9 March 2014

Wetlands and Flood Mitigation: The 10 Percent Solution

Posted by John Freeland

Following the Great Flood of 1993, an official report called for more research to find ways to prevent flood devastation, apparently unaware the problem had been mostly solved over a decade earlier. But the implementation of a national wetland restoration program has faced intractable political and economic obstacles.

The Problem
Given the current extensive snow cover across the United States, communities are bracing for worse than average floods. According to the National Weather Service, flood costs in the U.S., averaged over 30-years, are $8.19 billion in damages and 89 lives per year.

Anyone who’s lived through a major flood probably knows those cost estimates are low because of all of the lost items that aren’t covered by flood insurance.

In the paper Learning from the Mississippi Flood of 1993:
Impacts, Management Issues and Areas for Research
, former Executive Director Gerald Galloway, Interagency Floodplain Management Review Committee, described the flood this way:

The costs to the nation from the flood were extensive. Thirty-eight deaths were attributed directly to the flood and estimates of fiscal damages ranged from $12 billion to $16 billion. Agriculture accounted for over half of these damages. More than 100,000 homes were damaged. Flood response and recovery operations cost more than $6 billion.

Many costs could not be quantified as impacts on businesses in and out of the basin were difficult to calculate and there was no accurate way to assess tax losses to governments. There also were impacts of the flood on the population’s physical and mental well being, but these too were difficult to sum

Although floodplain soils are some of the most fertile in the world, the Great Flood of 1993 caused extensive soil redistribution. Again, from the Galloway report (emphasis added):

Damage from scour and deposition affected 455,000 acres on the Missouri River floodplain representing 20 percent of the flooded cropland along the Missouri and Mississippi rivers. Drainage ditches were filled with sediments, and other agricultural infrastructure was destroyed. Almost 60,000 acres had sand deposition more than 24 inches  thick and reclamation costs to restore fertility to damaged cropland were estimated at $190/acre. If cropland restoration required removal of sand, it cost approximately $3,200 to remove each acre-foot of sand. Over $10 million was required to remove sediment and debris from ditches (US Department of Agriculture, 1993).

“The Midwest flood caused extensive damages to water and wastewater treatment plants and other public facilities. Damages to utilities, including water and wastewater treatment facilities and stormsewer systems, exceeded $85 million. Water treatment plants often are located in floodplains to be near well fields or the surface water that supplies the system. In addition, water supply lines must cross floodplains to serve floodplain residents. The EPA  identified 200 municipal water systems impacted to some degree by the flood. The most prominent example was the Des Moines Water Works that serves the City of Des Moines and adjoining communities. The plant was flooded and remained out of operation for 12 days and water from it was not safe to drink for another seven days. In addition to physical damages of $12 million, significant impacts were felt in the service area. Businesses and government offices closed because of lack of fire protection, and bottled water and portable toilets had to be provided for residents. The economic impact of the shutdown exceeded the cost of repair of the physical damage.

Wastewater treatment plants also tend to be located in floodplains, which are generally the lowest point in a community and offer the advantage of gravity flow. Furthermore the effluent from these plants is discharged into major rivers or streams… A total of 388 wastewater facilities were impacted by the flood (Knight, 1993).

The EPA determined that 59 Superfund sites experienced flooding; however, impacts to the sites were minimal and corrective measures were completed on sites requiring them. In addition, 73 solid waste treatment, storage, and disposal sites were flooded. Large propane tanks that were dislodged floated downriver and created the potential for massive explosions. Besides the large propane tanks, states collected over 18,000 orphaned drums — each with a potential hazardous or toxic substance — and a large amount of household hazardous wastes (US EPA, 1994). Daily loads of agricultural chemicals (herbicides and nitrates) transported by the Mississippi River were large relative to previous years; record flooding did not dilute the concentrations of herbicides. Concentrations of two herbicides (atrazine and cyanazine) in some samples from the Mississippi River exceeded health-based limits for drinking water. (Goolsby and Battaglin, 1993). The cumulative impact of any flood-related releases of hazardous materials, including pesticides, herbicides, and other toxic materials has not been yet established.”

The 10 Percent Solution
So, with a rough idea of the problems caused by major floods, the question becomes what to do about it. How can this trouble be reduced to a tolerable level?

In the 1971 USGS publication Estimating Magnitude and Frequency of Floods in Wisconsin, hydrologist Duane Conger used the Log Pearson Type III to develop a flood model that reflected the effects of different watershed characteristics. The characteristics included in the model included the following:

Drainage Area
Main Channel Slope
Lake and Marsh area (%)
Main Channel Length
Forest Cover Area (%)
Mean Annual Precipitation
Mean Minimum January Temperature
Mean Annual Snowfall
Soil Index (infiltration capacity similar to Hydrologic Soil Group)
Mean Frost Depth on February 28
Mean Snow Depth on February 28
Precipitation Intensity Index (2-year 24 hour rainfall)
Areal Factors differences between observed and computed statistical values and related to soils, geology and physiography.

In conclusion, Conger wrote (emphasis added), “Of the several basin characteristics used in this study, only drainage area, main channel slope, lake and marsh area, and areal factors were found to be statistically significant at the 99 percent effectiveness level for all flood frequencies.”

Building on Conger’s work, USGS hydrologist Richard P. Novitzki published Hydrology of Wisconsin Wetlands (1982) and related watershed wetland and lake area to flood flows and sediment yields. In his Figure 12, Novitski shows a dramatic reduction in flood flows when watershed lake and wetland areas approach 10 percent.


His Figure 14 shows the positive effects of wetland and lake area on sediment yields, again when the percentage of watershed wetland and lake area approaches 10 percent.

Certainly, optimizing the positive wetland functions of reducing flood peaks and pollution requires consideration of other factors beside areal percentage. For example, the placement of the wetlands in the watershed between human activities allows the wetland to collect runoff and pollution before in reaches the stream.

In their paper on wetland restoration, Hey and Philipi (1995) wrote

“The 1993 flood verifies the need for additional wetlands: the amount of excess water that passed St. Louis during the 1993 flood would have covered a little more than 13 million acres —half of the wetland acreage drained since 1780 in the upper Mississippi Basin. By strategically placing at least 13 million acres of wetlands on hydric soils in the basin, we can solve the basin’s flooding problems in an ecologically sound manner.”

The “10 Percent solution” is being applied in select watersheds in the Upper Mississippi reagion, for example, The Wetlands Initiative is working on a project in Illinois that would cut nitrogen runoff nearly in half by placing 7.7 percent of the Big Bureau Creek watershed into online (adjacent to streams) wetlands.

The 10 Percent Solution of restoring and creating wetlands for flood reduction and pollution control has tremendous merit, but implementation remains illusive. The problems are both political and economic. Some see a national wetland program as over-reach by the federal government. The cost of wetland restoration, including design and construction, and forgone revenue from land taken out of production needs to be paid for, somehow.

Until all of that’s worked, replacing lost wetlands will continue on a piecemeal basis. Perhaps, given enough time and pressure, like a slow geological process, local and regional groups can accomplish the task.