Reducing Flood Damage Study

Too much drainage, too much damage

Wetlands needed to reduce flooding disasters

Despite a century of massive investment in flood control structures throughout the Upper Mississippi River Basin, flood damages have increased. Indeed, it could be argued that it is because of our emphasis on structural solutions, rather than ecological solutions, that we now face these damages. Clearly new solutions need to be explored.

The Wetlands Initiative believes the solution requires new thinking about the way we use our floodplains. Rather than immediately discharging water downstream after a precipitation event, we need to store that water on and in the ground for days, if not weeks. We need to restore the natural hydrology of the unleveed 100-year flood zone while reconnecting much of the leveed floodplain to its parent river. In short, we should be returning the floodplain to its basic functions—holding floodwaters, improving water quality, and supporting rich, biodiverse habitats.

To assess what it would take to implement this solution, the Wetlands Initiative (TWI) and its partners conducted a study, funded by the McKnight Foundation, to determine how much of the 100-year flood zone was existing or drained wetlands. Called Flood Damage Reduction in the Upper Mississippi River Basin: An Ecological Alternative, the study estimated the potential storage capacity of the floodplain if leveed lands and restored wetlands were pressed into providing the services of flood storage.

TWI began with an analysis of the 100-year floodplain within 77 counties of Iowa, Illinois, Minnesota, Missouri and Wisconsin. Within this study area, we identified the hydric soil, wetland, land use, and leveed areas. Our study area covered 24% of the total 100-year flood zone of the five states. These data were then used to extrapolate landscape characteristics for the entire area of the five states in the basin. We concluded that:

• Almost 7 million acres (37%) of the 18.4 million acres in the 100-year flood zone in the five-state region were wetlands prior to European settlement.
• Approximately 4 million acres of these former wetlands have now been drained for row crops (3 million acres) and for pasture (1 million acres). This means that nearly 60% of the former wetlands on the 100-year flood zone are now used for agriculture.
• Federal levees isolate approximately 2.3 million acres (13%) of floodplain from their parent rivers.

 Restoration Strategy

Based on these findings, TWI estimates that restoring the 4 million acres of former wetlands—land now used for row crops or grasslands—could significantly relieve flood problems in the basin.

By holding 3 feet of water in restored wetlands as well as in existing wetlands, we could acquire approximately 16.5 million acre-feet of flood storage. [An acre-foot is the volume of water that fills one acre of land at a depth of one foot.] We also could use land currently behind levees to store floodwaters. By adding spillways to the top of existing levees, 23 million acre-feet of floodwaters could be stored, assuming that they hold 10 feet of water during a flood event.

Hence, by replumbing our landscape—restoring water-holding wetlands on unleveed land and using our leveed areas to receive floodwaters—we could increase flood storage in the five state area by almost 40 million acre-feet of water. This volume is comparable to the 39 million acre-feet (as measured at St. Louis) that is credited with causing the damage in the Mississippi River Flood of 1993.

Our strategy is possible, but would require changes to our land use and subsidy practices. Rather than paying landowners to recover from damage after flooding, we would need to develop mechanisms that pay landowners to receive floodwaters. At the same time, urban or residential development behind leveed areas would need to cease. Although some farming on the floodplain might continue, rather than assuming that flooding is a rare event that needs to be avoided, farmers would treat flooding as an event to be expected and one from which benefits can be gained. Our study estimated that the economic benefit from converting cropland in the floodplain to wetland flood storage would be $500 million per year!¹

In addition to alleviating flooding and flood damage, restored wetlands would provide important water quality and wildlife benefits. For example, restored wetlands could support more than 150 bird species, including 60 that are of high conservation interest, whereas farm fields in our study supported only 50 bird species—only 8 of high conservation concern. Similar increases in other wildlife are likely.

For further reading:

Considering an ecological means to reduce  flood damages in the Upper Mississippi River Basin (TWI research study):  Executive summary (PDF, 6 pgs) or complete study (PDF, 44 pgs)

Mississippi Floods Can Be Restrained with Natural Defenses (National Geographic, 5/3/2011)

¹Economic benefit was computed as the savings from reduced crop damage and subsidy payments, plus income from other wetland benefits (e.g., recreation), minus the loss of rental income for cropland, plus wetland construction and operating costs.