Enchente do rio Caí em Bom Princípio em julho de 2020

Urban flooding is the inundation of land or property in a

built environment The term built environment refers to human-made conditions and is often used in architecture, landscape architecture, urban planning, public health, sociology, and anthropology, among others. These curated spaces provide the setting for human ...

, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems, such as

storm sewers A storm drain, storm sewer (United Kingdom, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain ( Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from impervious surface ...

. Although sometimes triggered by events such as flash flooding or

snowmelt In hydrology, snowmelt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snowmelt is an important part of the annual water cycle in many part ...

, urban flooding is a condition, characterized by its repetitive and systemic impacts on communities, that can happen regardless of whether or not affected communities are located within designated floodplains or near any body of water.Center for Neighborhood Technology, Chicago IL
"The Prevalence and Cost of Urban Flooding"
May 2013 Aside from potential overflow of rivers and lakes, snowmelt,

stormwater Stormwater, also spelled storm water, is water that originates from precipitation (storm), including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil ( infiltrate) and become groundwater, be stored on depressed land ...

or water released from damaged water mains may accumulate on property and in public rights-of-way, seep through building walls and floors, or backup into buildings through sewer pipes, toilets and sinks. In urban areas, flood effects can be exacerbated by existing paved streets and roads, which increase the speed of flowing water. Impervious surfaces prevent rainfall from infiltrating into the ground, thereby causing a higher surface run-off that may be in excess of local drainage capacity. The flood flow in urbanized areas constitutes a hazard to both the population and infrastructure. Some recent catastrophes include the inundations of Nîmes (France) in 1998 and Vaison-la-Romaine (France) in 1992, the flooding of

New Orleans New Orleans ( , ,New Orleans

Rockhampton Rockhampton is a city in the Rockhampton Region of Central Queensland, Australia. The population of Rockhampton in June 2021 was 79,967, Estimated resident population, 30 June 2018. making it the fourth-largest city in the state outside of ...

, Bundaberg,

Brisbane Brisbane ( ) is the capital and most populous city of the states and territories of Australia, Australian state of Queensland, and the list of cities in Australia by population, third-most populous city in Australia and Oceania, with a populati ...

during the 2010–2011 summer in

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , established_ ...

(Australia). Flood flows in urban environments have been studied relatively recently despite many centuries of flood events. Some recent research has considered the criteria for safe evacuation of individuals in flooded areas.

Background

Banks Street Kayakers - Mid-City New Orleans flooding July 2019

There are several types of flooding, including pluvial (flooding caused by heavy rain),

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluv ...

(caused by a nearby river overflowing its banks), and

coastal flooding Coastal flooding normally occurs when dry and low-lying land is submerged by seawater. The range of a coastal flooding is a result of the elevation of floodwater that penetrates the inland which is controlled by the topography of the coastal land ...

(often caused by

storm surge A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones. It is measured as the rise in water level above the ...

s). Different types of urban flooding create different impacts and require different mitigation strategies.

Links to climate change

Many of the common causes of urban flooding, including storm surges, heavy precipitation, and river overflow, are expected to increase in frequency and severity as climate change intensifies and causes increases in ocean and river levels. In particular, erratic rainfall patterns are expected to increase the frequency and severity of both pluvial flooding (as excessive amounts of rainfall in urban areas and cannot be adequately absorbed by existing drainage systems and pervious areas) and fluvial flooding (as excessive rainfall over a river can cause flooding and overflow, either where it occurs or downstream along the path of the river). The frequencies and severities of extreme storm events, including hurricanes and other types of

tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dep ...

s, are also expected to increase, raising the risk of storm surges and the potential for heavy rainfall and increasing flooding-related damages by up to US$54B annually. Additionally, due to the geographic distribution of developing urban areas, the land area potentially exposed to climate change-related flooding is expected to increase significantly.

Case studies

One of the most well known at-risk urban areas in the United States is

New Orleans New Orleans ( , ,New Orleans

Hurricane Katrina caused more than 1800 deaths and US$170B in damages. After Katrina, additional flood protections were built with a changing climate in mind; these protections have proved effective in reducing damages due to subsequent extreme weather events, such as Hurricane Ida. During the summer of 2021, Hurricanes Henri and Ida caused significant flooding in many cities along the east coast of the United States. In particular, New York City experienced record levels of rainfall, prompting many to question whether the city should implement additional flood protection measures in anticipation of potential future flood events. In September 2021, the New York City mayoral office released a new rainfall preparedness plan.

Impacts

Some of the most obvious impacts of urban flooding are those to human life and to property damage. In 2020, floods caused an estimated 6,000 deaths and caused US$51.3B in damages globally. Flood and its related disasters are caused by excessive volumes of water (runoff) which are not absorbed by the ground. Residents at low-elevated regions are often at risk of inundation, financial loss, and even the loss of lives. As the pace of urbanisation accelerates around the world, flash flood damage takes place more frequently. Between 1961 and 2020, nearly 10,000 cases were reported with 1.3 million deaths and a minimum of USD 3.3 trillion of financial losses at an equivalent loss rate of almost USD 1800 per second. On average, the total reported deaths worldwide were around 23,000/year for the past 6 decades at an equivalent rate of one death every 24 min. Urban flooding also impacts critical public services, including public transportation systems. Traffic congestion can be worsened by urban flood events, impacting ease of access to transportation, as well as the ability of emergency services to operate effectively. Urban flooding can also create far-reaching supply chain issues, which can create significant interruptions in the availability of goods and services, as well as financial losses for businesses.

