Equations For Estimating Synthetic Unit-Hydrograph .

INTRODUCTIONDesign of stormwater management facilitiesand other hydraulic structures (culverts, bridge waterways), determination of flood plain boundaries, andassessment of the safety of structures in rivers typicallyinvolve the application of a design hydrograph. Thesedesign hydrographs are computed on the basis ofdesign storms of a specified probability of occurrencedetermined from standard references, such as theU.S. Weather Bureau Technical Paper Number 40(TP40) (Hershfield, 1961) or the Illinois State WaterSurvey Bulletin 70 (Bulletin 70) (Huff and Angel,1989). Abstractions from rainfall resulting from interception, depression storage, and infiltration are thendetermined on the basis of available data from theliterature and considering the effects of the soil type,land cover/land use, and antecedent moisture conditions. Typically, the Soil Conservation Service (SCS,now known as the Natural Resources ConservationService) (1985) curve-number method is appliedto determine the abstractions. By subtracting theabstractions from the design rainfall, the precipitationexcess, which approximately equals the direct runoff(effective precipitation) resulting from the designstorm, is obtained. The precipitation excess is thentransformed into a hydrograph at the outlet of thewatcibhcu utilizing a synthetic unit hydrograph. If alarge area is studied, it is subdivided into a numberof subwatersheds and the runoff hydrographs fromeach of these subwatersheds is routed to the watershedoutlet with hydrologic or hydraulic routing methods.For example, Snider (1971) recommended that a singleSCS dimensionless unit hydrograph should not be usedfor watersheds greater than 20 mi .The primary advantage of utilizing syntheticunit hydrographs is that the complete unit hydrographmay be determined with the specification of one ortwo hydrograph parameters. Further, many studieshave shown that relations between these hydrographparameters and watershed and storm characteristicsmay be developed. Synthetic unit hydrographs maybe derived for ungaged watersheds utilizing therelations between hydrograph parameters, andwatershed and storm characteristics as long as theungaged watersheds are hydrologically similar to thegaged watersheds for which the relation was developed. Hydrologic similarity includes similarity intopography, geomorphology, soil types, land cover/land use, and climate. For some synthetic unit hydrographs, the relation between hydrograph parameters,and watershed and storm characteristics had beendeveloped for application on a national (or even global)basis if suitable relations for local conditions are notavailable. The relations for estimating the time ofconcentration and traveltime for the SCS TechnicalRelease 55 (TR55) (Soil Conservation Service, 1986)are examples of national relations. Application ofnational relations may result in substantial errors in aspecific region.The Clark (1945) unit-hydrograph method asimplemented in the U.S. Army Corps of Engineers(1990) Rood Hydrograph Package HEC-1 and theSCS dimensionless unit-hydrograph method as implemented in TR55 are the most commonly appliedsynthetic unit-hydrograph methods in Illinois. In addition to utilizing a unit-graph-lag parameter, the SCSdimensionless unit-hydrograph method applies a fixedrelation between unit-hydrograph peak discharge andwatershed area and time-to-peak. This peak factor hasbeen found to be substantially high in a number of areasin the United States (Woodward and others, 1995).Lake County, 111., is undergoing rapid urbanization and management of the resultant increases instormwater runoff and flooding is an important activityin the county. Design hydrographs are needed forstorrnv/ater management planning, flood plain delineation, and stormwater-mitigation structure design inLake County. The U.S. Geological Survey (USGS)has operated an extensive network of rainfall andstreamflow gages in and near Lake County sinceDecember 1989 in cooperation with the Lake CountyStormwater Management Commission (LCSMC). Thedata from this network are sufficient to develop relations between hydrograph parameters, and watershedand storm characteristics for stormwater managementon small watersheds (less than 25 mi2) in Lake County.Therefore, the USGS, in cooperation with the LCSMC,began a study to develop relations between hydrographparameters (unit-graph lag, time of concentration,hydrograph-time lag, and watershed-storage coefficient), and watershed (area, main channel length andslope, percentages of impervious cover, forest cover,and open water) and storm (effective-precipitationdepili, duration, and intensity) characteristics for usein stormwater management in Lake County, 111. Theapplicability of the peak factor utilized in the SCSdimensionless unit hydrograph for small watershedsin Lake County also was evaluated.Equations for Estimating Synthetic Unit-Hydrograph Parameter Values for Small Watersheds in Lake County, Illinois

