NHESSNatural Hazards and Earth System SciencesNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus PublicationsGöttingen, Germany10.5194/nhess-19-299-2019Assessing the tsunami mitigation effectiveness of the planned Banda Aceh Outer
Ring Road (BORR), IndonesiaAssessing the tsunami mitigation effectiveness of the planned BORRSyamsidiksyamsidik@tdmrc.orgsyamsidik@unsyiah.ac.idhttps://orcid.org/0000-0002-0124-5822TursinaSuppasriAnawatAl'alaMusaLuthfiMumtazComfortLouise K.https://orcid.org/0000-0003-4411-1354Tsunami and Disaster
Mitigation Research Center (TDMRC), Syiah Kuala
University, Gampong Pie, Banda Aceh 23233,
IndonesiaInternational Research Institute of Disaster Science (IRIDeS), Tohoku
University, Aramaki Aza-Aoba 468-1, Aoba-ku, Sendai 980-0845, JapanCivil Engineering Department, Syiah Kuala
University, Jl. Syeh Abdurrauf No. 7, Banda Aceh 23111, IndonesiaCivil Engineering Department, Faculty of Engineering, Syiah Kuala
University, Jl. Syeh Abdurrauf No. 7, Banda Aceh 23111, IndonesiaGraduate School of Public International Affairs, University of
Pittsburgh, Pittsburgh, USASyamsidik (syamsidik@tdmrc.org, syamsidik@unsyiah.ac.id)31January201919129931224September201823October20185January20197January2019This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://nhess.copernicus.org/articles/19/299/2019/nhess-19-299-2019.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/19/299/2019/nhess-19-299-2019.pdf
This research aimed to assess the tsunami flow velocity and height
reduction produced by a planned elevated road parallel to the coast
of Banda Aceh, called the Banda Aceh Outer Ring Road (BORR). The road will
transect several lagoons, settlements, and bare land around the coast of
Banda Aceh. Beside its main function to reduce traffic congestion in the
city, the BORR is also proposed to reduce the impacts of future tsunamis. The Cornell
Multi-grid Coupled Tsunami Model (COMCOT) was used to simulate eight
scenarios of the tsunami. One of them was based on the 2004 Indian Ocean
tsunami. Two magnitudes of earthquake were used, that is, 8.5 and
9.15 Mw. Both the earthquakes were generated from the same
source location as in the 2004 case, around the Andaman Sea. Land use data of
the innermost layer of the simulation area were adopted based on the 2004
condition and the land use planning of the city for 2029. The results of this
study reveal that the tsunami inundation area can be reduced by about 9 %
by using the elevated road for the earthquake of magnitude 9.15 Mw and
about 22 % for the earthquake of magnitude 8.5 Mw. Combined with
the land use planning 2029, the elevated road could reduce the maximum flow
velocities behind the road by about 72 %. Notably, the proposed land use
for 2029 will not be sufficient to deliver any effects on the tsunami
mitigation without the elevated road structures. We recommend the city to
construct the elevated road as this could be part of the co-benefit
structures for tsunami mitigation. The proposed BORR appears to deliver a significant reduction of impacts of the smaller intensity tsunamis compared
to the 2004 Indian Ocean tsunami.
Introduction
Tsunami mitigation by means of structural measures is not always affordable
in the case of developing countries. In contrast, the threats posed by
tsunamis are real and have the potential to deliver severe impacts on the
coastal area and the community at risk. Banda Aceh is one of the most
severely affected cities due to the 2004 Indian Ocean tsunami; however, it is
difficult to follow the guidelines demonstrated by advanced countries
that develop massive physical structures to mitigate the future impacts
of tsunamis. This is still beyond the financial capacity of the city. In
contrast, based on the probabilistic tsunami hazard assessment, this area
could potentially be affected by a tsunami larger than 0.5 m, that is, about
10 % higher, annually (Horspool et al., 2014). Therefore, seeking
alternative and economic ways to mitigate the impacts of tsunamis could help
the city in creating a more resilient region. Modifying the morphology and
land use of the coastal front of the area can reduce the tsunami wave energy
(Ohta et al., 2013). The nonlinearity effects generated on the inland tsunami
wave run-up are closely related to the local topography of the area (Mori et
al., 2017). The key parameters of reducing damages due to tsunami waves are
decreasing the wave velocity and the inundation depths (Kreibich et al.,
2009; Yamamoto et al., 2006). These are better represented by a quadratic
Froude number (Fr2) (Ozer and Yalciner, 2011). Constructing a
high seawall is costly. In the case of Banda Aceh, the estimated maximum
tsunami height based on the 2004 Indian Ocean tsunami was 15 m (Lavigne et
al., 2009). Only the sea walls higher than 5 m could contribute to reducing
the destructive effects of tsunamis as in the case of the 2011 Tohoku tsunami
(Nateghi et al., 2016). The cost of the structure is unarguably expensive.
