Theoretical Background - Response of Flood Events to Land Use and Climate Change - page 33

Table 2.3 (continued)

Years Changes Source

After 1950 Spatial fluctuation of forest-channel borders occurred. [ 141 ]

1957 Flood 1957 impacts had been largely attenuated below the braided section upstream from

Barcelonnette. The exceptional dimensions of the floodplain in this sector greatly contributed to
attenuating the flood, favoring bed load deposition. Thus, the millennial flood caused
comparatively little damage to the riparian vegetation in the study area, excepting the upstream
sector which is characterized by specific changes. Conversely, dykes downstream were
destabilized by a 0.5-1 m bed incision. The 1957 flood also reduced channel slope and hydraulic
power upstream from Barcelonnette, which explains the adjustment of vegetation and bed
geometry over the ensuing decades. These adjustments are ongoing at this site, whereas they
have not been observed since 1982 downstream from Barcelonnette.

[ 143 ]

1948-1990 In fact, sector analysis from 1948 to 1990 shows a complex geographical evolution. The forested

areas were almost stable as tree units went from 209 ha to 196 ha show a slight reduction of 6 %
(Fig. 2.10 ). The active channel was also restricted during this period mostly due to human
installations developed in the corridor downstream from Barcelonnette.

[ 141 ]

1970-1990 In contrast the 1970-1990 periods is characterized by reduced peak flow and vegetative

recolonization (+5 %) of the active channel (Fig. 2.11 ).

[ 141 ]

The 1973-1990 The sector located upstream from Barcelonnette showed major continuous vegetation colonization of

the active channel. Whereas downstream, these changes were discontinuous and were only
apparent between 1973 and 1982. The forest progression and additional restriction of the active
channel (Fig. 2.12 ). The forest gained only 9 ha, whereas the active channel lost 40 ha, as a
result of human modification (water treatment stations, camping grounds, and public dump sites).

[ 141 ]

1982 and 1990 A great part of the active channel was colonized by pioneer units. These units were observed in the

1982 aerial photographs, (+19 ha).

[ 141 ]

Early twentieth century Changes were observed in the riparian forest active channel contacts over shorter time periods and in

smaller areas. Forest covered only 10.3 % of the watershed in the early nineteenth century, but
from twentieth century, occupies 33 % of the watershed area. This recolonization has affected all
the local districts, as 70 % of them have more than doubled in forested area (Figs. 2.13 , 2.14 ).

[ 141 ]

(continued)