Learn about flood control projects and how gray infrastructure works to alleviate flooding.
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About gray infrastructure
Gray infrastructure is an engineered infrastructure that is built from hard, impervious materials such as concrete. Often, gray structural types are used in conjunction with one another and/or in tandem with green infrastructure projects to reduce flood risk. There are multiple kinds of gray flood infrastructure, six of these structure types are discussed below.
Levees are man-made structures that are located along rivers, streams, creeks, and tributaries as well as lakes and on the coast. Levees are embankments that are typically made of earthen materials such as clay and other soils that are impervious. These structures work to protect surrounding areas from certain levels of flooding; once this level is exceeded levees can be overtopped. Levees are built parallel to waterways to keep them from flooding the land. Land behind levees is still subject to flooding from waterways as these structures can fail or be overtopped depending on the flood scenario.
The anatomy of a levee
There are 45,645 levees in the US Army Corps of Engineers National Levee Database covering 28,093 miles in the U.S. Levees require upkeep especially due to the fact that the average age of the structures is 57 years old. Only a small fraction are accredited by FEMA. For a levee to be accredited, it must meet federal design, construction, operation, and maintenance requirements. These requirements state that the levee must adequately reduce the risk of a 1-percent-annual storm. In the U.S. Army Corps of Engineers (USACE) Levee Safety Program, a majority of levees, 85%, are maintained and owned by local entities. The remainder is overseen by USACE or another federal or state agency. Levees can protect against storms larger than the 1-percent-annual storm such as the 0.2-percent-annual storm and they can also protect against smaller, more frequent storms such as the 20-percent-annual storm. The cost of levees varies on the size and level of protection of a levee and how much funding a community is able to get from federal and/or state governments.
Risks of levees
Conditions in a community and levee conditions change over time so it is important to note that whether a levee is accredited or not, building levees does not mean a community is flood-proof. Levees reduce the risk of flooding but they do not eliminate this risk. Levees are subject to being overtopped and breached and they also can have other structural deficiencies such as seepage, piping issues, encroachments, and erosion. When looking up a property on Flood Factor®, if a property is within the service area of a levee, there will be a flag under this property stating that it is protected by a levee. A property owner can then go to the National Levee Database to look up their community and see if there is further information on the levee protecting their neighborhood. Researchers may be interested in looking up this information on the First Street API’s adaptation dataset. It is important to know the level of protection a levee provides and understand community emergency plans should a levee fail. In 2019, levees in Illinois, Iowa, and Missouri were breached during a historic storm on the Mississippi River. Also in 2019, levees were breached and overtopped along the Missouri River in Kansas, Missouri, Nebraska, and Iowa. A levee breach happens when the levee gives way allowing for flood waters to surge into protected areas behind the levee. A breach can occur gradually or suddenly. During large storm events when water levels are high and moving quickly, levees are at risk of being breached or they can also be overtopped. A levee is overtopped when floodwaters exceed the height of the levee.
2011 levee breach near Hamburg, Iowa.
Seawalls are structures located parallel to shorelines that are built to protect the land behind them from storm surges and wave action. Seawalls are made of concrete, masonry, or sheet pile and typically are placed between the transition from the beach to the mainland. The heights of seawalls vary depending on the height of storm surge they protect for. Seawalls typically at least reach the height of the mainland elevation and add a few feet of freeboard above the mainland elevation to protect against waves overtopping. These structures vary in length and the number of properties that they protect; individuals can build a seawall to protect their property or cities can build a seawall that stretches across miles of properties such as the ones in Virginia Beach, Stamford, and Galveston.
Galveston Seawall - Galveston, Texas
As with levees, it is also important to understand the risks associated with seawalls. Seawalls do not provide total protection from flood risk. They can be overtopped, breached, or suffer other issues such as seepage if they are not kept properly. Cracks can develop over time which leads to water seeping which then erodes the land around the seawall and eventually leads to seawall failure. Seepage and erosion cause instability of the seawall and during a large storm, the seawall can collapse causing flooding.
