Wetlands - Water Storage for Flood Water and Storm Runoff

During rain storms and snow melt events, the amount of water running over the surface of the land increases, and in severe storms, flooding may result. Many wetlands, particularly floodplain wetlands, have the capacity to temporarily store flood waters, during high runoff events. Although wetlands have often been referred to as natural sponges that soak up water, they actually function more like natural tubs, storing either flood waters that overflow riverbanks or surface water that collects in isolated depressions. As flood waters recede, the water is released slowly from the wetland soils. By holding back some of the flood waters and slowing the rate that water re-enters the stream channel, wetlands can reduce the severity of downstream flooding and erosion.

In watersheds where wetlands have been lost, flood peaks may increase by as much as 80 percent. On the Charles River in Massachusetts, the floodplain wetlands were deemed so effective for flood control by the U.S. Army Corps of Engineers that they purchased them rather than build expensive flood control structures to protect Boston. Wetlands within and upstream of urban areas are particularly valuable for flood protection. The impervious surface in urban areas greatly increases the rate and volume of runoff, thereby increasing the risk of flood damage. The drainage of wetlands, the diversion of the Mississippi and Missouri Rivers from their original floodplains, and the development allowed in the floodplains over the past 100 years were partly responsible for the billions of dollars in damage to businesses, homes, crops, and property that occurred as a result of the Midwest flood of 1993 (OEP 1993).

According to the Vermont Wetland Rules (pdf, 84 KB), wetlands that provide for the temporary storage of floodwater or stormwater runoff to the extent that they make an important contribution to: reducing risks to public safety, reducing damage to public or private property reducing downstream erosion or enhancing the stability of habitat for aquatic life, are significant wetlands. In determining whether a wetland is significant for the Water Storage for Flood Water and Storm Runoff, the Vermont Natural Resources Board shall, at a minimum, consider the extent to which the wetland meets the following criteria:

• Reduces either the magnitude or frequency of risks to public safety or of damage to public or private property due to flood water or stormwater runoff after considering:
• Its significance relative to other water storage capacity in its own watershed or in the watershed of any watercourse to which it is tributary;
• Whether it is contiguous to a lake or pond which would provide storage benefits independent of the wetland;
• The extent of development in any affected downstream flood plain;
• Its rating for flood storage using the Wetland Evaluation Technique;
• The extent to which a wetland's water storage capacity is created by beaver dams and similar temporary conditions;
• Wetland whose surface constitutes less than one percent of the watershed upstream of its outlet will be presumed to not be significant for reducing the magnitude or frequency of downstream flooding.
• Reduces the scouring and erosion of stream banks by attenuating flood peaks and reducing water velocities.
• Maintains the stability of important habitat for aquatic life by either attenuating peak flows of flood waters or stormwater runoff, or reducing the scouring and erosion of stream banks.
• Hydraulic and hydrologic analysis of the extent to which a wetland serves this function shall utilize average annual, 10 year, 50 year and 100 year storm frequencies in generating hydrographs for the wetland's inlet, outlet and at critical locations downstream.

• Wetlands and Floods: the University of Wisconsin discusses the role of wetland loss in flooding.
• Environmental Protection Agency: information on the role of wetlands on flood control.

Updated: January 2002