures to mitigate erosion. |
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