Figure 2 Elevation points (left) used to create the topographic (Topo) DEM (right) using emperical Bayesian kriging.
N
Cambodia
Mekong river Vietnam
meter (amsl)
-0.5 0 1 2 3 6
0 50 100
kilometers Elevation Gulf of
G
Thailand T
South China
Sea
A B
C
China Vietnam
Laos
Ca
C mbodia Thailand
Mekkongg River
Cambodia
Me Mekongg
de deltltaa
Elevation (m)
< -2
> 10 4
Population density 2000 (persons per km2)
0 1-4 5-24 25-249 250-999 1,000 +
1 meter 2 meter Low Elevated Coast Zone
1 meter
Sea level rise inundation
Elevation 2-5 m asl
2 m 3 m 4 m 5 m
Gulf of Thailand
South China
Sea
Elevation (m)
-1.0 - 0.0 0.1 - 0.4 0.5 - 0.8 0.9 - 1.1 1.2 - 1.5 1.6 - 2.0 2.0 - 2.5 2.5 - 3.0 3.0 - 3.5 3.5 - 5.0 5.0 - 10.0 11.0 - 20.0
> 20.0
A
A´
B´
B
B B´
A
TOPO DEM
Topo DEM
km Mean deltaplain elevation: 0.82 m
Mean prole elevation SRTM: 1.8 m
MERIT: 3.1 m Topo: 0.67 m
50 100 150 200 km
SRTM DEM 500 m binned
meter
10 8 6 4 2 0 -1
meter
11°N
10°N
9°N
105°E 106°E
A B´
105°E 106°E
A´
MERIT DEM
Mean deltaplain elevation: 3.3 m
A B´
105°E 106°E
A´
Mean deltaplain elevation: 2.6 m
SRTM DEM
N
0 50 100
kilometers
-1 4.5 8 meter (amsl)
B B
A´
50 100 150 200 250 300 km
MERIT DEM 500 m binned
Mean prole elevation SRTM: 2.6 m
MERIT: 3.4 m Topo: 1.15 m
Count (-)
12 10 8 6 4 2
0-5 -2.5 0 2.5 5 7.5 10 Residual (m)
Topo DEM
MD: 0.2 m MAD: 0.6 m SD: 0.7 m n=69
0 2.5 5 7.5 10 -5 -2.5
Count (-)
12 10 8 6 4 2 0
MERIT DEM
MD: 3.0 m MAD: 3.0 m SD: 1.3 m n=69
Count (-)
12 10 8 6 4 2
0-5 -2.5 0 2.5 5 7.5 10
Count (-)
12 10 8 6 4 2 0
SRTM DEM
Residual (m)
MD: 2.0 m MAD: 2.6 m SD: 2.9 m n=69
Residual (m)
5 10 15 20 25
Mean elevation (m)
3.0
1.5 2.0
1.0 2.5
0.5
0.0
Topo DEM elevation
5 10 15 20 25
Flood occurrence Well-correlated to ood occurrence
Increase in ood occurrence SRTM DEM elevation
Weakly-correlated to ood occurrence
MERIT DEM elevation
Well-correlated to ood occurrence
Figure 1 A) Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) of the Mekong delta in Vietnam and B1-C2) inundation maps following sea-level rise based on the SRTM DEM containing effects of striping and other height errors.
Figure 3 SRTM, MERIT3 and Topo Digital Elevation Models (DEM) of the Mekong delta with two levation profiles.
Figure 8 Deviation between independent elevation benchmarks and Topo DEM.
Figure 9 Histograms of differences between elevation of the SRTM (left), MERIT (center) and the Topo (right) DEMs and national benchmarks in the Mekong delta.
Figure 4 Geomorphological map of the Vietnamese Mekong delta4.
Tidal
at Sand
spit Relict beach
ridge and/or sand dune
Mangrove marsh Salt
marsh Coastal plain
Marsh (inland)
Natural levee River Channel bar Abandoned
channel belt
Swamp Flood basin
Alluvial landscape
Coastal landscape
Sea
Benchmark elevation Benchmark elevation
PM
PM
Active river system
A
B
Figure 5 Schematic profiles with geomorphological units of the alluvial and coastal landscapes.
Table 1 Elevation of geomorphological units in each DEM.
Figure 6 Inundation occurrences in the Mekong delta in the period 2007-20115.
