Increased Precipitation

One of the most direct causes of increased flooding is increased precipitation. On average, the contiguous United States have seen an increase in precipitation of 0.2 inches per decade since 1901 [17]. Some parts of the country have seen much greater increases: Northeastern Illinois, for example, has seen a 30 percent increase in total annual precipitation since the beginning of the twentieth century [17]. More localized precipitation trends can also be caused by factors such as the increased atmospheric turbulence caused by the “rough” surfaces of city skylines. A 2018 modeling study suggested that urbanization increased the rainfall magnitude of Hurricane Harvey when it made landfall in Houston, Texas, exacerbating the impacts of climate change on the storm’s magnitude [18].

Increased storm intensity is an even more important flood trigger than increasing annual precipitation. Higher intensity storms mean that water doesn’t have sufficient time to infiltrate into the ground, resulting in increased runoff and flooding. There has been a trend of an increasing share of precipitation coming in the form of extreme one-day events; the amount of rain falling in the heaviest downpours has increased 20 percent in the last century [19]. The National Climate Assessment shows that heavy downpours have increased in both frequency and intensity since 1901. Increasing amounts of heavy precipitation are particularly concentrated in the Pacific Northwest, the Ohio River Basin, the Great Lakes, and parts of the Missouri River Basin [49].

Increased Runoff

When rain falls, some is trapped in the canopies of trees (interception), in potholes in the street, or in natural rills in the ground surface. This water is returned to the atmosphere through evaporation and is known collectively as initial abstraction. Some water infiltrates into the ground, where it either travels down toward the saturated zone (see this module for more information on groundwater) or laterally as part of unsaturated subsurface interflow. Some infiltrated water in the unsaturated zone may eventually be taken up by plant roots and returned to the atmosphere through transpiration. Any water that is not part of initial abstraction or infiltration becomes surface runoff [21].

Surface runoff starts as sheet flow before deepening along temporary ridges and gullies into shallow concentrated flow and eventually finding its way into local waterways as open channel flow. Although both sheet flow and shallow concentrated flow can cause localized flooding if mismanaged, hydrologists generally evaluate the impact of excess runoff on a watershed. A watershed is the land surface area that drains to a fixed outlet point (typically the mouth of a stream or river). Watersheds can be evaluated at multiple scales: small tributary streams will have localized watersheds, with those watersheds combining to form the watershed of the larger river that the streams flow into. In general, the larger the watershed, the greater the lag time that a single storm event will have in impacting it. The science of flood modeling simulates how water flows within watersheds and how those flows of water might cause different types of flooding [22]. (See map below and U.S. Geological Survey's "Science in Your Watershed"). 

Any process that decreases either initial abstraction (particularly interception) or infiltration will increase runoff—and downstream flooding. An example of this is urban flooding. By substituting natural soils and vegetation with paved surfaces, cities increase the runoff potential of a site by decreasing the surface’s infiltration capacity. Many urban areas also decrease interception by removing tree cover.

The capacity of soil to infiltrate water is highly dependent on both its composition and its structure: soils that have been desiccated, compacted, or lost their topsoil through erosion will not be as effective in infiltrating water. Deforestation and wildfires can thus indirectly contribute to long-term flooding issues because the reduced vegetation cover leads to topsoil erosion and compaction of the remaining soil through the force of direct raindrop impact. Long-term drought also reduces the capacity of a soil to retain moisture, leading to cycles of drought and flooding within the same geographical area [21].

Erosion, Subsidence, and Sedimentation

Erosion is the transport of fine-grained topsoil through wind and water. Soil erosion exacerbates the risk of flooding by reducing the ability of the remaining soil to retain water, contributing to increased runoff. The eroded sediments wash into local streams and rivers and accumulate within their channels, altering their bed geomorphology in a process called sedimentation. This process changes the waterways’ flow path over time, lessening their conveyance capacity and leading to increased overbank flooding [23]. 

Sedimentation can also occur on the floodplain as part of the natural flooding process. However, the deposition of sediments in the floodplain reduces its long-term flood storage capacity, increasing the vulnerability of downstream locations. 

In certain landscapes, such as river deltas, sediment deposit from rivers forms an important aspect of local geomorphology. Rivers that have been significantly channelized or dammed (like the Mississippi) may face changes to their natural sediment loads that lead to lower rates of sediment deposit in their deltas. This phenomenon is known as sediment starvation [24]. New Orleans has suffered from a combination of sediment starvation and land subsidence caused by the drainage of local soils for farming and oil and gas pipeline channels. As a result, nearly 50 percent of the city is now below sea level, leaving it vulnerable to flooding from events such as hurricanes (see Vintage Assets, Inc. v. Tennessee Gas Pipeline Company, 2016) [25]. 

