Effects of the 1993 flood on the determination of flood magnitude and frequency in Iowa - Floods in the upper Mississipp

U .S . GEOLOGICAL SURVEY CIRCULAR 1120-K U.S . DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary

U.S . GEOLOGICAL SURVEY

Mark Schaefer, Acting Director

1997 Free on application to the U .S . Geological Survey Information Services Box 25286, Federal Center Denver, CO 80225

FOREWORD

During spring and summer 1993, record flooding inundated much of the upper Mississippi River Basin . The magnitude of the damages-in terms of property, disrupted business, and personal traumawas unmatched by any other flood disaster in United States history . Property damage alone is expected to exceed $10 billion . Damaged highways and submerged roads disrupted overland transportation throughout the flooded region . The Mississippi and the Missouri Rivers were closed to navigation before, during, and after the flooding . Millions of acres of productive farmland remained under water for weeks during the growing season . Rills and gullies in many tilled fields are the result of the severe ero sion that occurred throughout the Midwestern United States farmbelt . The hydrologic effects of extended rainfall throughout the upper Midwestern United States were severe and widespread . The banks and channels of many rivers were severely eroded, and sediment was deposited over large areas of the basin's flood plain . Record flows submerged many areas that had not been affected by previous floods . Indus trial and agricultural areas were inundated, which caused concern about the transport and fate of indus trial chemicals, sewage effluent, and agricultural chemicals in the floodwaters . The extent and duration of the flooding caused numerous levees to fail . One failed levee on the Raccoon River in Des Moines, Iowa, led to flooding of the city's water treatment plant . As a result, the city was without drinking water for 19 days .

As the Nation's principal water-science agency, the U.S . Geological Survey (USGS) is in a unique position to provide an immediate assessment of some of the hydrological effects of the 1993 flood . The USGS maintains a hydrologic data network and conducts extensive water-resources investigations nation wide . Long-term data from this network and information on local and regional hydrology provide the basis for identifying and documenting the effects of the flooding . During the flood, the USGS provided continuous streamflow and related information to the National Weather Service (NWS), the U.S . Army Corps of Engineers, the Federal Emergency Management Agency (FEMA), and many State and local agencies as part of its role to provide basic information on the Nation's surface- and ground-water resources at thousands of locations across the United States . The NWS has used the data in forecasting floods and issuing flood warnings . The data have been used by the Corps of Engineers to operate water diversions, dams, locks, and levees . The FEMA and many State and local emergency management agencies have used USGS hydrologic data and NWS forecasts as part of the basis of their local flood-response activities . In addition, USGS hydrologists are conducting a series of investigations to document the effects of the flooding and to improve understanding of the related processes . The major initial findings from these studies will be reported in this Circular series as results become available .

U.S . Geological Survey Circular 1120, Floods in the Upper Mississippi River Basin, 1993, consists of individually published chapters that will document the effects of the 1993 flooding . The series includes data and findings on the magnitude and frequency of peak discharges ; precipitation ; water-qual ity characteristics, including nutrients and man-made contaminants ; transport of sediment ; assessment of sediment deposited on flood plains ; effects of inundation on ground-water quality ; flood-discharge volume ; effects of reservoir storage on flood peaks ; stream-channel scour at selected bridges ; extent of floodplain inundation ; and documentation of geomorphologic changes .

Mark Schaefer Acting Director

CONTENTS Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa

By David A. Eash

To evaluate the effects of the 1993 flood in the upper Mississippi River Basin on the determi nation of flood magnitude and frequency, dis charges that had recurrence intervals of 10, 25, 50, and 100 years computed from data through the 1992 water year were compared with those computed from data through the 1993 water year for 62 selected streamflow-gaging stations in Iowa . On the basis of the flood-frequency analysis computed from data through the 1993 water year, a flood that was greater than or equal to a 10-year recurrence-interval discharge occurred during 1993 at all 62 gaging stations, and a flood greater than or equal to a 100-year recurrence-interval discharge occurred at 11 of the gaging stations . Results of the comparison indicated that inclusion of the 1993 flood in the data base resulted in an increase in the magnitude of dis charges for all selected recurrence intervals at the 62 streamflow-gaging stations in Iowa . A larger percentage increase in the magnitude of discharge was computed for the larger recurrence intervals than for the smaller recurrence intervals for most of the selected gaging stations . As a result of including the 1993 peak discharge in the flood-frequency analysis, three gaging stations had an increase in the 100-year recurrence-interval discharge that was greater than 30 percent . Several factors, which included recurrence intervals for the 1993 peak discharges and the effective record lengths for 1993, were investigated for the 62 selected streamflow gaging stations to evaluate their possible effect on the computed flood-frequency discharges . The combined effect of these two factors on the com puted 100-year recurrence-interval discharges was significant . Gaging stations were grouped into four discrete categories on the basis of recurrence intervals for the 1993 peak discharges and the effective record lengths for 1993 . Of the 28 gaging stations that had small flood magnitudes in 1993 and long record lengths, the difference between the 1992 and the 1993 flood-frequency analyses for 100- year recurrence-interval discharges at 22 gaging stations was less than 5 percent . Of the 10 gaging stations that had large flood magnitudes in 1993 and short record lengths, the increase in 100-year recurrence-interval discharges at 9 gaging stations was greater than 15 percent .