Purpose and ScopeThis report (1) describes the procedures utilizedto develop and test the relations between hydrographparameters, and watershed and storm characteristics,and (2) illustrates the accuracy and application of therelations developed for computation of synthetic unithydrographs for small, ungaged watersheds in LakeCounty, 111. The development of the relations includeddetailed analysis of rainfall and runoff data to identifystorms suitable for hydrograph-parameter determination; computation of hydrograph-time lag by manualanalysis; determination of unit-graph lag, time ofconcentration, and watershed-storage coefficient bycalibration of HEC-1; and determination of therelations by multiple nonlinear regression. Testing ofthe relations included comparison of hydrographparameter values determined for verification stormsto values estimated with the relations and comparisonof hydrographs computed utilizing the estimatedvalues of the hydrograph parameters to measuredhydrographs for verification storms. The accuracy ofthe relations is inferred from the results of HEC-1calibration, nonlinear regression, and relation verification.time step of 5 minutes. This network of rain gageswas installed to develop rainfall-runoff relations forsimulation of streamflow for watersheds in LakeCounty as described by Duncker and others (1995).When the project to develop rainfall-runoff relationswas completed on September 30, 1993, four of therain gages were discontinued and a new rain gage wasinstalled in Highland Park, 111. Thus, data from a totalof 24 rain gages in and near Lake County (fig. 1) wereavailable at various times (table 1) to assist in thedevelopment of the estimation equations. The arealextent of the rain-gage network results in reliable rainfall-depth and temporal distribution data for determination of hydrograph parameters for watersheds withstreamflow gages in Lake County.Streamflow data are available at 14 gages onstreams draining watersheds, primarily in LakeCounty, during the period of detailed rainfall data(December 1989-September 1993). These streamflowgages are listed below.StationnumberStation name05527940Tempel Farms Ditch near Old MillCreek, 00Bull Creek near Libertyville, 111.Terre Faire Ditch at Libertyville, 111.Indian Creek at Praire View, 111.Green Lake Ditch at Buffalo Grove, 111.Buffalo Creek near Wheeling, 111.North Branch Chicago River atDeerfield, 111.Skokie River at Lake Forest, 111.Skokie River near Highland Park, 111.West Fork of North Branch of ChicagoRiver at North Brook, 111.Squaw Creek at Round Lake, 111.Lakeview Plaza Ditch atLake Zurich, 111.Flint Creek near Fox River Grove, 111.Description of Study AreaThe objective of this study is to develop relationsbetween hydrograph parameters, and watershed andstorm characteristics for computation of synthetic unithydrographs for small, ungaged watersheds in LakeCounty. Therefore, the areas studied are the watershedsin Lake County for which detailed rainfall and runoffdata are available. The network of rainfall gages operated by the USGS in and near Lake County during thestudy period is shown in figure 1 and the station numbers, names, and periods of record of the rain gages arelisted in table 1. The watersheds utilized to develop theestimation equations are shown in figure 2. The factorsaffecting the selection of these watersheds aredescribed below.The network of rain gages in and near LakeCounty was started in December 1989 with theinstallation of eight gages at various locationsthroughout the county. The network increased to14 rain gages in February 1990, to 18 rain gages inApril 1990, and finally to 23 rain gages in May 1991.All installations included tipping-bucket rain gagescapable of measuring rainfall depths of 0.01 in. at ive of these watersheds and streamflow gageswere not considered in this study for the followingreasons. Tempel Farms Ditch drains a small 0.492-mi2watershed consisting of 100 percent pervious landcover in the form of agricultural pasture. As a resultof the pervious land cover, and corresponding highinterception and depression storage in the watershed,Introduction

3 07' 30"87 52' 30"\23, 42 30'-r\V WISCONSIN \IT*"" ILLINOIS \I1Old MillKg1222Grovey 3Iii ,! Lindenhurst21i42 22' 30Gurnee* 1 ?// Waukegan -ns CCZ iX--6!\\ Michigan Lake;t\ ForestM\42 15'14\ \S \V- i « \\16High Sand Park* \RiverGrove);"iiy*./"""9,s ,\XBase from U.S. Geological Survey1:100,000 Digital DataAlbers Equa-Area Conic ProjectionStandard paralleles 33 and 45 , central meridian -89 6 MILESii 036 KILOMETERSEXPLANATION1Figure 1.4U.S. GEOLOGICAL SURVEY RAIN GAGEAND SITE NUMBER-See table 1Location of Lake County in northeastern Illinois and U.S. Geological Survey rain gages.Equations for Estimating Synthetic Unit-Hydrograph Parameter Values for Small Watersheds in Lake County, Illinois

