The effect of urbanization on floods of different recurrence interval

American Geophysical Union (AGU) | June 3rd, 1975

Studies have shown that the urbanization of a catchment can drastically change the flood characteristics of a river. Published results are synthesized to show the g

Studies have shown that the urbanization of a catchment can drastically change the flood characteristics of a river. Published results are synthesized to show the general relationship between the increase in flood flows following urbanization and both the percentage of the basin paved and the flood recurrence interval. In general, (1) floods with a return period of a year or longer are not affected by a 5% paving of their catchment, (2) small floods may be increased by 10 times by urbanization, (3) floods with a return period of 100 yr may be doubled in size by a 30% paving of the basin, and (4) the effect of urbanization declines, in relative terms, as flood recurrence intervals increase.

The Governance Gap: Climate Adaptation and Sea-Level Rise in the San Francisco Bay Area

University of California, Davis (UC Davis) | June 27th, 2017

This report summarizes the results of an extensive study of governance for climate adaptation and sea-level rise in the San Francisco Bay Area (SF Bay Area), wh

This report summarizes the results of an extensive study of governance for climate adaptation and sea-level rise in the San Francisco Bay Area (SF Bay Area), where the concept of sea-level rise adaptation also includes coastal flooding from high tides and extreme storm events. We focus on the “governance gap” that exists between the problem of sea-level rise and the implementation of adaptation solutions that increase resilience. We take a broad view on governance as the set of institutional arrangements, governmental authorities, and public stakeholders who collectively plan and implement adaptation activities, including identifying funding strategies and the agreeing on the information basis for decision making. While most stakeholders recognize the risk of sea-level rise and other climate impacts, and have good ideas about potential on-the-ground solutions, realizing these solutions requires overcoming a series of governance challenges. All of these governance challenges are barriers to stakeholder cooperation at multiple geographic levels—local, regional, state, and federal. There are many different governmental and non-governmental actors facing the same sea-level rise problem (in “the same boat”), and coordination is required to have them “row in the same direction.” The imperative for multi-level cooperation becomes more poignant as the science on sea-level rise continues to advance, and considers the possibilities of higher average levels of sea-level rise and rapid and extreme sea-level rise due to accelerated melting of the Antarctic ice sheet. This report summarizes the major challenges, identifies the menu of possible solutions, and develops some specific recommendations on the next steps needed to remedy the governance gap and enhance the adaptive capacity of the SF Bay Area. The governance analysis is a component of the larger UC Davis/Berkeley research project called Resilience of Infrastructure as Seas Rise (RISeR), See Figure 1, which is funded by National Science Foundation to analyze the interactions between sea-level rise adaptation, transportation infrastructure, and governance in the SF Bay Area. The governance analysis was conducted in Fall 2016-Winter 2017, and is based on a qualitative case study that combines in-person interviews, focus groups, document analysis, and content analysis of interview responses. The draft report was made available to study participants in Spring 2017, along with two webinars for receiving feedback. Participants included leaders of governmental and non-governmental organizations across the SF Bay Area region. The research focused mainly on sea-level rise and coastal flooding, but also linked to broader issues of climate adaptation when appropriate. Focus groups deliberated on the best possible solutions to solving the identified governance challenges. Respondents identified the overarching governance challenge as the imperative for multi-level cooperation among all sea-level rise adaptation stakeholders. Sea-level rise adaptation entails interdependencies, where the vulnerabilities and adaptation decisions of local actors impose regional costs and benefits. While regional cooperation is beginning to emerge, most stakeholders see a critical need for shared learning, coordination and planning. The recommendations provided in this report are intended to accelerate the processes of learning and cooperation needed to address the increasingly urgent problem of sea-level rise and coastal flooding.

