Midwest Flood: Information on the Performance, Effects, and Control of Levees
Published: 1995-08-07T00:00:00

Fast Facts:

Highlights:
section_title: Why GAO Did This Study
paragraphs: Pursuant to a congressional request, GAO examined the operations of levees involved in the 1993 Midwest flood, focusing on: (1) whether the Army Corps of Engineers' flood control levees prevented flooding and reduced damage or increased the flooding and added to the damage; and (2) the governments exercise control over the design, construction, placement, and maintenance of nonfederal levees.
section_title: What GAO Found
paragraphs: GAO found that: (1) 157 of the 193 Army Corps of Engineers levees found in the areas affected by the 1993 flood prevented rivers from flooding and $7.4 billion in damages; (2) 32 of the Corps levees withstood floodwaters until the water rose above the levees and overtopped them; (3) 4 of the Corps levees allowed water into protected areas before their design capacity was exceeded; (4) the Corps estimated that the breaching of these levees caused about $450 million in damages; (5) although levees allow floodwater to rise higher than it normally would because they confine a flood to a portion of a floodplain, the Corps believe that its levees have the net effect of reducing flooding; (6) no federal programs specifically regulate the design, placement, construction, or maintenance of nonfederal levees, however, flood insurance and disaster assistance programs may exercise control over certain levees; and (7) 17 states and various local governments have programs to regulate levees, many of which are in response to the requirements of the National Flood Insurance Program.

section_title: Letter
paragraphs: This report responds to your request that we examine the performance, effects, and control of levees involved in the 1993 Midwest flood.
As agreed with your offices, unless you publicly announce its contents earlier, we plan no further distribution of this report until 30 days from the date of this letter. At that time, we will send copies to the Secretaries of Defense, Agriculture, and the Interior; the Director, Office of Management and Budget; and other interested parties. We will make copies available to others on request.
Please contact me at (202) 512-7756 if you or your staff have any questions. Major contributors to this report are listed in appendix VII.
subsections: section_title: Introduction
paragraphs: The 1993 Midwest flood, termed “the Great Flood of 1993,” was unprecedented in the United States in terms of the amount of precipitation, the recorded river levels, the duration and extent of the flood, the damage to crops and property, and the economic impact. The intense rainfall that deluged the upper Mississippi River basin in the spring and summer of 1993 caused the largest flow ever measured at St. Louis. Affecting nine midwestern states, the rainfall generated record-high flood levels at 95 measuring stations on the region’s rivers. Because of the catastrophic flooding, 38 people died, millions of acres were inundated, property and agriculture sustained heavy damage, tens of thousands of people were evacuated from their homes, and transportation, business, and water and sewer services were disrupted. President Clinton declared 505 counties to be federal disaster areas, and estimates of the damage have ranged from $12 to $16 billion.
subsections: section_title: The Great Flood of 1993
paragraphs: The 1993 flood affected most of the upper Mississippi River basin. The basin drains all or part of 13 states and encompasses about 714,000 square miles, or 24 percent of the contiguous United States. The upper basin includes the Mississippi River from its source in Minnesota to its confluence with the Ohio River at Cairo, Illinois. Its principal tributary is the Missouri River, which joins the Mississippi at St. Louis, Missouri. Other major tributaries include the Minnesota, Wisconsin, Iowa, Des Moines, and Illinois rivers. Figure 1.1 shows the Mississippi River basin and the area of the 1993 flooding, and figure 1.2 compares two satellite images of the St. Louis, Missouri, area at the confluence of the Illinois, Mississippi and Missouri rivers during a severe drought and during the 1993 flood.
subsections: section_title: Saturated Soil, Heavy Rainfall Led to Flooding
paragraphs: The conditions that produced the flood began in the summer of 1992. According to the Department of Commerce’s National Weather Service (NWS), July, September, and November 1992 were much wetter than normal in the upper Mississippi River basin. Winter precipitation was near normal, but a wet spring followed. The period from April to June 1993 was the wettest observed in the upper basin in the last 99 years. As a result, soils were saturated, and many streams were flowing well above normal levels when summer rains began.
A persistent atmospheric pattern during the summer of 1993 caused excessive rainfall across much of the upper Mississippi River basin. Major flooding resulted primarily from a series of heavy rainfalls from mid-June through late July. A change in the upper air’s circulation pattern created drier conditions in late July and early August, but heavy rainstorms brought more flooding to parts of the upper basin in mid-August.
The rainfall over the upper Mississippi River basin from May to August 1993 is unmatched in the historical records of the central United States. Generally, rainfall from the Dakotas to Missouri and Illinois was well above normal. Figure 1.3 locates the heaviest concentrations of rainfall from January through July 1993 in the flood region.
Rivers rose above flood levels at about 500 measurement points in the nine-state region, NWS reported. Record flooding occurred at 95 measurement points in the Upper Midwest—44 on the upper Mississippi River system, 49 on the Missouri River system, and 2 on the Red River of the North system. Water flow rates along major parts of the upper Mississippi and lower Missouri rivers equalled or exceeded floods with an annual probability of 1 percent—commonly called a 100-year flood. Figure 1.4 shows where the heaviest flooding occurred.
Extreme flooding of major river systems like the Mississippi and Missouri rivers seldom occurs in the summer. During a typical midwestern summer, a few localized heavy rains are scattered throughout the region. In 1993, the rare combination of closely timed and record-level rainfall occurred on both the lower Missouri and upper Mississippi basins, causing a record flood at St. Louis.
NWS reported that the extended duration of the flood was also extremely rare. Typically, periods of above-average rainfall during a midwestern summer last from 2 to 5 weeks, sometimes persisting up to 8 weeks. In 1993, major flooding continued throughout the summer along the Missouri and Mississippi rivers. For example, as of September 1, 1993, Hannibal, Missouri, had experienced 153 consecutive days, or about 22 weeks, of water above flood level. Flooding continued through the middle of September in many regions along the Mississippi River.
section_title: Flooding Caused Widespread Damage
paragraphs: The administration established the Interagency Floodplain Management Review Committee to evaluate the performance of existing floodplain management programs in light of the 1993 flood. Their review of existing damage estimates for the flood found that these estimates ranged from $12 billion to $16 billion. The available estimates are from such federal agencies as NWS and the Federal Emergency Management Agency (FEMA), which develop estimates for specific program purposes, such as disaster response and assistance. The Committee estimated that $4 billion to $5 billion in damage was to crops in upland areas outside the floodplain, which were destroyed by the heavy precipitation there. The Committee attributed about $2.5 billion in agricultural damage directly to the flooding.
Other significant damage occurred to about 100,000 residences, more than 5,000 businesses, many bridges, hundreds of miles of roads and railroads, and 33 airports. The flood also closed the major rivers to navigation and affected about 200 municipal water systems, 388 wastewater facilities, and other public facilities, such as public buildings and parks.
FEMA reported that about 6.6 million acres in the floodplain were flooded in 1993, of which 63.4 percent were agricultural lands and 2.5 percent were urban areas. The remaining acres in the floodplain were normally covered by water, were wetlands, and/or were used for other purposes.
section_title: Federal Involvement in Flood Control
paragraphs: The primary federal agency involved in flood control is the U.S. Army Corps of Engineers (the Corps). The Department of Agriculture’s Natural Resources Conservation Service (NRCS) is indirectly involved in flood control when it addresses the effects of flooding in agricultural watersheds.
subsections: section_title: U.S. Army Corps of Engineers
paragraphs: After a series of disastrous floods affected wide areas, the Congress enacted the Flood Control Act of 1936. This act established a nationwide policy that (1) flood control was in the interest of the general public and (2) the federal government would cooperate with the states and local entities to carry out flood control activities. The Corps’ flood control programs are designed to reduce the susceptibility of property to flood damage and to relieve human and financial losses.
The Corps has invested over $23 billion in flood control projects nationwide. It has constructed more than 600 projects, including reservoirs and about 10,500 miles of levees and floodwalls. Flood control reservoirs often provide the capacity to store water for multiple uses, including municipal and industrial water supplies, navigation, irrigation, production of hydroelectric power, conservation of fish and wildlife, maintenance of water quality, and recreation. Levees and floodwalls are usually turned over to local sponsors for operations and maintenance.
The Corps is also authorized to perform emergency activities, such as fighting floods, repairing and restoring flood control works, and supplying emergency clean water to communities. It also performs emergency assistance work requested and funded by FEMA. Permanent repairs to levees and other flood control facilities are provided under a levee rehabilitation program. Five of the 37 Corps districts performing civil works activities were involved in the 1993 flood: St. Paul, Minnesota; Rock Island, Illinois; St. Louis and Kansas City, Missouri; and Omaha, Nebraska.
