2019-09-19_REVISION - M2002120 ‎(17)‎

• Permit No: M2002120
• IBM Index Class Name: REVISION
• Doc Date: 9/19/2019
• Doc Name: Adequacy Review Response #2
• From: Civil Resources
• To: DRMS
• Type & Sequence: AM1
• Email Name: ECS
• Media Type: D
• Archive: No

CIVIL RESOURCES, LLC
ENGINEERS & PLANNERS
TECHNICAL MEMORANDUM
Bernhardt Spillway Design
TO: DRMS
Bernhardt Gravel Mine Adequacy Review Comments 2
FROM: Civil Resources, LLC
DATE: September 13, 2019
RE: Spillway Design
This memorandum discusses the proposed spillway designs at the Bernhardt Gravel Mine. Reservoirs 2 and 3 have been fully
mined out and Reservoir 4 is being mined to date. Reservoir 5 is still very similar to existing/historic conditions. Using floodplain
mapping from the Flood Insurance Rate Map indicates that all of the reservoirs associated with the project will be fully submerged
during the 100-year flood conditions except Reservoir 2 and the model clearly showed a significant drop across the water surface
from west to east through the Site with a less pronounced water surface drop during lesser storms. 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.
➢ Atypical 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 Big Thompson 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 and up to a six-inch per hour rise in flood water. The results are
presented under each reservoir discussion below, calculation tables are also attached to this memo with a drawing of
atypical section. The spillway equation: Q=CLH1'was used to determine the flow rate over the spillway.
Reservoir 3: 100 percent of Reservoir 3 is within the 100-year floodplain. The following information was considered in designing
the spillways and erosion protection:
➢ A historic drain area exists in the northwest corner of Reservoir 3 as this will direct flows towards the reservoir until fully
inundated. The perimeter is relatively consistent in elevation and water will recede through the inflow spillway. The
spillway will be set one-foot lower than the existing grade at that point of the reservoir.
➢ The reservoir is 388 acre-feet in volume and the spillway has 565 cubic feet per second (cfs) capacity at 1 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 under 3 hours,where-as a six inches
per hour rise would fill the reservoir in one hour. 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.
Reservoir 4: 100 percent of Reservoir 4 is within the 100-year floodplain. The following information was considered in designing
the spillways and erosion protection:
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