The human factor in water disasters

Nature Portfolio (Springer Nature) | December 14th, 2023

When water inundated parts of New York City in September 2023, 28 people had to be rescued from their cars and basement apartments. Thankfully, no one died this time. In

When water inundated parts of New York City in September 2023, 28 people had to be rescued from their cars and basement apartments. Thankfully, no one died this time. In 2021, flooding in New York killed 11 people. Neighbourhoods in the city also flooded in 2020 and 2022, and it’s not just New York. Floods are becoming increasingly frequent and severe globally, as are droughts. Steve Bowen, chief science officer for reinsurance firm Gallagher Re in London, described the most recent New York floods on X (formerly Twitter) as “the latest example of ageing infrastructure built for a climate that no longer exists”. Such sentiments are common, and frequently followed by calls for more infrastructure: bigger levees and seawalls, larger pipes and stormwater tanks, and more dams, aqueducts and desalination plants.

The importance of imperviousness

Watershed Protection Techniques, Center for Watershed Protection | December 1st, 1994

The emerging field of urban watershed protection often lacks a unifying theme to guide the efforts of its many participants—planners, engineers, landscape architects

The emerging field of urban watershed protection often lacks a unifying theme to guide the efforts of its many participants—planners, engineers, landscape architects, scientists, and local officials. The lack of a common theme has often made it difficult to achieve a consistent result at either the individual development site or cumulatively, at the watershed scale. In this article a unifying theme is proposed based on a physically defined unit: imperviousness. Imperviousness here is defined as the sum of roads, parking lots, sidewalks, rooftops, and other impermeable surfaces of the urban landscape. This variable can be easily measured at all scales of development, as the percentage of area that is not “green.” Imperviousness is a very useful indicator with which to measure the impacts of land development on aquatic systems. Reviewed here is the scientific evidence that relates imperviousness to specific changes in the hydrology, habitat structure, water quality and biodiversity of aquatic systems. This research, conducted in many geographic areas, concentrating on many different variables, and employing widely different methods, has yielded a surprisingly similar conclusion: stream degradation occurs at relatively low levels of imperviousness (~10%). Most importantly, imperviousness is one of the few variables that can be explicitly quantified, managed and controlled at each stage of land development. The remainder of this article details the relationship between imperviousness and stream quality.

The key role of dry days in changing regional climate and precipitation regimes

Nature Portfolio (Springer Nature) | March 13th, 2014

Future changes in the number of dry days per year can either reinforce or counteract projected increases in daily precipitation intensity as the climate warms. We

Future changes in the number of dry days per year can either reinforce or counteract projected increases in daily precipitation intensity as the climate warms. We analyze climate model projected changes in the number of dry days using 28 coupled global climate models from the Coupled Model Intercomparison Project, version 5 (CMIP5). We find that the Mediterranean Sea region, parts of Central and South America, and western Indonesia could experience up to 30 more dry days per year by the end of this century. We illustrate how changes in the number of dry days and the precipitation intensity on precipitating days combine to produce changes in annual precipitation, and show that over much of the subtropics the change in number of dry days dominates the annual changes in precipitation and accounts for a large part of the change in interannual precipitation variability.

The Ocean and Cryosphere in a Changing Climate

Intergovernmental Panel on Climate Change (IPCC) | September 24th, 2019

All people on Earth depend directly or indirectly on the ocean and cryosphere. The global ocean covers 71% of the Earth surface and contains about 97% of the Earth’s w

