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Dames & Moore, 1999 - USDA Forest Service

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and 3 was determined to be 10 to 1 1 feet per second, with a maximum estimated velocity between 13 and 15<br />

feet per second during the hypothetical 100-year event.<br />

where:<br />

where:<br />

where:<br />

Va = average channel velocity in feet per second<br />

D50 = 2.2 feet<br />

(Source: USDOT, 1970)<br />

D50 = 1.7 feet<br />

D50 = 0.0122(Va)"2.06<br />

(Source: Blodgett and McConaughy, 1986)<br />

D50 = 3.5 feet<br />

D50 = 0.0 l(Va)"2.44<br />

(Source: Blodgett and McConaughy, 1986)<br />

Based on the above equations, the average D5O riprap size should be on the order of 2 to 3 feet in order to<br />

protect the lowermost tailings pile slopes from erosion by Railroad Creek.<br />

SCS Method<br />

Utilizing the SCS method, the following riprap rock size was calculated:<br />

Input Data:<br />

Channel side slope = 1.5 to 2.5 to 1, average 2 to 1<br />

Channel slope = 1.5 percent<br />

Average channel depth = 5 feet<br />

Channel width = 50 feet<br />

Channel discharge = 3,500 cfs<br />

G:\WPDATA\W3WEPORTSWOLDEN-2W4-0,DOC<br />

17693-005-019Vuly 19. <strong>1999</strong>:4:51 PM:DRAFT FINAL RI REPORT

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