FLOOD04285

• Designation Number: 270
• County: Adams
• Community: Westminster
• Stream Name: Big Dry Creek & Tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek & Tributaries
• Date: 11/1/1986
• Designation Date: 12/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

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TABLE 6 (cont'd)
SUB-BASIN PARAMETERS FOR BIG DRY CREEK ANALYSIS

Developed Loss Ra tes
Drainage Length Ia Ia
Sub-Basin Area L Lca % Impervious % Impervious * Ct Pervious Impervious Ini tia 1 Final
Flooding Source Designa tion (Sq. Mi.) (Mi) (Mi) Exis ting Developed Prog. Cp Retention Retention in/hr in/hr Decay Rate Comments
S. Hylands Creek 456 1.59 2.38 1.00 39.00 39.00 0.32 0.54 0.3 0.1 3.5 0.53 .0018
457 2.17 3.33 1.76 34.50 40.80 0.37 0.60 0.4 0.1 3.4 0.53 .0018 includes 457
454 0.02 0.38 0.19 5.00 54.50 0.19 0.37 0.4 0.1 3.0 0.50 .0018
Middle Hylands C . 461 0.02 0.24 0.14 5.00 49.80 0.11 0.35 0.4 0.1 3.7 0.55 .0018
460 0.71 1. 76 1.05 35.50 38.50 0.25 0.48 0.4 0.1 3.7 0.55 .0018 includes 461
458 0.07 0.57 0.29 10.80 32.50 0.16 0.29 0.4 0.1 3.7 0.55 .0018
N. Hylands Creek 463 0.08 0.38 0.29 30.30 43.00 0.13 0.38 0.4 0.1 3.7 0.55 .0018
462 0.49 1.57 1.05 2.15 46.20 0.23 0.53 0.4 0.1 3.7 0.55 .0018 includes 463
Walnu t Creek 672 5.00 5.71 2.19 34.10 34.10 0.36 0.56 0.4 0.1 3.1 0.51 .0018
671 0.51 1.14 0.67 12.50 20.50 0.24 0.29 0.4 0.1 3.0 0.51 .0018
670 0.22 1.14 0.62 5.00 33.80 0.30 0.35 0.4 0.1 3.1 0.51 .0018
668 0.72 1.90 1.19 13.90 61.20 0.24 0.67 0.4 0.1 3.1 0.51 .0018
676 0.65 2.19 1.05 9.30 9.30 0.39 0.27 0.4 0.1 3.0 0.50 .0018
675 0.42 2.14 1.14 25.10 49.60 0.26 0.54 0.4 0.1 3.2 0.51 .0018
663 0.19 0.76 0.38 16.80 46.40 0.18 0.46 0.4 0.1 3.6 0.54 .0018
662 0.28 1.48 0.67 14.80 67.50 0.19 0.61 0.4 0.1 3.8 0.56 .0018
. 660 0.27 1.14 0.68 16.80 45.30 0.28 0.48 0.4 0.1 3.7 0.55 .0018
Countryside Cr. 667 0.17 1.14 0.48 34.10 57.90 0.16 0.53 0.4 0.1 3.1 0.51 .0018
666 0.43 0.86 0.57 24.40 48.60 0.19 0.54 0.4 0.1 3.0 0.50 .0018
665 0.33 1.38 0.71 24.40 42.50 0.27 0.50 0.4 0.1 3.1 0.51 .0018
N. Walnut Cr. 674 0.62 1.57 0.48 5.00 5.00 0.31 0.29 0.4 0.1 3.0 0.50 .0018
673 1.14 3.86 1.05 5.80 15.10 0.36 0.30 0.4 0.1 3.1 0.51 .0018 includes 674
Ketner Lake** 567 0.39 0.95 0.57 35.20 54.50 0.18 0.58 0.4 0.1 3.0 0.50 .0018
566 0.22 0.95 0.43 24.00 46.10 0.20 0.47 0.4 0.1 3.1 0.51 .0018
Ranch Creek** 216 0.11 0.43 0.24 38.80 80.00 0.12 0.57 0.4 0.1 3.0 0.05 .0018
215 0.69 1.34 0.67 39.20 60.00 0.20 0.66 0.4 0.1 3.1 0.51 .0018 includes 216
213 0.59 1.04 0.76 24.80 53.60 0.22 0.60 0.3 0.1 3.6 0.54 .0018
S. Ranch Creek** 218 0.42 1.09 0.57 32.90 41.00 0.20 0.47 0.4 0.1 3.4 0.53 .0018
217 0.70 1.52 0.76 26.90 42.50 0.23 0.52 0.4 0.1 3.4 0.53 .0018 includes 218

* Ct adjusted for CUHP-D computer program for application of the 1982 version of the Colorado Urban Hydrograph Procedure

** Floodplain not delineated in this study - hydrology only

UNIT HYDROGRAPH EQUATIONS (CUHP-D

Ct (LLca) 0.18
Ct (program) =
SoO.24

Applica tion)
Tp = 60tp + 0.5 t"
Qp = qpA

in which,
A = drainage area of basin (in sq. mi.)
L = hydraulic length of basin in (miles)
Lca= centroidal length of basin in (miles)
So = average stream slope in (ft/ft)
Ct = unit hydrograph coefficient reflecting
time to peak (Figure 4-2)
peaking parameter related to I
percent impervious of basin a

C =
p

0.15
Cp = PCtA
tp Ct (LLca) 0.3
qp 640 cp/tp

tp=
qp=

unit hydrograph coefficient reflecting
peak discharge
time to peak of unit hydrograph in
(cfs/sq. mi.)
unit peak of unit hydrograph in
(cis/sq. mi.)
time of unit rainfall duration (min)
time to peak of unit hydrograph (min)
peak discharge of unit hydrograph (cfs)

7

p =
I =
a

t =
u
T =
QP=
P