2017-03-16_REVISION - M1977141

• Permit No: M1977141
• IBM Index Class Name: Revision
• Doc Date: 3/16/2017
• Doc Name: Adequacy Response No. 3
• From: Aggregate Industries - WCR, Inc.
• To: DRMS
• Type & Sequence: TR2
• Email Name: MAC
• Email Name: WHE
• Media Type: D
• Archive: No

FN
Memo
Date: Tuesday, February 28, 2017
Project: Aggregate Industries—Lyons Quarry 112d Reclamation Permit Operations
To: John Ballegeer, PE, HDR
From: Brinton Swift, PE, HDR
Subject: St.Vrain Creek Scour Analysis and Scour Mitigation Measures
I. Introduction
The purpose of this technical memorandum is to provide an overview of scour analysis and preliminary
scour mitigation design recommendations in support the request to the Colorado Division of Reclamation
Mining and Safety (DRMS) for a Technical Revision JR) to the 112d Reclamation Permit Operations for
Lyons Quarry (File number M-1977-141).
II. Hydrology
Design flow rates are based on flow rates used in the Preliminary Basis of Design Report for South St.
Vrain Creek Restoration at Hall Ranch prepared by Matrix Design Group for Boulder County, Colorado in
September of 2016("the Report"). Various sources of hydrologic data were evaluated in the Report. It was
recommended that CDOT Hydrologic Evaluation of the St. Vrain Creek Watershed flow rates be used for
less frequency design storms, while the St. Vrain Creek Channel Flood Recovery Design-Build Services be
used for more frequent design storms. A summary of flow rates in cubic feet per second (cfs) for various
design storms can be seen in Table 1.
Table 1:Summary of Flow Rates for Various Design Storms from the Report
Design Storm ears 2 1 5 1 10 1 25 1 50 1 100 500
Flow Rate cfs 681 1 1063 1 1605 1 3168 1 4933 1 7234 14748
It should be noted that Table 1 flows are greater than Federal Emergency Management Agency (FEMA)
Flood Insurance Study(FIS)flow rates. These flows were used in this analysis as a conservative approach
and are assumed to be more representative of actual conditions. However, these flows will not correlate
with existing FEMA floodplain mapping. It is also important to note that project grading will not occur within
the existing FEMA regulatory floodplain as it is mapped. However, the local floodplain administrator should
be consulted to verity assumptions.
III. Hydraulics
A. ID Hydraulic Modeling
Matrix Design Group created a 1D hydraulic model using HEC-RAS Version 4.1.0 to
determine flow depths and velocities across the channel reach. The existing conditions
cross sections were evaluated using post-2013 flood topography. HDR received this model
on February 22, 2017 and ran the existing model in HEC-RAS Version 5.0.3 to determine
hydraulic parameters for scour analysis within the project location. Cross sections used to
evaluation hydraulic conditions within the project location are: 8021 to 17202. See Figure
1 HEC-RAS 1D Model Cross Sections for a layout view of cross section locations.
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