A nine-State area of the upper Mississippi River Basin was flooded from late March through September 1993 (fig . 1) . From mid-June through early August

1993, the flooding was severe as a result of the wide

areal extent, large peak discharges, long duration, and overall destructiveness . Record or near-record peak discharges were recorded in 1993 at streamflow gaging stations in Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota, and Wisconsin after intense and persistent rainfall on soils saturated from excessive precipitation (Parrett and oth ers, 1993) . Spring 1993 was wetter than average, and weather patterns that persisted from early June through July caused an unusually large amount of precipitation to fall in the upper Midwest (Wahl and others, 1993) . From April through September, more than 50 inches of rain fell in parts of Iowa, Kansas, and Missouri (fig . 2) .

The intense rain from the storm systems that tracked across the general area of flooding from April through September resulted in sustained high-flood volumes . Flood volumes at many gaging stations in the flooded area were significantly larger than previous maximums and more than twice the mean flow-volumes for April through September (Southard, 1995) .

Knowledge of the magnitude and frequency of floods is essential for the effective management of flood plains and for the economical planning and safe design of bridges, dams, levees, and other structures located in flood plains . Flood-frequency analyses are computed for streamflow-gaging stations by using annual peak discharges . As each additional annual peak discharge is added to the record of a gaging station, an updated flood-frequency analysis can be computed, and revised discharges for various frequencies of exceedance or recurrence intervals can be determined . Thus, flood-frequency statistics can be recalculated each year, and as additional annual peak discharges are collected and used in the analyses, these statistics become more reliable . One important aspect of the analysis and description of the 1993 flooding in the upper Mississippi River Basin must be considered-the effect the addition of 1993 peak discharges to the records of gaging stations will have on the determination of flood magnitude and frequency and, subsequently, on esti mates of recurrence intervals of the 1993 flood .

Purpose and Scope

The purpose of this report is to compare quanti tatively discharges that had recurrence . intervals of 10, 25, 50, and 100 years computed from data through the 1992 water year to those computed from data through the 1993 water year and to evaluate the effects of the 1993 flooding on the computed flood-frequency statis tics in Iowa . Iowa was selected for this study because a large number of streamflow-gaging stations recorded significant flooding in 1993, and a large data set could, therefore, be compiled for comparative flood-frequency analyses .

Acknowledgments

The information contained herein is based on data collected by the National Weather Service, the U.S . Army Corps of Engineers, the U.S . Geological Survey, and several State and local agencies . Appreci ation is expressed to the personnel in these agencies who were involved in the collection of data . The flood data used in this study often were collected during adverse conditions, and the efforts of these individuals made this analysis possible .

FLOOD-FREQUENCY ANALYSES FOR IOWA

Engineers and planners often design structures or regulate development on flood plains for which dam age may be incurred by occasional floods of varying magnitude (Dalrymple, 1960) . By using flood-frequency analyses to design structures or to regulate development on flood plains to either a specific proba bility or a specific calculated risk, such as a 1- or a 2- percent chance that a given flood magnitude will be exceeded in any one year, engineers and planners are able to standardize the risk factors involved with esti mating flood-frequency discharges .

Selection of Streamflow Data

In Iowa, 83 unregulated streamflow-gaging sta tions that had at least 11 years of systematic, continuous-record data through the 1993 water year were initially considered for use in this study. Peak discharges in 1993 at these gaging stations were initially compared with previously published flood-frequency discharges computed from data through the 1990 water year (Eash, 1993) ; on the basis of this comparison, 62 of these gaging stations that had a greater than or equal to 10-year recurrence-interval flood in 1993 were selected for this study.

