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<br />: <br /> <br />has been altered greatly by man-made constrictions and gravel <br />mining. The effects of these alterations has been given major <br />consideration in determining the results shown. It is emphasized <br />that the results are based on the assumption that the geometry of <br />the valley and its constrictions will be substantially the same <br />during future floods as it was in 1961. Also, there is ample <br />evidence that major floods will themselves cause drastic changes <br /> <br />to channel and constriction geometry unless positive protective <br /> <br />measures are taken. If these changes occur, the boundaries of <br /> <br />inundated areas and depths and velocities of flow may be substan- <br />tially different from those shown. The effects of further alter- <br />ations, whether they be made by floods or by man, have not been <br />completely evaluated because of the large number of possible changes <br />and the even larger number of their possible combinations, but the <br />individual effects of the more important ones can be discussed <br />generally. <br />Any combination of failure of the fills north of the Hover <br />Road and Sunset Drive bridges and the levee north of the main channel <br />upstream from the railroad bridge will allow more flow on the left <br />flood plain between the point of failure and a section near the <br /> <br /> <br />mouth of Lefthand Creek. The flow on the right flood plain will <br /> <br /> <br />be correspondingly less. Failure of the Hover Road and Sunset Drive <br /> <br /> <br />highway embankments will result in major damage on the left flood <br /> <br /> <br />plain from floods as small as the 25-year flood. Embankments sometimes <br /> <br />26 <br /> <br />