Economic impacts

Urban flooding has significant economic implications. In the US, industry experts estimate that wet basements can lower property values by 10%-25% and are cited among the top reasons for not purchasing a home."The Prevalence and Cost of Urban Flooding."
May 2013, Center for Neighborhood Technology, Chicago IL. According to the U.S Federal Emergency Management Agency (FEMA), almost 40% of small businesses never reopen their doors following a flooding disaster. In the UK, urban flooding is estimated to cost £270 million a year in

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...

and

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

; 80,000 homes are at risk. A study of

Cook County, Illinois Cook County is the most populous county in the U.S. state of Illinois and the second-most-populous county in the United States, after Los Angeles County, California. More than 40% of all residents of Illinois live within Cook County. As of 20 ...

, identified 177,000 property damage insurance claims made across 96% of the county's ZIP codes over a five-year period from 2007-2011. This is the equivalent of one in six properties in the County making a claim. Average payouts per claim were $3,733 across all types of claims, with total claims amounting to $660 million over the five years examined. Despite concerted efforts, many communities lack the funds to fully address these issues and often seek funds elsewhere. Numerous watersheds within

Los Angeles County, California Los Angeles County, officially the County of Los Angeles, and sometimes abbreviated as L.A. County, is the most populous county in the United States and in the U.S. state of California, with 9,861,224 residents estimated as of 2022. It is th ...

do not meet state water quality standards, despite spending $100 million a year on clean water programs to combat issues such as urban runoff. To combat this problem, officials have introduced a measure that would assess a fee to homeowners and local businesses in an attempt to raise $290 million for effective urban runoff management.

Modeling

Flood modeling is often conducted in a very localized fashion, with

hydrological Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...

models created for individual municipalities and incorporating details about buildings, infrastructure, vegetation, land use, and drainage systems. This localized modeling can be very useful, especially when paired with historical data, in predicting which specific locations (e.g. streets or intersections) will be the most impacted during a flood event and can be helpful in designing effective mitigation systems specific to local needs. Flood flows in urban environments have been investigated relatively recently despite many centuries of flood events. Some researchers mentioned the storage effect in urban areas. Several studies looked into the flow patterns and redistribution in streets during storm events and the implication in terms of flood modelling. Some recent research considered the criteria for safe evacuation of individuals in flooded areas. But some recent field measurements during the

2010–2011 Queensland floods A series of floods hit Queensland, Australia, beginning in November 2010. The floods forced the evacuation of thousands of people from towns and cities. At least 90 towns and over 200,000 people were affected. Damage initially was estimated at ...

showed that any criterion solely based upon the flow velocity, water depth or specific momentum cannot account for the hazards caused by the velocity and water depth fluctuations. These considerations ignore further the risks associated with large debris entrained by the flow motion. Global Climate Model

Modeling of climate impacts, on the other hand, is often done from a "top-down", global perspective. While these models can be helpful in predicting worldwide effects of global warming and in raising awareness about large-scale impacts, their spatial resolution is often limited to 25 km or more, making them less helpful for local planners in mitigating the effects of climate change on a street-by-street scale. Some advocate for an integration of localized hydrological modeling with larger-scale climate modeling, claiming that such integration allows the benefits of both forms of modeling to be realized simultaneously and creates the potential for modeling flooding due to climate change in a way that allows planners to design specific strategies to mitigate it at the local level. The curve number (CN) rainfall–runoff model is widely adopted. However, it had been reported to repeatedly fail in consistently predicting runoff results worldwide. Unlike the existing antecedent moisture condition concept, one of the recent studies preserved the parsimonious curve number runoff predictive basic framework for model calibration according to different watershed’s saturation conditions under guidance from inferential statistics. The study also showed that the existing CN runoff predictive model was not statistically significant without recalibration. CN runoff predictive model can be calibrated according to regional rainfall-runoff dataset for urban flash flood prediction.

Management

Mitigation

Gray infrastructure

One traditional urban flooding management strategy is gray infrastructure, which is a set of infrastructure types (including dams and seawalls) traditionally constructed of concrete or other impervious materials and designed to prevent the flow of water. While gray infrastructure can be effective in preventing flooding-related damage and can be economically valuable, some models suggest that gray infrastructure may become less effective at preventing flood-related impacts in urban areas in the future as climate change causes flooding intensity and frequency to increase.

Green infrastructure

An alternative to gray infrastructure is

green infrastructure Green infrastructure or blue-green infrastructure refers to a network that provides the “ingredients” for solving urban and climatic challenges by building with nature.Hiltrud Pötz & Pierre Bleuze (2011). Urban green-blue grids for sustainab ...

, which refers to a set of strategies for absorbing and storing stormwater at or close to the location where it falls. Green infrastructure includes many types of vegetation, large open areas with pervious surfaces, and even rainwater collection devices. Green infrastructure may prove to be an effective and cost-efficient way to reduce the extent of urban flooding.

Drainage systems

One way urban flooding is commonly mitigated is via urban drainage systems, which transport storm water away from streets and businesses and into appropriate storage and drainage areas. While urban drainage systems help municipalities manage flooding and can be scaled up as population and urban extent increase, these systems may not be sufficient to mitigate additional future flooding due to climate change.

Land use

Since the ratio of pervious to impervious surfaces across an area is important in flooding management, understanding and altering land use and the proportion of land allocated to different purposes/use types is important in flood management planning. In particular, increasing the percent of land dedicated to open, vegetated space can be helpful in providing an absorption and storage area for storm runoff. These areas can often be integrated with existing urban amenities, such as parks and golf courses. Increasing the pervious surface fraction of an urban area (e.g. by planting green walls/roofs or using alternative pervious construction materials) can also help de-risk climate-linked flood events.

References

{{reflist Flood control Stormwater management Drainage