Abstract
Anthropic interventions at catchment level modify the hydrological
morphology and fluxes, especially in the last 50-60 years. In the
North-Eastern Romania hilly area for various reasons (the dry conditions
- 400-500 mm multiannual rainfall amount – and the need for water, for
wetland to arable land conversion and for flooding control), especially
in the last 60 years, a big number of reservoirs were built (about 1000
in an 8000 km2 area). These reservoirs were mostly shallow (under 5 m
depth) and small in volume (under 10 mil m3) and were filled in 10 to 30
years. The decommissioning of these reservoirs was made mainly through
dam breaching in the median part in order to recover the filed reservoir
bottom for pastures. This intervention created the conditions of channel
incision of these flat surfaces and the development of gully system
which tend to evolve toward continuous channels. In the present study we
present a database of gullies affecting the dry bottom of decommissioned
anthropic lakes from Jijia Hills, north-eastern Romania. The gullies
were manually extracted from a Digital Terrain Model (DTM) with 0.5 m
cell size. The extraction was performed using shading maps, with support
from contour lines maps, slope maps and maps with the edge detection of
slope. The gully delineation was also verified through 3D perspective
view and through topographic sections, in order to obtain good
delineation. We delineated more than 500 gullies, which are
predominantly found on dry bottom reservoirs of 2d and 3rd Strahler
order catchments. The morphometry of the gullies (length, depth, volume)
is directly related to the dimension of the filled reservoir. Thus,
there are lengths of 1-2 m for newly created gullies, and up to 800-1000
m for the most developed ones, with big variations of the width as a
function of the shape of the gully channel and the evolution stage of
gully (newly incised, evolving, fully developed). The depth of gullies
ranges from 20-30 cm to 4-5 m and depends on the initial size of the
reservoir, the depth of sediment fill and on the dam height. The volume
of eroded sediments varies from 20 m3 to up to 70 000 m3 for individual
gullies, with a total volume of eroded deposits over 1,000,000 m3.
Considering the resulted volumes these types of gullies are one of the
most important sediment sources in Jijia Hills region. The bottom
reservoir gullies inventory and the estimated sediment volumes provide a
database of hot-spots of sediment sources in the north-eastern Romania
lowland. This aspect has a great practical importance considering that
the majority of these gullies are not fully developed, and their further
evolution implies the production of important quantities of sediments in
the fluvial system. Furthermore, the same study area is defined by the
presence of many dry abandoned reservoirs where the studied types of
gullies can appear in the near future in the context of climate change
that will increase the torrential rainfall. The presented study is a
case of anthropic induced morphology and sediments accumulation (the
filled reservoirs) which, through further induced anthropic
decommissioning, generated gully morphology and produced sediments
through erosion.