The depth of flood water and the duration of flooding have a significant impact upon property damage. Together flood depth and duration have a significant impact on property damage in Natomas Basin. The Corps of Engineers estimated flooding will last for 30 days. This is an extraordinary amount of time when compared to the flood duration of less than 5 days for other parts of the Sacramento floodplain. The Corps also estimated damages to buildings and contents would reach 100 percent where flood depths exceed 13 feet.10 Figure 5 shows that approximately 59 percent of Natomas Basin's land area could flood to depths exceeding 13 feet and 32 percent could flood to depths of 8 to 13 feet. A total of 91 percent of the land area could flood to depths exceeding 8 feet.

The Corps estimated 13,730 structures and the contents are valued at $2.351 billion and would suffer flood damage estimated at $1.592 billion, which is over 67 percent of the market value. "Natomas is the only reach (part the Sacramento floodplain) where flood damages would exceed 50 percent of the market value of the structure." 11

Even if a higher level of flood control is provided, the depth and duration of flooding in Natomas Basin is the same. Whenever the next larger flood breeches the levees, Natomas Basin fills with water. The Corps of Engineers observed:

"Flooding from levee failure would be similar in Natomas . regardless of the frequency of the flood event because (1) the ground elevation adjacent to the levees in these locations is lower than the water surface in the river, and (2) the volume of water in the American River . . . and Sacramento River in the case of Natomas ... would fill the floodplains to similar depths.'

» 12

To illustrate the Corps point, Figure 6 is a Natomas Basin levee profile along the American River. It shows the highest river water level during the February 1986 storm was 13 feet above the ground level in Natomas Basin. If the levee had failed with the river at the highest level, Natomas Basin could have filled up. As shown in Figure 6, a single story house would have been completely covered by water. 13 The Corps of Engineers concluded that the Natomas Basin will experience severe flooding whenever the flood control levees are breached. The only thing which changes is the probability, or frequency, of when flooding will occur. Severity of flooding in Natomas Basin is the same no matter the frequency of flood. In other words, severity of flooding in Natomas Basin is the same no matter the probability, or probable frequency, of flooding.

10 "For the Natomas reach, duration is 30 days, and the average velocity is less than 0.5 feet per second. Duration has a significant effect on the flood damages for this area. Damages to structure and content for each land use category [i.e., residential, commercial, industrial, public, and mobile home] were set at 100 percent for depths greater that [sic] 13 feet. For the remaining reaches in the floodplain, duration is less than 5 days, and the average velocities range from 0.5 to 5 feet per second. Neither duration nor velocity had any significant effect on damages." Source: Corps Feasibility Report, Appendix C, page C-22, (emphasis added).

11 Ibid., page C-25.

12 Ibid., Main Report, page III-18, (emphasis added).

13 Figure 6 is a graphic illustration of Natomas Basin flood depths and what this much water does to a single story house constructed on grade. Building on grade is the typical method used in much of California. The house is completely covered. Destruction is total for building and contents.

SOURCE: US Army Corps of Engineers, American River Watershed Investigation. California Feasibility Report. (Sacramento District, December 1991). Main Report, p. III-18.

Drawn from US Army Corps of Engineers, American River Watershed Investigation. California: Easikality Report. (Sacramento District, December 1991). Appendix M. Chapter 2, Appendix M-2-A, Levee and Flood Profiles Sections 5 through 9, Section 7.

PROBABILITY OF FLOODING

What is the probability of Natomas Basin flooding? To answer the question, consider an incident from the Great Flood of 1993:

"For various points along the Mississippi, the Army Corps of Engineers has calculated the probability of floods of varying heights. The smallest probability they bothered to calculate was a '500-year flood' one so unlikely it can be expected to happen only once in five centuries. . . . [A]t Hannibal, Mo., a 500-year flood was determined to be 30 feet, so the city built its new levee a foot higher, at 31. It was finished in April after two years of work, and when the waters rose in early July residents heaped three feet more of sandbags atop it. When the crest reached Hannibal, the river was at 32 feet." 14

This incident helps us to understand the uncertainty of predicting and controlling flood events. Also it shows the confusion caused by the terms used to describe flood events. The Corps of Engineers calculates the probability of floods of varying heights, or volumes. Each calculated flood event has an estimated water height which is caused by an estimated water volume. As can be expected, greater volume results in greater height. As the volume of water increases, the probability decreases because larger floods occur less often.

To name different flood events, engineers use a shorthand description based upon the probability of when each flood event will occur. Engineers take historical records of past floods and apply statistical calculations to predict the probability or chance of flooding. For example, the "500-year flood" is the calculated flood of a certain volume or height which has a 1 in 500 chance of occurring in any year. The 500-year flood has a one-fifth of one percent (0.20 percent) of occurring in any year. Over 30 years the 500-year flood has a 6 percent chance of occurring during the 30 years. The "500-year flood" DOES NOT mean this size of flood will only occur once in 500 years. As can be seen from Table B, over 100 years the 500-year flood has a 18 percent chance of occurring while the 100-year flood has a 63 percent chance of occurring during the same 100 years.

The terms 500-year flood and 100-year flood can be misleading. Such floods may be experienced in any year. Since the National Flood Insurance Program was established, there have been numerous instances where communities have experienced two, and even three, 100-year or greater floods within a span of several years. A notable example is Pennsylvania in the 1970's where within five years after Tropical Storm Agnes (1972) battered the State another 100-year flood occurred.1

15

Table B presents the percent chance or probability of different flood events happening in any year and over various numbers of years. The chance of the 1 in 100 flood, or the 100-year flood, occurring over the life of a 30-year mortgage is 26 percent, or about one in four. The question for the homeowner is whether or not to purchase flood insurance. By contrast, during the term of a 30-year mortgage, there is only a 1 percent chance of suffering a fire loss.16 What level of risk is acceptable? What level of risk is unacceptable?

14 Jerry Adler, "Troubled Waters," Newsweek, July 26, 1993, page 21.

15 Federal Emergency Management Agency, Mandatory Purchase of Flood Insurance Guidelines, FEMA 186, Effective on July 13, 1989, p. 7.

16 Ibid.