Floods in New England
Throughout the history of New England, water has played an important and defining role in shaping the landscape and how people live. Over the last 30 years, flooding in Massachusetts has resulted in losses estimated to cost $346 million. Nature is unpredictable, and there will always be a risk of flood. Yet we can reduce the risk of floods to communities, economies and environments by effectively communicating lessons from past floods and applying advancements in knowledge and technology in our planning and preparation processes. Below we provide a summary of answers to critical, underlying questions relevant to rivers and floods.
Why think about the full river system?
A river system includes the entire land area that drains into a river (this is called the catchment), often including many small streams and brooks. In New England, human activity has dramatically changed the paths of our rivers. Rivers have been channeled and straighten for logging and transportation; dammed for flood protection and hydropower; dredged for gravel and sand; and otherwise altered for farming and human settlements. As a result of these activities, the river body itself has often been cut off from its floodplains (the low-lying lands near the water) and riverbanks armored. Rivers adjust to such changes particularly during large storm events. Seemingly stable river channels may move, breach or erode their banks and large amounts of sediment may be moved or deposited along the river or in flooded areas. Much of the flood damage in New England is associated with these changes to the river and nearby lands. Such erosion processes threaten public infrastructure, houses, businesses, agricultural lands, and other private investments.
What factors contribute to floods?
Though there are many contributing factors, rainfall is a primary factor leading to flooding. When rain falls on a catchment, the amount of rainwater that reaches the river depends on the characteristics of the catchment, particularly its size, shape and land use. Some rainfall is ‘captured’ by soil and vegetation, and the remainder enters the river as flow. River characteristics such as size and shape, the vegetation in and around the river, and the presence of structures in and adjacent to the river all affect the level of water in the river. Flooding in New England can be attributed to four interrelated processes:
- Spring Thaw– During the spring, frozen land prevents melting snow or rainfall from seeping into the ground. Each cubic foot of compacted snow or ice contains multiple gallons of water and once the snow melts, it can result in the overflow of streams, rivers, and lakes. Add spring storms to that and the result is often serious spring flooding.
- Heavy Rains– The Northeast is at high risk from heavy rains produced by large storm events, including Nor’easters and Hurricanes. This excessive amount of rainfall can happen throughout the year. Large rain storms may inundate storm-water management infrastructure. Rapid accumulation and flows of large quantities of water (i.e., a flashflood) can roll boulders, tear out trees and destroy buildings and bridges. The combination of their strength and their unpredictability make flash floods are the #1 weather-related killer in the U.S.
- Levees, Dams, & Other Flood-Control Infrastructure– Under extreme loads, structures designed to retain and channelize floodwater can and do fail. Weakening of levees and other flood-control infrastructure over time, or as a result of weather events exceeding the infrastructure’s level of support, can cause it to be overtopped or breached, thus increasing the chance for flooding. During large rain events, dam managers may need to purposefully release floodwaters to protect the dam from catastrophic failure. This may increase the likelihood of flooding downstream.
- New Development& Land Use Change – Construction and shifts in landscapes can change natural drainage patterns and create new flood risks. The impermeable surfaces created by new buildings, parking lots, roads, and soil compaction mean less water is absorbed into the ground or stored during precipitation event
What are the consequences of floods?
Floods impact both individuals and communities and have social, economic, and environmental consequences. The consequences of floods vary greatly depending on the location and extent of flooding, and the vulnerability and value of the natural and constructed environments they affect.
How do we manage flood risks?
Flood risk is determined both by the likelihood that a flood event will take place and by its potential impact. A combination of risk assessment, land use planning, building codes, infrastructure, ecosystems-restoration/preservation and emergency operations planning can reduce risk. Risk assessment aids in identification of hazards and locations of concern. Land use planning involves strategic decisions regarding how land will be used in order to minimize damage. Building codes specify standards for structures in and near floodplains that may protect public health and property damage should a flood occur. Infrastructure can help move or drain water, block flows, or prevent erosion damage. Ecosystems restoration/preservation may provide flood protection by preventing river sedimentation and floodplain erosion during flood events. Riparian ecosystems provide bank stabilization and increases floodwater infiltration to more quickly reduce floodwaters. Lastly, emergency operations planning puts in place systems for dealing with and minimizing the effects of floods during and in the immediate aftermath of a flood.