Hamed R. Moftakhari

Cumulative hazard: The case of nuisance flooding

The cumulative cost of frequent events (e.g., nuisance floods) over time may exceed the costs of the extreme but infrequent events for which societies typically prepare. Here we analyze the likelihood of exceedances above mean higher high water and the corresponding property value exposure for minor, major, and extreme coastal floods. Our results suggest that, in response to sea level rise, nuisance flooding (NF) could generate property value exposure comparable to, or larger than, extreme events. Determining whether (and when) low cost, nuisance incidents aggregate into high cost impacts and deciding when to invest in preventive measures are among the most difficult decisions for policymakers. It would be unfortunate if efforts to protect societies from extreme events (e.g., 0.01 annual probability) left them exposed to a cumulative hazard with enormous costs. We propose a Cumulative Hazard Index (CHI) as a tool for framing the future cumulative impact of low cost incidents relative to infrequent extreme events. CHI suggests that in New York, NY, Washington, DC, Miami, FL, San Francisco, CA, and Seattle, WA, a careful consideration of socioeconomic impacts of NF for prioritization is crucial for sustainable coastal flood risk management.

Increased nuisance flooding along the coasts of the United States due to sea level rise: Past and future

Author(s): Moftakhari, HR; AghaKouchak, A; Sanders, BF; Feldman, DL; Sweet, W; Matthew, RA; Luke, A | Abstract:

Compounding effects of sea level rise and fluvial flooding

Significance Population and assets in coastal regions are threatened by both oceanic and fluvial flooding hazards. Common flood hazard assessment practices typically focus on one flood driver at a time and ignore potential compounding impacts. Here we outline a unique bivariate flood hazard assessment framework that accounts for the interactions between a primary oceanic flooding hazard, coastal water level, and fluvial flooding hazards. Using the notion of “failure probability,” we also assess coastal flood hazard under different future sea level rise scenarios. The results show that, in a warming climate, future sea level rise not only increases the failure probability, but also exacerbates the compounding effects of flood drivers. Sea level rise (SLR), a well-documented and urgent aspect of anthropogenic global warming, threatens population and assets located in low-lying coastal regions all around the world. Common flood hazard assessment practices typically account for one driver at a time (e.g., either fluvial flooding only or ocean flooding only), whereas coastal cities vulnerable to SLR are at risk for flooding from multiple drivers (e.g., extreme coastal high tide, storm surge, and river flow). Here, we propose a bivariate flood hazard assessment approach that accounts for compound flooding from river flow and coastal water level, and we show that a univariate approach may not appropriately characterize the flood hazard if there are compounding effects. Using copulas and bivariate dependence analysis, we also quantify the increases in failure probabilities for 2030 and 2050 caused by SLR under representative concentration pathways 4.5 and 8.5. Additionally, the increase in failure probability is shown to be strongly affected by compounding effects. The proposed failure probability method offers an innovative tool for assessing compounding flood hazards in a warming climate.

Projecting nuisance flooding in a warming climate using generalized linear models and Gaussian processes

Nuisance flooding corresponds to minor and frequent flood events that have significant socio-economic and public health impacts on coastal communities. Yearly-averaged local mean sea level can be used as proxy to statistically predict the impacts of sea level rise (SLR) on the frequency of nuisance floods (NF). In this study, we use Generalized Linear Models (GLM) and Gaussian Process (GP) models combined to (i) estimate the frequency of NF associated with the change in mean sea level, and (ii) quantify the associated uncertainties via a novel and statistically robust approach. We calibrate our models to the water level data from eighteen tide gauges along the coasts of United States, and after validation, we estimate the frequency of NF associated with the SLR projections in year 2030 (under RCPs 2.6 and 8.5), along with their 90% bands, at each gauge. The historical NF-SLR data is very noisy, and shows large changes in variability (heteroscedasticity) with SLR. Prior models in the literature do not properly account for the observed heteroscedasticity, and thus their projected uncertainties are highly suspect. Among the models used in this study the Negative Binomial Distribution GLM with GP best characterizes the uncertainties associated with NF estimates; on validation data ≈ 93% of the points fall within the 90% credible limit, showing our approach to be a robust model for uncertainty quantification. This article is protected by copyright. All rights reserved.

