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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The Albuquerque District investigated discharge-frequency relationships for the stream gages
on the Los Pmos River between Vallecito Reservoir and La Boca. Because the stream gages do not
record natural and unregulated flow, a hydrologic analysis supported completely by frequency-
discharge relationships computed for the gages proved unsatisfactory. Consequently, the
Albuquerque District developed a computer model of the watershed using the Flood Hydrograph
Package Computer Program HEC-I, developed by the Hydraulic Engineering Center (HEC) in
Davis, California, to model the 70 square mile drainage system between Vallecito Reservoir and the
Southern Ute Indian Reservation Boundary near Bayfield, Colorado, to determine the 0.20/0-, 1.0%-,
2.Wo-, and 10.00/0-chance (500-, 100-, 50-, and 10-year) flood peak discharges at selected locations.
The watershed map is presented as Exhibit I, and the HEC-l Summary is in Appendix A.

Unit Hydrograph Computation

The Albuquerque District adopted Snyder's synthetic method to compute unit hydrographs
for the subareas in the watershed model. Snyder's method draws a relationship between measurable
physical characteristics of a drainage basin and the basin's unit hydrograph. The U.S. Army Corps
of Engineers' Engineering Manual EM 1110-2-9021, Flood Runoff Analysis, dated II October 1991,
describes Snyder's unit hydro graph method and provides the following equation to compute the time
to peak (tp):

tp = C, x (L X Lca)O.3
L = Total travel length in miles
Lca = Travel length to the centroid of the basin in miles
Ct = Time-to-peak coefficient

The C,-versus-slope curve generated by the Albuquerque District for Rio Puerco and Rio
Salado basins served as the basis for development of the Ct coefficient and the Cp (peaking)
coefficient. The HEC-I model generates unit hydro graphs based on the tp and Cp values. This Ct-
versus-slope curve has been used successfully for the San Juan River watershed and, therefore, is
considered appropriate for the Los Pinos River watershed.

Channel Routing

The Albuquerque District used the Muskingum-Cunge routing method to model the Los
Pinos River channel in the HEC-I model. The stream routing method employs typical eight-point
cross sections to represent each channel and overbank reach. Based on field investigations, a
roughness coefficient ('n' value) of 0.035 is considered appropriate for the Los Pmos River channel
and a Manning's 'n' value of 0.050 represents the undeveloped overbank areas.

Frequency Rainfall

The Albuquerque District calculated the frequency rainfall based on the National Oceanic
and Atmospheric Administration (NOAA) Atlas II, Volume III (1973), for Colorado. To imitate a
general storm centered over the entire watershed, the point rainfall was reduced for the 70-square-

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