Electronic Material Properties - und Geowissenschaften ...

granite/gneiss basement rocks in the south and west are highly erodible due to deep
weathering and low infiltration capacity, the overlying carbonate rocks outcropping in the
north and east are less prone to erosion due to better soil structure and rapid infiltration
in the karst system which prevents excessive overland flow (Fig. 2). Around the lake of
Lagoa Santa, the Neoproterozoic carbonate rocks are overlain by siltite rocks with a
very low infiltration capacity and weak soil and saprolite cohesion. In these highly
vulnerable hills, many gullies can be observed already, most but not all of them related
to urban areas. On the mountain tops in the middle of the study area, laterite crusts
protect the underlying soils from erosion and thus these areas are assigned low
erodibility values.
Fig. 2: Erosivity of the study area, based on
lithology and the existence of laterite crusts.
Fig. 3: Erosion hazard of the study area
derived by overlay of erosivity and topographic
index.
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A map showing the natural erosion
hazard in the area has been created
by overlying the erodibility map with a
map showing a topographic index
based on the length slope factor used
in the universal soil loss equation. The
resulting map (Fig. 3) highlights the
areas of the steep siltite hills around
the city of Lagoa Santa in the east and
the hilly landscape on Archean
gneiss/granite rocks in the south and
west. In an ongoing study existing
gullies in Lagoa Santa are evaluated
with regard to their catchment area,
steepness, flow concentration along
roads and the amount of sealed area
in the catchment in order to estimate
local threshold values for gullying.
Additionally to regional erosion hazard
estimation, a local case study on land
use change due to sand and gravel
extraction and urbanisation in the
alluvium of the Riberão da Mata has
been performed. This area has been
subject to sand, clay and gravel
extraction during the last decades, and
many of the small scale mining sites
have been simply left open after the
extraction had been banned due to the
Brazilian legislation that puts a buffer
zone along rivers under protection or
as sand extraction at low costs above
the groundwater level was not viable
anymore. Still, it is one of the few
areas close to the city where sand and
gravel for construction purposes can
be found. A visual analysis of aerial
photos and satellite images covering