261
lagoon ecosystem before ocean swells and tsunamis, as was demonstrated with the
far field tsunami of Japan 2011, the ocean swell in August 2015 and the tsunami
occurred in September 2015. In the case of El Yali, coastal dunes were doubly
affected: first, by the impact and second, by the creeping mechanism; water eroded
a volume of sediments from the area which was no quantified but estimated as significant when comparing the current dune field with pre-tsunami images. However,
when analyzing historic geographic charts from the Military Geographic Institute
and the recent evolution of the dune field, it is possible to appreciate the instability
and fragility of the system since 1923 until present days (Fig. 8.11). The coastal
lagoon is located where El Yali brook mouth used to be. This mouth moved to the
south. Presumably, in many opportunities during the past these dunes were destroyed
by intense tsunamis and/or ocean swells. However, they were reconstructed with the
important sediment load from Rapel River. After Rapel dam was constructed (which
allowed damming Rapel river in 1968), imposing a definitive cutoff of this continuous sand supply, the current situation suggests that this system lacks its natural
regenerative capacity or requires a longer time to recover, since the volume of sediments that Rapel river used to supply experienced a sharp decrease after the dam.
Rapel hydroelectric central (34°02′S; 71°35′W) consists of a concrete vault
which upper part has a curvature radius of 174 m and 350 m long. The wall height
is 112 m and the dam can content 832 million m
3
, with a maximum depth of 90 m
(Vila et al. 1986). The importance of this reservoir for El Yali wetland arises because
the material forming the dunes and the long beach surrounding it corresponds to
material originally coming from Maipo River (Paskoff and Manríquez 2004).
Because the river was dammed, this process does not longer occur in a natural fashion since 1968 (Paskoff et al. 2000).
The morphological evolution of the Albufera seems to react to the occurrence of
tsunamis that affect the wetland. For instance, when comparing Table 8.2 with
Fig. 8.11, it is observed that the most drastic changes in the shape of this wetland
coincidently occurred right after a tsunamigenic earthquake. The segmentation that
the Albufera suffered in 2010 was well documented (Contreras 2014), but also drastic changes in the shape of the river and the Albufera after tsunamis of 1960 and
1985 can still be observed. However, even when the tsunami of 1960 must have had
similar characteristics to the tsunami of 2010, Rapel River was not dammed yet and
therefore the beach and dunes recovered as rapidly as other similar systems, as
Mataquito river, did after the earthquake of 2010. In 1985, a new tsunami occurred
in this area and produced a 50 cm vertical co-seismic raising (Quezada et al. 2012b).
Somehow, this co-seismic raising, unlike the sinking observed in 2010, mitigated
the lack of sedimentary material discharged by Rapel River. On the other hand, due
to the high precipitations occurred before and after 1985, the dam gates were opened
in several occasions (Fig. 8.12.)
It is presumed that the poor recovery of the coastal dune and beach that used to
protect the wetland is related to the closure of the dam gates since 2009 (Fig. 8.12).
On the contrary, the recent recovery of this waterbody seems to respond to the opening of the dam gates, allowing supply of sedimentary material. The monthly evolution of Matanza lagoon and the Albufera surfaces is shown in Fig. 8.13.
8 El Yali National Reserve: A System of Coastal Wetlands in the Southern…
lagoon ecosystem before ocean swells and tsunamis, as was demonstrated with the
far field tsunami of Japan 2011, the ocean swell in August 2015 and the tsunami
occurred in September 2015. In the case of El Yali, coastal dunes were doubly
affected: first, by the impact and second, by the creeping mechanism; water eroded
a volume of sediments from the area which was no quantified but estimated as significant when comparing the current dune field with pre-tsunami images. However,
when analyzing historic geographic charts from the Military Geographic Institute
and the recent evolution of the dune field, it is possible to appreciate the instability
and fragility of the system since 1923 until present days (Fig. 8.11). The coastal
lagoon is located where El Yali brook mouth used to be. This mouth moved to the
south. Presumably, in many opportunities during the past these dunes were destroyed
by intense tsunamis and/or ocean swells. However, they were reconstructed with the
important sediment load from Rapel River. After Rapel dam was constructed (which
allowed damming Rapel river in 1968), imposing a definitive cutoff of this continuous sand supply, the current situation suggests that this system lacks its natural
regenerative capacity or requires a longer time to recover, since the volume of sediments that Rapel river used to supply experienced a sharp decrease after the dam.
Rapel hydroelectric central (34°02′S; 71°35′W) consists of a concrete vault
which upper part has a curvature radius of 174 m and 350 m long. The wall height
is 112 m and the dam can content 832 million m
3
, with a maximum depth of 90 m
(Vila et al. 1986). The importance of this reservoir for El Yali wetland arises because
the material forming the dunes and the long beach surrounding it corresponds to
material originally coming from Maipo River (Paskoff and Manríquez 2004).
Because the river was dammed, this process does not longer occur in a natural fashion since 1968 (Paskoff et al. 2000).
The morphological evolution of the Albufera seems to react to the occurrence of
tsunamis that affect the wetland. For instance, when comparing Table 8.2 with
Fig. 8.11, it is observed that the most drastic changes in the shape of this wetland
coincidently occurred right after a tsunamigenic earthquake. The segmentation that
the Albufera suffered in 2010 was well documented (Contreras 2014), but also drastic changes in the shape of the river and the Albufera after tsunamis of 1960 and
1985 can still be observed. However, even when the tsunami of 1960 must have had
similar characteristics to the tsunami of 2010, Rapel River was not dammed yet and
therefore the beach and dunes recovered as rapidly as other similar systems, as
Mataquito river, did after the earthquake of 2010. In 1985, a new tsunami occurred
in this area and produced a 50 cm vertical co-seismic raising (Quezada et al. 2012b).
Somehow, this co-seismic raising, unlike the sinking observed in 2010, mitigated
the lack of sedimentary material discharged by Rapel River. On the other hand, due
to the high precipitations occurred before and after 1985, the dam gates were opened
in several occasions (Fig. 8.12.)
It is presumed that the poor recovery of the coastal dune and beach that used to
protect the wetland is related to the closure of the dam gates since 2009 (Fig. 8.12).
On the contrary, the recent recovery of this waterbody seems to respond to the opening of the dam gates, allowing supply of sedimentary material. The monthly evolution of Matanza lagoon and the Albufera surfaces is shown in Fig. 8.13.
8 El Yali National Reserve: A System of Coastal Wetlands in the Southern…