consent2.pd

• Board Meeting Date: 7/11/2007
• Description: Flood Protection Section - Designation and Approval of 100-year Floodplain Information
• Board Meetings - Doc Type: Memo

Consent A!!enda Item 2
Floodplain I'esolution number: FPR 07-601
Affected communities: City of Las Animas, Bent County
Name of study to be fOl'mally acted on: "Arkansas River at Las Animas, Hydrology Review", dated
March 2007, by Ayres Associates.
Studied streams: Arkansas River
Technical Issues: This analysis is documented by a letter transmitted by Ayres Associates that
summarizes the findings of a hydrologic analysis for the Arkansas River at Las Animas. Recent flood
events and known problems with channel aggradation have raised some concerns regarding the adequacy
of the U.S. Anny Corps of Engineers levee that protects a portion of Las Animas. The cunentIy used peak
flood flow value is 113,000 cfs. This value is published in the Flood Insurance Study and is based on a
1967 report by the Corps of Engineers. According to the Corps, the levee at Las Animas was initially
designed for a flow of 140,000 cfs with four feet of freeboard. Due to significant channel aggradation and
due to tamarisk, other vegetation, and high sediment loads from upstream, the estimated levee capacity has
dropped to approximately 80,000 cfs. The new hydrology study was commissioned by CWCB to
determine a reasonable and up-to-date 100-year flood flow value for the reach of interest. The
CWCB/Ayres study indicates that a 100-year flow value of 53,800 cfs is more appropriate for this reach.
The significant reduction is due to a longer period of record, more reliable methodology, and better
modeling of the flood control benefits of the Pueblo Reservoir.
Community response: No written request for designation has been received, but a meeting was held on
August 25, 2006, in which this study was discussed and ultimate designation was described and verbally
requested.
Staff findings: CWCB staff has determined that the subject 100-year hydrologic information for the
studied stream reach is in confOlmance with the CWCB's mles and regulations for floodplain designation
and approval. The CWCB staffhas relied on the technical competence of Ayres Associates, who produced
the study, but also performed an independent technical review of the report. CWCB staff therefore
endorses this study as containing the most cunent hydrologic information available and urges tIle affected
community to adopt said study for land use regulation and further floodplain mapping purposes pursuant to
statutory authority.

Staff recommendation fOl' FPR 07-601: Staff recommends that the Board: 1) designate and approve the
100-yeat' hydrologic infOlmation contained in said report within Bent County and the City of Las Animas
and 2) authorize staff to prepare a floodplain resolution to be signed by the Director and transmitted to
Bent County, the City of Las Animas, the u.s. Anny Corps of Engineers, and the u.s. Geological Survey.
This action is recommended in order to meet statutory requirements.

Flood Protection. Water Project Planning and Financing. Stream and Lake Protection

Water Supply Protection. Conservation Planning

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