2022-04-04_PERMIT FILE - M2022013 ‎(38)‎

• Permit No: M2022013
• IBM Index Class Name: Permit File
• Doc Date: 4/4/2022
• Doc Name Note: Exhibit G - Flood Control Mitigation Plan
• Doc Name: Application
• From: Varra Companies, Inc.
• To: DRMS
• Email Name: RDZ
• Email Name: MAC
• Media Type: D
• Archive: No

Two Rivers Gravel Pit Mine
Riverside Berm Failure Analysis
and Flood Control Mitigation Plan
' ' ' January 22,2020
* Interpolated values
USGS Gage Data
1. Big Thompson Stafiorr 06741510 at LCR 13C
40 22'43"N,105 03-38"W
Peak not recorded;maximum discharge of 4250 cfs at 1845 hrs on Sept 12 2013
USGS B6741510 BIG THOMPSON RIVER AT LOVELAND, CO,
5000
v 4BBB
c 3BBB
m 2BBB
N
a 1BAB
iA
Y
Y
u
.y
7
u
lee
L
JR
.y
� 2e
Sep Sep Sep Sep Sep Sep Sep Sep
16 11 12 13 14 15 16 17
2013 2813 2013 2013 2013 2013 2013 2013
---- Provisional Data Suhject to Revision ----
Median daily statistic (34 years) ' Measured discharge
Discharge
NOTES
Methodology.
This procedure done so that HG for WinDAM C can be based on an actual flood.The September 2013 Big Thompson flood HG was used because, (1) it was available, (2)was produced
by a large rainstorm in the foothills, and (3) Qp similar to 100-yr flood which is used for WinDAM C design flood.
A flood hydrograph for the September 2013 South Platte River flood near Greeley(closest to the site) is not available. The closest gage is at Kersey but was damaged during the flood
on September 14, 2013 ("Flooding in the South Platte River and Fountain Creek Basins in Eastern Colorado, September 9-18, 2013, Robert Kimbrough and Robert Holmes,Jr.) . Due to
such, and for the purpose of this study, it was assumed that the Big Thompson River hydrograph mentioned above is a reasonable representation of the South Platte River.The Big
Thompson River, in addition to several similar rivers that flow from the foothills into the South Platte River, are tributary to the South Platte River which would reflect their hydrographs.
The hydrograph ordinates were proportioned to the FEMA FIS 100-yr peak discharge for the South Platte River at Highway 85 which is assumed to be the discharge at Two Rivers for
100-yr flood; i.e., WinDAM C design flood.
(1) Dates per USGS graph above.
(2) Hour increments so can be entered in WinDAM as such approximates linear, ordinates were spaced several hours apart for simplicity.
(3) Qs from USGS graph above. Linear extrapolation on both rising and falling limb.
(4) Column (4) proportioned to design Q of 32,500 cfs at Two Rivers (Q100 for South Platte R at HWY 85/Q100 for Big Thompson R at Loveland per graph above (32,500/6,000)). Peak
discharge for Big Thompson R at Loveland not recorded and extrapolated from graph.
(5) Discharge that enters site is (4) 100-yr Q less 10-yr discharge for both Big Thompson and S. Platte Rivers. It is assumed that flows less than 10-yr floods are controlled by berm that
surrounds the site. BTR Q10 = 2,500 cfs; SPR Q10 = 11,000 cfs;Total Q10 = 13,500 cfs.
(6) Q proportioned between Big Thompson floodplain and South Platte floodplain. BT Qp= 8,000 cfs at confluence with SP. SP Qp= SP Q100 at Hwy 85 =32,500 cfs.Then BT Qp at
confluence=32,500 -8,000 = 24,500 cfs. Ratio of Qp= 24,500/8,000 =3.1 (say, 3).Then, Q ordinates= (5)/3. p9 28(7) (5) -(6)•
(8) (7) -6,000 cfs. Based on 1,000 cfs entering each of 6 groins. Inflow channel to groin is 1,000 cfs as in Coulson. Values about the same as for NW Pit runs, so left NW Pit HG in Central
Pit Berm Sizing WinDAM (file copied from NW Pit Best) because its output more reasonable.