2020-02-05_REVISION - M1998013 ‎(27)‎

• Permit No: M1998013
• IBM Index Class Name: REVISION
• Doc Date: 2/5/2020
• Doc Name: Request For Amendment To Permit
• From: Bestway Concrete Co.
• To: DRMS
• Type & Sequence: AM3
• Email Name: ECS
• Email Name: MAC
• Media Type: D
• Archive: No

CIVIL RES'20URCES, LLC
ENGINEERS & PLANNERS
TECHNICAL MEMORANDUM
Vogl Amendment - Spillway Design
TO: DRMS
Shore Gravel Mine—Vogl Amendment AM03
FROM: Civil Resources, LLC
DATE: January 29, 2020
RE: Spillway Design
This memorandum discusses the proposed spillway designs at the Vogl Amendment Gravel Mine. Final Reservoirs 2a and 3
are located in the floodplain whereas Cell 1 is not located in the floodplain and does not require a spillway. Using floodplain
mapping from the Flood Insurance Rate Map indicates that Reservoirs 2a and 3 will be fully submerged during the 100-year
flood conditions. Subsequently, the inflow spillways proposed were primarily placed near the upstream ends of each facility.
The capacity for each spillway is discussed below. The inflow spillways will be protected with riprap in order to deliver flood
water in a controlled manner into the respective reservoirs.
➢ A typical 100-year storm in the Front Range is 3-inches of rainfall per hour. During a storm event the major contributing
factors to runoff are travel time, location of the storm and ground cover conditions. For example during the 2013 flood
that lead to major flooding was caused by a strong orographic effect as the storm started in the foothills west of the site
which resulted in longer travel times. The mountain area tributary to the St.Wain River received approximately fifteen
plus inches of rain over a week. As the flood wave propagated downstream flood water rose more slowly. Should a
storm cell"sit'over the site the time to peak will be greater. As a result the site spillways were modeled considering a
one-inch per hour rise in flood water. The results are presented under each reservoir discussion below and the spillway
equation: Q=CLH15 was used to determine the flow rate over the spillway as presented on the reclamation maps.
Reservoir 2a: 100 percent of Reservoir 2a is within the 100-year floodplain. The following information was considered in
designing the spillways and erosion protection:
➢ The spillway was set on the upstream end approximately two-foot lower than the upstream max water surface elevation.
This inflow point is the natural low point.
➢ The reservoir is approximately 337 acre-feet in volume and the spillway has 1,047 cubic feet per second(cfs)capacity
at 2 foot of overtopping depth. Each spillway was designed to be able to fill the reservoir so the banks are less
susceptible to major washouts. A one-inch per hour rise in flood water would fill the reservoir in over 4 hours. This is
adequate to protect the non-reinforced banks from severe erosion as the reservoir would be full by the time the banks
would overtop and potentially erode.
➢ The reservoir was also equipped with a discharge spillway at the downstream low point. The discharge spillway will
help direct flow as flood waters recede and limit erosion to localized areas. The outflow spillway will potentially serve
as an inflow point in conjunction with the upstream spillway depending on how the flood waters rise in the vicinity of
these pits.
Reservoir3., 100 percent of Reservoir 3 is within the 100-year floodplain. The following information was considered in designing
the spillways and erosion protection:
323 FIFTH STREET 0 FREDERICK, COLORADO 80530 0 PHONE: [phone] FAX: [phone]