EPA: Wetlands and Flood Protection

Wetlands are important everywhere, but they are especially important in urban areas for their role in stormwater management and flood mitigation. This underscores the need for infill to tread lightly when urban greenspace is at stake. The Environmental Protection Agency states (emphasis added):

Wetlands function as natural sponges that trap and slowly release surface water, rain, snowmelt, groundwater and flood waters. Trees, root mats, and other wetland vegetation also slow the speed of flood waters and distribute them more slowly over the floodplain. This combined water storage [and] braking action lowers flood heights and reduces erosion. Wetlands within and downstream of urban areas are particularly valuable, counteracting the greatly increased rate and volume of surface-water runoff from pavement and buildings. The holding capacity of wetlands helps control floods and prevents water logging of crops. Preserving and restoring wetlands, together with other water retention, can often provide the level of flood control otherwise provided by expensive dredge operations and levees. The bottomland hardwood-riparian wetlands along the Mississippi River once stored at least 60 days of floodwater. Now they store only 12 days because most have been filled or drained.
An EPA brochure, Wetlands: Protecting Life and Property from Flooding (PDF), goes on to say:

A one-acre wetland can typically store about three-acre feet of water, or one million gallons. An acre-foot is one acre of land, about three-quarters the size of a football field, covered one foot deep in water. Three acre-feet describes the same area of land covered by three feet of water. Trees and other wetland vegetation help slow the speed of flood waters. This action, combined with water storage, can actually lower flood heights and reduce the water’s destructive potential. (Source: EPA)

…In Minnesota, an additional study by The Wetlands Initiative noted that flood peaks and damage costs would be decreased by restoring the natural hydrology of the floodplain. The cost of replacing the flood control function of the 5,000 acres of wetlands drained each year in Minnesota alone would be $1.5 million, compared to the potentially millions of dollars lost to flooding. Preserving wetlands in the first place and restoring some of those that have been drained could help reduce future flood losses. (Source: The Wetlands Initiative)

…Along the Charles River in Massachusetts, the U.S. Army Corps of Engineers (the Corps) has acted to utilize wetlands in preventing flood damage. It was calculated that loss of all wetlands in the Charles River watershed would have caused an average annual flood damage cost of $17 million. The Corps concluded that conserving wetlands was a natural, less expensive solution to controlling flooding than the construction of dikes and dams alone, and they proceeded to acquire 8,103 acres of wetlands in the Charles River basin for flood protection. (Source: U.S. Army Corps of Engineers – Charles River Natural Valley Storage Area)

…The Vermillion River has always flooded. It has a narrow channel and flows slowly, making it “flood prone.” Thousands of years ago, this part of South Dakota was scoured by glaciers that carved out shallow depressions which remain today and seasonally fill with water. These “prairie potholes” are intermittent, seasonal wetlands which dot the landscape. They quickly thaw in spring and provide habitat for a multitude of migratory birds and other water fowl.

For hundreds of years, the rain and snow melt in the watershed were held in these wetlands, and runoff across the prairie was slowed. As South Dakota became populated, many prairie potholes were filled to facilitate farming. While these wetlands are small, they are numerous and can hold a significant amount of flood water. As more wetlands were filled, flooding increased.

The Great Flood of 1993 was devastating to the area. To combat future flooding, structural flood controls were put in place, but they were not sufficient. In response to this problem, the National Park Service and the Federal Emergency Management Agency formed a partnership with the South Dakota Division of Emergency Management and Turner-Lincoln-Clay Counties Water Project District. Working together, this coalition assessed the area and condition of the remaining network of potholes. They developed a plan to protect the remaining wetlands and restored some of those that had been filled. (Source: National Park Service, “Floods, Floodplains and Folks”, 1996, The Vermillion River: Managing the Watershed to Reduce Flooding, Federal Emergency Management Agency)

…Preserving wetlands, along with other flood control measures, can offer a degree of protection against flooding that is often more effective and costs less than a system of traditional dikes and levees. If more communities protect existing wetlands and increase the quantity of wetlands through restoration projects, we will be better protected against the consequences of floods.

See also:

The Why Files: Wetlands and floods
Wetlands are major flood-control agents in the watershed of the Charles River, which flows through Boston. Larson observes that when the Corps of Engineers arrived to investigate the flooding problem, it was raining heavily, and they saw the marshes filling and “acting as natural flood controls. The rain was after a dry period–and the water went downstream over three weeks rather than three days.”

After calculating the cost of creating that amount of storage with dams, Larson says the Corps concluded it “would be a lot less expensive to buy 8,000 acres of natural wetland, and to use conservation restrictions to buy up development rights” on other floodplain acreage.

Northampton’s Flood and Natural Hazard Mitigation Plan: Floyd Flood Damage Reported Behind View Avenue; Avoid Building on Filled Wetlands
In a table of Existing Mitigation Strategies, the plan includes a “100 foot buffer around wetlands and the wetland resource area itself…” It says this strategy has been “Effective”, and says that an option to improve it would be to “Strengthen Wetland Ordinance”.

As Hurricane Threat Builds, Has Complacency Set In about Flooding?
Infill sounds great on paper, but when it means paving over green space in downtown Northampton, it runs contrary to sound flood mitigation practice. The reality is that much of the remaining green space in downtown is in low-lying areas that are most susceptible to flooding. It makes sense to go along with the collective wisdom of the past 350 years and leave them undeveloped.