Of the 251 Corps levees located in these districts, 193 were in the flooded area.
The Corps operates 98 reservoirs in the upper Mississippi River basin to reduce flood damage. Of these, 22 were constructed by the Bureau of Reclamation. While not all of the reservoirs were in the flooded area, most had some impact on the flood because they stored water. For example, Corps headquarters officials said the reservoirs stored more than 20 million acre-feet of floodwater on August 1, 1993, reducing flood levels throughout much of the flood area—for example, lowering the crest of the Mississippi River at St. Louis on that day by 5 feet. In addition to the reservoirs, the Corps has built or improved more than 2,200 miles of levees for the protection of communities and agriculture in the basin.
subsections: section_title: Corps’ Floodplain Management Assessment
paragraphs: After the 1993 flood, the Congress funded a broad 18-month effort by the Corps to assess floodplain management in the upper Mississippi River and lower Missouri River basins. This effort, which was separate from the work of the interagency committee, describes the existing resources in the floodplain, identifies alternatives for the future use of the floodplain, and suggests policy changes and areas for further study. The assessment was conducted in collaboration with numerous federal, state, and local governments and interested parties. The Corps presented its findings and conclusions in a report published on June 30, 1995.
section_title: Natural Resources Conservation Service
paragraphs: NRCS’ programs are designed to protect and prevent flooding in small watersheds, repair or relocate agricultural levees that are damaged in flooding, and convert cropland to wetland reserves. The Small Watershed Program authorized by the Watershed Protection and Flood Prevention Act of 1954 (P.L. 83-566) provides for NRCS to install land conservation measures and flood damage reduction works nationally. NRCS traditionally works on smaller projects affecting watersheds of fewer than 400 square miles; the Corps addresses needs in larger watersheds. In addition, NRCS has authority under the Flood Control Act of 1944 (P.L. 78-534) for a flood prevention program for 11 watersheds.
In the nine midwestern states affected by the 1993 flood, NRCS has performed soil and water conservation work on 3 million acres, installed 2,964 reservoirs, and worked on 818 miles of channel. NRCS estimated that its watershed projects prevented $400 million in damage from the 1993 flood.
When a disaster strikes, NRCS implements the Emergency Watershed Protection Program under section 403 of the Agricultural Credit Act of 1978 (P.L. 95-334). This program provides assistance—including repairs to damaged levees—to reduce hazards to life and property.
section_title: Flood Control Levees in the Upper Mississippi River Basin
paragraphs: Levees are linear earthen embankments whose primary purpose is to prevent high water from reaching the floodplain. They normally extend from high ground along one side of a floodplain and around it to another area of high ground. Levees protect the area between the levee and the high ground.
For stability, an earthen levee is normally constructed so that its bottom width is several times its height; hence, a levee requires considerable land area. In urban areas where space is limited, the Corps builds masonry floodwalls. A long levee system may include a combination of several segments of earthen levees and floodwalls. Figure 1.5 represents a cross-sectional view of a typical earthen levee.
Levees reduce but do not eliminate flooding in the floodplain because levees may be overtopped by floods larger than those for which they are designed. Generally, the Corps analyzes the risks, costs, and benefits of constructing a levee to various heights; determines, with the participation of a local cost-sharing sponsor, how much protection the levee should provide; and proposes a plan to the Congress. After reviewing the Corps’ analyses, the Congress can authorize and fund the plan.
The height of a levee is based on the maximum flow (discharge) of floodwater, measured in cubic feet per second (cfs), associated with flooding of a particular frequency, or average recurrence level, at the levee’s location. For example, the Kaskaskia Island levee in the Mississippi River in Illinois, which is designed to withstand a flood with an average frequency (or recurrence interval) of 50 years, was built to withstand a water level of 45.7 feet and a flow of 1,010,000 cfs. At other locations, floods of the same frequency will be associated with different heights and flows. For example, the Des Moines-Mississippi River levee in Missouri, which also protects against a flood with an average recurrence interval of 50 years, was built to withstand a water level of 24 feet and a flow of 371,000 cfs. Both levees provide the same degree, or level, of protection but have different performance criteria because the river’s channel and flows differ greatly at these levees.
Engineers have accounted for uncertainties in the water level and for unknown factors—such as wave action, bridge openings, and the effects of urbanization—by adding 1 to 3 feet to the overall elevation of a levee’s design; this addition is known as freeboard. A levee should withstand floods up to and including the flood for which it was designed. Floods larger than the design flood may overtop or breach the levee—that is, cut a hole through it. The ways that floods can damage levees—by overtopping, piping, saturation, and underseepage—are depicted in figure 1.6.
Hydrologists have several ways of describing the size of a flood at a specific location. They may refer to the water’s level and flow, or they may refer to the average interval between occurrences of that particular water level and flow. They also refer to the annual probability that the same water level and flow may occur. Because floods occur randomly, the interval between extreme water levels of the same height are far from uniform—a large flood in one year does not preclude the occurrence of an even larger flood the next year. For example, a flood with an average recurrence interval of 100 years has a 1-percent chance of being equaled or exceeded every year. This means that a “100-year” flood may occur several times within a 100-year period, or it may not occur at all.
As communities and farms have grown on the floodplains of the upper Mississippi and Missouri rivers since the early to mid-1800s, levees have been constructed by various nonfederal entities, ranging from cities to individuals, to protect floodplains from seasonal flooding. Owners have wanted to protect the floodplain from flooding because it has often contained the most fertile land for farming. While no inventory of nonfederal levees exists, the Interagency Floodplain Management Review Committee estimated that such levees extend over 5,800 miles in the upper Mississippi River basin.
The Corps estimated that about 1,100 of the 1,358 nonfederal levees in the area covered by the five Corps districts involved in the 1993 flood failed to keep the flood out of the areas they were designed to protect or were otherwise damaged. Corps officials told us, however, that this estimate was approximate and incomplete.
Corps and nonfederal levees protect nearly all of the floodplain in the upper Mississippi River basin. Above Rock Island, Illinois, the Mississippi River floodplain is narrow and is filled largely with navigation pools. The remaining floodplain contains wildlife refuges, some farmland, and a few levees; scattered towns are protected by urban levees. Below Rock Island, the floodplain widens to as much as 6 miles, and because the extensive floodplain is used for crops, the river is almost continuously lined with Corps agricultural levees to Fort Madison, Iowa, and from Keokuk, Iowa, to Cairo, Illinois. In addition, many cities and towns, including St. Louis, are protected by levees and floodwalls in this section of the river.
Missouri River floodplains, used predominantly for agriculture, are protected to varying degrees by levees. Between Omaha and Kansas City, Missouri, the river is heavily lined with Corps agricultural levees. Between Kansas City and St. Louis, the Missouri River has four Corps levees, but the river is heavily lined with nonfederal levees.
Developed floodplains with larger urban areas—such as Omaha/Council Bluffs, Kansas City, and St. Louis—are largely protected by Corps urban levees. Near Kansas City and St. Louis, several residential, industrial, and commercial areas are built on floodplains behind levees.
section_title: Objectives, Scope and Methodology
paragraphs: The Ranking Minority Member of the Subcommittee on Water Resources and Environment, House Committee on Transportation and Infrastructure, and Representative William L. Clay of Missouri asked GAO to review the extent to which (1) the Corps’ flood control levees prevented flooding and reduced damage during the event; (2) these federal levees increased the height of the flooding and added to the damage; and (3) federal, state, and local governments exercise control over the design, construction, placement, and maintenance of nonfederal levees.
To address these objectives, we obtained information from the Corps and other federal agencies, state agencies, and other public and private organizations. We also interviewed officials and obtained documents from these agencies and organizations, as well as from individuals. Appendix I provides further details on our scope and methodology.
We conducted our review between November 1993 and July 1995 in accordance with generally accepted government auditing standards. We discussed the facts in our report with responsible officials of the five agencies primarily involved: the Chiefs of the Readiness, Hydraulics and Hydrology, Central Planning Management, and Policy Development branches in the Corps’ Civil Works Directorate; the Director of FEMA’s Program Implementation Division; the Deputy Chief for Natural Resources Conservation Programs and the Acting Director of the Watershed Projects Division at NRCS headquarters; the Chief of the Science and Applications Branch and staff from the Office of Surface Water at USGS headquarters; and the Chief of NWS’ Hydrological Service Branch. Generally, these officials agreed with the basic information provided but offered comments, corrections and suggestions to improve the accuracy and clarity of the report. We made changes to the report where appropriate.
section_title: Most Corps Levees Performed as Designed and Prevented Significant Damage
paragraphs: According to the Corps, 157 (81 percent) of the 193 Corps levees located in the area affected by the 1993 flood prevented rivers from severely flooding about 1 million acres. However, some of these acres were flooded by smaller streams behind the levees and by seepage under the levees. Nevertheless, the Corps estimated that the 157 levees prevented about $7.4 billion in damage during the flood.
