FLOOD00734

• Designation Number: 213
• County: Adams
• Community: Thornton
• Stream Name: Brantner Gulch and Tributaries
• Title: Flood Hazard Area Delineation - Brantner Gulch and Tributaries
• Date: 1/1/1983
• Designation Date: 5/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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8

Table 2 presents the basin parameters and the computed

peak flood flows. The parameters in Table 2 represented each

sub-basin or combination of sUb-basins, as indicated by the

notes. Where two peak flood flows are seemingly tabulated for

one basin, the first is the flow computed for the individual sub-

basin; the second is the peak flow in the channel determined by

an accumulation of the sub-basins contributing flow at the

specific hydrologic point location.

Information was acquired from the City of Thornton and

Adams County Planning Department concerning forecasted land use

within the basin at future dates. The information W?S presumed

to represent "fully developed" land use. Parameters mch as

infiltration rate, depression storage, detention sto 1ge, and

percentage of impervious ground cover were utilized ~0 determine

the amount of "effective rainfall" which would be discharged

from each sub-basin. Additional parameters of tributary area,

channel reach length, and distance to basin centroid were calcu-

lated and used to compute the runoff hydrographs which would

result from effective precipitation on each sub-basin. Para-

meters were determined for describing each basin area as they

accumulated along the channel. The CUHP program and appropriate

input data are all based on Snyder unit hydrograph theory.

Typical hydrographs are illustrated on Figures 1 through 5.

Figure 1 illustrates hydrographs computed along Brantner Gulch

TABLE 1

BRANTNER GULCH ~~D REGIONAL PARK TRIBUTARIES
l20-MINUTE DESIGN STORM

Rainfall Distribution in Accordance with I-Hour
Time NOAA Rainfall
(Hinutes) 2-year 5-year 10-year 50-year 100-year
5 0.019 0.027 0.032 0.030 0.027
10 0.038 0.050 0.059 0.081 0.080
15 0.080 0.117 0.131 0.116 0.122
20 0.152 0.205 0.240 0.185 0.213
25 0.238 0.335 0.400 0.347 0.372
30 0.133 0.174 0.192 0.578 0.665
35 0.060 0.078 0.090 0.277 0.372
40 0.048 0.059 0.069 0.185 0.213
45 0.029 0.048 0.061 0.116 0.165
50 0.029 0.048 0.051 0.092 0.133
55 0.029 0.040 0.051 0.069 0.106
60 0.029 0.040 0.051 0.069 0.106
65 0.029 0.040 0.051 0.069 0.106
70 0.019 0.040 0.051 0.051 0.053
75 0_019 0.029 0.051 0.051 0.053
80 0.019 0.029 0.040 0.042 0.032
85 0.019 0.029 0.030 0.042 0.032
90 0.019 0.029 0.030 0.030 0.032
95 0.019 0.029 0.030 0.030 0.032
100 0.019 0.020 0.030 0.030 0.032
105 0.019 0.020 0.030 0.030 0.032
110 0.019 0.020 0.030 0.030 0.032
115 0.010 0.020 0.030 0.030 0.032
120 0.010 0.017 0.021 0.030 0.032
Total
Rainfall 1.104 1. 543 1. 851 2.610 3.074