Environmental Engineering Reference
In-Depth Information
Figure 17.4 An rill erosion network established in tephra deposits around Ol
Doinyo Lengai volcano, Tanzania. Image by Tom Pfeiffer /
www.volcanodiscov-
ery.com
(
©
).
channel (rill) networks (
Figure 17.4
; Collins and Dunne,
1986
), which stabilize to
fewer and deeper master rills with time (Manville
et al
.,
2009
). Slope erosion
processes lead to the preferential denudation of tephra deposited on steep topog-
raphy, leading to the concentration of ash in valleys and the choking of freshwater
lakes and rivers. This can lead to changes in river courses for systems as big as
the Blue Nile (Adamson
et al
.,
1982
) and can lead to catastrophic break-out
oods
if catchments become dammed (
Figure 17.3H
; Manville,
2002
).
17.2.4 Ecosystem recovery
The recovery of ecosystems depends on the severity of the ash deposition. Recov-
ery of
flora will be slower at higher latitudes due to the shorter growing period.
Studies of the 74 ka Younger Toba Tuff deposits in India (see
Figure 17.1
)
indicated that tephra remained mobile for several years prior to burial and
re-vegetation (Jones,
2010
). Ecological recovery after the complete destruction
of proximal vegetation from the 1883 eruption of Krakatau (Indonesia) took
15 years to re-establish trees and shrubs, and considerably longer to return to pre-
eruption conditions (Bush,
2006
;
Figure 17.5
). The re-establishment of pioneer
species requires seeding by wind or animal transport, so in cases where large
areas are completely deforested then the recovery of
flora must begin at the
ash-blanket edges. The speed of recovery can be further restricted or even halted
by feeding insects (Fagan and Bishop,
2000
; Knight and Chase,
2005
).
In extreme cases where ecosystems are particularly vulnerable the recovery time
can take millennia (Kilian
et al
.,
2006
).