Data Analyses

In this study, the method described in the Interagency Advisory Committee on Water Data (1982) was used to compute the magnitude and frequency of floods at each of the 62 selected streamflow-gaging stations in Iowa . Two separate flood-frequency curves were devel oped for each gaging station by fitting a Pearson Type-III distribution to the logarithms (base 10) of the annual peak discharges by means of the U.S . Geological Survey's WATSTORE flood-frequency-analysis program (Kirby, 1981) . The flood-frequency analysis, which was based on data through the 1992 water year, is here after termed the "1992 analysis," and the flood-frequency analysis, which was based on data through the 1993 water year, is hereafter termed the "1993 analy sis ."

Table 1 (at end of report) lists the computed discharges that had recurrence intervals of 10, 25, 50, and 100 years for the 1992 and the 1993 analyses ; the loca tions of these streamflow-gaging stations and ranges in recurrence intervals for their 1993 peak discharges, which were based on the 1993 analysis, are shown in figure 3 . Table 1 also lists information on the drainage area; the discharge, date, and unit runoff of the 1993 peak discharge (Southard and others, 1994) ; the differences between discharges from the 1992 and the 1993 analyses ; the recurrence interval for the 1993 peak discharge, which was interpolated from the 1992 and the 1993 analyses ; the annual peak discharge period of record (listed as the WATSTORE peak flow record) ; the previous annual maximum discharge and date ; use of historical data ; and the effective record length of the gaging station for the 1993 analysis . The effective record length is an estimate of the record length when historical flood data are weighted with the systematic record length .

The record of annual peak discharges for a streamflow-gaging station includes the water years dur ing which it was operated, which is termed the "period of systematic record ." This record also may include historical peak discharges during water years outside the period of systematic record . Annual peak discharges, which are maintained in theWATSTORE Peak Flow File data base (Lepkin and others, 1979), were used to perform the flood-frequency analyses described in this report .

For the 1992 and the 1993 analyses, extremely small discharge values (low outliers) were censored and adjusted for, historical data were used to make adjustments for extremely large discharge values (high outliers), and the coefficient of skew was weighted for each streamflow-gaging station with skew values obtained from a statewide skew analysis . Whenever possible, historical flood data were used to extend the flood record for gaging stations . Flood-frequency analyses for 41 of the 62 selected gaging stations were adjusted for historical data, whereas flood-frequency analyses for the other 21 were based only on their period of systematic record .

Assumptions of Data Analyses

The accuracy and reliability of flood-frequency analyses are dependent on several assumptions about the data (Interagency Advisory Committee on Water Data, 1982) . The fundamental assumption is that the record of past flood discharges is an accurate and reliable indicator of the range of flood discharges that could occur in the future . This assumption implies the following assumptions :

The flood-generating mechanism is time-stationary, this is, the meteorologic and hydrologic processes that generate future floods will be the same as they were during the period of the past flood record . This implies that the climatic and meteorologic conditions, hydrologic conditions created by land use and land cover in the watershed, and hydraulic conditions of the stream channel and flood plain remain constant through time . The flood record is an accurate and representative depiction of the floods that occurred during the period of record . This implies that measurements of flood discharges included in the record are accurate and that any special flood-risk conditions associated with a site were properly identified and considered in the flood-frequency analysis . The hydrologic process of flood occurrence can be represented mathematically as a sequence of inde pendent annual peak discharges that are randomly sampled from a population of all possible flood discharges .

Flood-Recurrence Interval

The magnitude and frequency of floods are com puted for a streamflow-gaging station by relating annual peak discharges to either annual exceedance probability or recurrence interval. Annual exceedance probability is expressed as the chance that a selected flood magnitude will be exceeded in any one year . Recurrence interval, which is the reciprocal of the annual exceedance probability, is the average number of years between exceedances of a selected flood magnitude . For example, if a theoretical flood magnitude. i s exceeded once on the average during any 100-year period (recurrence interval), then it has a 1-percent chance (annual exceedance probability equals 0.01) of being exceeded during any one year. This flood, which is commonly termed the. "100-year flood," is the theo retical peak discharge against which actual flood peak discharges generally are compared to measure their severity. Although the recurrence interval represents the long-term average period between floods of a spe cific magnitude, rare floods could occur at shorter intervals or even within the same year. For example, the South Skunk River near Ames streamflow gaging station (fig . 3, site 26) has a theoretical 100- year recurrence-interval discharge of 9,090 cubic feet per second as computed from the 1993 analysis (table

1) . During 1993, however, a flood peak discharge of

11,100 cubic feet per second occurred on July 9, and another flood peak discharge of 11,200 cubic feet per second occurred on August 16 . Thus, two floods that, theoretically, each had less than a 1-percent chance of occurring during any one year occurred at this site in the same year.