Another 32 Corps levees withstood flood flows until the water exceeded their design capacity and overtopped the levees. Three other levees were breached without being overtopped by floodwaters, and an opening in an urban floodwall for railroad tracks was not closed in time to prevent flooding. The Corps estimated that flooding at the 36 levees caused about $450 million in damage.
To assess the levees’ performance, we compared information on the levees’ design values for either flows or water levels (design capacity) with the flows and water levels recorded during the 1993 event. Data were not available on either the levees’ design capacity and/or the 1993 flows or water levels for 12 of the 193 levees. Data on the other 181 levees show that 177 withstood the flows and water levels at least as well as designed and 4 did not. The flood eventually overtopped some of the levees. Nevertheless, local flood-fighting efforts at some locations permitted levees to withstand flows and water levels that exceeded the levees’ design capacity. Most levees also withstood saturation far longer than they were designed to do.
subsections: section_title: Levees’ Performance Is Measured Against Design Criteria
paragraphs: Corps officials told us that generally three basic design criteria apply to each levee. Two of these are flood level, expressed in feet, and flow, expressed in cfs. The five Corps districts involved in the 1993 flood used data on either flood level or flow or on both criteria to judge the performance of levees. The third design criterion is the extent to which a levee can be saturated and still withstand its design flood.
The Corps designs levees to withstand saturation. However, according to Corps district officials, the time required to reach a levee’s maximum saturation point varies by flood. A Corps manual (No. 1110-2-1913, Mar. 31, 1978) provides that levees are expected to be exposed to flood flows for only a few days or weeks per year. Embankments that will be exposed to flows for longer periods must meet more stringent criteria for earthen dams.
Water standing against a levee for an extended period may move through or under the levee, leading to problems such as sinkholes or sandboilson the landward side. The higher pressure of the floodwater will eventually overcome the materials within the levee and its foundation, and a breach may occur.
To determine whether a levee performed to its design capacity, we attempted to compare its design flow capacity with actual or estimated flows during the flood. If the levee’s flow capacity was not available, we used the levee’s flood level capacity. Corps district officials agreed that these measures were acceptable bases for assessing a levee’s performance. As an additional criterion, we considered the length of time the levee withstood saturation from flooding.
We asked the Corps to give us the design capacity of, and the flow rates or water levels at, the 193 levees involved in the flood. District personnel told us that all or some of the data were not readily available for 12 levees. They said that Corps field staff or local officials advised them that the land behind the 12 levees was not flooded. Therefore, they said they can reasonably assume that these 12 levees performed within their design capacity. None of the 12 levees were designated as overtopped on Corps levee repair schedules. Appendix II lists the levees for which insufficient data were available for our comparison.
section_title: Most Corps Levees Performed to Their Design Capacity
paragraphs: Of the 181 levees for which comparative data were available, 177 clearly performed up to their design capacity and sometimes exceeded it during the 1993 flood. Many levees withstood flows that, in some cases, were greater than those for which the levees had been designed because flood-fighting efforts extended their performance by raising their height. In addition, many levees experienced saturation far longer than they were designed to do.
Of the 177 levees that clearly met performance criteria, 145 prevented the river from entering the protected floodplain. The flood eventually exceeded the design capacity of the remaining 32 levees and overtopped them. Only four levees allowed floodwater to enter the protected floodplain before the levees were overtopped. Appendix III lists the 145 Corps levees in the flood area that the Corps said prevented flooding.
Figure 2.1 displays the location of the 193 levees in the flooded area and identifies the levees that met design criteria but were overtopped, as well as the levees that were breached or otherwise failed without first being overtopped.
subsections: section_title: Flood-Fighting Efforts Extended the Performance of Some Levees
paragraphs: Corps officials said that, in some instances, flood-fighting at levees prevented water from entering protected areas when the water reached flow rates or elevations beyond the levees’ design capacity. They said that workers prevented overtopping by piling sandbags and building other makeshift barriers on the top and landward sides of the levees. In these cases, a levee exceeded its design capacity in three ways. First, the added height permitted the levee to continue holding back the flood even when the water rose above the top of the levee. Second, the base of the levee withstood the additional water pressure created by extending the height. Third, because flood-fighting prevented or delayed overtopping, the levee withstood saturation far longer than anticipated.
Flood-fighting techniques effectively increased the design capacity of many levees. For other levees, such as those at St. Louis or North Kansas City, the success or failure of flood-fighting determined whether the levees were able to meet their original design capacity. Examples of flood-fighting efforts during the 1993 flood are described in appendix IV.
section_title: Floodwaters Exceeded the Design Capacity of Some Levees
paragraphs: According to the Corps, flooding caused 32 Corps levees to be overtopped. These levees were designed to protect against floods whose average recurrence intervals ranged from 20 to 500 years. Of the 32 levees, 26 were on the Mississippi and Missouri rivers where the flood was greatest. Five of the other six levees were located on the Illinois River, and one was located near the Missouri River. Table 2.1 lists, by Corps district and by river, the 32 levees that the flood overtopped, the design capacity of each levee, and the flood’s estimated flow or water level at each levee.
section_title: Levees Were Overtopped for Various Reasons
paragraphs: We compared either the design flow capacity of the levee with the flood flow recorded at the gauge nearest the levee, or the design height of the levee with the flood level recorded at the gauge nearest the levee.
For 29 of the 32 levees, either the flood flow exceeded the design flow capacity of the levee or the flood level exceeded the design height of the levee. For example, the peak flood flow for the South River levee along the Mississippi River just south of Hannibal, Missouri, was 524,000 cfs, far above the levee’s design flow capacity of 349,000 cfs.
The flood overtopped three levees, identified in table 2.1, even when the data indicated that the flood flow did not exceed the design flow capacity of the levees. Corps officials said that these cases may be generally explained by (1) a decline from the levee’s design flow capacity, which they attribute to a change in the relationship between the flood level and the flow rate at the levee that resulted in higher flood levels for the same flow rate; (2) the distance between the levee and the gauge used to measure the flood flow, which resulted in an inaccurate flood flow estimate for the levee location; and (3) the location of the overtopping. For example, the main portion of Missouri River Levee Unit R-520 was not overtopped, just as the data indicate. However, the levee was overtopped at a point along a creek where the levee’s design flow capacity was much lower.
The 32 levees whose design capacity was exceeded by the flood were concentrated in six general areas: above St. Louis, below St. Louis, the lower Illinois River, southeast Nebraska/northwest Missouri, north of Kansas City, and central Missouri. With one exception, the levees in these areas were built by the Corps to protect agricultural lands from floods. One levee was built to protect Elwood, Kansas, a community of about 1,000 residents, which lies across the river from St. Joseph, Missouri.
section_title: Four Corps Levees Were Breached or Failed
paragraphs: Three Corps levees, two on the Mississippi River and one on the Missouri River, were breached without first being overtopped by floodwaters. In addition, a railroad opening in a floodwall along the Raccoon River was not closed to prevent flooding in the city of Des Moines. Table 2.2 lists the design criteria and flood flows for these four levees.
Corps officials said that they would not classify the breaches of the Bois Brule and Kaskaskia Island levees as performance failures for two reasons. First, these breaches occurred when the water reached the freeboard area of the levee, which is the safety zone above the levee’s design height. They said that when water rises above a levee’s design height, the levee cannot be expected to continue holding against a flood. Second, because the duration of the 1993 flood far exceeded the design standard for levees, some Corps officials believe that the breaches should not be characterized as performance failures.
According to Corps district staff, the breach in the third levee, Missouri River Levee Unit L-550, was caused by the use in the construction of the levee of a material that allowed underseepage. Corps staff told us that the fourth levee failed to prevent flooding because a railroad opening in the floodwall protecting the city of Des Moines was not closed in time. Each of these situations is described in appendix V.
section_title: Corps’ Estimates of Flood Damage Are Imprecise
paragraphs: In its April 1994 report to the Congress on flood damage for fiscal year 1993, the Corps qualified the accuracy and completeness of estimates of the damage prevented and incurred because of the broad scope of the damage and the rapid compilation of preliminary estimates. Given their methodologies for estimating flood damage, the Corps and NWS said that their estimates of both the damage prevented and the damage incurred, presented in this report, are probably understated.
The Corps estimated that its levees prevented the flooding of about 1 million of about 1.4 million acres protected. The overtopping and breaching of levees caused about 400,000 acres to be flooded. However, in areas behind levees that held, some flooding still occurred because of heavy seepage through and under the levees and heavy flows from streams draining areas behind the levees. Corps officials said that they have not estimated the number of acres flooded from these sources.
