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<br />The Town of Lyons does not have any flood plain management program <br />in effect, but does have a restrictive building code for structures <br />located within the flood plain. <br />, <br /> <br />, <br /> <br />3.0 ENGINEERING METHODS <br /> <br />, <br />," <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equalled or exceeded ~ on the average during any <br />10-, '50-, 100-, or SOO-year period (recurrence interval) have been <br />selected as having special significance for flood plain management and <br />for flood insurance premium rates.' These events, commonly termed the <br />io-, 50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent <br />chance, respectively, of being equalled or exceeded during any year. <br />Although the recurrence interval represents the long term average period <br />between floods of a specific magnitude, rare floods could Occur at short <br />intervals or even within the same year. The risk of experiencing a rare <br />flood increases when periods greater than 1 year are considered. For <br />example, the risk of having a flood which equals or exceeds the 100-year ' <br />flood (1 percent of annual occurrence) in any 50-year period is approxi- <br />mately 40 percent (4 'in 10), and, for any 90-year period, the risk increases <br />to approximately 60 percent (6 in 10). The analyses reported here <br />reflect flooding potentials based on conditions existing in the community <br />at the' time of "completion 'of' this 'study: 'Maps' and- flood elevationswilf <br />be amended periodically to reflec~ future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />. <br />, <br /> <br />HydrOlogic analyses were carried out to establish the peak discharge- <br />frequency relationships 'for floods of the selected recurrence inter- <br />vals for each stream studied 'in detail in the community. <br /> <br />The Lyons streamflow gage, located on the left bank of St. vrain <br />Creek 0.4 mile downstream from the confluence of North St. Vrain <br />Creek and South St. Vrain Creek, has been in operation since 1895. <br />The flows recorded are partly regulated by small diversions above <br />the gage station. The peak flood discharges and stages recorded <br />during this period are presented in Table 1 (Reference 3). <br /> <br />, <br /> <br />, <br /> <br />l,! <br />