Estimation of watershed lag times and times of concentration for the Kansas City Area.

PDFTech ReportKS-16-01C2027WatershedsStreamflowRunoffMissouriKansas. Dept. of Transportation. Bureau of ResearchMcEnroe, Bruce M.Young, C. BryanGamarra Zapata, Ricardo A.University of Kansas. Dept. of Civil, Environmental and Architectural EngineeringKansas. Dept. of Transportation. Bureau of ResearchJohnson County Public Works and Infrastructure. Stormwater Management ProgramUS Transportation CollectionLag time (TL) and time of concentration (TC) are two related measures of how quickly a stream responds torunoff-producing rainfall over its watershed. In this report, a general relationship for lag time is derived from theManning equation for frictional resistance, the rational formula for bankfull discharge, and a generalized rainfallintensity-duration relationship. This relationship accounts for the length, average slope, and average Manning n valuefor the longest flow path and three watershed characteristics: average width, a rational runoff coefficient, and areference rainfall intensity. A simpler form of this relationship is derived for regional calibration. The regional form ofthe lag-time equation accounts for the impacts of urbanization through two inputs: the fraction of the longest flow paththat is paved or enclosed, and the fraction of the drainage area covered by impervious surfaces. The relationshipsindicate that lag time is strongly related to the length, slope, and roughness of the main channel and less stronglyrelated to other watershed characteristics that affect the bankfull discharge in the channel.The regional lag-time equation was calibrated for the Kansas City area with observed lag times and channeland watershed characteristics for 30 gage sites in the Automated Local Evaluation in Real Time (ALERT) flash-floodwarning system. Average lag times for the gaged watersheds were determined from archived rainfall and water-levelrecords for significant runoff events. The new lag-time equation for the Kansas City area performs better than the 2001KU-KDOT equation for urban lag time, which was developed from a study of 14 gaged watersheds in Johnson County(McEnroe & Zhao, 2001). The main advantage of the new lag-time equation is that it accounts directly for the highervelocities in the paved and enclosed segments of the flow path. The new equation\ue2\u20ac\u2122s solid theoretical foundation is alsoan advantage.Because time of concentration cannot be determined from gaging data, direct calibration of an equation for TCis not possible. However, time of concentration and lag time are closely related, so TC can be estimated from TL. TheNatural Resources Conservation System (NRCS, 2010) recommends the approximation TC = 5/3 TL. Therecommended time-of-concentration equation for the Kansas City area incorporates this approximation