According to the Corps, the levees prevented about $7.4 billion in flood damage during the 1993 flood. To calculate the damage prevented, the Corps uses damage curves that employ the principles of hydrology and economics to graphically depict the estimated costs of the damage that would occur if a Corps levee were not protecting the area. Economists compare curves depicting the estimated damage with and without a levee. The difference is the estimated value of the damage prevented by the project.
Corps staff said that the severity and duration of the 1993 flood in the St. Louis district were so great that the existing damage curves could not be used to estimate the damage accurately. As a result, Corps personnel extrapolated damage estimates for the 1993 flood from damage estimates for the large 1973 flood and used their professional judgment.
Corps staff pointed out that because the dollar values on the damage curves tend to be outdated, the estimates of flood damage derived from these curves are probably understated. They said that some of the dollar values they developed when they constructed the levee have not been updated to reflect the value of development that has since occurred behind the levee. They also mentioned that obtaining information about urban land values requires extensive fieldwork and research, which are labor-intensive, expensive, and time-consuming. Corps officials said that because their resources are limited, they give priority to updating information on the largest urban centers where the greatest threat of damage exists.
The four Corps districts where levees were overtopped and breached estimated that about $450 million in damage was incurred behind the levees. According to the Corps, it did not have sufficient funding to complete on-site field surveys to estimate the damage incurred from the 1993 flood and, as an alternative, used a variety of techniques to estimate the damage.
In the Rock Island District, for example, estimates were based on data from county-level sources that, according to Corps district officials, were rough estimates and were specific to only a few sites. The Omaha and St. Louis districts used computer programs and actual river elevations to compute the damage incurred, supplementing these sources with information from surveys of businesses, local and state government officials, and field personnel from the Department of Agriculture. The Kansas City District used water levels and damage curves to estimate the damage incurred.
NWS makes the overall estimates of flood damage suffered each year that the Corps reports to the Congress. For fiscal year 1993, NWS estimated that all floods caused more than $16 billion in damage. According to staff from NWS’ Office of Hydrology, loss estimates can be considered only approximate because they are developed as an ancillary function to NWS’ primary mission of forecasting weather and floods. They said the quality of resulting estimates is uneven because of insufficient resources, inconsistent methods and sources, incomplete data collection by field offices, and early reporting deadlines.
section_title: Conclusions
paragraphs: While data on the design flow capacity of the Corps levees and the actual flows during the 1993 flood are incomplete, the Corps levees in the area affected by the 1993 flood generally performed as designed. In fact, some levees withstood significantly greater flood flows and elevations than they were designed to withstand, especially when the duration of the flood is considered. Where the levees did not prevent water from entering protected areas, the Corps levees were overwhelmed by the size of the Great Flood of 1993.
section_title: Levees Increase Flood Levels, but Other Factors Also Affect Extent of Flooding
paragraphs: By confining floodwaters within a smaller portion of a floodplain than they would otherwise occupy, levees pressure the waters to rise higher and flow faster than they would do without restraint. Whether levees significantly increase flood levels varies by location. Computer simulations of the 1993 flood estimated that the nearby Corps levees added up to 2.7 feet to the flood crest at St. Louis and up to 7.3 feet to the flood crest at other locations.
Corps officials acknowledge that levees increase flood levels and induce some flooding. However, they emphasized that the net effect of levees, reservoirs, and navigation structures in the upper Mississippi River basin is to reduce flood levels and damage. There is no consensus among researchers, however, about the long-term effects of these structures on flood levels.
Many factors besides levees help determine the peak level of a flood. These include the amount of water entering a river from precipitation, the size and shape of the river’s channel and floodplain, and other natural factors. Human activities, such as clearing the floodplain for cultivation, have also affected flood levels.
Studies show that cumulative changes within the basins have caused higher flood levels for Mississippi and Missouri river flows. In addition, available evidence suggests that precipitation in the upper Mississippi basin may be increasing. If these trends continue upward, rather than cycling downward, more frequent and more extensive flooding will occur and future damage from flooding may increase.
subsections: section_title: Experts Agree That Levees Contribute to Higher Flood Levels
paragraphs: Available studies and the experts we spoke with agreed that levees generally contribute to higher flood levels above and within the levied part of a river. They said that the construction of a levee in a floodplain forces the water into an artificial floodway, causing the water to back up, just as the loss of a traffic lane on a busy highway causes heavy traffic to become congested. This restriction raises the level of the water both upstream of the levee and at the levee itself. It also forces the water to flow faster than it would if it were permitted to spread out across the whole floodplain. Figure 3.1 depicts a cross-sectional view of a floodplain and the floodway created by the construction of a levee.
subsections: section_title: Impact of Levees on Flood Levels Varies
paragraphs: According to the results of a 1994 study conducted by a federal interagency team, a levee can increase flood levels from a few inches to several feet. Factors such as the size of the flood, the height of the levee, and the dimensions of the floodway compared with those of the natural floodplain account for much of the variation.
The U. S. Army Engineer Waterways Experiment Station, a Corps research facility, identified the impact of some of these factors in a 1991 study requested by FEMA. Using a model developed by NWS, the Corps simulated a range of hypothetical floods and levees for 150 miles along the Missouri River from Jefferson City to Waverly, Missouri. This portion of the river is lined by mostly small private and other nonfederal agricultural levees that do not allow for the floodway recommended by FEMA. These levees are built to varying heights and, as a result, do not provide a consistent level of protection. Overall, these levees prevent only small floods—those with average frequency intervals below 10 years—from entering the protected floodplain.
The 1991 study found that the impact of levees on flood levels increases with the size of the flood until the flood is large enough to overtop the levees. It also estimated that raising the height of the existing levees to contain the 100-year flood would further increase flood levels by about 5 inches. Moving the existing levees back to FEMA’s recommended floodway boundary would reduce peak flood levels for the 100-year flood by an average of about 1.2 to 1.5 feet. The study concluded that flood levels are more sensitive to the size of the available floodway than to the height of the levees.
section_title: Mississippi River Basin Levees Caused Some Increases in Flood Levels but Generally Prevented Damage in 1993
paragraphs: The results of three studies show that Corps levees and nonfederal levees contributed to the flood levels experienced during the 1993 flood. These studies used computer modeling to simulate flows and water levels in part of a river during the 1993 flood, with and without levees. Observed differences, therefore, can be attributed directly to the presence of levees. However, the accuracy of a model’s results depends on the accuracy of the information describing the flood and the floodplain. It also depends on how well mathematical equations in the model represent actual hydrologic processes. These studies are the only modeling efforts on the 1993 flood to date and use models created by or accepted by the Corps. Because of the extremely complex nature of hydrologic computer models, we did not review the accuracy of the models or their results.
In 1993, the St. Louis Post-Dispatch commissioned an associate professor of civil engineering at the University of Illinois to simulate the 1993 flood. According to the researcher, the simulation included the Mississippi River beginning just north of St. Louis and extending 50 miles downstream to Prairie Du Rocher, Illinois. He used a Corps model that, for simplicity, assumes that floodwaters flow at a steady rate rather than at varying actual rates. The simulation estimated that Corps agricultural levees added about 1.0 to 1.5 feet to the flood crest at St. Louis.
The flood’s crest passed St. Louis just as two large downstream agricultural levees were being overtopped and the land behind them flooded. The Post-Dispatch simulation indicates that if these levees had not been overtopped or if they had been overtopped at another time, they would have added from 1.4 to 1.8 feet to the crest. This would have brought the crest to within a foot of the design capacity of the St. Louis floodwall.
After the 1993 flood, the administration established an interagency Scientific Assessment and Strategy Team to provide data and an analysis of the 1993 flood. At the request of the Interagency Floodplain Management Review Committee, the team simulated the flood. The simulation included the Mississippi River and major tributaries from near Hannibal, Missouri, to Cairo, Illinois, and the Missouri River from Hermann, Missouri, to the mouth at St. Louis, Missouri. This area contains Corps urban and agricultural levees and smaller private and other nonfederal levees. The team used a recently developed model capable of simulating the varying rates of flow that occur during a flood. The simulation estimated that Corps agricultural levees added a few inches to 7.3 feet to the flood crests at 14 locations. For St. Louis, it estimated that the levees added a maximum of about 1.5 feet to the flood crest. These results were consistent with the results simulated for the Post-Dispatch.
