PERMFILE114603

• Permit No: C1981012
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 21 Purgatoire River Diversion
• Media Type: D
• Archive: No

8. The Division has required that a 100 yeah event (7910 cfs) be used to demonstrate
hydraulic stability of the proposed diversion. WFC must provieie the necessary "
desif;n criteria and calculations for this event.
The response to this question updates and modifies Section II, Part 1 related to the
Middle Fork of the Purgatoire River diversion. As shown by Figure II. 1-5 (revised)
the xoposed diversion will be an engineered channel approximately 800 feet long
having an overall longitudinal slope of 1.756. The channel has three horizontal
curves. The first two curves at the upstream end of the channel have centerline
radii of about 100 feet, and the third curve, where the relocated diversion joins the
existing Middle Fork channel, has a centerline radius of about 200 f~:et.
Figure E-8 shows a cross section of the proposed channel, which will have 2H:1 V side
slopes and a 30 foot wide bottom. In addition, a berm will be maintained on the
south bank approximately 5 feet above plant grade with a 10 foot wide crest and
2H:1 V side slopes. Minimum cross sections for the new channel as shown in the
original permit revision (Wyoming Fuel Company, 1984), will handle the flow of
approximately 12,000 cfs with 5 feet of freeboard. The 100-y~:ar storm event,
estimated at 7,910 cfs, was used to size rip rap revetment for the 2H:1V side slopes
of the diversion channel. Hydraulic and rip rap calculations for the 100-year flood
are attached.
The 7,910 cfs design discharge has a depth of approximately 9.6 feet and an average
velocity of 16.8 feet per second. Rip rap design calculations were ~fone utilizing the
U.S. Army Corps of Engineers Manual EM1110-2-1601. (Corps of Engineers, 1970).
The rip rap revetment design utilizes the boundary shear approach. Several steps ,
were used to size the median (D50) rip rap diameter. First, the boundary shear
stress at the bottom of the channel was estimated, and then this stress was adjusted
for the 2H:1V side slopes, and the 100 and 200-foot radius curves. Also, super-
eleva.tion of the water surface at the outside of the 100 and 200-foot radius curves
was =alculated and rip-rap sized for these locations. Table E-2 summarizes the
median (D50) rip rap diameter, the recommended rip rap thickness, and the height of
rip rasp at various locations along the proposed diversion channel. Except for the 100
and ::00-foot radius curves, the median rip rap diameter is 1.2 feet and has a 2 foot
thickness. The rip rap would extend approximately 9.6 vertical feet from the
bottom of the channel which is the water surface of the 100-year design discharge.
The rip rap would extend three feet below the channel bottom. On the outside
2H:1 J slopes of the 100 foot radius curves, the median rip rap diameter incre::ses to
2.6 feet and a thickness of 4.5 feet. Because of super-elevation of the water
surface on the outer 2H:1V slope at the curve, the rip-rap should extend to 15.6
feet above the channel bottom. For the 200 foot radius curve, ':he outside slope
should have a median rip rap diameter of about 1.9 feet and a thickness of 3.5 feet
and extend about 12.6 feet above the channel bottom. At the inside of the curves,
the rip rap median diameter may be the same as the rest of the channel, chat is, a
median diameter of 1.2 feet and a thickness of 2 feet and extend 9.6 feet above the
channel bottom. Typical channel cross-sections for these channel locations are
shown on Figure E-8, referenced to revised Figure II. L-5.
The proposed gradation of the rip rap is given in the design calculations along with
the proposed gradation of the bedding (filter) to be placed beneath the rip rap. The
existing refuse may be used as a bedding if its gradation approximate that
recommended for the bedding material. ,