Subsidence can also occur in relation to glacial isostatic adjustment. Glaciers reached their greatest extent during the last ice age about 22,000 years ago; since that time, Earth’s crust has been “rebounding,” i.e., adjusting to the disappearance of these massive ice sheets, resulting in the elevation of land that was once under ice and the sinking of land previously on the edge of ice sheets [26]. Certain U.S. coastal areas (for example in New Jersey and along the northwestern Atlantic coast) fall into the latter category, and as such are subject to even greater sea level rise than would otherwise be expected, exacerbating coastal flood hazards [27].  

Downstream Blockages and Backups

Floods can be caused by natural or manmade blockages in a river channel. In cold climates, ice jams can lead to localized flooding due to the backup of water behind the jam, while a jam breach can lead to catastrophic flooding downstream. Jam breach floods can be particularly destructive because the rushing water carries with it large chunks of ice capable of doing substantial damage to downstream structures. Ice jams are created in the spring when snow and ice begins to melt. The river can carry large chunks of floating ice on its surface, which can become stuck in areas where the river suddenly narrows, such as near a bridge pier. Ice jams can also occur where a river’s slope suddenly becomes less steep, or where the river intersects an area with a natural buildup of stationary ice, such as the outlet of a lake [28] [21]. 

Culverts are used to route flow where the path of the water intersects with a structure such as a road or railway. Because most culverts are smaller in cross-section than the waterways they convey, there is usually some increase in water elevation in the area upstream of the culvert (known as a backwater effect), which is accounted for in the design process. However, culverts designed to carry runoff many years ago may not be able to manage increases in runoff due to increased precipitation or land use changes. Undersized culverts become hydraulic pinch-points during storm events, resulting in local flooding upstream, overtopped roadways, and downstream erosion due to the high velocity of water exiting the culvert. Smaller culverts are at higher risk of getting clogged by debris. Culverts should be regularly maintained to prevent upstream flooding [29]. 

Infrastructure Failure

Manmade dams are used for a variety of purposes, including recreation, flood control, domestic water supply, and irrigation. Some dams are used to form ponds or impoundments as part of the treatment process for mining tailings or coal ash (See Water Quality: Common Sources of Pollutants). Most dams in the United States are embankment dams, made of natural soil, rock, and waste from mining operations. The next most common type of material used in dam construction is concrete, typically reinforced with steel.

Importantly, though dams are regulated through a combination of state and federal enforcement, they are mostly privately owned and operated [30].

Water escapes from a dam in a number of ways:

• Spillways serve as a primary mechanism for water to escape the dam when the levels get too high. The specific type of spillway (free-overflow, gated, pipe-and-riser, or shaft) depends on the material out of which the dam is constructed and the amount of flow being managed. For example, dams built into a steep rock canyon, like the Hoover Dam, require a shaft spillway, located some distance upstream of the dam.
• Larger dams will typically also have an auxiliary spillway as an emergency release mechanism during conditions with high reservoir levels and high levels of inflow.
• Outlet works such as culverts and pipes are used to draw down the water in the reservoir for repairs and maintenance. Dams with uses dependent on controlled releases of water (such as hydroelectricity generation or flood control) may have gates or valves on these outlets to control the rate of flow.
• Water can also flow through or under a dam through seepage. Excessive seepage can be a sign of internal erosion (known as piping) and structural instability and requires careful monitoring and maintenance.
• Dams can overtop if the water levels in the reservoir get too high. While some concrete dams are designed to overtop, excessive overtopping can lead to degradation over time. Unintended overtopping as a result of inadequate spillway design (or spillway blockage) accounts for 34 percent of dam failures in the United States [31].

Dam failures are typically not directly caused by storm events. Rather, dam breaches are mostly caused by structural, mechanical, or hydraulic failures within the infrastructure itself.

Even without failure, dam operations can cause downstream flooding. Dams release water either in response to emergencies (such as during extreme events, when threatened overtopping would endanger the integrity of the structure) or in planned releases for flood management. Although many dams do have sirens to warn people downstream of an impending release, they can still cause extensive flood damage (see Arkansas Fish and Game Commission v. United States) [30].

Other Geographical Features

Certain geographic features are uniquely vulnerable to flooding. Alluvial fans occur in dry, mountainous landscapes, such as those common in the Western states, and in Kentucky, Tennessee, and West Virginia. They are made up of deposits of rock and soil eroded by water from the mountainside, which spreads out when they reach the valley floor in a quintessential fan shape. Although alluvial fans do contain natural drainage pathways, these channels are mutable and poorly defined. When a storm hits, runoff carries loose debris into the channels, filling them up and causing widespread flooding across the extent of the fan. Alluvial fan floods are fast-moving (flow speeds of up to 30 feet per second have been measured) and can cause widespread damage due to their capacity to carry large quantities of rock and other debris [21].