COMPARISON OF 1992 AND 1993 FLOOD-FREQUENCY DISCHARGES FOR IOWA

Differences between discharges that had recurrence intervals of 10, 25, 50, and 100 years for the 1992 and the 1993 analyses were evaluated to determine the effects of the 1993 flood on the computed flood magnitude and frequency (table 1) . Differences were calculated as follows : the difference between the 1993 recurrence-interval discharge and that of 1992 was divided by the 1992 recurrence-interval discharge, and this value was multiplied by 100 . The ranges in differences between the discharges are shown in figure 4, and the summary statistics of the ranges are listed in table 2 . The overall trend indicated by the differences in discharges for selected recurrence intervals (fig . 4 ; table 2) is that smaller differences were computed for discharges that had smaller recurrence intervals, and larger differences were computed for discharges that had larger recurrence intervals .

The mean and median statistics for differences listed in table 2 for each recurrence interval show this trend of larger differences for larger recurrence intervals . The mean differences range from 5.9 percent for the 10-year recurrence interval to 9 .6 percent for the 100-year recurrence interval . The median differences range from 4.6 percent for the 10-year recurrence interval to 6.2 percent for the 100-year recurrence interval .

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Figure 4 shows the number of streamflow-gaging stations that had differences of greater than or equal to 15 percent for the selected recurrence intervals-3,10 year (fig . 4A) ; 6, 25 year (fig . 4B) ; 10, 50 year (fig . 4C) ; and 13,100 year (fig . 4D) . The spatial distribution of ranges in differences between the 1992 and the 1993 analyses for 100-year recurrence-interval discharges is shown in figure 5 .

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H A comparison of the computed discharges for the 1992 and the 1993 analyses listed in table 1 indicates that inclusion of the 1993 flood resulted in larger dis charges for all selected recurrence intervals at the 62 selected streamflow-gaging stations . Including the 1993 flood in the flood-frequency analysis had a greater effect on the larger recurrence-interval discharges for the majority of these gaging stations . Differences between the 1992 and the 1993 analyses were slightly larger at the 10-year recurrence interval than at the 100- year recurrence interval at five of the gaging stations listed in table 1 (sites 18, 19, 38, 51, 56) . On the basis of the 1993 analysis, recurrence intervals for 1993 peak discharges at these five gaging stations ranged from 10 to 14 years .

At three streamflow-gaging stations (sites 1, 29,

59) listed in table 1, the increase in the 100-year recur-

rence-interval discharge was greater than 30 percent as a result of including the 1993 flood in the flood-fre quency analysis . The maximum difference between the 1992 and the 1993 analyses (tables 1, 2) was 34 .9 percent for the 100-year recurrence-interval discharge for the Upper Iowa River near Dorchester (site 1) . This site had a large flood magnitude in 1993 (recurrence interval of 70 years based on the 1993 analysis) and a fairly short record length (computed effective record length of 25 years) .

FACTORSTHAT AFFECTTHE COMPUTED FLOOD-FREQUENCY DISCHARGES

The following factors were investigated to eval uate their possible effect on the differences between discharges computed from the 1992 and the 1993 anal yses (table 1) : drainage area, 1993 peak discharge, 1993 peak unit runoff (the 1993 peak discharge divided by the drainage area), recurrence interval of the 1993 peak discharge based on the 1993 analysis, and effective record length based on the 1993 analysis . Sum mary statistics for these factors are listed in table 3 .

Differences between the 1992 and the 1993 analyses for 100-year recurrence-interval discharges were correlated with the factors . The 100-year recur rence interval was selected because the differences between the 1992 and the 1993 analyses were largest (fig . 4 ; table 2) . Results of the correlations are listed in table 4 as Pearson's product-moment correlation coefficients and Spearman's rank correlation coeffi cients . The Pearson's product-moment correlation coefficient is computed by using a parametric correlation analysis on the data, and the Spearman's rank correla tion coefficient is computed by using a nonparametric correlation analysis on the ranks of the data.

