An official website of the United States government
Abstract Natural levees form through sediment delivery from channels, dispersal onto floodplains, and storage at channel margins. When levees breach, they release water and sediment onto the floodplain, occasionally causing river avulsions. Despite their significance, levee growth remains poorly understood, and no existing models capture the dynamic channel‐levee evolution systems. A common assumption is that levee and channel bed aggradation rates are coupled or equal; however, this cannot be true because levees do not accumulate everywhere along aggrading channel belts. Using a one‐dimensional numerical model, we investigate levee growth decoupled from channel bed aggradation under flood scenarios wherein the flooded level: (a) exceeds the levee crest height (i.e., front loading); or (b) is lower than the levee crest partially inundating distal levee deposits (i.e., back loading). Front loading events initially aggrade the levee crest, which confines the channel, increases bankfull depth, and reduces flooding. During confinement, levee growth restricts flooding, and minor back loading events are more common. Over this period, the channel bed aggrades until bankfull depth decreases sufficiently to trigger larger floods. This channel‐releasing process increases flood likelihood and enhances overbank accumulation, promoting front loading and re‐confining the channel. Our findings suggest aggradational channels may experience confined‐release phases characterized by episodic levee growth and fluctuating bankfull depth. Rapid in‐channel aggradation increases flood frequency and variability with more confined‐release cycles. These results imply that river avulsions and associated floods might preferentially occur when the channel bed aggrades faster than adjacent levees, whereby the channel becomes shallower and destabilized.
more »
« less
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.
more »
« less
- Award ID(s):
- 1810855
- 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
More Like this
-
-
There are growing concerns regarding the increase in flood risk due to climate change and land use/land cover changes. In light of these changes, levees play an increasingly critical role in safeguarding communities, infrastructure, and the natural environment. However, the average age of levees in the United States is 60 years, with the majority operating under marginal conditions. The most common failure mode of earthen levees is breach due to overtopping. Existing methodologies evaluate the site-specific probability of levee overtopping and do not provide a holistic view of flood risk across a wider area. Here we present a regional-scale overtopping model for levees using a data-driven overtopping model that uses five variables: (1) levee construction classification, (2) overtopping depth, (3) overtopping duration, (4) erosion resistance classification, and (5) duration of levee loading before overtopping. The overtopping model is applied to levee systems in Wilton, California. The probability of breach due to overtopping is presented for three distinct scenarios (overtopping duration) during a 50-year flood event. The results show the probability of breach for the Wilton Levee ranges from 0.32 to 0.91 for overtopping durations of 6 hours and between 6 to 24 hours. For durations exceeding 24 hours, the probability of breach increases to a range of 0.73 to 0.98. The proposed framework offers a viable tool for performing regional-scale levee risk assessment, offering broader implications for enhancing preparedness, response, and recovery strategies in the face of escalating flood risks.more » « less
-
Inland levees can amplify flood risk in unprotected communities by altering floodwater levels away from their location. While these nonlocal effects of levees, which we term flood teleconnections, have been studied for specific river segments, their impact on flood risks along a river network remains underexplored. By combining data-driven, hydrodynamic, and economic models, we quantify the magnitude, spatial distribution, and economic damages associated with flood teleconnections for a large river network system with extensive levees. We find that due to levees, the 100-year flood inundation extent grows by 25% of the total levee-protected area regionally, and the flood inundation depth increases by up to 2 m at specific locations. Levees also increase the vulnerability of unprotected, marginalized communities to flooding. Our results demonstrate that flood teleconnections are spatially widespread, involve unaccounted costs, and can lead to flood inequities. These findings will be critical to climate adaptation efforts in flood-prone regions.more » « less
-
null (Ed.)Earthen levees protecting coastal regions can be exposed to compound flooding induced by multiple drivers such as coastal water level, river discharge, and precipitation. However, the majority of flood hazard analyses consider only one flood driver at a time. This study numerically investigates the performance of an earthen levee in Sherman Island, Sacramento, CA, under compound flooding induced by fluvial and pluvial flooding. A finite element model is built for fully coupled 3D stress-flow simulations of the levee. The finite element model is then used to simulate the hydro-mechanical response of the levee under different flood scenarios. Fluvial flood hydrographs for different scenarios are obtained using a bivariate extreme analysis of peak river discharge and peak ocean level while accounting for the significance of correlation between these two variables. Pluvial flooding is characterized using intensity-duration-frequency (IDF) curves of extreme precipitations for the study area. The fluvial and pluvial flood patterns for different recurrence intervals are used in the finite element model to simulate the hydro-mechanical response of the levee. Results show that considering compound flooding leads to 8.7% and 18.6% reduction in the factor of safety for 2 and 50-year recurrence intervals, respectively.more » « less
-
Abstract River deltas will likely experience significant land loss because of relative sea‐level rise (RSLR), but predictions have not been tested against observations. Here, we use global data of RSLR and river sediment supply to build a model of delta response to RSLR for 6,402 deltas, representing 86% of global delta land. We validate this model against delta land area change observations from 1985–2015, and project future land area change for IPCC RSLR scenarios. For 2100, we find widely ranging delta scenarios, from +94 ± 125 (2 s.d.) km2yr−1for representative concentration pathway (RCP) 2.6 to −1,026 ± 281 km2yr−1for RCP8.5. River dams, subsidence, and sea‐level rise have had a comparable influence on reduced delta growth over the past decades, but if we follow RCP8.5 to 2100, more than 85% of delta land loss will be caused by climate‐change driven sea‐level rise, resulting in a loss of ∼5% of global delta land.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/nhess-22-4087-2022