Furthermore, the tsunami wave has long-wave characteristics, whereby blocking
the wave will only indicate the delayed time of the wave to reach a certain
area behind the seawall due to the scouring process (Chen et al., 2016).
Inequalities of the hydrostatic forces generated around the seawalls and the
process of overflowing will occur and destroy the structures (Ozer et al.,
2015); however, this could reduce the tsunami wave energy (Guler et al.,
2018).
Another way to reduce the tsunami wave energy is by using an elevated road.
The elevated road can function as an inland tsunami defense structure
that could stop the tsunami wave or reduce its intensity as revealed in the
case of the 2011 Great East Japan Earthquake and Tsunami (GEJET; Goto et
al., 2012a). In the GEJET case, the Tobu highway in Sendai was the maximum
limit of the tsunami inundation area in Sendai of Miyagi Prefecture in Japan
(Abe et al., 2012; Goto et al., 2012b; Sugawara et al., 2012). A new 6 m
elevated road is now being constructed in Sendai, the idea of which was
adapted from the tsunami mitigation effects revealed by the Tobu highway
structure during the 2011 Tohoku tsunami (Suppasri et al., 2016). Japan is an
exemplary nation that promotes a tsunami multilayered defense system, either
by structural or nonstructural mitigation. A tsunami multi-defense system is a set of
structures to mitigate the impacts of tsunamis. The concept was introduced in
the Tohoku region of Japan during the rehabilitation and reconstruction process
following the 2011 tsunami. The structures consist of sea walls, coastal
forests, a canal that is parallel to the coastline, escape hills, and elevated
roads. Concerning structural mitigation, the GEJET-affected areas have been
developing several massive structures to prevent future tsunami losses
(Strusinska-Correira, 2017; Koshimura et al., 2014; Pakoksung et al., 2018).
As a result of its population and economic growth, Banda Aceh is planning to
construct a road transect as a response to the traffic demands of the city.
One of the most recently introduced plans is a road that will circle the city
from its periphery. The proposed road is named the Banda Aceh Outer Ring Road
(BORR). Initially, the road was only introduced by a road transect and the
detailed structure of the road is yet to be decided. This is part of a
long-term development program as stipulated in its spatial planning that aims
to regulate the city planning until 2029 (Government of Banda Aceh, 2009). In
the spatial planning, no new significant tsunami mitigation infrastructure
has been included. The structural mitigation facilities that were developed
between 2005 and 2010 include four escape buildings, one tsunami museum that
also functioned as a tsunami escape building, and several escape routes.
A 7 km revetment structure was constructed between 2006 and 2010 to reshape
the city's coastline and to prevent further coastal erosion problems
(Syamsidik et al., 2015).
To address the gap as stated earlier, this research aimed to investigate the
reduction of potential tsunami destructive impacts through an elevated road
structure parallel to the coastline of Banda Aceh (BORR). The Cornell Multi-grid
Coupled Tsunami Model (COMCOT), a two-dimensional horizontal model, was
utilized to numerically simulate the tsunami characteristics as well as to
evaluate the reduction impacts of BORR. Two types of land use maps in 2004
and 2029 were used to evaluate the mitigation effect of future tsunamis. The
evaluation of the performance of the elevated road to reduce the tsunami
wave energy may contribute to a better city planning of Banda Aceh in a
long-term development program.
The study area. The elevated road (yellow line) is part of the city's
development planning documents. There are 139 points of the 2004 tsunami
heights measured by NOAA (red dots) (NOAA, 2018) and 56 locations of tsunami
poles representing water marks based on eyewitness accounts (yellow
triangles).
