English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Flood-peak discharges and frequencies are presented for 394 gaged sites in Illinois/ Indiana, and Wisconsin for recurrence intervals ranging from 2 to 100 years. A technique is presented for estimating flood-peak discharges at recurrence intervals ranging from 2 to 500 years for nonregulated streams in Illinois with drainage areas ranging from 0.02 to 10/000 squares miles. Multiple-regression analyses/ using basin characteristics and peak streamflow data from 268 of the 394 gaged sites/ were used to define the flood-frequency relation. The most significant independent variables for estimating floodpeak discharges are drainage area/ slope/ rainfall intensity/ and a regional factor. Examples are given to show a step-by-step procedure in calculating a 50-year flood for a site on an ungaged stream/ a site at a gaged location/ and a site near a gaged location. INTRODUCTION The purpose of this report is to provide updated station flood-peak discharges and frequencies and to provide improvement to the previous techniques for estimating flood-peak discharges and frequencies of floods for sites on most streams where flood discharges are not significantly affected by regulation or urbanization. Flood-peak discharges and frequencies are presented for 394 gaging stations in Illinois/ Indiana/ and Wisconsin for recurrence intervals of 2/ 5/ 10/ 25/ 50/ and 100 years. A technique using drainage area (A)/ slope (S)/ rainfall intensity (I)/ and regional factor (Rf) was developed for estimating flood-peak discharges at ungaged sites in Illinois. Equations using these variables are applicable for estimating flood-peak discharges for recurrence intervals of 2 to 500 years for drainage areas ranging from 0.02 to 10/000 square miles (mi 2 ) on nonregulated rural streams. Estimates of future floods are necessary for the proper design of engineering projects such as bridges/ culverts/ highways/ and flood-control structures; for establishment of actuarial flood-insurance rates; and for proper flood-plain management by State and local agencies. Previous techniques for estimating flood-peak discharges and frequencies in Illinois have been provided by Mitchell (1954)/ Speer and Gamble (1965)/ Wiitala (1965)/ Patterson and Gamble (1968)/ Ellis (1968)/ Cams (1973)/ Curtis (1977a)/ and Alien and Bejcek (1979). Techniques were developed by Cams (1973)/ Curtis (1977a)/ and Alien and Bejcek (1979) using ordinary least squares multiple-regression analyses as recommended by Thomas and Benson (1970). Additional data and improved analytical methods used in this report increase the confidence in estimating techniques over those published in earlier reports. This report was prepared under a cooperative agreement between the State of Illinois, Department of Transportation, Division of Water Resources, and the U.S. Geological Survey (Survey). Streamflow data were collected in cooperation with the U.S. Army Corps of Engineers and State and local agencies. TECHNIQUE FOR ESTIMATING FLOOD-PEAK DISCHARGES Annual peak discharges from gaging stations having a minimum of 10 years of record through the 1985 water year were used to define station floodfrequency relations. Locations of these stations are shown in figures 1 and 2. The map number, identification number, name, geographic location, and station flood-peak discharges for the stations are listed in table 1. All figures and tables are grouped in the back of the report for easy reference. Station flood-frequency relations were defined using the Hydrology Subcommittee of Interagency Advisory Committee on Water Data (1982), formerly U.S. Water Resources Council, guidelines. These guidelines outline procedures to fit the logarithms of observed annual peak discharges to the Pearson Type III frequency distribution. Peak discharges of various recurrence intervals and basin characteristics for gaging stations were used in multiple-regression analyses to develop estimating equations for flood-peak discharges and frequencies on nonregulated rural streams in Illinois. Data from stations affected by either urbanization or by regulation were not included in the regression analyses. Relations were developed for estimating flood-peak discharges corresponding to the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence-interval flood (T-year flood or Qrp). The regression analyses indicated that the independent variables drainage area (A), slope (S), rainfall intensity (I), and regional factor (Rf) are the most significant variables to use in estimating flood-peak discharges for Illinois streams. One estimating equation for each recurrence interval provides a straightforward technique to compute flood-peak discharges for both small and large Illinois streams. Flood-peak discharges and frequencies, basin characteristics and other pertinent data, and regional factors are tabulated in tables 1, 2, and 3. In table 1, two sets of station flood-peak discharges are presented for two stations (Nos. 385 and 389) on the Big Muddy River. The