Environmental Engineering Reference
In-Depth Information
measures to minimize future disaster impacts and the proactive adoption of an array
of components, such as levees, sea walls, relocation to higher ground, well-considered
land use and building code enforcement, for a sustainable recovery in the Tohoku
region. To be specifi c, the Council presented fi ve schematics of future land use
patterns for future tsunami impact reduction, each refl ecting different geographical
and damage characteristics, with a mix of relocation, land raising, and levee strate-
gies. The land use patterns were offered for consideration in areas where: (1) urban
functions were totally destroyed in low elevation areas; (2) urban functions were
partially destroyed in low elevation areas, but others had been saved in higher
elevation areas; (3) urban functions are concentrated in fl atlands and surrounded
by hilly geography; (4) fl atland areas along the coast; and (5) inland areas that were
destroyed and experienced liquefaction.
Level one and two tsunami protection: Minimizing future tsunami damage.
The concept of level one (L1) and level two (L2) tsunamis and the ways to protect
inland assets and lives were also discussed and defi ned during this phase. First, the
frequency of different magnitude tsunamis was differentiated between L1 and L2
by the Japan Society of Civil Engineers (
2011
) and MLIT. Based on historic tsunamis
in the Tohoku region, a L1 tsunami was defi ned as an event occurring once every ten
to one hundred years (or having less than a one percent annualized probability of
occurrence), and an L2 tsunami was described as an event that happens once
every several hundred to one thousand years (or having a greater than one percent
annualized probability of occurrence). The tsunami caused by the GEJE was
classifi ed as a L2 tsunami since similar magnitude tsunamis had occurred with less
frequency historically than the L1 tsunamis. A policy consensus also emerged that
structural measures, i.e. levees, would be designed and built to defend and protect
land and people against L1 tsunamis, while non-structural measures, mainly land
use patterns and evacuation plans, would be implemented in addition to levee
defenses to secure human lives against L2 tsunamis.
Prefectures are the government level responsible for coastal management and
they had responsibility for deciding levee heights after the L1 and L2 concepts were
developed. In the end, Iwate Prefecture established levee heights for rebuilding in
24 bays, Miyagi Prefecture in 22 bays, and Fukushima Prefecture in 14 bays
(MLIT
2011
). Mainly, L1 tsunami heights were used as the basis for the decisions,
unless other historic storm surge heights were found to be higher than a L1 tsunami.
Levee heights in many bays turned out to be higher than the heights of levees that
existed prior to the GEJE. In some cases, the recommended levee heights were as
high as 15.5 m in Iwate Prefecture and as low as 2.4 m in Fukushima Prefecture
(MLIT
2011
).
Land use policies for rebuilding.
The Tohoku-wide damage assessment and land
use survey led by MLIT targeted 62 disaster affected local governments in six
prefectures and developed basic land use recommendations for rebuilding for 32
local coastal governments. National consultants were hired to support the local
governments in developing land use plans that incorporated the L1 and L2 tsunami
protection concepts, together with the spatial strategies envisioned in the national
recovery vision. The work mainly proceeded in two steps. First, the national consultants
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