The study area
Banda Aceh is situated in the northern part of the island of Sumatra and is the
largest city in Aceh Province. Figure 1 presents the city location. The
topography of the city is flat with no hilly regions. The nearest hilly region is
located around 7 km outside the city's borders. There are several coastal
lagoons situated in the northern part of the city. The city was severely
damaged by the 2004 Indian Ocean tsunami, which caused the deaths of about
90 000 people (Doocy et al., 2007). Prior to the 2004 tsunami, no knowledge
was available regarding the potential tsunami that resulted in zero
prevention of the hazard. During the rehabilitation and reconstruction
process led by Aceh-Nias Rehabilitation and Reconstruction Agency
(BRR Aceh-Nias), the city faced serious challenges in relocating its
people to a safer area. This resulted in several houses being built in the
coastal area. Initially, it was proposed that about 500 m is left from its
coastal line to any settlement, with the aim of using this space for coastal vegetation as a
part of the tsunami mitigation; this was named the “green belt” area. This was
mentioned in the Master Plan for Rehabilitation and Reconstruction composed
by the Indonesia Development and Planning Agency (BAPPENAS, 2005). The 14-year rehabilitation and reconstruction process until 2018 has
failed to make it happen.
At present, the coastal population of the city is growing significantly due
to return migration from the affected community and more affordable land
prices and house rent fees in the coastal area compared to other places in the
city (Syamsidik et al., 2017). Figure 1 presents the study area of this
research. In Fig. 1, several tsunami flow depth data for this city, published
by NOAA, are presented by red dots (NOAA, 2018); data from Tsuji et al. (2006) are
indicated by blue dots. Tsunami poles are represented by yellow triangles. These flow depths were later incorporated in the tsunami
numerical results' validation. Figure 2 presents the conditions of the coastal
area of Banda Aceh based on an aerial image captured by a drone in February
2018. There is a 7 km revetment structure constructed along the city coast
to immediately recover the eroded coastline and to create a barrier between
the sea and ponds. The revetment was completed in 2010. Later in 2015, the
government constructed a road transect at the leeward of the revetment;
however, since the revetment is often being overtopped by waves, the road is
frequently damaged by the waves. Figure 3 presents the revetment structure
and the road behind the revetment.
The situation of the coastal area of Banda Aceh based on an aerial image
taken in February 2018.
New spatial planning and regulation of the city was released in 2009. This
was modified in 2012 to accommodate the tsunami reconstruction process along
with a few ideas to mitigate the impacts of disasters such as tsunamis; however, no
concrete measures were included in the spatial planning document to
structurally mitigate the tsunami impacts.
Under the revision process of the spatial planning in 2012, the government of
Banda Aceh set up a new plan to construct a road due to the traffic
congestion in the city. The road was proposed to aid the mobility of people from the periphery of the city and was named the Banda Aceh Outer Ring
Road (BORR). The Japan International Cooperation Agency (JICA) has studied the project. At present, the road project has been put on hold due to an increase
in land prices, but it is still in the formal city development document. A series of discussions were conducted to include the tsunami
mitigation measures in the new planned road. There is an opportunity to
modify the design of the road to an elevated road. Some alternatives have
been drawn. One of the most intense discussions was to elevate the road to 3 m
from the initial ground; however, the impacts of the planned elevated road on
tsunami wave energy are not clear. The BORR transect is presented in Fig. 1.
The BORR will pass some area of salt marshes where ponds existed as the major
land use type before the tsunami. After the tsunami, large areas of the fishponds
were damaged and have never been recovered. In the new spatial planning
regulation of the city, the area will be kept as it is and only minor changes
are proposed.
A 7 km embankment along the coast of Banda Aceh and a road which was
constructed at its leeward side (date photo taken:
February 2018).
MethodsTsunami numerical simulations
To measure the impacts of the tsunami waves on the city, two scenarios of the
coastal morphology were considered, that is, (1) without BORR and (2) with
BORR. Tsunami simulations were performed using the Cornell Multi-grid Tsunami
Coupled Model (COMCOT). The COMCOT is a hydrostatic model that uses a leapfrog
finite difference method to solve the shallow water equations (SWEs) with a
staggered scheme. Both nonlinear and linear shallow water equations can
be selected in the model. COMCOT is a two-dimensional horizontal model that
calculates the depth-averaged velocities. The linear shallow water equations
in the spherical coordinate system used in COMCOT are as follows:
∂η∂t+1Rcosφ∂P∂ψ+∂∂φcosφQ=-∂h∂t,∂P∂t+ghRcosφ∂η∂ψ-fQ=0,∂Q∂t+ghR∂η∂φ+fP=0.
