FLOOD04241

• County: Broomfield
• Community: Broomfield
• Stream Name: Nissen Res., City Park, Gay Res., West Lake Basins
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 6/1/1979
• Prepared For: UDFCD
• Prepared By: Wright-McLaughlin Engineers
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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APPENDIX B

HYDROLOGICAL APPENDIX

TABLE B-1
INCREMENTAL RAINFALL DATA
(All values in inches)
Time I ncrementa I Rainfal I for Various Frequencies
(Minutes) 2-Year 5-Year 10-Year 100-Year
0 0 0 0 0
10 0.05 0.06 0.10 0.16
20 0.08 0.10 0.13 0.18
30 0.16 0.23 0.31 0.48
40 0.44' 0.58 0.77 1. 20
SO 0.15 0.19 0.22 0.3S
60 0.08 0.10 0.13 0.19
70 0.07 0.08 0.06 0.08
80 0.05 0.05 0.05 0.06
90 0.03 0.05 0.04' 0.06
100 0.03 0.03 0.04' O.OS
110 0.03 0.03 0.03 0.05
120 0.02 0.02 0.03 0.04'

RAINFALL
Depth-duration-frequency curves were developed for the basin to represent
the various frequencies of design rainfall, These curves were developed
from Volume III of the NOAA Atlas (Ref, 20) and are shown in Figure B-1.
The time incremented rainfall is derived from these curves, rearranged to
a design rainfall for each of the various frequency events and presented
in Table B-1.

DETERMINATION OF DESIGN FLOWS
The Urban Drainage and Flood Control District established a digital model
of each basin using the Environmental Protection Agency Stormwater Management
Model (SWMM) to calculate peak flows for the 2, 5, 10, and 100-year flood
events. These flows are presented in Figures 111-1 through III-S under
Section III of this report.

SWMM is a digital computer model which calculates storm runoff from an input

rainfall event, The model has the capability of similating the water quality

of storm runoff; however, this capability was not utilized in the course

of this study. The drainage area is modeled by a series of catchment (over-

land flow) elements and stream (gutter, pipe, and channel) elements, The

rainfall event is applied uniformly over these elements. If catchments are

not totally impervious, SWMM calculates the amount of water which infiltrates

into the ground or is captured on the surface by vegetation and small surface

depressions. The remainder of the rainfall (rainfall excess) flows off the

drainage area. This runoff is simulated by the mathematical equations which

descirbe shallow flow (overland) and flow in streams.

The catchments and streams are described by physical parameters which can

be obtained from maps, photos, soil surveys, and field investigations. The

main parameters which describe a watershed area are: drainage area, flow

length, slope infiltration characteristics, surface resistance character-

istics, and channel or pipe sizes,