The Corps, as part of its 1995 Floodplain Management Assessment, also ran computer simulations of the 1993 flood. Its analysis, which included the lower Missouri River and the middle and upper Mississippi River basins, was performed on a systemwide basis using the model employed by the Scientific Assessment and Strategy Team. One simulation estimated that agricultural levees added up to 7.2 feet to the flood levels recorded at 11 locations in the Corps’ St. Louis District, including about 2.7 feet at St. Louis. Similarly, it estimated that agricultural levees added from a few inches to 4.7 feet to the flood levels along the Missouri River with one exception: Agricultural levees reduced the 1993 flood levels at Hermann, Missouri, by about 1 foot.
No evidence is available to show the extent of the damage brought about by the addition to the flood’s height attributable to the levees, but Corps officials acknowledge that damage was caused by the levees. The Corps also points out that its levees provided substantial benefits in 1993 by preventing flooding on about 1 million acres in the developed floodplain. The Corps estimates that its levees in the Kansas City, Omaha, Rock Island, St. Louis, and St. Paul districts prevented about $7.4 billion in flood damage. Although unprotected areas within reach of levee backflows are subject to greater flooding than would occur if no levees existed, Corps officials believe that the damage prevented by levees greatly outweighs that induced by levees. In addition, they said that the capacity of reservoirs to store floodwater compensates for the increases in flood levels caused by levees. For example, the Scientific Assessment and Strategy Team found that storing water in flood control reservoirs reduced the peak flood level at St. Louis by 5 feet in 1993. Thus, levees and reservoirs at St. Louis achieved a net reduction in flood levels.
section_title: Debate Continues Over Long-Term Effects of Navigation and Flood Control Structures on Flood Levels
paragraphs: Levees are one type of navigation or flood control structure that can affect flood levels. While the impact of levees on a particular event, such as the 1993 flood, has been estimated with sophisticated models, the long-term effects of such structures on flood levels in the Mississippi River basin is less certain. Over the years, researchers have used trend analysis to assert a relationship between long-term increases in flood levels and these structures. However, the value of this analysis is limited by a lack of accurate information about historic flood flow rates, as well as by the conflicting results these studies have yielded.
subsections: section_title: Historical Records of Floods Are Not Accurate
paragraphs: Much of the research is based on flow rates estimated for extreme floods at St. Louis. Some of the flow rates for extreme floods in the historical record are estimates based on observations of water levels recorded at the time of the floods. Other flow rates in the historic record are based on methods or equipment now shown to be inaccurate. In any case, measuring flow rates during extreme floods is very difficult and sometimes impossible. Not only can a flood destroy recording equipment, but it can also prevent access to the best measurement sites. As a result, 7 of the 10 highest flow rates recorded for St. Louis before 1993 are estimates rather than actual measurements.
A more accurate and now generally used device for measuring flow, the Price current meter, was not used exclusively at St. Louis until 1931.Before that time, various methods of measuring flow rates were used. Some researchers question the accuracy of these methods for very high flows because of the findings of two studies conducted at the University of Missouri at Rolla.
The first study, conducted in 1976, found that estimates of very high flow rates are subject to large errors and that flows above the banks of the river could not be estimated satisfactorily. The second study, conducted in 1979, tested the accuracy of measurement devices used at St. Louis before 1931. Although the author concluded that most of the pre-1931 measurements were valid for use in analyses, he found that 57 percent of the test measurements taken for flows above the banks of the river using double floats exceeded the measurements taken with a Price current meter by more than 10 percent. Double floats were used at St. Louis from October 1881 through December 1930. On the basis of his findings, the author recommended that historical estimates of high flow rates be used only for the relative ranking of floods.
In 1985-86, researchers at the Corps’ Waterways Experiment Station found a further indication of errors in pre-1933 flow rate information. Using a physical model of the Mississippi River, the researchers found that they could reproduce the high water marks at St. Louis for the 1844 and 1903 floods using flow rates 33 percent and 23 percent lower than historic flow estimates published by USGS.
section_title: Researchers’ Findings Vary
paragraphs: Three studies performed during the 1970s analyzed the trends in water levels for similar flows at specific locations and produced dissimilar results. In addition, two of the studies used pre-1939 historical data that are of questionable accuracy. Given the data and/or methodological problems associated with these studies, no conclusions can be drawn from them about the long-term impact of navigation structures (such as dikes) or flood control structures (such as levees) on flood levels. The following paragraphs outline the results of each study.
In a 1974 study, researchers at Colorado State University addressed the impact of constructing levees and channelizing the river for navigation on water levels and flow rates at St. Louis before and after 1900. Their comparison found that water levels were higher for all flows above 300,000 cfs, but maximum annual water levels and average and maximum annual flow rates remained unchanged. They also found that lower water levels existed after 1900 for all flows below 300,000 cfs. The researchers asserted that the construction of navigation dikes and levees between 1900 and 1940 caused both decreases and increases in the water level’s relationship to the flow rates at St. Louis.
Another study, which was published in 1975 by an associate professor of geology at St. Louis University and was widely cited during the 1993 flood, compared the relationship between maximum annual water levels and flow rates at St. Louis in 1973 with the same relationship during a base period from 1861 through 1927. This study found that the 1973 flow rate of about 851,100 cfs produced a flood elevation about 7.9 feet higher than it did during the base period. The researcher attributed the rise in water level per flow rate to a combination of navigation works, levees, and riverbed sedimentation. He has since refined his results and attributes about 4 to 5 feet of the total increase to levees, about 2 to 3 feet to navigation works, and about 1 foot to riverbed sedimentation.
The Corps has questioned the results of both studies because the studies used the suspect historical data. In addition, a USGS headquarters hydrologist who specializes in the statistical analysis of hydrologic information told us that modeling is better than trend analysis for identifying the effects of navigation dikes and levees. The USGS hydrologist said that trend analysis can never prove what caused the changes identified. Thus, changes in water levels and flow rates that occur after the construction of navigation dikes and levees may suggest but do not prove that these structures are the source of the changes.
In 1975-76, researchers from the University of Missouri at Rolla attempted to study the effects of levees on flood levels. Unlike the authors of the 1974 and 1975 studies, they used only post-1930 data to avoid questions about the accuracy of historical data. As a result, they concluded that the Mississippi River had not experienced enough floods of sufficient size from 1930 through 1976 to evaluate the effect of levees on floods.
Their study also examined the effect of navigation dikes on water levels in the middle Mississippi River. They found that changes in water level per flow rate between 1934 and 1974 at three major gauging stations were dissimilar. The University of Missouri researchers concluded that changes in water level per flow rate at the study gauges showed no association with dike construction. The study stated that although the constriction of the channel caused by building an individual dike must have at least a temporary effect on the relationship between the water level and the flow rate, the dissimilar findings at the three gauging stations suggest that the effect may be restricted to the area immediately around the dike. Therefore, they said data on water levels and flow rates cannot be extrapolated from a single point of record to an entire reach of the river.
section_title: Other Factors Affect River Flood Levels
paragraphs: Although floods result from heavy rainfall during a short time or above-normal rainfall over a long time—sometimes in combination with snowmelt—this precipitation interacts with the atmosphere, land topography, vegetation, soils, channel geometry, and human activities to determine the amount of runoff. The chief determinants of a flood’s peak level at a particular location in a river are the amount of water reaching the river as runoff, the size and shape of the river channel, and the size of the floodway. Assessing the impact of a single factor, such as levees, on water level is very difficult because hydrologic models can only approximate the complex processes that move and store water.
Variations in flood levels under like conditions are not uncommon. According to the Corps, records of the relationship between high flows and water levels at St. Louis show about a 5-foot variation in water levels for like flows. For example, two floods passed St. Louis within a month in the spring of 1983 with similar flows but with crests whose height differed by 2.7 feet. According to the Corps, no accurate accounting for this variation exists.
Over time, some of the factors that help determine a flood’s peak level also shift the range of water levels produced by like flows. Several studies have addressed the factors, both natural and man-made, that affect flood levels.
subsections: section_title: Many Natural Factors Affect Flood Levels
paragraphs: Natural variables that help determine a flood’s peak level include (1) the flood’s duration and whether it is rising or receding, (2) the seasonal level of vegetation in the floodway, (3) the way the flood carries sediment, and (4) the water’s temperature. Long-term changes in the river’s channel from erosion, past floods, and earthquakes, as well as the growth of vegetation in the floodplain, particularly in the floodway, also affect peak flood levels.
A 1994 study on the relationship between flow rate and flood level simulated hypothetical floods of 4.5 days, 9 days, and 13.5 days with the same peak flow rate through a channel approximating the Mississippi River at St. Louis. The study found that the speed with which a flood reaches its peak flow and the duration of that peak flow help determine the flood’s peak water level. For instance, in 1993, flow rates of 1,030,000 cfs at St. Louis on 2 consecutive days increased the water level by half a foot on the second day.