Meanwhile, for nonlinear shallow water equations, COMCOT applies the
following equations:
∂η∂t+1Rcos∅∂P∂ψ+∂∂∅cos∅Q=-∂h∂t,∂P∂t+1Rcos∅∂∂ψP2H+1R∂∂∅PQH+gHRcos∅∂η∂ψ-fQ+Fx=0,∂Q∂t+1Rcos∅∂∂ψPQH+1R∂∂∅Q2H+gHR∂η∂∅+fP+Fy=0,f=Ωsinφ,Fx=gn2H7/3PP2+Q21/2,Fy=gn2H7/3QP2+Q21/2,H=η+h.
Here, P is the volume fluxes in the x direction (east–west direction), which
is equal to hu, and Q is the volume fluxes in the y direction
(south–north direction), which is equal to hv, where h is the
depth at the grid to the mean sea level and (u,v) are the velocities in the x and y direction, respectively. Furthermore, η is the water
surface elevation, (φψ) are the latitude and longitude for the spherical coordinate system, R is the earth radius, g is gravitational
acceleration, and h is the water depth at the grid. The component of
-∂h/∂t denotes the effect of transient seafloor
motion; the Coriolis force coefficient due to the earth's rotation is
expressed as f. Meanwhile, Ω is for the rotation rate of the
earth; H is the total water depth. Fx and Fy represent the
bottom friction in the ψ and φ direction, respectively; and
n is Manning's roughness coefficient. A complete explanation of the COMCOT
module can be referred to in Wang (2009).
Six simulation layers and the size of the grids (written in each
layer) applied in COMCOT.
Information on the setup of the six layers for the COMCOT simulations.
LayerLatitudeLongitudeNumberRatioGridTimeManning's roughnessSWE typeof gridsize (m)step (s)coefficients10.188.1177218560.1noneLinear14.93102.82391192029280.05noneLinear1010034.0892.0538993309.330.017noneLinear8.9897.9845.270894.5131373103.110.006noneLinear6.69595.9955.595.141426334.370.002noneLinear5.6995.3965.51595.2352362311.50.001Variable Manning's roughnessNonlinear5.61595.378coefficients (see Table 3)Computational regions
We applied six layers of simulation domains, starting from Layer 1 that
covers the largest numerical domain including the tsunami source in the
Andaman Sea. The innermost layer was Layer 6 that encompasses the city of Banda Aceh and has the smallest size of the grid. The nested grid system also
allows us to include the nonlinear effects of the tsunami waves in the COMCOT
simulation. Details of the grid specification are listed in Table 1. All
layers in the simulation apply the spherical coordinate system. Fig. 4 presents
the simulation layers applied in this study.
Bathymetry data for Layers 1–4 were adopted from the GEBCO data with a resolution of 1 min for all scenarios. Meanwhile, for Layers 5 and 6, we
used the bathymetry data measured by the Geospatial Information Agency of
Indonesia for the case of the 2004 tsunami. For the scenarios of 2029, we used
the bathymetry data measured by the Aceh Public Works Department in
2007. Topography data measured by the Japan International Cooperation Agency
(JICA) in 2005 were used for land topography data. The data were later
updated by the Banda Aceh Development and Planning Agency. For the elevated
road, the topography data along the transect were altered to 5 m above mean sea level. The elevations were considered affordable in terms of the
construction cost for the city. The structure of the elevated road was
assumed to sustain the tsunami wave forces. For this, no scouring processes or altered
ground elevation were done due to the tsunami wave forces.
Initial wave forms of generated by the 9.15 Mw earthquake as proposed by
Koshimura et al. (2009) (a) and by a
hypothetical earthquake of 8.5 Mw(b).