first set of discharges are for periods of nonregulated flow and were used in the regression analyses. The second set of discharges are for periods of regulated flow and were not used in the regress: .on analyses. The reliability of flood-frequency estimates is uncertain for very large recurrence intervals. Because of this uncertainty, the 500-year flood discharges are omitted from table 1. An estimating equation for the 500-year flood is provided primarily for planners who are required to compute this event for special purposes such as flood-insurance studies. Only those stations used in the regression analyses are listed in table 2. The flood-frequency and the regression analyses, used to develop the estimating technique, are defined in detail in the data-analyses section. Flood-peak discharge equations, applicable statewide, for estimating Qrp on nonregulated rural streams are as follows: Q2 40.3 A0 ' 790 S°' 481 (1-2.5)°' 677 Rf (1) Q5 66.4 A0 ' 786 S°' 513 (1-2.5)°' 719 Rf (2) Q 10 83.0 A0 ' 785 S°' 532 (1-2.5)°' 742 Rf (3) Q25 = 103 A0 * 786 s°* 552 (1-2.5)°' 768 Rf (4) Q50 = 118 A0 * 786 s°* 566 (1-2.5)°' 786 Rf (5) Q 100 132 A0 ' 787 S°' 578 d-2.5)°' 803 ^ (6) Q500 = 162 A0 ' 789 S°' 601 (1-2.5)°' 838 Rf (7) The four variables required to solve the equations are drainage area (A), slope (S), rainfall intensity (I), and regional factor (Rf). Drainage area and slope are determined from topographic maps. Drainage area is the area contributing to surface runoff. Slope is determined between points 10 percent and 85 percent of the total distance measured along the low-water channel from the site to the basin divide. The rainfall intensity is determined from figure 3. The regional factor is determined by first selecting the region number from figure 4 and then the appropriate regional factor from table 3. Flood-peak discharge equations for recurrence intervals between 2 and 100 years, other than those in equations 1 to 7, may be developed by interpolating the regression constant and coefficients from the graphs in figure 5. APPLICATION OF ESTIMATING TECHNIQUE The technique for estimating flood-peak discharges and frequencies is applicable to either ungaged or gaged nonregulated rural streams. Figure 6 shows the sequence to follow for estimating a flood-peak discharge at a site. Step-by-step procedures for applying the estimating technique are given in the examples that follow. Site on Ungaged Stream Flood frequency estimates at sites on ungaged streams are calculated using equations 1 to 7. Example 1: Computation of the 50-year recurrence interval flood at a site on an ungaged stream: 1. Determine the size of contributing drainage area (A), in square miles. The area can be planimetered on topographic, county, or other maps suitable for delineating the basin boundary. For this example, assume A = 625 mi^. 2. Determine the slope (S), in feet per mile (ft/mi). Slope is based on the difference of elevations divided by distance between points 10 percent and 85 percent of the total distance measured along the low-water channel of the stream from the site to the basin divide. For this example, assume S = 2.5 ft/mi. 3. Determine the rainfall intensity (I), in inches, from figure 3. The value of I should be an average for the basin. For this example, assume I = 3.1 inches. 4. Determine the region (R) and the regional factor (Rf) from figure 4 and table 3, respectively. For this example, R is III and Rf is 0.862. 5. Select equation 5 from page 3 and compute the flood magnitude. Q50 = 118 A0 786 s°' 566 (1-2.5)°' 786 Rf = (118)(625)°786 (2.5)°566 (3.1-2.5)°786 (0.862) = (118)(157.6)(1.68)(0.669)(0.862) = 18,000 ft 3 /s. Site at Gaged Location Flood frequency estimates at gaged sites are combinations of the gaging station frequency curve and the equation estimates. The equivalent years of record concept (Hardison, 1971) was used to obtain weighted estimates of peak flow at gaged sites using estimates obtained from station records and from equations 1 to 7. This procedure was described by the Hydrologic Subcommittee (1982) and is expressed in the equation Om Yrs of record (log sta. QT ) + Eg yrs record (log regional QT ) t Yrs of record + Eq yrs record In equation 8, station QT is obtained from the first line of discharge values in table 1 and converted to a logarithm (log). mined from table 2. The regional Qip is computed using the desired regional estimating equation on page 3 or obtained from The years of record are deterthe second line of discharge values in table 1 and then transformed into logs. The station equivalent years of record (Eq yrs record) for the equation are also given in table 2. The antilog of the result from equation 8 is the weighted estimate of the station flood discharge. Example 2: Computation of the weighted 50-year recurrence interval flood at the gaging station 05572000 Sangaraon River at Monticello, Illinois (map No. 307) : Yrs of record (log sta. Qcn) + Eq yrs record (log eq

Technique for estimating flood-peak discharges and frequencies on rural streams in Illinois