2019-05-24_PERMIT FILE - M2019028 ‎(13)‎

• Permit No: M2019028
• IBM Index Class Name: PERMIT FILE
• Doc Date: 5/24/2019
• Doc Name: Application
• From: J-2 Contracting Co.
• To: DRMS
• Email Name: ECS
• Email Name: MAC
• Media Type: D
• Archive: No

InUT Consulting, Inc.
May 13, 2019
Division of Reclamation, Mining, & Safety
1313 Sherman Street, Room 215
Denver, CO 80203
RE: J-2 Contracting Company— DPG Pit
Floodplain Study
To whom it may concern,
We have prepared this letter to address the effects of mining activities at the DPG Pit on the
Cache la Poudre River and South Platte River 100-year floodplain. The floodplain has been
modeled to determine the effects resulting from temporary stockpiles located within the pit
boundary.
The mining area lies within the 100-year floodplain as shown on the Colorado Hazard Mapping
Program (CHAMP) floodplain review web site. The floodplain maps in Appendix A depict the
location of the 100-year floodplain "shown on the CHAMP web site, and locations of the HEC-
RAS cross-sections used in our modeling for both the Cache la Poudre River and South Platte
River. -The effect the mining operation has on the 100-year floodplain is the temporary
stockpiling of aggregate products generated by processing the sand and gravel deposit through
the processing facility and the stockpiling of overburden and topsoil that is stripped from the site
prior to mining. We assumed the largest block-outs in the floodplain by the stockpiles to
determine the worst-case effect on the floodplain.
The topography shown on the floodplain maps was developed from a photogrammetric drone
survey done by Mr. Rob Valentine, with a photo date of January 16, 2019. Ground control points
for the survey were provided by American West Land Survey Co. 1-foot interval topography was
developed from the photo. This topography, along with the CHAMP HEC-RAS models were
used to model the effects of the stockpiles on the floodplain. The CHAMP HEC-RAS models
were obtained from AECOM for the area in the immediate vicinity of the mining activities. The
study area for this modelling included the reach from Sections 12+34.80 through 88+03.94 of
the Cache la Poudre River HEC-RAS model; 88+03.94 being the upstream study limit, and
12+34.80 being the downstream study limit, and the reach from Sections 13965+52 through
14041+82 of the South Platte River HEC-RAS model; 14041+82 being the upstream study limit,
and 13965+52 being the downstream study limit.
Two models for each river reach were developed for the site using HEC-RAS 5.0.7 software:
1) A duplicate effective floodplain model was developed showing the limits and elevations
of the 100-year floodplain (base flood) and floodway using the HEC-RAS files obtained
from AECOM. The results from this model were compared to the results shown in
original AECOM model to ensure that they matched.
V 305 Denver Avenue—Suite D•Fort Lupton CO 80621 •Ph:303-857-6222•Fax:[phone]