Earthquake scenarios
We used two magnitudes of the earthquake in the simulations, that is,
magnitude 8.5 and 9.15 Mw. Based on the probabilistic tsunami
hazards' assessment, the magnitude 8.5 Mw could occur once in
about 200–300 years (Sengara et al., 2008; Suppasri et al., 2012a, b), or in
another study, it was said to have an exceedence return period of 100 years
(Burbidge et al., 2009). Here the 8.5 Mw earthquake has a
focal depth of 10 km, with a displacement of 8.3 m and dip and slip
angles of 8 and 110∘, respectively. The strike angle was set to
305∘ and the slip angle was 110∘. Furthermore, the
8.5 Mw fault scenario was set to give maximum impacts on the Banda
Aceh coast. Therefore, the location of the fault was moved along the fault
lines to find the right location to deliver the maximum impacts. The
magnitude 8.5 Mw was calculated as a single fault. Meanwhile,
the multifault method was adopted for 9.15 Mw. The fault details
of the 9.15 Mw followed Koshimura et al. (2009). Dimensions of
the rupture area were calculated using Wells and Coppersmith formulae (Wells
and Coppersmith, 1994). Deformation of the seafloor caused by the rupture
area was calculated following the formulae suggested by Masinha and Smylie
(1971) and Okada (1985). Initial sea surface levels as a result of earthquake
generation are presented in Fig. 5. The land use for Layer 6 was adopted
based on two conditions, that is, (1) land use of the city in 2004 before the
Indian Ocean tsunami and (2) land use of the city as in the Banda Aceh spatial
planning regulation for 2029. The impacts of the elevated road by imposing
two scenarios of the road were then compared, that is, (1) with BORR and
(2) without BORR.
Data and validation
There are eight simulations in total, as listed in Table 2. Validation of the
simulation was done by comparing the scenario no. 211 with the heights of
the tsunami inundation in Banda Aceh as marked by several tsunami poles in
the city (Sugimoto et al., 2010). We used Aida functions to validate the
numerical results (Aida, 1978) that are based on K and κ as
follows:
logK=1n∑i=1nlogKi,logκ=1n∑i=1nlogKi2-logK2,Ki=Hobs-iHsim-i,
where Hobs-i is the observed tsunami inundation height or
depth at point i and Hsim-i is
the tsunami inundation height or depth based on the simulation at point i.
The value of κ represents the variance of Ki. Meanwhile, K
represents the mean of Ki. Takeuchi et al. (2005) suggested that the
model results are in good agreement if 0.8≤K≤1.2 and κ≤1.60. Another study also suggests
that if the value of κ is valid based on the given criteria and the value of K is
slightly > 1.05, the results can also be classified as “good
enough” (Koshimura et al., 2009).
Variations in the land use were included by modifying the Manning roughness
coefficients based on land cover of the area. Table 3 presents the values of
the Manning coefficients included in the simulations as suggested by Li et
al. (2012). Distribution of the Manning coefficients used in the two types of
land use, that is, the 2004 and 2029 land use, is presented in Figs. 6 and 7,
respectively.
Manning's coefficients based on land cover of the area (Li et al.,
2012).
Land useManning's roughnesscoefficient (n)Coastal vegetation0.035Fish ponds0.017Building0.04Sea0.013Soil0.02
Distribution of Manning's coefficients used in the simulation for land
use types in 2004 (before the 2004 Indian Ocean tsunami).
ResultsValidation of the 2004 Indian Ocean tsunami
To validate the result, we used the 2004 Indian Ocean tsunami case with the land
use form adopted from the situation before the tsunami (without BORR) or
scenario no. 211 as listed in Table 2. Validations of the initial wave
forms and offshore tsunami wave propagation have been done in several studies
(Koshimura et al., 2009; Suppasri et al., 2011; Suppasri et al., 2010). The studies
used the water level around a transect in the Andaman Sea captured by the
Jason-1 satellite about 2 h after the 9.15 Mw earthquake on
26 December 2004. The agreement of the simulated offshore tsunami wave
heights was found to be good in the two aforementioned studies. For
the tsunami inundation heights and depths, the results of the validation are
presented in Table 4 using Aida parameters calculated based on Eqs. (11)–(13). Based on the results, we confirmed that the simulated
reports are in accordance with the observed data provided by the NOAA data
and tsunami poles in the city.
Distribution of Manning's coefficients used in simulations for land use
types as described in the Banda Aceh spatial planning regulation aimed to be
implemented until 2029.
The validation results of the simulation using Aida parameters for
scenario no. 211.
Model resultsAida parameters KkNOAA data (n=139)1.181.42Tsunami pole data (n=56)0.791.50Tsuji et al. (2006) (n=50)0.681.61Impacts of the elevated roads
Using the two magnitudes of earthquakes to generate tsunami waves, the
impacts of BORR were tested. The reduction of tsunami velocity due to
obstacles, both natural and man-made structures, has been proven correct by
previous research (e.g., Nandasena et al., 2012; Matsutomi and Okamoto,
2010). Sea walls as well as other types of onshore structures will reduce
the energy of the tsunami, mainly by reducing the wave's velocity. Froude numbers
will be reduced as the tsunamis hit natural barriers or other solid man-made
structures. The following section elucidates a series of comparisons of the
maximum wave run-up in Banda Aceh.
