FLOOD10395 ‎(2)‎

• County: Montezuma
• Community: Dolores
• Stream Name: Dolores River
• Title: Dolores River near Dolores, Colorado, Flood Insurance Study
• Date: 5/1/2004
• Prepared For: Dolores
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

Dolores Lower (Dolores River from the confluence of the West Dolores to the Dolores River at

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Dolores gage), and Lost Canyon Creek at gage (Table 6).

6.4 Loss Rates. The Initial and Constant Loss Rate method was used in both the HEC-l

and HMS models. Flood optimization HEC-l runs made in 1976 showed that the basin initial

loss could vary from as great as 2.5 inches, for at !least two weeks of antecedent dry conditions, to

as low as 0.7 inch for continuous antecedent stomlS. The optimized range for constant loss was

0.15 to 0.20 inch per hour. These loss rates are basically the same as those from other regional

studies conducted by the Corps, Los Angeles District and the U.S. Bureau of Reclamation

(USBR). In the HMS model an initial loss of 0.8 inch and constant loss of 0.19 inch per hour

were used to reconstitute the observed hydro graph at the Dolores gage and the observed peak

flow at the "below Rico" gage on 6 September 1970. The Dolores basin received between 2 and

4.5 inches of rainfall during August with most occurring two weeks before the September 6

storm. This is slightly above average for the basin (see Table 3) and sets up a loss rate at the lower

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range for initial loss.

6.5 Baseflow. The recession method was used to compute baseflow for both the HEC-l

and HMS models. This method is suitable for watersheds where precipitation events strongly

influence the volume and time of base flow. The parameters are: initial baseflow (initial Q),

recession constant, and threshold baseflow (threshold Q). Initial baseflow is the subbasin flow at

the start of the flood event, in cfs per square mile (cfs/mi2). The initial baseflow of 0.6 for the

1970 flood reproduction is based on a flow around 300 cfs observed at the Dolores gage at the

start of5 September 1970. The recession constant, describing the rate of base flow decay, is the

ratio of present baseflow to baseflow one day earlier. Threshold Q, the point on the recession

limb of the hydro graph where baseflow n::places overland flow as source of flow for the subbasin,

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