FLOOD11632 ‎(2)‎

• County: Costilla
• Community: Costilla County
• Stream Name: Rito Seco
• Basin: Rio Grande
• Title: Approximate Floodplain - Rito Seco
• Date: 1/5/2007
• Prepared For: Costilla County
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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1~ ikure 2- Survey P~-ints
Hydr~io~y
~~e hydrology methodology used for establishing the tlood flows was obtained from thc
report produced by the USGS called the Analytiis of the Magnitude and Freyuency of
Fl~x~ds in Cc~lc~rado, ?(Hx). This report establishes regression cyuations for various floocl
frequenry tlows for different hydrologic areac in Colorada. 'The variables that are
reyuired are drainage area and mean batiin annual precipitation. Thr Rito Seco drainage
basin area is 29.6 syuare miles (Figure 3) and ha~ .~ mean annual precipitatiun ~~f 2O
inches. The flows that were calculated from this methodolc~gy are listed in the Table I.
The tlows were also checked against the CWCB methodology for establishing the 100-
year flow rate. The two methodologies aligned well giving more confidence to the flow
rates used in this study. All hydrolc~gy calculations are included in the appen~ix.
Area - 29.59 sq. mi.
Mean Annual
Precipitation= 20 in
Recurrence Interval (yrs)
2 a= 0.03'(A)^0.979'(P)^1.615 - 104 cts
5 O- 0.12"(A)^0.940'(P)^1.384 = 183 cfs
10 O- 0.25'(A)^0.914'(P)^1.277 = 254 cfs
25 Q- 0.52'(A)^0.884'(P)^1.117 - 295 cfs
50 O- 0.8t'(A)^0.864'(P)^1.121 - 435 Cfs
San Luis, Colorad~i - Rito Seco Approximatc Floodpluin