Comparisons between measured tsunami wave heights and simulation
results.
Comparison of maximum tsunami inundation depths generated by the 8.5 Mw earthquake
with conditions without BORR (a) and with BORR(b).
Maximum tsunami wave depths based on the 9.15 Mw earthquake
without BORR (a) and with BORR (b). The simulations are
shown for the land use type before the 2004 Indian Ocean tsunami.
Comparisons of tsunami inundation area based on the simulations.
MagnitudeLand use typeTotal area ofArea of% of total% ofofinundationinundationdecreasedecrease forearthquake(ha)deeper than 2 marea deeper(ha)than 2 m8.5 Mw2004 without BORR1591.73998.89-21.33-25.282004 with BORR1252.20746.382029 without BORR1553.03979.58-22.51-24.292029 with BORR1203.47741.679.15 Mw2004 without BORR4654.273722.60-8.60-43.022004 with BORR4254.172121.172029 without BORR4592.603561.26-9.66-44.092029 with BORR4148.911991.13Magnitude 8.5 Mw
The distribution of the tsunami flow depths caused by the 8.5 Mw earthquake is presented in Fig. 9. Due to the BORR structure, the area of the
inundation could be reduced by about 22 %. Table 5 provides comparisons
for all the scenarios for the tsunami inundation area. It is observed that the
impacts of the land use changes are not significant enough to further reduce the
tsunami inundation area. The 2029 land use, if combined with BORR, will only
further reduce the tsunami inundation area by about 1.2 %. The BORR
coupled with land use changes can reduce the inundation area that is deeper
than 2 m by about 25 %.
Figure 12 provides comparisons of the tsunami wave heights for the three
transects indicated in Fig. 1 that are relatively perpendicular to the
coastline. At all transects, we could observe that the magnitude
8.5 Mw could still generate tsunami heights of about 3 m along
the coastline. Tsunamis could cover the BORR structure, in particular, at the
area around transect B. Interestingly, the tsunami inundation area behind the
BORR structure at transect B is mostly located around the salt marsh area where
no inhabitants reside. At the other transects the tsunami waves could be
stopped by the BORR structure, provided that the structure can sustain the
stability test produced by the waves. Without BORR (scenarios no. 111 and
no. 112), the tsunami wave could reach about 2 km from the coastline as
presented in transect A (Fig. 12). With BORR (scenarios no. 121 and no. 122),
the tsunami run-up could be reduced to an area of about 0.8 km from the
coastline (see transect B in Fig. 12). For the area where the bridges are
located, the tsunami waves could travel about 6 km along the main rivers.
Considering that the river embankment is higher than 1.5 m from the original
soil surface, the tsunami wave along the river will be able to stay within the river's main channel. At present, the river embankment along the city
is 3 m above the soil surface as a result of several projects undertaken
between 1989 and 1992.
This was also proven true in the case of dike impacts on reducing the
tsunami wave heights during the 2011 Tohoku earthquake and tsunami in the city of Ishinomaki in Japan (Takagi and Bricker, 2014). Interestingly, the
inland structures as represented by the elevated road managed to stop the
tsunami inundation. This was possible as the elevated road could reduce the
velocity of the tsunami wave.
Maximum tsunami wave depths based on the 9.15 Mw earthquake
without BORR (a) and with BORR (b). The simulations are based on the 2029
land use planning of Banda Aceh.
Maximum velocities after the elevated road structures.
TransectMax. velocities for 8.5 Mw (m s-1) Max. velocities for 9.15 Mw (m s-1) Without BORR With BORR Without BORR With BORR 20042029200420292004202920042029A4.523.800.000.004.854.982.851.40B3.201.300.250.224.425.151.901.51C0.450.410.000.004.924.602.351.25Magnitude 9.15 Mw
Figure 10 presents a comparison of the maximum tsunami inundation depths
based on the land use types as in the conditions before the 2004 Indian Ocean
tsunami without BORR to the conditions with BORR for the earthquake of magnitude 9.15 Mw. The comparison clearly indicates the changes made by
the BORR in terms of tsunami inundation depths. In front of BORR, the tsunami
waves could be higher compared to the landward area of the road. In contrast,
the tsunami inundation area could be 8.60 % smaller if the road were
constructed.