Tide gates operate to prevent backflow during high tides. Tide gates are typically placed in areas where streams and other riverine waterways meet the ocean. The most common way a tide gate operates is that it is installed on the downstream end of a culvert. The tide gate then opens or closes due to water level differences between the upstream and downstream ends of the culvert. Since they only open one way, the tide gate opens for downstream flow and closes for tidal flow thus preventing backflow during high tides.
A system of tide gates - West River, New Haven, Connecticut
Tide gates risk
Tide gates are built to withstand high tides but as with all flood infrastructure, this level can be exceeded causing flooding or infrastructure failure. If hinges are improperly installed or become corroded over time, the tide gate can fail and remain closed. Tide gates can also suffer damage from debris. These structures, like all gray infrastructure, must be maintained and checked regularly to ensure that they operate properly.
Pump stations work to reduce tidal flooding and flooding from stormwater. A pump station reduces the amount of standing water in low-lying areas. Pump stations can also be used in tandem with levees and other flood barriers to providing extra capacity to move water that floods the dry side during storm events out of communities. They are placed in areas where water cannot be moved by gravity; the energy from the pump station is needed to move the water out of these areas. Pump stations are connected to drainage systems in order to quickly and efficiently remove water from drainage pipes so that the system is not overwhelmed and does not flood.
Pump station at the West Closure Complex - Belle Chase, Louisiana
Pump station risks
Pump stations can fail in large storm events if the power goes out and there is not an emergency diesel-powered backup generator. They can also be overwhelmed in large storm events that produce water amounts beyond what the infrastructure is built to handle.
Channelization projects occur in areas across the U.S. This gray infrastructure type is used to prevent bank erosion and control flooding in existing waterways. Rivers and other waterways are engineered to be wider or deeper in order to increase the capacity of water flow volume. This then leads to less damage to banks because water can move more efficiently through the channel. Channels must be maintained to prevent debris and other material from building up and causing jams that lead to flooding. Federal channelization projects are done by the USACE across the U.S.
Flood channel example image
In the Southwest, channels also known as arroyos are used for intense rain events that cause flash flooding. These channels are dry until there is a rain event and directs water back into a waterway. Depending on the length and size of the project, channel project costs can vary and so can the duration of the project.
Channels are built to convey a certain amount of water and when that amount is exceeded, channels are overtopped causing flooding. Debris or ice can build up in channels causing the channels to become clogged which can also lead to flooding.
Dams provide flood control and also serve many other purposes: water storage, recreation, navigation, electrical generation, and irrigation. Flood control dams keep floodwaters impounded and either release floodwaters in controlled amounts downstream to the river below or store or divert water for other uses. According to the National Inventory of Dams, there are over 90,000 dams in the U.S. and roughly 18% of these dams are used explicitly for flood control. The average age of dams is 56 years old.
US Army Corps tests Hartwell Dam spillway gates, Savannah River, South Carolina, and Georgia
Dam failures can be catastrophic for areas lying adjacent to dams. Dams are built to withhold hundreds of thousands of gallons of water but if dams are not maintained and operated properly, they can fail. These structures also fail during large storm events that overwhelm dams beyond what they can hold. Some dams have emergency spillways for when they are overwhelmed to divert excess water away quickly.
As of 2017, over 2,000 high-hazard risk dams were deficient and in need of maintenance. It is estimated that it would take $45 billion to repair high-risk dams. The catastrophic damage of dam failure was seen recently in Michigan when the Edenville Dam located on the Tittabawassee River failed causing thousands of people to evacuate. In March 2020, the dam failure caused flooding that damaged thousands of homes downstream. The 96-year-old dam did not meet Michigan safety standards and its emergency spillway had inadequate capacity.
The National Inventory of Dams shows the location of the nation’s dams. If a community lies downstream of a dam, make sure the dam is being properly operated and maintained, and check local community pages to see what the emergency response is, should a dam fail.