According to the Scientific Assessment and Strategy Team’s study, the vegetation in the floodway affects flood levels because it obstructs and slows the flow of water, causing the water to rise. Consequently, the water level in an area covered with shrubs and trees would be higher than in an area covered with grass. Similarly, the same flood can be higher during the summer than during the late fall, winter, or early spring because of the summer foliage.
Researchers have found that swiftly moving floodwater can cause intense erosion and sedimentation. The transport and deposition of sediment during a flood can increase or decrease water levels at various locations. In addition, changes in water temperature affect the amount and shape of sediment in the river. Cold water carries more sediment and enlarges the size of the sediment particles, increasing the friction that, over time, can scour the channel and increase its flood-carrying capacity, reducing all water levels.
section_title: Human Activities Affect Flood Levels
paragraphs: Floodplains reduce flood levels by providing space for the temporary storage of floodwaters until natural drainage can carry them away. They also reduce flood velocities. In addition to flood control, human activities in the last 175 years in agriculture, navigation, and urban development have altered the floodplains in the upper Mississippi River basin. These activities have altered water flow rates, the width and depth of the river channel, the size of the floodway, the pattern of erosion and sedimentation, the level of vegetation, and the speed with which precipitation flows into streams.
subsections: section_title: Changes for Agriculture
paragraphs: Early development in the upper Mississippi River valley was closely tied to the rivers. By the late 1800s, settlers had cleared millions of acres in the floodplain for cultivation. Vegetation in the unaltered floodplain, especially in wetlands, created resistance to flow. Researchers have shown with modeling that removing resistance reduces flood levels and increases flow velocities and erosion. They speculate that clearing the floodplain for agriculture had the same effects.
Between 1780 and 1980, an estimated 57 percent of the original wetlands in the nine midwestern states affected by the 1993 flood were converted to other uses, mainly agriculture. Wetlands temporarily store and hold some floodwater for later drainage. According to the 1994 report from the Interagency Floodplain Management Review Committee, the loss of wetlands contributes to higher flood levels for smaller, more frequent floods, like 25-year or smaller floods.
Agricultural land management practices affect the processes of erosion, sedimentation, and runoff. For example, the Illinois Natural History Survey found that planting crops in rows and plowing with moldboards (which lift and turn the soil) increased the rate at which Illinois lakes were filling with sediment. Although researchers have observed the influence of agricultural land management practices on small watersheds, the influence of these practices on major rivers is still largely speculative.
section_title: Changes for Navigation
paragraphs: Travel to and commerce with early settlements along the rivers created a demand for improved navigation on the rivers. The Congress first approved a plan for improving the Mississippi River’s channel in 1881. However, most channel improvements for navigation on the Mississippi and Missouri rivers were made between 1927 and 1944. The improvements generally narrowed the natural channels and shortened the rivers.
The Corps has constructed about 3,100 wing dikes to create and maintain navigation channels on the Mississippi and Missouri rivers. Wing dikes are embankments built in the river perpendicular to the shoreline to increase channel depths by reducing channel widths and increasing flow rates. The dikes help keep sediment from accumulating in the main channel and trap it along the shoreline. The previously cited 1974 Colorado State University study found that, between 1888 and 1968, wing dikes decreased the average width of the middle Mississippi River by 2,100 feet, or by about 40 percent. The report also stated that degradation of the riverbed has occurred along the middle Mississippi River whenever the channel has been narrowed.
The Corps also stabilizes and dredges hundreds of miles of river bank and channel, respectively, to maintain open water navigation on the Missouri and Mississippi rivers. Stabilizing the banks reduces shoreline erosion, and dredging deepens the channel. In addition, the Corps has built and operated a system of locks and dams on the upper Mississippi River since the 1930s. This system converted the upper river into a series of pools to maintain channel depths at low and normal flows. A 1988 study of the long-term effects of the oldest lock and dam on the upper Mississippi River found that initially the river’s width and volume had increased behind the dam. However, the long-term impact has been to trap sediment. As a result the river has steadily lost both width and volume and returned to near pre-dam water levels and flow.
section_title: Changes Due to Urban Development
paragraphs: Studies have shown that the growth of urban areas increases the speed of water running off the land into streams. Rain that falls on paved, tiled, or other impervious surfaces and runs into storm drainage systems is delivered to streams more quickly than it could run off porous surfaces. Hence, urban runoff produces higher, sharper flood peaks on small rivers and streams than rural runoff. However, as the water from each small stream joins the water in larger streams, the effects of urbanization on flood levels are diluted. One expert told us that researchers have not been able to measure the impact of urban development on the flooding of the Mississippi or Missouri rivers because the effects of urbanization are too small to isolate.
section_title: The Upper Mississippi River Basin May Have to Contend With Increasing Flood Damage
paragraphs: Recent analyses show that water levels for high flows may be increasing for some locations in the upper Mississippi River basin. A continuing Corps study of Missouri River water levels shows that flow rates that once nearly filled the channel have been producing higher flood levels since the late 1920s. Similarly, a 1994 study of flow rates on the Mississippi and Missouri rivers found that flood levels for like flow rates have increased over time. Evidence also suggests that precipitation in the upper Mississippi basin may be increasing. These trends concern the Corps because they increase the frequency and extent of flooding, thereby increasing the damage from flooding.
subsections: section_title: Water Levels at High Flow Rates Are Increasing
paragraphs: According to the Corps’ Missouri River Division, seven of nine gauges on the Missouri River have slowly and consistently produced higher water levels for the same high flow rates since about 1927. The rising trends are most noticeable at Nebraska City and Omaha, Nebraska, and at St. Joseph, Missouri. Table 3.1 shows the approximate increases in water levels for selected high flow rates.
Statistical studies performed in 1994 at the Environmental Management Technical Center at Onalaska, Wisconsin, also examined water levels and flow rates for the period of record at six gauges: St. Louis, Chester, and Thebes on the Mississippi River; and St. Joseph, Waverly, and Hermann on the Missouri River.
The studies found that water levels for like flood-level flow rates have been increasing at all six gauges at an average of about 1.2 inches annually. Over the periods of record, which range from 51 years at Chester to 132 years at St. Louis, increases in water levels at the six gauges ranged from about 3 to 9 feet. Table 3.2 shows the approximate increase at each gauge.
The studies also found that, except at Thebes, the trend for maximum annual water levels is increasing. The studies found no changes in trends for flow rates.
section_title: The Climate of the Upper Mississippi River Basin May Be Changing
paragraphs: According to the 1994 Natural Disaster Survey Report issued by NWS, the duration and size of the 1993 Midwest flood and the wet conditions leading up to it suggest a significant variation in climate. An air circulation feature, called El Niño/Southern Oscillation, driven by abnormal sea surface temperatures occurred in both 1992 and 1993. Preliminary NWS modeling using the temperatures associated with the El Niño episode produced large-scale atmospheric results resembling the abnormal precipitation and temperature pattern experienced in 1993. However, NWS stated that it requires more in-depth and thorough analyses to understand the role played by El Niño in the extreme precipitation.
According to a 1993 report on the Midwest flood by the Illinois State Water Survey, increases in the volume of water flowing down the Mississippi River are most closely related to the overall climate and precipitation. The report states that climate and resulting precipitation exert such a strong impact on streamflow that it masks changes from other sources, such as physical changes to the basin.
The Illinois State Water Survey reports that, for most locations along the Mississippi River, average streamflows for the 28-year period since 1965 have been the highest on record. On the basis of an 11-year moving average, the Survey calculated that average flow rates at Clinton and Keokuk, Iowa, and St. Louis, Missouri, have increased by about 25 to 33 percent over the long-term average since the mid-1960s. An 11-year moving average is used to describe the trend because it smooths out natural fluctuations in the data. Also, average streamflows at Keokuk and St. Louis between 1965 and 1992 are about 17 and 13 percent, respectively, above the long-term average streamflows based on over 100 years of record. Average streamflows for the same period for the tributaries between Clinton and Keokuk, Iowa, for the Des Moines River, and for the Illinois River are even further above their long-term average streamflows. According to officials of the Survey, these deviations are significant because, historically, average streamflow has remained remarkably consistent with long-term average streamflow.
USGS headquarters officials cautioned that because weather is cyclical and variable, any trends in climate are difficult to distinguish from normal weather cycles of higher precipitation and drought.
section_title: Rising Water Levels Are Causing Concern
paragraphs: For the 10-year period from 1983 through 1992, NWS estimates that damage to the United States from flooding totaled about $20.5 billion, or an average of about $2.1 billion annually, unadjusted for inflation. With the inclusion of data for 1993, the total estimated flood damage became $36.9 billion, or about $3.4 billion annually over the 11-year period.