Table 1.Rainfall data-collection sites in and near Lake County, III.[Site identifiers correspond to those in figure 1; present refers to September 10, 1996]SiteidentifierStationnumberStation Southwest Fork of South Branch of Ravine 10 at Highland Park, 111.Des Plaines River at Russell, 111.Tempel Farms Ditch near Old Mill Creek, 111.Des Plaines River near Gurnee, 111.Bull Creek near Libertyville, Dl.12/07/93-present12/05/89-present077 11/91 804005528475055285000553450005535070Terre Faire Ditch at Libertyville, 111.Green Lake Ditch at Buffalo Grove, 111.Buffalo Creek near Wheeling, 111.North Branch Chicago River at Deerfield, 111.Skokie River near Highland Park, 111.05/08/91 -present04/25/91 1113088042200Squaw Creek at Round Lake, 111.Nippersink Creek near Spring Grove, 111.Lakeview Plaza Ditch at Lake Zurich, 111.Flint Creek near Fox River Grove, 111.Lake Zurich Wastewater Treatment Facility at Lake Zurich, 111.12/06/89-11/21/9303/1 3/91 533088084600422118088014700422315088091800Vernon Hills Rain Gage at Praire View, 111.Diamond Lake Wastewater Treatment Facility at Diamond Lake, 111.Wauconda Wastewater Treatment Facility at Wauconda, 111.Grayslake Wastewater Treatment Facility at Grayslake, 111.Fox Lake Rain Gage at Fox Lake, 00Waukegan Airport at Waukegan, 111.Lindenhurst Wastewater Treatment Facility at Lindenhurst, 111.Paddock Lake Wastewater Treatment Facility at Paddock Lake, Wis.Kenosha Airport Rain Gage at Kenosha, 6/9302/06/90-presentno storms produced more than the target level of directrunoff volume for hydrograph analysis (discussed inthe "Determination and Evaluation of Synthetic UnitHydrograph Parameters" section). Mill Creek drainsa 59.6 mi primarily rural watershed with substantialwetland areas along the main stem. The wetlands andsemipermanent debris jams along Mill Creek result insubstantial attenuation of the runoff hydrograph forthis watershed, which is not representative of conditions on small (less than 25 mi2), ungaged watershedsin Lake County. The Buffalo Creek watershed waseliminated from consideration because of a floodcontrol reservoir about 1 mi upstream from thestreamflow gage. The West Fork of the North Branchof Chicago River watershed was eliminated fromconsideration because of two off-line flood-controlreservoirs in the watershed that substantially alterthe natural rainfall-runoff process for larger storms.Finally, Lakeview Plaza Ditch drains a 0.0055-mi2watershed consisting of nearly 100 percent imperviousarea in the form of a commercial shopping mall andadjacent parking lot. The hydrograph parameters forthis watershed are not useful for development of relations for general application because of the extremelyPeriodof recordsman size and nign imperviousness 01 tne walersneu.Because the goal of this study was to develop estimation equations suitable for watersheds with areas lessthan 25 mi2, data from Indian and Flint Creeks couldhave been omitted from the analysis. However, datafrom these watersheds were retained to include a widerrange of watershed conditions in the regression analysis. Thus, the study area consists of nine watersheds inand near Lake County, 111. (fig. 2).Lake County lies entirely within the WheatonMorainal Region (Leighton and others, 1948). Underthe more natural, nonurban conditions present in 1954,Mitchell (1954, p. 335) noted that the WheatonMorainal Region is characterized by flat slopes, long,narrow basins, and large storage in lake and swampareas. In the mid 1980's, rapid urbanization began inrural areas in Lake County. Since then the presence ofimpervious areas and drainage structures (storm sewersand swales) has substantially affected the rainfallrunoff process and resulting hydrographs in LakeCounty. The primary geomorphologic and land-covercharacteristics of the watersheds utilized for determination of the parameters for synthetic unit hydrographsand development of relations for estimating theseIntroduction

87 52' 30"3 07' 30"42 30'JA/ISCONSJN L.k.ILLINOIS\ j\ \ Ola Mill\ } Creek\ . xjKji! :'Lindenhurst'(/ Lake./"j42 22' 30"Xlo. rifoJ O'.,'-"* )' - !Lake - /-' : Round" lLa «T \iV:-,A/,' A \\ « * Waukegan\GuVnee l!/ n-*"Grays!a //La/ceM/c/7/ an42 15'42 07' 30Base from U.S. Geological Survey1:100,000 Digital DataAlbers Equal-Area Conic Projectionstandard parallels 33* and 45", central meridian -89 01234SMILESl I I Ti01 2345 KILOMETERSEXPLANATIONWATERSHED BOUNDARY-For Skokie River,northern watershed is Skokie River at LakeForest, and the entire watershed is SkokieRiver near Highland ParkTFDGLDTERHE FAIRE DITCHGREEN LAKE DITCHFigure 2. Location of the watersheds in Lake County, III., utilized to develop, verify, and illustrate equationsfor estimation of synthetic unit-hydrograph parameters.6Equations for Estimating Synthetic Unit-Hydrograph Parameter Values for Small Watersheds in Lake County, Illinois

parameters are listed in table 2. The watersheds utilizedin this study include a representative range of thewatershed characteristics likely to substantially affecthydrograph shape for watersheds in Lake County.The land-cover percentages listed in table 2 weredetermined on the basis of remotely sensed thematicmapping and side-looking-airborne-radar imagery. Theprocedure for determining the percentage of impervious area was specially calibrated to data from theGreen Lake Ditch watershed and verified for the BullCreek watershed as described in Duncker and others(1995, p. 17). The percentage of forest area for theSkokie River and North Branch Chicago River watersheds appear relatively high. However, these high percentages reflect Forest Preserve land along each riverand the large residential estates with extensive woodedareas in each watershed.AcknowledgmentsTwelve municipalities in and near Lake Countya

7. Time-to-peak for the Snyder unit-hydrograph method estimated with the method of Singh (1981) compared with the minimum, mean, and maximum values determined from calibration for all storms on selected watersheds in Lake County, 111., utilized to develop and verify the equations for estimation of Cited by: 11Publish Year: 1997Author: Charle