All people on Earth depend directly or indirectly on the ocean and cryosphere. The global ocean covers 71% of the Earth surface and contains about 97% of the Earth’s water. The cryosphere refers to frozen components of the Earth system. Around 10% of Earth’s land area is covered by glaciers or ice sheets. The ocean and cryosphere support unique habitats, and are interconnected with other components of the climate system through global exchange of water, energy and carbon. The projected responses of the ocean and cryosphere to past and current human-induced greenhouse gas emissions and ongoing global warming include climate feedbacks, changes over decades to millennia that cannot be avoided, thresholds of abrupt change, and irreversibility. Human communities in close connection with coastal environments, small islands (including Small Island Developing States, SIDS), polar areas and high mountains are particularly exposed to ocean and cryosphere change, such as sea level rise, extreme sea level and shrinking cryosphere. Other communities further from the coast are also exposed to changes in the ocean, such as through extreme weather events. Today, around 4 million people live permanently in the Arctic region, of whom 10% are Indigenous. The low-lying coastal zone8 is currently home to around 680 million people (nearly 10% of the 2010 global population), projected to reach more than one billion by 2050. SIDS are home to 65 million people. Around 670 million people (nearly 10% of the 2010 global population), including Indigenous peoples, live in high mountain regions in all continents except Antarctica. In high mountain regions, population is projected to reach between 740 and 840 million by 2050 (about 8.4–8.7% of the projected global population). In addition to their role within the climate system, such as the uptake and redistribution of natural and anthropogenic carbon dioxide (CO2) and heat, as well as ecosystem support, services provided to people by the ocean and/or cryosphere include food and water supply, renewable energy, and benefits for health and well-being, cultural values, tourism, trade, and transport. The state of the ocean and cryosphere interacts with each aspect of sustainability reflected in the United Nations Sustainable Development Goals (SDGs). Over the last decades, global warming has led to widespread shrinking of the cryosphere, with mass loss from ice sheets and glaciers (very high confidence), reductions in snow cover (high confidence) and Arctic sea ice extent and thickness (very high confidence), and increased permafrost temperature (very high confidence).

The Radical Groundwater Storage Test

Scientific American | October 19th, 2017

IN BRIEF California could better survive flood years and drought years by capturing floodwaters and using them to recharge underground aquifers that farmers and muni

IN BRIEF California could better survive flood years and drought years by capturing floodwaters and using them to recharge underground aquifers that farmers and municipalities now pump from when rains are scarce. To do so, water users and managers have to regard surface water and groundwater as one connected resource. A variety of test projects are proving that this approach can work. For example, farm fields can be flooded even when crops are growing so the water can percolate down into aquifers. Revising water rights and paying landowners to allow flooding can help the techniques work across California and the world.

The Rainfall Intensity-Duration Control of Debris Flows After Wildfire

American Geophysical Union (AGU) | May 23rd, 2023

The role of climate and population change in global flood exposure and vulnerability

Nature Portfolio (Springer Nature) | February 3rd, 2025

Flooding is a pervasive natural hazard with wide-ranging impacts on society. Using a high-resolution global flood model considering coastal, fluvial, and pluvial hazards,

Flooding is a pervasive natural hazard with wide-ranging impacts on society. Using a high-resolution global flood model considering coastal, fluvial, and pluvial hazards, we clarify the role of climate effects versus population growth effects in changing flood exposure. Between 2020 and 2100, the population likely exposed to 1% annual risk (100-year) flood hazard will increase from 1.6 to 1.9 billion people. Of this change from the 2020 exposure, we attribute 21.1% to climate change, 76.8% to population change, and 2.1% to both climate and population change. The largest driver of uncertainty in exposure is population change, while climate change remains a smaller but still important driver. The global increase in exposure between 2020 and 2100 is primarily driven by low-GDP regions, and by 2100 the lowest GDP areas will make up 63% of the exposure both overall and in urban areas. Urban areas are especially vulnerable in nearly all global regions, and urban areas sensitive to extreme events are expected to see a 33% increase in population exposure. This study highlights the vast inequities in flood exposure, and future work should direct resources and strategies toward sustainable risk mitigation in these areas.

The Science and Prediction of Post-Fire Debris Flows in the Western United States

U.S. Forest Service (USDA) | August 1st, 2017

Debris flows are among the most destructive hydrological consequences of fires in steep watersheds. The high likelihood of catastrophic wildfires in the western U

Debris flows are among the most destructive hydrological consequences of fires in steep watersheds. The high likelihood of catastrophic wildfires in the western United States and the encroachment of human activities into steep fire-prone areas have created the need to better understand, predict, and mitigate these hazards. This article highlights recent advances in understanding post-fire debris-flow generation and provides an overview of the evolution of hazard assessments in the western United States. Specific emphasis is placed upon free, publicly available toolsdeveloped by the U.S. Geological Survey (USGS) and available for analyses in support of Burned Area Emergency Response (BAER) and Emergency Watershed Protection (EWP) program activities. These tools consist of empirical models that predict the likelihood, potential volume, and the rainfall intensity-duration thresholds for debris flows in recently burned watersheds.