Quantifying Anthropogenic Stress on Groundwater Resources

This study explores a general framework for quantifying anthropogenic influences on groundwater budget based on normalized human outflow (hout) and inflow (hin). The framework is useful for sustainability assessment of groundwater systems and allows investigating the effects of different human water abstraction scenarios on the overall aquifer regime (e.g., depleted, natural flow-dominated, and human flow-dominated). We apply this approach to selected regions in the USA, Germany and Iran to evaluate the current aquifer regime. We subsequently present two scenarios of changes in human water withdrawals and return flow to the system (individually and combined). Results show that approximately one-third of the selected aquifers in the USA, and half of the selected aquifers in Iran are dominated by human activities, while the selected aquifers in Germany are natural flow-dominated. The scenario analysis results also show that reduced human withdrawals could help with regime change in some aquifers. For instance, in two of the selected USA aquifers, a decrease in anthropogenic influences by ~20% may change the condition of depleted regime to natural flow-dominated regime. We specifically highlight a trending threat to the sustainability of groundwater in northwest Iran and California, and the need for more careful assessment and monitoring practices as well as strict regulations to mitigate the negative impacts of groundwater overexploitation.

What Is Nuisance Flooding? Defining and Monitoring an Emerging Challenge

Nuisance flooding (NF) refers to low levels of inundation that do not pose significant threats to public safety or cause major property damage, but can disrupt routine day-to-day activities, put added strain on infrastructure systems such as roadways and sewers, and cause minor property damage. NF has received some attention in the context of low-lying coastal cities exposed to increasingly higher high tides, a consequence of sea level rise, which exceeds the heights of coastal topography. However, low levels of flooding are widespread and deserve greater attention. Here a simple, quantitative definition of NF is proposed based on established flood intensity thresholds for flood consequences (e.g., pedestrian safety, property damage, and health risks). Based on a wide range of literature including hydrology, transportation, public health risk, and safety impacts, we define NF based on depth >3 cm and <10 cm, regardless of the source. This definition of NF is not limited to high tide flooding but rather is inclusive of all possible flood drivers including pluvial, fluvial, and oceanic and can capture trends in NF resulting from trends in, and compounding effects of, flood drivers. Furthermore, we also distinguish between NF as a process and NF as an event, which is important for linking NF to societal impacts and developing effective policy interventions and mitigation strategies. Potential applications and implications of NF monitoring are also presented.

How do natural hazards cascade to cause disasters

Track connections between hurricanes, wildfires, climate change and other risks, urge Amir AghaKouchak and colleagues.Track connections between hurricanes, wildfires, climate change and other risks, urge Amir AghaKouchak and colleagues.

Multihazard Scenarios for Analysis of Compound Extreme Events

California Energy Commission [500-15-005]; National Science Foundation Hazards-SEES Program [DMS 1331611]; National Oceanic and Atmospheric Administration Ecological Effects of Sea Level Rise Program [NA16NOS4780206]

Translating Uncertain Sea Level Projections Into Infrastructure Impacts Using a Bayesian Framework: Impact Assessment of SLR Projections

Author(s): Moftakhari, H; AghaKouchak, A; Sanders, BF; Matthew, RA; Mazdiyasni, O | Abstract: ©2017. American Geophysical Union. All Rights Reserved. Climate change may affect ocean-driven coastal flooding regimes by both raising the mean sea level (msl) and altering ocean-atmosphere interactions. For reliable projections of coastal flood risk, information provided by different climate models must be considered in addition to associated uncertainties. In this paper, we propose a framework to project future coastal water levels and quantify the resulting flooding hazard to infrastructure. We use Bayesian Model Averaging to generate a weighted ensemble of storm surge predictions from eight climate models for two coastal counties in California. The resulting ensembles combined with msl projections, and predicted astronomical tides are then used to quantify changes in the likelihood of road flooding under representative concentration pathways 4.5 and 8.5 in the near-future (1998–2063) and mid-future (2018–2083). The results show that road flooding rates will be significantly higher in the near-future and mid-future compared to the recent past (1950–2015) if adaptation measures are not implemented.