Correlation coefficients are statistics that provide a measure of the strength of the linear relation between two variables . The correlation coefficient ranges between -1 .0 and +1 .0, and the closer the value is to ±1 .0, the stronger is the linear relation . A positive value for the correlation coefficient indicates that as one variable increases, the other variable also increases . A negative value for the correlation coefficient indicates that as one variable increases, the other variable decreases .

The two factors with the strongest correlations with differences between the 1992 and the 1993 analy ses for 100-year recurrence-interval discharges were selected for further investigation . Results of the corre lation analyses listed in table 4 indicate that the two factors with the strongest correlations are recurrence interval of the 1993 peak discharge and 1993 effective record length. These two factors were investigated fur ther to evaluate their possible effect on the computed flood-frequency discharges .

Effect of 1993 Flood Magnitude

Of the five factors investigated in the correlation analyses, 1993 peak discharge, 1993 peak unit runoff, and recurrence interval of the 1993 peak discharge are related to the magnitude of the 1993 flood. Of these three, the recurrence interval of the 1993 peak dis charge most strongly correlated with the difference between the 1992 and the 1993 analyses for 100-year recurrence-interval discharges (table 4) . Thus, on the basis of the 1993 analysis (table 1), recurrence intervals of the 1993 flood peaks were used to eval uate the effect of 1993 flood magnitude on the com puted flood-frequency discharges in Iowa .

. and fig-Figure 3 shows the spatial distribution ure 6A shows the frequency distribution of ranges in recurrence intervals for the 1993 peak discharges for the selected streamflow-gaging stations . In table 5, summary statistics for each of the selected recurrence intervals for the 62 selected gaging stations are grouped on the basis of recurrence intervals for the 1993 flood of greater than or equal to 50 years and recurrence intervals of less than 50 years . The mean and median statistics for the two groups indicate that gaging stations that had large flood magnitudes (recurrence intervals of 50 years) in 1993 generally had greater than larger differences between the 1992 and the 1993 flood-frequency analyses for discharges of all selected recurrence intervals .

Effect of 1993 Record Length

Effective record lengths that were based on the 1993 analysis were used to evaluate the effect of record lengths on the computed flood-frequency discharges in Iowa . The effective record length is an estimate of the record length for a streamflow-gaging station when historical flood data are weighted with the systematic record length. The increase in the effective record length over the. systematic record length provides an estimate of the value of the historical data . If a gaging station record only contains systematic data, then the effective record length is the same as the systematic record length . Effective record lengths calculated for gaging stations that have historical flood data can, therefore, be compared with the record lengths of those that have systematic data .

The effective record length of a streamflow gaging station is based on an empirical analysis made by Gary D . Tasker (U .S . Geological Survey, written commun ., 1992) of results reported in Tasker and Thomas (1978) and Stedinger and Cohn (1986) . Table 1 lists the effective record lengths that were calculated for each gaging station .

Figure 6B shows the frequency distribution of the effective record lengths for the selected streamflow-gaging stations . As indicated in table 4, the correlation is neg ative between effective record length and the difference between the 1992 and the 1993 flood-frequency analyses for 100-year recurrence-interval discharges . This correlation is negative because as the effective record length of a gaging station increases, the influence of any given annual peak discharge decreases even though an annual peak discharge may be very large . In table 6, summary statistics for each of the selected recurrence intervals for the 62 selected gaging stations are grouped on the basis of effective record lengths for 1993 of greater than or equal to 50 years and effective record lengths for 1993 of less than 50 years . The mean and median statistics for the two groups indicate that gaging stations that had short record lengths (effective record lengths of less than 50 years) through 1993 generally had larger differences between the 1992 and the 1993 flood-frequency analyses for discharges of all selected recurrence inter vals .

