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| Titel |
Short-term vs. medium-term monitoring for detecting gully-erosion variability in a Mediterranean environment: Addressing the time-scale problem in gully dynamics |
| VerfasserIn |
Irene Marzolff, Johannes B. Ries, Jean Poesen |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055585
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| Zusammenfassung |
| Gullies are amongst the most important sediment sources in small to medium-scale
catchments and show highly distinctive spatio-temporal dynamics. This study investigates
how medium-term gully-development data differ from short-term data, and which factors
influence the spatial and temporal variability of gully development using 9 selected actively
retreating bank gullies situated in Spanish basin landscapes (the Ebro, Guadalentín, Baza and
Guadix Basins).
Small-format aerial photography using unmanned, remote-controlled platforms was taken
at the gully sites in short-term intervals of usually 1 or 2 years over medium-term periods of
7-13 years between 1995 and 2009. This enabled to document the erosion processes at gully
headcuts and sidewalls with the spatial and temporal resolutions corresponding
to the process magnitude and frequency. Gully change during each monitoring
period was determined using photogrammetry and GIS software, and various linear,
areal and volumetric parameters of gully retreat were computed for each gully and
monitoring period. Results show a high variability of annual gully retreat rates
both between individual gullies and between observation periods. The mean linear
headcut retreat rates range between 0.02 and 0.26 m a-1. Gully area loss (approx. 0.8
to 22 m2 a-1) varies by a factor of 25 and gully volume loss (approx. 0.5 to 100
m3 a-1) by a factor of 200. For these, sidewall erosion may play a considerable
part: for some gullies, the ratio of headcut to sidewall volume change is as low as
0.5.
A non-linear relationship between catchment area and medium-term gully headcut
volume change was found for these gullies, confirming the result of earlier studies in the
same region. The short-term gully volume changes observed at the individual gullies show
very high variability: on average (median of all gullies), the maximum headcut change
observed in 7-13 years was 14.3 times larger than the minimum change, but the
degree of fluctuation varies strongly between the gullies. Not much difference in
variability could be found for the headcut and sidewall parts of the gullies, although
they clearly are subject to different erosion processes, and sidewall changes show
no or much less dependency on precipitation than headcut retreat. The varying
negative and positive influence of land-use and human activities – especially on
connectivity of surface flow to the headcut – appears to play a dominant role in these
study areas, both for short-term variability and medium-term difference in gully
development.
The study proves the value of capturing spatially continuous, high-resolution 3D data
using small-format aerial photography for detailed gully monitoring. Results confirm that
short-term data are not representative for longer-term gully development and demonstrate the
necessity of medium- to long-term monitoring. However, short-term data are still required to
understand the processes causing fluctuations in gully erosion rates. In particular, human
activity on varying time scales and its positive or negative effects on runoff production and
connectivity need to be considered as an important factor for gully-erosion variability. |
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