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Graph showing annual exceedance-probability curves for Bloody Run Tributary near Sherrill, Iowa (05414605), showing the difference between expected moments algorithm (EMA/MGB) and Bulletin 17B/GB annual exceedance-probability analyses for a crest-stage gage (CSG) with four annual peak discharges below the minimum recording threshold ..
An unusually wet winter and spring of 2007 to 2008 resulted in extremely wet antecedent conditions throughout most of Iowa. Rainfall of 5 to 15 inches was observed in eastern Iowa during May 2008, and an additional 5 to 15 inches of rain was observed throughout most of Iowa in June. Because of the severity of the May and June 2008 flooding, the U.S. Geological Survey, in cooperation with other Federal, State, and local agencies, has summarized the meteorological and hydrological conditions leading to the flooding, compiled flood-peak stages and discharges, and estimated revised flood probabilities for 62 selected streamgages. Record peak discharges or flood probabilities of 1 percent or smaller (100-year flooding or greater) occurred at more than 60 streamgage locations, particularly in eastern Iowa. Cedar Rapids, Decorah, Des Moines, Iowa City, Mason City, and Waterloo were among the larger urban areas affected by this flooding. High water and flooding in small, headwater streams in north-central and eastern Iowa, particularly in June, combined and accumulated in large, mainstem rivers and resulted in flooding of historic proportions in the Cedar and Iowa Rivers. Previous flood-peak discharges at many locations were exceeded by substantial amounts, in some cases nearly doubling the previous record peak discharge at locations where more than 100 years of streamflow record are available.
The Basin Characteristics System (BCS) has been developed to quantify characteristics of a drainage basin. The first of four main BCS processing steps creates four geographic information system (GIS) digital maps representing the drainage divide, the drainage network, elevation contours, and the basin length. The drainage divide and basin length are manually digitized from 1:250,000‐scale topographic maps. The drainage network is extracted using GIS software from 1:100,000‐scale digital line graph data. The elevation contours are generated using GIS software from 1:250,000‐scale digital elevation model data. The second and third steps use software developed to assign attributes to specific features in three of the four digital maps and analyze the four maps to quantify 24 morphometric basin characteristics. The fourth step quantifies two climatic characteristics from digitized State maps of precipitation data.
Compared to manual methods of measurement, the BCS provides a reduction in the time required to quantify the 26 basin characteristics. Comparison tests indicate the BCS measurements are not significantly different from manual topographic‐map measurements for 11 of 12 primary drainage‐basin characteristics. Tests indicate the BCS significantly underestimates basin slope. Comparison‐measurement differences for basin slope, main channel slope, and basin relief appear to be due to limitations in the digital elevation model data.
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