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<br /> <br />19 <br /> <br /> <br />constriction geometry and roughness are those outl~ned by Kindsvater <br /> <br />and others (1953), Carter (1957), Dalrym~le (written communication, <br />March 16, 1955), and the standard step method of computing backwater <br />profiles. <br />The following assumptions were made: <br />1. That the geometry of the valleys and their constrictions <br />will be practically the same during future floods as it was in the <br />fall of 1961. This assumption presupposes that highway and railroad <br />embankments and levees will not be changed, either artifically or by <br />erosion, that bridges will not be plugged by debris, that further <br />encroachment on the flood plains will be negligible, and that gravel <br />mining will have small additional effect on the characteristics of <br />the main channel and flood plains. <br />2. That the patterns of regulation and diversion will not be <br />altered greatly. For example, the relation between magnitude and <br />frequency of floods on St. Vrain Creek may be affected substantially <br />if a storage reservoir is constructed on the creek or its tributaries <br /> <br />upstream from Longmont. <br /> <br />3. Other assumptions that affect the computations have been <br /> <br /> <br />made at specific sites. Justification for such assumptions are <br /> <br /> <br />contained in the file of computations for the report. <br /> <br /> <br />St. Vrain Creek <br /> <br /> <br />Description of Constrictions <br /> <br /> <br />The valley of St. Vrain Creek and its constrictions in the <br />