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1. What Causes Floods in Iowa?
- University of Iowa Press
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All rivers flood. In fact, they flood with surprising regularity—almost every year or two (Leopold et al. 1964). Some floods are harmless and almost go unnoticed , with water barely spilling out of the river’s banks. Others are natural disasters that draw national attention. Iowa experienced such a disaster not long ago. Floodwaters destroyed homes and businesses, shut down city services, disrupted travel in the region, and damaged farms and cropland. That was 1993. In its aftermath, many Iowans probably assumed they had witnessed the biggest flood that would occur in a very, very long time. And then came 2008, a second major flood disaster in fifteen years. WhatdothesefloodstellusaboutthenatureoffloodinginIowa?Thischapter explores some general truths about floods—when they typically occur in Iowa and why. It also examines the common traits that big floods share. The meteorological and hydrological causes of floods in the Midwest are well known. Floods occur whenever more water runs off the landscape than the river can hold within its banks. Too much runoff is caused by excessive rainfall or melting snow and ice—water amounts far greater than can soak into 1 What Causes Floods in Iowa? A. Allen Bradley, Jr. 8 rising rivers, spreading waters the ground. If the ground is saturated or frozen and cannot hold more water, it makes flooding that much easier. Are there certain times of the year when a flood is more likely? The flood history of Iowa (see figure 1-1) shows that spring and summer define the flood season; about 90 percent of all floods occur from March through August. In contrast, floods rarely occur in the fall and winter. Iowa’s climate plays a dominant role in the timing of floods. Fall floods are rare because soils are so dry; by fall, the summer sun and growing plants have depleted the water stored in the ground. Although big rainstorms still do occur, FIGURE 1-1 Occurrences of floods in Iowa by month and according to the river’s size (as defined by its drainage area). The flood season is from early spring (March) through summer (August). There is an early springtime peak (March) and a late spring/early summer peak (June) within the flood season. Springtime flooding is more common on larger rivers, while summertime flooding is more common on smaller rivers. Illustration by the author, based on annual peak discharge data obtained online from the U.S. Geological Survey at waterdata.usgs. gov/nwis. Flood occurrences are for 167 sites in Iowa with at least 30 years or more of operation. Sites along the Mississippi and Missouri Rivers are excluded. For each river, its bankfull discharge is approximated by the annual peak discharge exceeded in two-thirds of the years, which corresponds to a 1.5-year flood recurrence interval (Leopold 1994). A flood occurrence is then defined as an annual peak discharge greater than its bankfull discharge. 60 50 40 30 20 10 0 All Flood Occurrences (%) Category Large Rivers Medium Rivers Small Rivers Drainage Area >1000 mi2 100 –1000 mi2 10 –100 mi2 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Date [54.160.133.33] Project MUSE (2024-03-28 23:36 GMT) what causes floods in iowa? 9 the ground can typically soak up a lot of the rainwater. Winter floods are rare because precipitation is often in the form of snow. While it can rain in winter, and rain on winter’s frozen ground can produce flooding, this is uncommon because rainfall intensity is usually much less than at other times of the year. Still, rain and snow during the fall and winter help replenish the moisture in the ground, leaving soils much wetter—and less able to soak up water—at the start of the flood season in spring. Spring floods occur in a variety of ways. The accumulation of snow during the winter months, and its melt over a relatively short period (of days and weeks), produces many spring floods (Soenksen et al. 1991). Spring weather, characterized by the clash of warm and cold air masses, marks the return of heavy rain with strong low-pressure systems. If widespread rain falls shortly after melting snow has saturated the ground, or if melting is caused by a warm period with rain on snow, a flood is often the result. Such events are responsible for the springtime (March) peak in Iowa floods (see figure 1-1). Latespringandsummerfloodsarecausedbythunderstorms(Soenksenetal. 1991). It is both the heavy rainfall that...