2008-12-31_HYDROLOGY - M1999120

• Permit No: M1999120
• IBM Index Class Name: HYDROLOGY
• Doc Date: 12/31/2008
• Doc Name: Weld County Flood Plain Development Plan Approval
• From: Weld County
• To: L.G. Everist, Inc.
• Permit Index Doc Type: Hydrology Report
• Email Name: ECS
• Media Type: D
• Archive: No

COLORADO
December 31, 2008
Lynn Mayer Shults
Regulatory Manager
L.G. Everist, Inc.
7321 East 88th Avenue, Suite 200
Henderson, CO 80640
PUBLIC WORKS DEPARTMENT
1111 H STREET, P.O. BOX 758
GREELEY, COLORADO 80632
WEBSITE: WWW.CO.WELD.CO.US
PHONE: [phone], EXT. 3750
FAX: [phone]
Subject: AmFHDP-355, L.G. Everist Fort Lupton Sand and Gravel Mine
Dear Lynn:
On September 29, 2008, you submitted evidence that the berms from the eastern section of the L.G.
Everist Fort Lupton Sand and Gravel Mine had been removed. The evidence consisted of documentation
from the Colorado Division of Reclamation, Mining, and Safety confirming that the berms had been
removed.
On December 11, 2008, the Public Works Department received the "Supporting Documentation for Fort
Lupton Sand and Gravel Mine FHDP". It was dated December 9, 2008 and was signed by Mark Severin,
PE. Included with the Report was a copy of the HEC-RAS computer model showing the impacts the mine
was having on the South Platte River Floodplain.
The HEC-RAS modeling was reviewed and several relatively minor errors were noted. A summary of
those errors is provided below:
1. Cross sections 80 and 90 did not extend completely across the floodplain. Therefore, the
program had to vertically extend the end of the cross sections in order to run.
2. Cross section 20 had to be vertically extended so that the model would run. It is likely that the far
eastern channel shown in the cross section should have been modeled as ineffective flow.
3. The pre-mining and existing model geometries were modified to remove natural and man-made
depressions. These depressions as well as the pit should have likely been modeled using
permanent ineffective flow areas.
4. The Manning's roughness values did not match the values used in the Flood Insurance Study
(FIS) that ends just upstream of the gravel pit. The FIS used roughness values of 0.03 for the
channel and 0.045 to 0.120 for the overbanks. No discussion was provided to show why these
values were not used.
Public Works adjusted the above mentioned discrepancies and reran the submitted model. Weld County
agrees that the Fort Lupton Sand and Gravel Mine does not appear to adversely impact adjacent
Page I of 2 December 31, 2008
MAPLANNING - DEVELOPMENT REVIEW\FHDP-Flood Hazard Development Permit\Am355 LG Everist 1no\AnTHDP-355 Compliance Letter 12-31-08.doc