According to the Corps, upward trends in water levels are of concern, whether they are caused by increased streamflow or by higher water levels for the same flow rates, because they increase the frequency of flooding and the area subjected to flooding. For instance, between 1928 and 1959, flows of 100,000 cfs at St. Joseph, Missouri, never exceeded water levels of 17 feet, the official level at which flooding begins. Since 1959, flows of 100,000 cfs have exceeded flood level 16 times. If changes in the climate of the upper Mississippi River basin increase precipitation in the future and if water levels for like flow rates continue to rise, then the damage from flooding will rise unless the ability of flooding to cause damage is mitigated.
section_title: The Interagency Floodplain Management Review Committee Addresses Levees
paragraphs: The administration’s Interagency Floodplain Management Review Committee was formed to identify the major causes and consequences of the 1993 Midwest flood, evaluate the performance of existing floodplain management programs, and recommend changes that make the programs more effective. Among the Committee’s findings and recommendations are many related to flood control activities, particularly levees. See appendix VI for a summary.
section_title: Conclusions
paragraphs: That levees increase flood levels is subject to little disagreement. Whether this increase is significant varies from location to location, but whether unprotected lands are more likely to be flooded than protected lands depends on the increase in flood levels after the construction of a levee. Proponents of levees point out that the impact of a levee should not be isolated because the net effect of all flood control projects has been to reduce flood levels and prevent billions of dollars in flood damage.
Levees are only one of many natural and man-made factors that help determine the peak level of a flood. Cumulative changes within the upper Mississippi River basin have caused higher water levels for similar flows. These trends could mean that the damage from flooding may increase in the future because higher water levels are associated with more frequent and more extensive flooding.
section_title: Federal, State, and Local Governments Exercise Some Control Over Nonfederal Levees
paragraphs: No federal program regulates the design, placement, construction, or maintenance of nonfederal levees. However, the federal government can exercise some control over nonfederal levees through programs that regulate navigable waters and wetlands and that provide flood insurance and disaster and emergency assistance.
Overall, 17 of the 50 states have specific programs for regulating levees. Five of the nine states involved in the 1993 flood have regulatory programs that, to varying degrees, affect nonfederal levees. Most often, states are responsible for the overall coordination of floodplain management activities within the state and across state lines. In some cases, states may regulate local land use when localities are unable or unwilling to take the actions needed to reduce the risk of flooding.
Local governments usually exercise control over nonfederal levees in response to requirements of the National Flood Insurance Program and state regulatory programs. However, in states without a regulatory program, local land use regulations generally affect the placement and construction of levees.
subsections: section_title: Some Federal Control of Nonfederal Levees Exists Under Federal Programs
paragraphs: Nonfederal levees are regulated to some extent under two federal programs that require permits to construct or modify levees affecting navigable waters and wetlands. In addition, the programs of three federal agencies that provide flood insurance and emergency and disaster assistance to repair flood-damaged levees affect nonfederal levees.
Under the National Flood Insurance Program, FEMA exempts communities from certain requirements of the flood insurance program if they can show that the levees protecting them are designed, constructed, located, and maintained according to specified criteria.
As part of its mission to provide disaster assistance, the Corps is authorized to repair levees through its levee rehabilitation program. The Department of Agriculture’s Natural Resources Conservation Service (NRCS) provides funds and technical assistance for the emergency repair of nonfederal levees that are damaged during a flood.
In all of these programs, the federal government exercises some direct or indirect control over the nonfederal levees’ design, placement, construction, or maintenance. However, all of these elements are not always affected by each program. Table 4.1 lists the federal programs affecting levees, and table 4.2 shows whether these programs affect a nonfederal levee’s design, placement, construction, or maintenance.
subsections: section_title: Legislation Requires Permits for Certain Levees
paragraphs: Section 10 of the Rivers and Harbors Appropriation Act of 1899 prohibits the obstruction or alteration of navigable U.S. waters without a permit from the Corps. Section 404 of the Clean Water Act requires a permit from the Corps when wetlands are to be altered. Under both of these authorities, the Corps may control the design, placement, and construction of a nonfederal levee when the levee affects areas regulated by these programs. However, the owner, sponsor, or builder is responsible for informing the Corps of the intent to construct a levee and for obtaining a permit. The Corps’ data do not indicate how many of the 15,000 permits requested for activities in wetlands or navigable waters in fiscal year 1994 were for constructing or modifying levees.
For the 1993 flood, the Corps issued a nationwide permit for the construction of temporary levees and emergency repairs to levees. This permit encompassed U.S. waters—including rivers, streams, lakes, and wetland areas—in the counties that had been declared flood disaster areas. The permit was specific and identified conditions where it applied. For example, the permit required any levee that was reconstructed or repaired to be maintained after the flood. Since some levees were being constructed in new locations, the Corps could also require that the former location of the levee be restored to its previous condition. The permit also required that any temporary levees built must minimize damage to U.S. waters and that measures must be taken to maintain near-normal downstream flows. Furthermore, all levees were to be designed and constructed so as to prevent the channel from being constricted or redirected and erosion from occurring upstream or downstream.
subsections: section_title: Corps’ Regulatory Authority May Extend to Additional Nonfederal Levees
paragraphs: Section 10 prohibits the obstruction of the navigable capacity of waters of the United States without a permit from the Corps. To clarify the extent of the Corps’ authority to regulate levees under this section, we asked the Corps to provide its interpretation of this authority. Specifically, we asked whether the Corps has the authority to regulate upland nonfederal levees. Currently, these levees are not regulated by the Corps.
In a November 1994 response, the Chief Counsel of the Corps indicated that the Corps might be able to assert jurisdiction over upland nonfederal levees on the basis of section 10. While this provision prohibits the obstruction of the navigable capacity of waters of the United States, it also prohibits the alteration or modification of the course, location, condition, or capacity of “any navigable water of the United States unless recommended by the Chief of Engineers and authorized by the Secretary of War” before work begins.
From his review of court cases dealing with section 10, the Chief Counsel opined that a prerequisite to the Corps’ exercising jurisdiction would be circumstances demonstrating an alteration or modification of the course, location, condition, or capacity of navigable U.S. waters. However, he noted that a court might require that jurisdiction be founded on a negative effect and that, arguably, nonfederal levees have an essentially beneficial effect on navigable capacity.
The Chief Counsel concluded that if the Corps exercised jurisdiction over upland federal levees under section 10 without clear authority and direction from the Congress, this action would likely be overturned upon appeal to the federal courts.
section_title: The National Flood Insurance Program Imposes Requirements for Levees
paragraphs: FEMA administers a levee certification program under the auspices of the National Flood Insurance Program. To be certified and to enable communities to enact less stringent building codes, levees must meet FEMA regulations specifying design, placement, construction, and maintenance standards. In all cases, the levee must, at a minimum, protect against a 100-year flood and, in most cases, have at least 3 feet of height (or freeboard) above the 100-year level of protection.
When FEMA maps a community, it designates the 100-year floodplain and floodway. When a community joins the insurance program, it must require that all buildings constructed or substantially improved be protected to the base flood elevation. This is usually accomplished by elevating the structure above the base flood elevation or, in some instances, by floodproofing. While increasing construction costs, these requirements make buildings more resistant to flood damage. FEMA officials said that experience confirms that elevated buildings are less susceptible to flood damage and, therefore, cost less to repair when a flood occurs. When FEMA certifies a levee as protecting against a 100-year flood, it exempts new construction and substantial improvements from the requirement to build above the base flood elevation. The effect of this exemption is to increase the costs of repairing buildings that are more susceptible to flood damage because they are not elevated.
section_title: Corps’ Program Repairs Flood-Damaged Levees
paragraphs: The Corps will repair, on a cost-shared basis, nonfederal levees that are damaged by floods if the levees meet the qualifying standards of the program and are in good standing with the program at the time of flooding. In order to be accepted into the Corps’ rehabilitation program for damaged nonfederal flood control levees, a sponsor’s levee must meet or be improved to meet the Corps’ minimum design standards before flood damage occurs. The owner may need to modify the levee in order for the levee to be admitted to the program. Once in the program, the levee must be maintained in accordance with the Corps’ criteria.
For levees that have qualified, the Corps provides 80 percent of the repair costs and the levee’s sponsor pays the rest. To qualify, the levee must (1) be publicly sponsored to ensure that the 20-percent share can be paid, (2) be a primary levee providing a 10-year level of protection for urban areas and a 5-year level of protection for agricultural areas, (3) be properly maintained and regularly inspected, and (4) provide benefits that equal or exceed the cost of the levee’s repair. The rehabilitation program includes levees maintained by local sponsors, such as levee districts, individual municipalities, or Indian tribes.
