Abstract River deltas all over the world are sinking
beneath sea-level rise, causing significant threats to natural
and social systems. This is due to the combined effects of
anthropogenic changes to sediment supply and river flow,
subsidence, and sea-level rise, posing an immediate threat
to the 500–1,000 million residents, many in megacities that
live on deltaic coasts. The Mississippi River Deltaic Plain
(MRDP) provides examples for many of the functions and
feedbacks, regarding how human river management has
impacted source-sink processes in coastal deltaic basins,
resulting in human settlements more at risk to coastal
storms. The survival of human settlement on the MRDP is
arguably coupled to a shifting mass balance between a
deltaic landscape occupied by either land built by the
Mississippi River or water occupied by the Gulf of Mexico.
We developed an approach to compare 50 % L:W isopleths
(L:W is ratio of land to water) across the Atchafalaya and
Terrebonne Basins to test landscape behavior over the last
six decades to measure delta instability in coastal deltaic
basins as a function of reduced sediment supply from river
flooding. The Atchafalaya Basin, with continued sediment
delivery, compared to Terrebonne Basin, with reduced
river inputs, allow us to test assumptions of how coastal
deltaic basins respond to river management over the last
75 years by analyzing landward migration rate of 50 %
L:W isopleths between 1932 and 2010. The average landward
migration for Terrebonne Basin was nearly 17,000 m
(17 km) compared to only 22 m in Atchafalaya Basin over
the last 78 years (p\0.001), resulting in migration rates of
218 m/year (0.22 km/year) and\0.5 m/year, respectively.
In addition, freshwater vegetation expanded in Atchafalaya
Basin since 1949 compared to migration of intermediate
and brackish marshes landward in the Terrebonne Basin.
Changes in salt marsh vegetation patterns were very distinct
in these two basins with gain of 25 % in the Terrebonne
Basin compared to 90 % decrease in the Atchafalaya
Basin since 1949. These shifts in vegetation types as L:W
ratio decreases with reduced sediment input and increase in
salinity also coincide with an increase in wind fetch in
Terrebonne Bay. In the upper Terrebonne Bay, where the
largest landward migration of the 50 % L:W ratio isopleth
occurred, we estimate that the wave power has increased
by 50–100 % from 1932 to 2010, as the bathymetric and
topographic conditions changed, and increase in maximum
storm-surge height also increased owing to the landward
migration of the L:W ratio isopleth. We argue that this
balance of land relative to water in this delta provides a
much clearer understanding of increased flood risk from
tropical cyclones rather than just estimates of areal land
loss. We describe how coastal deltaic basins of the MRDP
can be used as experimental landscapes to provide insights
into how varying degrees of sediment delivery to coastal
deltaic floodplains change flooding risks of a sinking delta
using landward migrations of 50 % L:W isopleths. The
nonlinear response of migrating L:W isopleths as wind
fetch increases is a critical feedback effect that should
influence human river-management decisions in deltaic
coast. Changes in land area alone do not capture how
corresponding landscape degradation and increased water
area can lead to exponential increase in flood risk to human
populations in low-lying coastal regions. Reduced land
formation in coastal deltaic basins (measured by changes in
the land:water ratio) can contribute significantly to
increasing flood risks by removing the negative feedback
of wetlands on wave and storm-surge that occur during
extreme weather events. Increased flood risks will promote
population migration as human risks associated with living
in a deltaic landscape increase, as land is submerged and
coastal inundation threats rise. These system linkages in
dynamic deltaic coasts define a balance of river management
and human settlement dependent on a certain level of
land area within coastal deltaic basins (L).
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A global open-source database of flood-protection levees on river deltas (openDELvE)
Abstract. Flood-protection levees have been built along rivers and coastlines globally. Current datasets, however, are generally confined to territorial boundaries (national datasets) and are not always easily accessible, posing limitations for hydrologic models and assessments of flood hazard. Here, we bridge this knowledge gap by collecting and standardizing global flood-protection levee data for river deltas into the open-source global river delta levee data environment, openDELvE. In openDELvE, we aggregate levee data from national databases, reports, maps, and satellite imagery. The database identifies the river delta land areas that the levees have been designed to protect. Where data are available, we record the extent and design specifications of the levees themselves (e.g., levee height, crest width, construction material) in a harmonized format. The 1657 polygons of openDELvE contain 19 248 km of levees and 44 733.505 km2 of leveed area. For the 153 deltas included in openDELvE, 17 % of the land area is confined by flood-protection levees. Around 26 % of delta population lives within the 17 % of delta area that is protected, making leveed areas densely populated. openDELvE data can help improve flood exposure assessments, many of which currently do not account for flood-protection levees. We find that current flood hazard assessments that do not include levees may exaggerate the delta flood exposure by 33 % on average, but up to 100 % for some deltas. The openDELvE is made public on an interactive platform (https://www.opendelve.eu/, 1 October 2022), which includes a community-driven revision tool to encourage inclusion of new levee data and continuous improvement and refinement of open-source levee data.
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- Award ID(s):
- 1810855
- NSF-PAR ID:
- 10387114
- Date Published:
- Journal Name:
- Natural Hazards and Earth System Sciences
- Volume:
- 22
- Issue:
- 12
- ISSN:
- 1684-9981
- Page Range / eLocation ID:
- 4087 to 4101
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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