Urban flood management

Urban flood management refers to the management of flood events in cities and surrounding areas. Urban flooding occurs for several reasons and has a wide variety of impacts, and there are correspondingly various strategies to mitigate those impacts.

Background[]

People kayaking down a street in Mid-City New Orleans following flooding in 2019

There are several types of flooding, including pluvial (flooding caused by heavy rain), fluvial (caused by a nearby river overflowing its banks), and coastal flooding (often caused by storm surges). 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.^[1] 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 cyclones, are also expected to increase,^[2] raising the risk of storm surges and the potential for heavy rainfall and increasing flooding-related damages by up to US$54B annually.^[3] 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.^[4]

Case studies[]

One of the most well known at-risk urban areas in the United States is New Orleans. Because of its coastal location and low elevation, the city is prone to flooding due to tropical storms, including cyclones and hurricanes and is particularly vulnerable to changes in sea level or storm frequency. In 2005, Hurricane Katrina caused more than 1800 deaths and US$170B in damages.^[5] 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.^[6]

During the summer of 2021, Hurricanes Henri and Ida caused significant flooding in many cities along the east coast of the United States.^[7][8] 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.^[9] In September 2021, the New York City mayoral office released a new rainfall preparedness plan.^[10]

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.^[11]

Urban flooding also impacts critical public services, including public transportation systems.^[12][13] Traffic congestion can be worsened by urban flood events,^[14] 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,^[15][16] which can create significant interruptions in the availability of goods and services, as well as financial losses for businesses.

Modeling[]

Flood modeling is often conducted in a very localized fashion, with hydrological models created for individual municipalities and incorporating details about buildings, infrastructure, vegetation, land use, and drainage systems.^[17] 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.

Global climate models usually do not include detailed local predictions

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.^[18]

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.^[19]

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^[20] and can be economically valuable,^[21] 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.^[22]

Green infrastructure[]

A schematic showing how green infrastructure and water management can be integrated

An alternative to gray infrastructure is green infrastructure, 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.^[23] Green infrastructure may prove to be an effective and cost-efficient way to reduce the extent of urban flooding.^[24]

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.^[25]

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.^[26][27] 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.^[28] 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.^[29][30]

See also[]

• Green infrastructure
• Integrated urban water management
• Urban flooding
• Climate modeling

References[]