Sea Level Rise

Around the world, sea levels are expected to rise due to oceanic thermal expansion and the accelerated melting of glaciers and ice sheets. At the regional level, sea level rise can also be impacted by local changes to the ocean’s circulation and density, as well as land subsidence. Relative mean sea level [♦] is expected to increase between 0.31 and 0.52 meters on average for the contiguous United States by 2050, with higher mean sea level increases on the East and Gulf coasts, compared to the West coast [32].

Increased sea levels amplify the effects of other causes of coastal flooding. Even the relatively small increases in mean sea level over the past few decades have increased the frequency of flooding along the eastern seaboard. In particular, the annual frequency of minor (nuisance) tidal flooding in cities such as Miami, Florida, New York City, New York, and Charleston, South Carolina, has doubled in the past two decades due to increased relative mean sea levels [33]. Areas affected by sea level rise are increasingly vulnerable to storm surges and extreme storm events, as higher sea levels increase the area inland that can be reached during these flooding events. Rising sea levels also reduce the effectiveness of stormwater and wastewater conveyance systems. Finally, higher sea levels also accelerate challenges associated with coastal erosion.

[♦] Relative mean sea level (RMSL) is the average elevation of sea level compared to a standardized base reference point on land. As a result, RMSL may also be impacted by vertical land motion, such as land subsidence.

[17] United States Environmental Protection Agency, “Climate Change Indicators: U.S. and Global Precipitation,” July 17, 2021. [Online]. Available: https://www.epa.gov/climate-indicators/climate-change-indicators-us-and-global-precipitation. [Accessed June 6, 2022].

[18] W. Zhang, et al., “Urbanization exacerbated the rainfall and flooding caused by hurricane Harvey in Houston,” Nature, vol. 563, p. 384–388, 2018.

[19] C. Kennedy, “Heavy downpours more intense, frequent in a warmer world,” Mar. 4, 2014. [Online]. Available: https://www.climate.gov/news-features/featured-images/heavy-downpours-more-intense-frequent-warmer-world. [Accessed June 6, 2022].

[21] D. Tarboten, “Runoff Generation Mechanisms,” in Rainfall Runoff Processes, Washington, D.C.: NOAA COMET and Utah State University, 2003.

[22] T. Cech, Principles of Water Resources: History, Development, Management, and Policy, 3rd ed., Wiley & Sons, 2009.

[23] M. Nones, “Dealing with sediment transport in flood risk management,” Acta Geophysica, vol. 67, p. 677–685, 2019.

[24] F. Dunn and P. Minderhoud, “Sedimentation strategies provide effective but limited mitigation of relative sea-level rise in the Mekong delta,” Communications Earth & Environment, vol. 3, no. 2, 2022.

[25] R. Campanella, “How Humans Sank New Orleans,” The Atlantic, 6 Feb. 2018.

[26] University of Colorado, “What is glacial isostatic adjustment (GIA), and why do you correct for it?,” 2022. [Online]. Available: https://sealevel.colorado.edu/presentation/what-glacial-isostatic-adjustment-gia-and-why-do-you-correct-it. [Accessed July 12, 2022].

[27] T. Frederikse, K. Simon, C. Katsman and R. Riva, “The sea-level budget along the Northwest Atlantic coast: GIA, mass changes, and large-scale ocean dynamics,” Journal of Geophysical Research: Oceans, vol. 122, no. 7, pp. 5486–5501, 2017.

[28] National Oceanic and Atmospheric Administration, “What Is an Ice Jam?,” [Online]. Available: https://scijinks.gov/ice-jams/. [Accessed June 6, 2022].

[29] L. Mays, Stormwater Collection Systems Design Handbook, 1st ed., McGraw Hill, 2001.

[30] Federal Emergency Management Agency, “Living with Dams: Know Your Risks,” FEMA, Washington, D.C., 2013.

[31] Association of Dam Safety Officials, “Dam Failures and Incidents,” [Online]. Available: https://damsafety.org/dam-failures. [Accessed June 6, 2022].

[32] W. Sweet, et al., “Global and Regional Sea Level Rise Scenarios for the United States: Updated Mean Projections and Extreme Water Level Probabilities Along U.S. Coastlines,” NOAA, Silver Spring, Md., 2022.

[33] United States Environmental Protection Agency, “Climate Change Indicators: Coastal Flooding,” March 17, 2021. [Online]. Available: https://www.epa.gov/climate-indicators/climate-change-indicators-coastal-flooding. [Accessed June 6, 2022].

[49] V. Lee and D. Wessel, “The Hutchins Center Explains: National Flood Insurance Program,” Oct. 10, 2017. [Online]. Available: https://www.brookings.edu/blog/up-front/2017/10/10/the-hutchins-center-explains-national-flood-insurance-program/. [Accessed June 6, 2022].