FLOOD02650

• County: La Plata
• Stream Name: Animas River
• Basin: San Juan/Dolores
• Title: Hydrology Addendum to FIS La Plata County Unincorporated Areas
• Date: 5/21/2002
• Prepared For: La Plata County
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrolol!:ic Analvsis

Table I lists all streams and stream reaches for which approximate or detailed floodplain
boundary information is contained in the May 21, 2001 AS. Hydrologic analysis for streams
studied by detailed methods in unincorporated La Plata County is based on one of three
approaches:

. Discharge-frequency analysis of stream gage records.
. Discharge-frequency analysis of stream gage records for similar streams.
. Hydrologic modeling using synthetic unit hydrograph methodologies.

Discharge rates for streams studied by detailed methods are presented in the AS for the 10-, 50-,
100- and 500-year events.

Hydrologic analyses and discharge rates for streams studied by approximate methods are not
documented by FEMA in the 2001 AS. As previously discussed, Colorado statutes require that
this information be provided in order for floodplain studies to be "designated" and thus made
legally usable for floodplain regulation. Table I presents the list of flooding sources in
unincorporated La Plata County studied by approximate methods in the 2001 AS. For the
purposes of this HA, 100-year flow rates for streams in La Plata County studied by approximate
methods in the 2001 AS have been determined using one of three information sources:

. CWCB regional regression equations.
. CWCB regional regression equations + dam outlet capacity
. 1999 USACE Floodplain Study for Los Pinos River.

Table 2 presents 100-year discharge rates determined for approximate streams in the 2001 AS
and sources of these computed values. Details of these information sources and their application
to approximate streams in La Plata County is provided in the following paragraphs.

Discharge rates for the Los Pinos River between Vallecito Reservoir and the Town of Bayfield
were derived from a recently completed detailed study by the U.S. Army Corps of Engineers
entitled Flood Plain Delineation Study, Los Pinos River Below Vallecito Dam to the Southern
Ute Indian Reservation Boundary, La Plata County, Colorado (1999). Flows in this study are
based upon the results of HEC-I model developed for the 70 sq mi drainage basin between
Vallecito Reservoir and Bayfield. The modeling assumes a base flow rate from the reservoir of
3,100 cfs representing the maximum release with the conservation pool full. Peak flow rates for
tributaries to the Los Pinos were determined using regional regression equations developed by
the CWCB and documented in Guidelines for Determining JOO-Year Flood Flows for
Approximate Floodplains in Colorado (CWCB, January 1999). Regression equations presented
in this publication compute 100-year peak discharge rates and are based upon data from
floodplain studies previously designated by the CWCB. Regression equations have been
developed for nine major river basins within the state and a number of subbasins. The

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