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<br />ADJUSTMENT OF PEAK DISCHARGE RATES <br />FOR URBANIZATION <br /> <br />By David L. Gundlach,' A. M. ASCE <br /> <br />INTRODUCTION <br /> <br />Various investigators (1.4,5,7) have shown that changes from rural to urban <br />conditions within a watershed, in general, significantly affect flood flows. <br />Recorded annual peak discharge rates (or peak discharge rates above a given <br />base) for a basin that has been undergoing a change in land-use conditions <br />represent a nonstationary time series. The series must be adjusted to a specific <br />land-use condition (typically present conditions) prior to performing a statistical <br />frequency analysis. It is the aim of this note to present a procedure utilizing <br />a single event rainfall-runoff model for transforming recorded peak discharges <br />at a gaging station to a consistent set that reflects existing land-use conditions. <br /> <br />ADJUSTMENT OF RECORDED PEAK DISCHARGE RATES <br /> <br />In general, when a single event rainfall-runoff computer program such as <br />the one in Ref. 2 is utilized to develop a calibrated basin model, the procedure <br />involves the reconstitution of several flood hydrographs at a particular streamflow <br />gaging station. If significant urbanization has taken place in the basin over <br />time, it becomes necessary to reconstitute flood events that have occurred in <br />a given time frame so that the effect of land-use change on the hydrologic <br />reglme within that time frame can be considered negligible. In most urbanizing <br />watersheds there is more hydrologic and physiographic data available for recent <br />years, such that it is normally expedient to calibrate a hydrologic basin model <br />for present (or existing) conditions. <br />Results obtained from Ref. 3 are used herein to illustrate the technique for <br />adjusting recorded peak discharge rates. Ref. 2 was utilized to reconstitute <br />three recent flood hydrographs at the United States Geological Survey recording <br />streamflow gage, at Rahway River near Springfield. In this case the resulting <br />calibrated hydrologic basin model represented 1970 conditions. <br />Note.-Discussion open until February I. 1979. To extend the closing date one month, <br />a written request must be filed with the Editor of Technical- Publications, ASCE. This <br />paper is part of the copyrighted Journal of the Irrigation and Drainage Division, Proceedings <br />of the American Society of Civil Engineers, Vol. 104, No. lRJ, September. 1978. Manuscript <br />was submitted for review for possible publication on December 13, 1977. <br />'Hydr. Engr., The Hydrologic Engrg. Center, Corps of Engrs., Dept. of the Army. <br />Davis, Calif. <br /> <br />1 <br />