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Inadequate Floodwater Storage Capacity

Due to climate change, our region is already getting more rain than was the case historically. Precipitation events are become more frequent and more intense but of a shorter duration, and our storm water infrastructure is simply not designed and sized to handle the amount of rainfall we are receiving.

According to The Significance of Rain Atlases for Aquatic Management ( Part 1): "The rain gauge data set in the Updated Bulletin 70 incorporates precipitation records from 1948-2017. Statistical adjustment methods were also applied in recognition of trends of increasing precipitation and more frequent extreme weather events. However, in 2018, 2019 and 2020 the Chicago area received annual rainfall in excess of 45 inches. Average rainfall historically had been 34 inches/year. Several years have been above 50 inches. This begs the question of whether we can continue to depend on historical data for purposes of ongoing design."
Wetlands have a great deal to offer in terms of floodwater storage capacity. Because process-based restoration leads to more complex river systems with wider corridors of riparian vegetation and wetlands, these systems are able to impound more storm water throughout a watershed.
According to "Shelter from the Storm: How Wetlands Protect Our Communities from Flooding," Spring 2015, Environment America: "The ability of wetlands to hold large amounts of water enables them to serve as a key protection against flooding. During times of heavy precipitation, wetlands act as a sponge – slowing the velocity of runoff and retaining excess water, thereby reducing the danger of flooding. Once captured by a wetland, excess water evaporates, settles into the soil to replenish groundwater, or is slowly released over time....The flood protection that wetlands provide is valuable. According to one study, inland wetlands in the United States provide over $237 billion in water flow regulation services annually."
Illinois Levee Infrastructure Is Aging
In its May 2020 report, "High-Risk Levees Along the Upper Mississippi River Report," the Environmental Policy and Law Center reviewed the
United States Army Corps of Engineers’ (USACE) records and identified the need for much more robust monitoring, reinforcement, and repairs of Illinois levees. There are levees at risk of breach all along the Mississippi River in Illinois, Iowa, and Missouri. The key takeaways for Illinois levee risks:
  • Four levees were assessed to be at “high-risk,” putting 154,200 people and $16.9 billion worth of property in danger.
  • Five levees were assessed to be at “moderate-risk,” putting 8,353 people and $1.9 billion worth of property in danger.
  • 27 levees had not been assessed for safety standards, putting 5,161 people and $1.04 billion worth of property in danger.
  • In 2019, local news articles show four levees in Illinois failing (two that were low risk and one that was never assessed), putting 312 people and $51 million worth of property in danger. 
A wider river-wetland corridor along these rivers would relieve a significant amount of the pressure off the aging levee systems.
Milwaukee River Watershed Study Demonstrates That Beaver Dams Upstream Decrease Flooding Downstream
A 2020 study of the Milwaukee River watershed, "Hydrological Impact of Beaver Habitat Restoration in the Milwaukee River Watershed,"  demonstrated that beaver dams on the upper reaches of a watershed significantly decreased flooding downstream. The study was conducted by Milwaukee Riverkeeper, the Milwaukee Metropolitan Sewerage District, and University of Wisconsin Milwaukee. The study used the The Beaver Restoration and Assessment Tool (BRAT) to estimate the likelihood of beaver dam building activity and beaver dam capacities in the Milwaukee River watershed, based on GIS analysis of the stream network, vegetation cover, and stream power under baseflow and high-flow conditions. The simulations showed that peak flow rates were reduced by 6% to 48%, and flood flow volumes were reduced by 14% to 48%, depending on the development stages of beaver dams, and actual storm characteristics. Two factors contribute to peak flow reduction: (1) flow interception by storage capacity of beaver dams makes the primary contribution; and (2) energy dissipation through dam overflow when the storage capacity is filled. 

For More Information on How Wetlands Mitigate Flooding
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