2004-09-30_REVISION - M1987064

• Permit No: M1987064
• IBM Index Class Name: Revision
• Doc Date: 9/30/2004
• Doc Name: 100-year flood info
• From: Frank J. Anesi
• To: DMG
• Type & Sequence: AM1
• Media Type: D
• Archive: No

Southwest Hydro-Logic
265 Ridge Road
Durango, Colorado 81303
Phone: [phone]
Mobile: [phone]
Email:
Web:
September 28, 2004
Frank Anesi
P.O. Box 2185
835 East Second Avenue, Suite 220
Durango, Colorado 81302
[e-mail]
wwwswhydrologic.com
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Subject: Response to the Colorado Division of Minerals and Geology review of the Sandco Pit
application, Mine ID# M-1987-064, regarding the Sandco Reach of the Animas River, and
comparison of the Southwest Hydro-Logic 100-year flood modeling to National Flood
Insurance Program maps.
The floodplain maps from the National Flood Insurance Program were obtained in response to a
review letter from the Division of Minerals and Geology (DMG) dated September 8, 2004. Two
versions of the flood plain maps are available-- (1) the original National Flood Insurance Program maps
(Community Panel Number 080097 0269B, dated December 15, 1981), and (2) Letter of Map Revision
(COMA) maps prepared by the Federal Emergency Management Agency (FEMA), dated September 27,
2002, in response to a request by the La Plata County Commissioners, October 31, 2001. The LOMR
and maps are attached as an appendix and are included on a CD-ROM. The flood hazard areas on the
1981 maps extend across the entire Animas River Valley; these areas were mapped to indicate a
1 percent probability of flood impacts. It is not cleaz how FEMA arrived at these flood elevations (the
contact person for this area has refved). The flood hazard areas on the 2002 LOMR maps show the
original 1981 shaded areas across the Animas River Valley and show updated 100-yeaz flood elevations
as equal-elevation lines across the map.
A request was made to the La Plata County Emergency Response office (Butch Knowlton, oral
common., September 23, 2004) regarding the data that were used for the LOMR maps. Mr. Knowlton
provided access to an engineering report produced by Goff Engineering and Surveying, Inc.that was
generated in 1992 in response to proposed development in the Animas River valley. The 100-year flood
water-surface elevations for a discharge of 24,600 ft3/sec were modeled by Goff using the U.S. Army
Corps of Engineers HEC-2 computer program. The modeled river stations are presented as river miles
above the mouth of the river; however, the locations of the river stations may be currently different than
in 1992 because of development on the west bank of the river during 1994 (Dave Dillon, oral common.,
2004) that filled in and realigned the river (figure I), Cross sections for input to the Goff model were
derived from surveyed cross sections (where available) and elevations derived from the topographic map