Figure 7 Mean MERIT, SRTM and Topo DEM elevation arranged according to
increasing tide-dominated flood occurrence in the southwestern part of
the Mekong delta.
Figure 6 Inundation occurrences in the Mekong delta in the period 2007-20114.
Topographical elevation points and Topo DEM
Introduction
Many densely populated deltas and coastal areas on Earth are located in data-sparse regions, forcing researchers and policy makers to use low-resolution, global elevation data obtained from satellite platforms to do sea-level rise impact assessments.
Using a new, high-accuracy elevation model of the Vietnamese Mekong delta, we show that the quality of such global data is insufficient. This may have profound implications for sea-level rise impact assessments worldwide, with elevation errors potentially larger than a century of sea level rise.
For correct assessments of elevation to local sea level, the vertical datum of DEMs need to be converted to local tidal datum. However, this crucial step is very often neglected, either due to lack of data on local tidal datums or as a result of lack of geodetic expertise.
As a result, the global geoid is wrongly assumed to represent local sea level.
This potentially leads to large vertical offsets with actual local sea level and this error is propagating to elevation above sea level and sea-level rise impact assessments.
Conclusions Data
Digital elevation model comparison
Inundation occurrence
Relative elevation validation: Geomorphology
Relative elevation: Inundation occurrences
Absolute elevation validation: national benchmarks
• The Mekong delta has an extremely low mean elevation of ~0.8 m above mean sea level, dramatically lower than the ~2.6 m suggested by analyses based on global,
satellite-based elevation data.
• This demonstrates that accuracy problems in global datasets of coastal elevation and offsets between vertical datum and actual local sea level may have profound
implications for coastal elevation to sea level and sea-level rise impact assessments worldwide, especially for data sparse regions, with elevation errors potentially larger than a century of sea level rise (confirmed by Kulp & Strauss 2019, October 2019).
Implications for sea-level rise impact assessments
Large errors in relative elevation and sea-level rise assessments of the world’s coastlines
Case for the Mekong delta, Vietnam
AGU Fall Meeting - 9-13 December 2019 - San Francisco, USA
1 Department of Physical Geography, Utrecht University, The Netherlands
2 Department of Subsurface and Groundwater Systems, Deltares Research Institute, Utrecht, The Netherlands
Figure 6 Inundation occurrences in the Mekong delta in the period 2007-20114.
Philip S.J. Minderhoud
1,2, L. Coumou
1, H. Middelkoop
1, G. Erkens
2,1, E. Stouthamer
1Published as Minderhoud et al., 2019 in Nature Communication in August 2019 (received July 2018)
1 Carew-Reid, J., 2008. Rapid Assessment of the Extent and Impact of Sea-level rise in Viet Nam, Climate Change. ICEM - Int. Cent. Environ. Manag.
2 Warner, K., Hamza, M., Oliver-Smith, A., Renaud, F., Julca, A., 2010. Climate change, environmental degradation and migration. Nat. Hazards 55, 689–715. doi:10.1007/s11069-009-9419-7
3 Yamazaki, D., Ikeshima, D., Tawatari, R., Yamaguchi, T., O’Loughlin, F., Neal, J.C., Sampson, C.C., Kanae, S., Bates, P.D., 2017. A high-accuracy map of global terrain elevations. Geophys. Res. Lett. 44, 5844–5853. doi:10.1002/2017GL072874
4 Nguyen, V.L., Ta, T.K.O., Tateishi, M., 2000. Late Holocene depositional environments and coastal evolution of the Mekong River Delta, Southern Vietnam. J. Asian Earth Sci. 18, 427–439. doi:10.1016/S1367-9120(99)00076-0
5 Kuenzer, C., Guo, H., Huth, J., Leinenkugel, P., Li, X., Dech, S., 2013. Flood mapping and flood dynamics of the mekong delta: ENVISAT-ASAR-WSM based time series analyses. Remote Sens. 5, 687–715. doi:10.3390/rs5020687
Figure 10 Area below sea level with sea-level rise (SLR) up to 1 meter based on the A) SRTM DEM, B) the MERIT DEM, C) the transposed MERIT DEM and the D) Topo DEM. The transposed MERIT DEM matches the mean delta elevation of the Topo DEM by subtracting 2.5 m from the MERIT DEM.
Table 2 Delta plain and estimated number of people below sea level (SL) for 0 and 1 meter sea-level rise (SLR)
