2016-09-01_REVISION - M2004031

• Permit No: M2004031
• IBM Index Class Name: Revision
• Doc Date: 9/1/2016
• Doc Name: TR to AM01 and Adequacy Response#2
• From: Aggregate Industries
• To: DRMS
• Type & Sequence: AM1
• Email Name: ECS
• Media Type: D
• Archive: No

N TETRA TECH TECHNICAL MEMO
To: Mr. Mike Refer, Aggregate Industries, Mr. Bryan Kohlenberg, P.E., Urban Drainage and Flood
Control District. and Mr. Greg Labrie,Adams County
From: Gary Linden, P.G.,Jeff Butson, P.E.
Date: August 30, 2016
Subject: Updated Design Memorandum, Hazeltine Mine Spillway, Adams County, Colorado
This memorandum summarizes Tetra Tech's design of the Hazeltine Spillway located between the
South Platte River and the Hazeltine Mine in Adams County, Colorado. The site is approximately
2,200 feet north of 104''Avenue on the east bank of the river.
BACKGROUND
Tetra Tech, on behalf of Aggregate Industries,prepared an application for an Amended Flood Plain
Use Permit submitted to Adams County. The amended permit would allow Aggregate Industries to
construct a spillway along the South Platte River in conjunction with Urban Drainage and Flood
Control District(UDFCD) standards. The spillway was originally designed by Tetra Tech and
approved by UDFCD in 2008. The original design was documented in a Design Memorandum by
Tetra Tech dated January 21,2008. The design memorandum documents the design of a spillway
with a length of 200 feet. Adams County referred review of the design to UDFCD for comment.
Subsequently,UDFCD raised concerns about the riprap size and Tetra Tech performed calculations
regarding riprap sizing that were documented in a letter to UDFCD dated April 7, 2008. Both of
these documents are attached to this memo for reference(Attachment 1).
UDFCD removed objections to the riprap sizing in a letter dated April 23,2008. On June 6, 2008,
Adams County approved the Amended Flood Plain Use Permit. The 2008,UDFCD approved design
was at elevation 5,039.8 feet included riverside and pitside concrete cutoff walls, a vegetated crest
between the cutoff walls, and buried Type H riprap below the cutoff walls. On the riverside,the
buried Type H riprap was designed for burial 5 feet below the river thalweg. On the pitside,the
buried Type H riprap was designed for burial 3 feet below the river thalweg. At a site visit in 2015,
UDFCD requested that the spillway crest be raised to elevation 5,040.8. Consequently the crest was
raised to 5,040.8. The above parameters and the spillway crest at 5,040.8 represent the normal mine
reclamation requirements of the DRMS.
Aggregate Industries wishes to construct the spillway in 2016. The City of Thornton has requested
that the spillway be additionally reinforced beyond the normal design considered appropriate for
normal mine reclamation. The current design was upgraded to include a concrete apron on the crest
and Type H buried riprap extending to the bottom of the mine. It is our understanding that Thornton
may require additional modifications beyond what is required for normal mine reclamation.
The purpose of this memorandum is convey applicable calculations utilized in the final design. The
final design was previously sent to UDFCD and Adams County in December 2015.
Tetra Tech
1900 South Sunset Street,Suite I-E,Longmont,CO 80501
Tel [phone] Fax [phone] www.tetratech.com