Similar effects of the BORR structures on the distribution of tsunami wave
depths are presented in Fig. 11 for the 2029 land use planning. Using the
2029 planned land use types with BORR, the wave heights could be decreased in the area behind the road. In contrast, if we compare
Fig. 10a and Fig. 11a, we notice that the impacts of changing land use
types as is planned for 2029 will not have any significant difference in
terms of the tsunami inundation depths and areas. Therefore, the changes of
the land use alone are not sufficient to reduce the adverse impacts of the
tsunami waves if the magnitude of the earthquake is 9.15. BORR coupled with
the land use change (for 2029) could reduce the tsunami inundation area by
about 9.7 %.
The comparison of tsunami wave heights at transects A, B, and C
based on scenarios with and without BORR.
All observed transects reveal similar effects of the BORR on the maximum
inundation depths. The depths could be decreased after the BORR structure.
Just at the leeside of the BORR structure, the depths will be decreased to
about 4 m with the structure for the earthquake of magnitude
9.15 Mw. This is about 28.5 % lower than the situation
without BORR. Figure 12 presents the comparison of maximum tsunami inundation
depths for all the simulation scenarios for transects A, B, and C.
Maximum wave velocities in the area behind the proposed elevated road are
listed in Table 6. The results proved that the structure could significantly
stop the tsunami in the case of the earthquake of magnitude 8.5 Mw.
For the earthquake of magnitude 9.15 Mw, the maximum velocities
can be reduced by about 50 % provided the land use is still the same as
in 2004 and about 72 % if the land use for 2029 is implemented. Therefore,
the modification of the land use combined with the BORR structure could
potentially reduce the damage of the tsunami waves by about 22 % compared to the land use of 2004.
Discussions and limitations of the study
The effects of elevated roads to limit tsunami inundation, demonstrated in
the case of the 2011 Great East Japan tsunami, has inspired this research for
Banda Aceh. This city was severely damaged by the 2004 Indian Ocean
tsunami. The inland structures and modification of the land use could help
mitigate the impacts of tsunami waves. In our study, the proposed elevated
road (BORR), planned to be constructed in Banda Aceh, which will be
relatively parallel to the coastline, is expected to reduce the tsunami wave
energy. This research found that the elevated road could effectively mitigate
the tsunami generated by the earthquake of magnitudes 8.5 and
9.15 Mw, generated around the Andaman Sea, with different
percentages of reduction. The larger the magnitude of the earthquake, the less
effective the reduction in the tsunami wave energy through BORR
coupled with land use changes will be. As land use is a dynamic variable, it is
important to note that certain land use controls to ensure the effectiveness of tsunami
reduction are necessary.
Based on the land use plan of Banda Aceh for 2029, the city will reclaim
a certain area around the coastal lagoons amd salt marshes and will preserve some
areas of the lagoons as they are at present (salt marshes with mangrove forest).
The lagoons play a significant role as they function as dug pools behind
the revetment structures. In the case of overflow, the lagoons have the
potential to reduce the tsunami wave energy, similar to that observed in
the Teizan canal of the Tohoku area during the 2011 tsunami (Tokida and Tanimoto,
2014). The mangrove forests are also crucial for the reduction of the energy of
tsunami waves, as proven by several research studies (see Yanagisawa et al., 2009;
Iimura and Tanaka, 2012; Tanaka et al., 2014; Strusińska-Correia et al.,
2013). In the case of an inland embankment structure (such as the BORR structure
in this study), the seaward coastal forest can reduce the possibility of
overflow events. Furthermore, the landward forest could reduce the drag force
behind the forest (Igarashi and Tanaka, 2018) and stop the tsunami debris.
Therefore, it is important to preserve the area of mangrove forests and
salt marshes.
Tsunami wave heights, as high as 3 m, can be reduced up to 1.5 m behind the
structure of the elevated road, provided that the road structure is not
breached. The concept of elevating the road to help mitigate impacts of the
tsunami could be regarded as a concept of co-beneficial development, simultaneously
integrating the traffic demands and tsunami mitigation. A similar concept was
observed in Sri Lanka to check the possibilities of elevating the train railway
to use an embankment type of railway to reduce the intensity of tsunamis
(Samarasekara et al., 2017). Adopting the principles of tsunami mitigation in
the existing plan of the structure could also derive other impacts, such as
the need to modify the city drainage system.