Combined Effect of the 1993 Flood Magnitude and Record Length

To evaluate the combined effect of the 1993 flood magnitude and the record length on the computed flood-frequency discharges in Iowa, the 62 selected streamflow-gaging stations were grouped into four dis crete categories on the basis of recurrence intervals of the 1993 flood and effective record lengths through 1993 as follows : A, the recurrence interval of the 1993 flood was greater than or equal to 50 years and the effective record length was less than 50 years ; B, the recurrence interval of the 1993 flood was greater than or equal to 50 years and the effective record length was greater than or equal to 50 years ; C, the recurrence interval of the 1993 flood was less than 50 years and the effective record length was less than 50 years ; and D, the recurrence interval for the 1993 flood was less than 50 years and the effective record length was greater than or equal to 50 years . The frequency distributions of recurrence intervals for the 1993 flood, effective record lengths for 1993, and the above four categories are shown in figure 6 . Table 7 lists summary statistics for differences between the 1992 and the 1993 analyses for 100-year recurrence-interval discharges for each of these four categories .

As indicated by the differences in the mean and median statistics listed in tables 5 to 7 for the 100-year recurrence-interval discharge, the combined effect of the 1993 flood magnitude and record length is greater than the effect of either individual factor on the com puted flood-frequency discharges in Iowa . Categories A andD best indicate the combined effect of these two factors on the computed 100-year recurrence-interval discharges . Large flood magnitudes coupled with short record lengths (category A) have mean and median statistics of 23 .5 and 24 .2 percent, respectively (table 7) . Conversely, small flood magnitudes coupled with long record lengths (category D) have mean and median statistics of4.0 and 3 .4 percent, respectively (table 7) . The contrast between the mean and median statistics of these two categories of streamflow-gaging stations indicates that the 1993 flood magnitudes and record lengths that were used in the 1993 analysis had a sig nificant combined effect on the computed 100-year recurrence-interval discharges .

The spatial and frequency distributions of the 62 streamflow-gaging stations were grouped on the basis of those that had a less than 5-percent difference, a 5 to less than 15-percent difference, and a greater than or equal to 15-percent difference between the 1992 and the 1993 analyses for 100-year recurrence-interval discharges, as shown in figures 7 and 8, respectively . The spatial and frequency distributions shown in these two figures are identified according to the previously defined categories in figure 6C and table 7 .

Figure 8 . Ranges in differences between the 1992 and the

interval discharges and selected categories of recurrence interval (RI) and effective record length (ERL), in years, for selected streamflow-gaging stations in Iowa . A, Streamflow gaging stations that have a less than 5-percent difference ; B, Streamflow-gaging stations that have a 5-to less than 15-percent difference ; C, Streamflow-gaging stations that have a greater than or equal to 15-percent difference .

Of the 28 streamflow-gaging stations in category D (fig . 6C), 22 had a less than 5-percent difference between 100-year recurrence-interval discharges for 1992 and 1993 (fig . 8A), and of the 25 gaging stations that had a less than 5-percent differ ence between 100-year recurrence-interval discharges for 1992 and 1993, 22 were in category D (figs . 7A and 8A) . Conversely, of the 10 gaging sta tions in category A (fig . 6C), 9 had a greater than or equal to15-percent difference between 100-year recurrence-interval discharges for 1992 and 1993 (fig . 8C), and of the 13 gaging stations that had a greater than or equal to 15-percent difference between 100-year recurrence-interval discharges for 1992 and 1993, 9 were in category A (figs.7C and 8C) .

EFFECT OF FLOOD-FREQUENCY DISCHARGES ON WATER-SURFACE ELEVATIONS

Associated with a specified flood-frequency discharge is a water-surface elevation that can be determined by using hydraulic principles of open-channel flow or from a stage-discharge rating curve . Water-surface elevations determined for flood-frequency discharges are used for the effective management of flood plains and for the safe design of bridges, dams, levees, and other structures located in flood plains .

To evaluate the effect that changes in flood-frequency discharges may have on water-surface elevations, two of the streamflow-gaging stations in Iowa that had large differences between the 1992 and the 1993 flood-frequency discharges were selected for investigation . The Upper Iowa River near Dorchester (fig . 3, site 1) and the Squaw Creek at Ames (fig . 3, site

27) had differences between the 1992 and the 1993

analyses for 100-year recurrence-interval discharges of 34 .9 and 26 .2 percent, respectively (table 1) .

The records of annual peak discharges for these two streamflow-gaging stations are shown in figures 9A and 10A . The 1992 and the 1993 flood-frequency curves that were computed by fitting a Pearson Type-III distribution to the logarithms of the annual peak discharges for each of these gaging stations (table 1) are shown in figures 9B and 10B . These figures show a larger difference in estimated discharges between the 1992 and the 1993 flood-frequency curves at the smaller annual exceedance probabilities or larger recurrence intervals .