The Corps received 546 requests for assistance under the levee rehabilitation program after the 1993 flood. Of these requests, as many as 345 either were declared ineligible for assistance because the levees did not meet the basic requirements identified above or had been repaired under other disaster assistance programs, such as NRCS’ program. As of October 1994, the Corps estimated that the costs of repairing the 201 levees that met its established criteria would be $250 million.
Also, during 1993, the Corps worked with the Department of Commerce’s Economic Development Administration (EDA) to identify and repair levees that were deemed to affect the economic development of the community where the levee was located. The Corps assisted EDA in assessing the levees that had not qualified for the Corps’ program in advance primarily because they lacked public sponsorship. Because one of EDA’s missions is to provide economic assistance to areas experiencing sudden and severe economic distress, the agency provided funding on a cost-shared basis to repair levees that protect critical public infrastructure.
Working with the Corps, EDA reviewed all applications and funded those that identified local sponsors to share costs and maintain the levees. EDA has funded the repair of 13 levees at a cost of about $4.2 million.
section_title: NRCS’ Program Affects Some Nonfederal Levees
paragraphs: NRCS’ Emergency Watershed Protection Program, authorized by the Agricultural Credit Act of 1978 (P.L. 95-334), funds repairs to levees. To be eligible for these funds, the levee project must protect property threatened by a watershed emergency and the owner must either have exhausted, or lack, the funds needed to remedy the problem. To be eligible for assistance after the 1993 Midwest flood, potential applicants were also required by NRCS to show that the levee restoration project protected property or life, that benefits exceeded the costs of repair, and that regular maintenance would be performed. Since the program supports the repair of nonfederal levees to their preflood condition, it does not control the original design, placement, or construction of nonfederal levees. However, because regular maintenance is required for future assistance, the program does set maintenance standards.
NRCS also received funding in a 1994 supplemental appropriation to make repairs to levees otherwise ineligible for assistance on condition that a sponsor share the cost and that the levee qualify for the Corps’ levee rehabilitation program. The levee also had to be environmentally sound and provide a 5-year level of protection.
While NRCS’ Emergency Watershed Protection Program provides for a smaller proportion of the funding for repairs than the Corps’ levee rehabilitation program (75 percent versus 80 percent), NRCS would provide 100 percent of the funding in the case of extreme need or hardship on the part of the levee sponsor. As of February 1, 1995, NRCS had approved repairs for 375 levees at a cost of $10.2 million under the Emergency Watershed Protection Program. NRCS had also determined that 16 levees were eligible for repairs, costing a total of $650,000, under the supplemental appropriation for fiscal year 1994. NRCS officials reported the agency is evaluating repairs for 26 more levees under the supplemental appropriation. Some of these repairs were for levees whose drainage area exceeded NRCS’ threshold of 400 square miles and would, therefore, normally have fallen under the Corps’ levee rehabilitation program. However, NRCS was given the responsibility for repairing these levees under the supplemental appropriation.
section_title: Some States Regulate Nonfederal Levees
paragraphs: In 1992, the Association of State Floodplain Managers issued a reportdocumenting the scope of floodplain management programs in the states and found that 17 states regulate levees as part of their floodplain management programs. Of the nine states involved in the 1993 flood, five states (Iowa, Kansas, Minnesota, North Dakota, and Wisconsin) have programs to regulate levees. These programs have standards regulating the design, construction, placement, and maintenance of levees. We judgmentally selected Iowa as an example of a state that regulates its levees.
subsections: section_title: Iowa Has a Regulatory Program for Levees
paragraphs: Iowa’s program for regulating levees addresses the design, placement, construction, and maintenance of nonfederal levees. The program, which is described in Iowa’s statute on floodplain management, is administered by the state’s Environmental Protection Division within the Department of Natural Resources and has been in existence since at least 1967.
In general, Iowa requires permits to construct, operate, and maintain levees in rural areas where the watershed drainage area exceeds 10 square miles and in urban areas where the watershed drainage area exceeds 2 square miles. Specific requirements must be met to comply with the statute’s criteria for level of protection, placement, interior drainage, freeboard, or design. For example, Iowa requires agricultural levees, at a minimum, to protect against a 10- to 25-year flood and urban levees to protect against a 100-year flood. Urban levees must provide at least 3 feet of freeboard above the design flood profile. Furthermore, any agricultural levee whose protection level is increased must comply with the state’s equal and opposite conveyance rule: An increase in the levee on one side of a river’s floodplain cannot cause a disproportionate increase in the flooding of the river’s opposite floodplain.
Iowa officials told us that only one FEMA regulation more strictly controls levees. Under this rule, development within FEMA’s identified 100-year floodway cannot increase the area designated as the 100-year floodway. The effect of this rule is that agricultural levees may be placed farther away from the river than the equal and opposite conveyance rule may initially require.
section_title: Some States Have Not Yet Established Regulatory Programs for Nonfederal Levees
paragraphs: Of the nine states involved in the 1993 flood, four (Illinois, Missouri, Nebraska, and South Dakota) have yet to develop specific regulatory programs for levees. All of these states have a floodplain management program that may (1) provide technical expertise to localities and individuals on complying with FEMA’s or other federal agencies’ requirements, (2) regulate local land use when localities are unable or unwilling to take needed actions to reduce the risk of flooding, or (3) coordinate local and regional floodplain management programs. We judgmentally selected Missouri as an example of a state that does not regulate levees.
Missouri does not currently regulate its levees but has identified areas of its floodplain management program that need to be improved. A task force established by Missouri’s governor to review and evaluate the state’s floodplain management program recommended the development of a levee oversight program to help decrease the risk to life and property from flooding.
The task force recommended in a July 1994 report that Missouri (1) adopt a permit program for constructing and modifying levees; (2) identify all existing levees for the purpose of developing design criteria and policy guidelines; (3) determine the need for setbacks, relocations, and construction standards; and (4) enact legislation that would make it easier for levee districts to form and obtain public sponsorship for participation in the Corps’ levee rehabilitation program. As of June 1995, the Missouri General Assembly had not yet enacted these recommendations into law.
section_title: Local Governments Often Exercise Control Over Nonfederal Levees Through Floodplain Regulations
paragraphs: Local governments generally exercise more control over local floodplains and levees than do the states or the federal government. This is often because FEMA requires the adoption of community floodplain regulations as a condition for joining its flood insurance program. The 1992 report by the Association of State Floodplain Managers noted that every state has granted its localities enough authority to meet the regulatory requirements of FEMA’s flood insurance program. As a result, localities may enact ordinances that allow them to (1) require certain building codes for development in the floodplain, (2) issue zoning regulations describing the types of land uses permitted in the floodplain, or (3) require that development incorporate improvements to alleviate potential flood hazards.
Two exceptions to this local authority exist, however: First, most localities cannot regulate federal or state property or development by other localities, and, second, some states have exempted from local control certain activities that are important to the state’s economy (i.e., transportation, agriculture, mining). Under these exceptions, a local government could be prevented from exercising control over a nonfederal levee if, for example, the state determined that such control would negatively affect the economy of the river’s transportation industry.
In a state that regulates levees, the locality is generally required to enforce and comply with the state’s standards for the design, placement, construction, and maintenance of levees. For example, in Iowa, local regulations must meet state requirements. In Minnesota, the state government may directly regulate levees and enforce compliance if the local government does not.
In states that do not regulate levees, such as Missouri, local governments are bound by the standards of FEMA’s flood insurance program if the community participates in the program. According to the city engineer in St. Louis, the city complies with FEMA’s standards for floodplain management but has not established local levee regulations.
section_title: Interagency Committee Calls for Better State Regulation of Nonfederal Levees
paragraphs: As part of its review of floodplain management in the aftermath of the 1993 flood, the Interagency Floodplain Management Review Committee reported that few states control the design, placement, construction, and maintenance of nonfederal levees. As explained in detail in appendix VI, the Committee found that the states’ involvement in many floodplain management activities—including issuing permits for levees, flood-fighting, and repairing levees—was highly variable and in need of enhancement. The Committee recommended that the states assume responsibility for regulating the location, alignment, design, construction, upgrading, maintenance, and repair of levees and flood-fighting at levees.
section_title: Conclusions
paragraphs: While no comprehensive federal program regulates the design, placement, construction, or maintenance of nonfederal levees, control over nonfederal levees is exercised through various federal programs for regulating navigable waters and wetlands and for providing flood insurance and disaster and emergency assistance. Not all aspects of levees are regulated in every program, and not all nonfederal levees are affected.
Five of the nine states involved in the 1993 flood have regulatory programs that, to varying degrees, affect the design, placement, construction, or maintenance of nonfederal levees. Most often, states are responsible for the overall coordination of floodplain management activities within the state and across state lines, and local governments exercise direct control over levees through land-use regulations. Local governments often impose regulations to comply with the requirements of the National Flood Insurance Program and state regulatory programs.