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<br />. <br /> <br />the 25-year flood profile at the cross-section. Estimated depths <br />of inundation by the 50-year flood at selected points are shown <br />in table 3. Depths of inundation by the 50-year flood in other <br />parts of the flood plain will generally be less than 2 feet. <br />6. Velocities of flow on the flood plain for both the 25- and <br />50-year floods generally will be low, except where the geometry <br />of constrictions .has appreciable effect on the flow pattern. <br />7. Alteration to the geometry of the valleys and their con- <br />strictions will result in boundaries of inundated areas and depths <br />and velocities of flow different from those shown. <br />There is ample evidence that the floods themselves will cause <br /> <br />drastic changes to the channel and constrictions of St. Vrain Creek <br /> <br />unless very positive protective measures are taken. Failure of <br /> <br />even the highway embankments north of the bridges on Hover Road <br />and Sunset Drive will result in major damage on the left flood plain <br />from floods as small as the 25-year flood. Plugging of bridges will <br />result in increased inundated areas and depths and velocities of flow <br /> <br />on the flood plains both upstream and downstream from the crossing. <br /> <br />Additional construction on the flood plain will increase inundated <br />areas and depths. If flood control structures are constructed and <br /> <br />properly operated on St. Vrain Creek or its tributaries, the dis- <br /> <br />charge of floods of given recurrence intervals will be decreased. <br /> <br />It seems unlikely that the channel of Lefthand Creek will be <br />altered sufficiently to impair its capacity seriously. <br /> <br />31 <br /> <br />