The computation of discharge records at a streamflow-gaging station is dependent upon the devel opment of a stage-discharge relation, or rating curve, between water-surface elevations, or stages, and the corresponding flow rates, or discharges . A rating curve is developed by measuring the discharge at a variety of stages, graphing the stage versus discharge points, and drawing a best-fit curvilinear line through the points .

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110"11" IIIIIIIIIII~~"~I The rating curves that were in use during the 1992 and the 1993 water years at the two selected streamflow-gaging stations are shown in figures 9 C and 1OC . Discharges that were computed for the 100-year recurrence interval for 1992 and 1993 (table 1) were used to interpolate gage heights (stages) from the respective rating curve for each gaging station . Differ ences in gage heights between the 1992 and the 1993 analyses for 100-year recurrence-interval discharges were interpolated to have increased 1 .77 feet for the Upper Iowa River near Dorchester gaging station (fig . 9C) and 1 .05 feet for the Squaw Creek at Ames gaging station (fig . 1OC) .

The gage heights shown in figures 9C and IOC for 100-year recurrence-interval discharges are consid ered to be only approximate because they do not take into consideration adjustments that may have been made to the rating curves during the flood ; consequently, they are presented only for illustrative purposes . The differences in gage heights between the 1992 and the 1993 analyses for 100-year recurrence interval discharges for these two streamflow-gaging stations illustrate how changes in flood-frequency discharges, which result from the inclusion of the 1993 peak discharges, can affect water-surface elevations . It should be noted that the differences in gage heights that are illustrated for these two gaging stations are larger than anticipated for most gaging stations given their large differences in 100-year recurrence-interval discharges .

To evaluate the effects of the 1993 flood on the determination of flood magnitude and frequency in Iowa, discharges that had recurrence intervals of 10, 25, 50, and 100 years computed from data through the 1992 water year were compared with discharges that had the same recurrence intervals computed from data through the 1993 water year for 62 selected streamflow-gaging stations in Iowa . On the basis of the 1993 flood-frequency analysis, a flood that was greater than or equal to a 10-year recurrence-interval discharge occurred during 1993 at all 62 gaging stations, and a flood that was greater than or equal to a l 00-year recurrence-interval discharge occurred at 11 of the gaging stations .

Results of the comparison indicated that inclusion of the 1993 peak discharges in the flood-frequency analysis caused an increase in the magnitude of discharges for all selected recurrence intervals at the 62 selected streamflow-gaging stations in Iowa . A larger percentage increase in the magnitude of discharge was computed for the larger recurrence intervals than for the smallerrecurrence intervals for most of the selected gaging stations . As a result of including the 1993 flood in the flood-frequency analysis, three gaging stations had an increase in the 100-year recurrence-interval discharge that was greater than 30 percent .

Several factors, which included recurrence intervals for 1993 peak discharges and effective record lengths for 1993, were investigated for the 62 selected streamflow-gaging stations to evaluate their possible effect on differences between 1992 and 1993 flood-frequency discharges . This investigation indicated that the 1993 flood magnitudes and record lengths that were used in the 1993 flood-frequency analysis had a signif icant combined effect on the computed 100-year recurrence-interval discharges . Gaging stations were grouped into four discrete categories on the basis of recurrence intervals for 1993 peak discharges and effective record lengths for 1993 . Of the 28 gaging stations that had small flood magnitudes in 1993 and long record lengths, the difference between the 1992 and the 1993 flood-frequency analyses for 100-year recurrence-interval discharges at 22 gaging stations was less than 5 percent. Of the 10 gaging stations that had large flood magnitudes in 1993 and short record lengths, the increase in 100-year recurrence-interval discharges at 9 gaging stations was greater than or equal to 15 percent .

Dalrymple, Tate, 1960, Flood frequency analyses, pt . 3 of Flood flow techniques in Manual of hydrology : U.S . Geological Survey Water-Supply Paper 1543-A, p.1-180 . Eash, D.A ., 1993, Estimating design-flood discharges for streams in Iowa using drainage-basin and channelgeometry characteristics : U.S . Geological Survey Water-Resources Investigations Report 93-4062, 96 p . Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency : Hydrology Subcommittee Bulletin 17B, 28 p . and appendixes .

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