This study has certain limitations. Our proposed elevated road structure is
an embankment type that has the elevation of 5 m above mean sea
level. This type of structure will soon be covered by the tsunami waves if
the magnitude of the earthquake is larger than 8.5 Mw. Since the
waves are characterized as long waves, scouring effects may occur immediately
after the overflowing process. Furthermore, the leeside of the embankment
will be easily damaged in the case of overflowing at a rubble-mound-type
embankment (Aniel-Quiroga et al., 2018). The extreme difference of the
hydrostatic pressures between the seaward and leeward direction of the BORR
should also be considered. This could destabilize the structure (Ozer et al., 2015). This study excluded the damage that occurred due to the overflow
process and scouring. Moreover, the density of the buildings was not
considered as a parameter that could fluctuate the Manning roughness
coefficients as suggested by previous research (Kotani et al., 1998;
Dutta et al., 2007).
Conclusions and recommendations
This study explores the possibility of mitigating the impacts of future
tsunamis on Banda Aceh based on eight scenarios of numerical simulations. We used two
magnitudes of earthquakes that generate tsunamis, that is, magnitudes 8.5 and
9.15 Mw. An elevated road and the land use planning for 2029 were
included in the simulations to test the possibility of adopting the concept of
a co-beneficial structure for tsunami mitigation. A tsunami multilayer defense
system as applied by the Tohoku region after the 2011 Great East Japan Earthquake
and Tsunami cannot be afforded in the tsunami-prone cities in developing
countries, such as Banda Aceh. There is a potential way to include structural tsunami mitigation by modifying the coastal area profile. One of
the possibilities for Banda Aceh is to elevate a planned road parallel to
the coast, namely, the Banda Aceh Outer Ring Road (BORR). Based on the
simulations, the elevated road, by reclaiming 5 m above mean sea
level, could reduce the inundation area by about 9 % and 22 % in the
case of 9.15 and 8.5 Mw earthquakes, respectively. The wave
heights and the wave velocities could also be reduced using the elevated road
structures. Notably, land use planning alone without BORR will cause
insignificant reduction in the tsunami wave heights and tsunami inundation
area. Therefore, the elevated road coupled with the 2029 land use planning is
expected to reduce the tsunami risks for the city, if implemented.
Based on the results, we recommend the city of Banda Aceh to conduct several
tsunami mitigation measures, as follows:
Control the increase in population and settlements around the coastal
area.
Control the land use of the coastal area, in particular, the area in
front of the planned BORR transect, and maintain it as a nonresidential
area.
Adopt the elevated roads in the BORR construction as this will
significantly help the city to cope with future tsunamis.
Preserve the salt marsh area around the coast, as this would also help
to reduce the tsunami impacts. The salt marsh area could also be planted
with mangroves or other brackish water vegetation that would increase the
Manning roughness coefficients of the area. This further will reduce the
speed of the tsunami waves.
Raw data belong to the Banda Aceh Municipality (BAPPEDA Banda Aceh).
Authors are not authorized to publicly share the data directly.
S is the principal investigator
of this research and led the analysis and writing process. T
simulated all the scenarios of numerical models. AS
helped to validate the simulation result and corrected this article.
ML and MA conducted bathymetry surveys and
digitized BA land use.
LKC is a US research collaborator for this research
project funded by PEER USAID.
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors are grateful for the research grant from the Partnership
Enhanced Engagement in Research (PEER) Cycle 5 sponsored by the USAID and
National Academies of Sciences, Engineering, and Medicines of United States
(NAS) under research grant no. 5-395, with the title “Incorporating climate
change induced sea level rise information into coastal cities' preparedness
toward coastal hazards”, with NAS subaward no. 2000007546. A visit of
Anawat Suppasri (co-author of this article) to Banda Aceh and fine-tuning of the paper took place under the World Class Professor Program
(WCP) Scheme B, promoted by the Ministry of Research, Technology, and Higher
Education of Indonesia (RISTEKDIKTI) in 2018 (contract no. 123.41/D2.3/KP/2018). Digitizing certain spatial data for land use and
elevated roads was done under the PKLN of RISTEKDIKTI Program grant no.
SK.60/UN11.2/SP3/2018 Year 2018, with the title “Mitigating Impacts Of Tsunami
Waves On Coastal Structures And Harbor Facilities”. The publication of this
paper is also funded by IRIDeS of Tohoku University, Japan. Edited by: Maria Ana Baptista
Reviewed by: two anonymous referees
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