HomeMy WebLinkAboutDRB140536 Floodplain Grading LetterFebruary, 2015 amec
Tom Kassmel foster
Town of Vail wheeler
Re: Hoyt Residence Floodplain Development
Dear: Mr. Kassmel
It is my professional opinion that the amount of fill added within the flood plain to Parcel A -
Stephens Subdivision, 2764 South Frontage Road, Vail, Colorado, does not significantly affect
the Base Flood Elevation (BFE).
Gore Creek is located immediately south of the property in question and a portion of the property
is inside the regulatory floodplain. The BFE is 7,824.5' on the east side of the property in which
the proposed fill is to be permanently located according to the Eagle County, Colorado Flood
Insurance Study effective December 2007. The proposed fill is completely outside the floodway
and thus is not subject to those regulations requiring a conditional letter of map revision
(CLOMR). (44 CFR 60.3 d) The project is located within the flood fringe which means a
CLOMR is not required but a Letter of Map Revision (LOMR) would be required if the fill caused
an increase in the BFE of more than 0.3 feet. (Colorado Water Conservation Board Rule 12 J as
stated below)
"Whenever a Stream Alteration activity is known or suspected to increase or decrease the
established Base Flood Elevation in excess of 0.3 vertical feet (or a more stringent standard
adopted by the local government authority), a Letter of Map Revision showing such changes
shall be obtained in order to accurately reflect the proposed changes on FEMA's regulatory
floodplain map for the stream reach. The local community is responsible for ensuring that this
process is pursued. This section herein does not require a Conditional Letter of Map Revision
(CLOMR) to be applied for, unless mandated by the local government having land use
authority."
An analysis of the proposed fill in the floodplain was performed by Amec Foster Wheeler. It was
found that it is situated mostly at an elevation above the regulatory BFE of 7,825'. It is also
noted that the fill is located in an area of the flood fringe that would be considered "ineffective",
or not contributing to the conveyance of the stream. The displaced water in the ineffective area
of the floodplain caused from the fill will result in a rise of 0.9 inches in Gore Creek which is well
below the 0.3 feet that would require a more extensive study.
Please feel free to contact me for questions or concerns regarding this matter.
Sincerely,
Coleman Miller, PE CFM
coleman.millerCamecfw.com / (303) 742-5316
Amec Foster Wheeler Environment & Infrastructure, Inc.
Amec Foster Wheeler
Environment & Infrastructure, Inc.
2000 South Colorado Blvd., Suite 2-1000
Denver, CO 80222
Tel: (303) 935-6505
Fax: (303) 935-6575
amecfw.com
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Supporting Documents
Figure 1: 100 -year Flood Profile
Figure 2: Cross Section locations
Page 2 of 3
Supporting Documents
17 100 -YR WSEL = 7,825'
DISPLACED FL40D WATER ELEV 7,824.5'
Figure 3: Cross Section of Proposed Fill
-ICP-ACED FLO E, WATE?
'R• --PO SEC P -F. , 4NE'JT
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Figure 4: Stream Cross Section Schematic (for discussion purposes only — not to scale)
i
Floodplain top width
220 ft
Length of proposed fill impacting flood water
50 ft
Depth of water at proposed fill
0.5 ft
Cross section area of displaced water
50 * 0.5 / 2 = 12.5 ft2
New Floodplain top width
220 ft — 50 ft = 170 ft
Potential rise in floodplain due to proposed fill
12.5 ft2 / 170 ft = 0.07 ft = 0.9 inches
Page 3 of 3